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REFLEX ACTION

Posted by Prakash
Saturday, February 13, 2010
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Of the many kinds of neural activity, there is one simple kind in which a stimulus leads to an immediate action. This is reflex activity. The word reflex (from Latin reflexus, “reflection”) was introduced into biology by a 19th-century English neurologist, Marshall Hall, who fashioned the word because he thought of the muscles as reflecting a stimulus much as a wall reflects a ball thrown against it. By reflex, Hall meant the automatic response of a muscle or several muscles to a stimulus that excites an afferent nerve. The term is now used to describe an action that is an inborn central nervous system activity, not involving consciousness, in which a particular stimulus, by exciting an afferent nerve, produces a stereotyped, immediate response of muscle or gland.

The anatomical pathway of a reflex is called the reflex arc. It consists of an afferent (or sensory) nerve, usually one or more interneurons within the central nervous system, and an efferent (motor, secretory, or secreto-motor) nerve.

Most reflexes have several synapses in the reflex arc. The stretch reflex is exceptional in that, with no interneuron in the arc, it has only one synapse between the afferent nerve fibre and the motor neuron. The flexor reflex, which removes a limb from a noxious stimulus, has a minimum of two interneurons and three synapses.

Probably the best-known reflex is the pupillary light reflex. If a light is flashed near one eye, the pupils of both eyes contract. Light is the stimulus; impulses reach the brain via the optic nerve; and the response is conveyed to the pupillary musculature by autonomic nerves that supply the eye. Another reflex involving the eye is known as the lacrimal reflex. When something irritates the conjunctiva or cornea of the eye, the lacrimal reflex causes nerve impulses to pass along the fifth cranial nerve (trigeminal) and reach the midbrain. The efferent limb of this reflex arc is autonomic and mainly parasympathetic. These nerve fibres stimulate the lacrimal glands of the orbit, causing the outpouring of tears. Other reflexes of the midbrain and medulla oblongata are the cough and sneeze reflexes. The cough reflex is caused by an irritant in the trachea and the sneeze reflex by one in the nose. In both, the reflex response involves many muscles; this includes a temporary lapse of respiration in order to expel the irritant.

The first reflexes develop in the womb. By seven and a half weeks after conception, the first reflex can be observed; stimulation around the mouth of the fetus causes the lips to be turned toward the stimulus. By birth, sucking and swallowing reflexes are ready for use. Touching the baby's lips induces sucking, and touching the back of its throat induces swallowing.

Although the word stereotyped is used in the above definition, this does not mean that the reflex response is invariable and unchangeable. When a stimulus is repeated regularly, two changes occur in the reflex response—sensitization and habituation. Sensitization is an increase in response; in general, it occurs during the first 10 to 20 responses. Habituation is a decrease in response; it continues until, eventually, the response is extinguished. When the stimulus is irregularly repeated, habituation does not occur or is minimal.

There are also long-term changes in reflexes, which may be seen in experimental spinal cord transections performed on kittens. Repeated stimulation of the skin below the level of the lesion, such as rubbing the same area for 20 minutes every day, causes a change in latency (the interval between the stimulus and the onset of response) of certain reflexes, with diminution and finally extinction of the response. Although this procedure takes several weeks, it shows that, with daily stimulation, one reflex response can be changed into another. Repeated activation of synapses increases their efficiency, causing a lasting change. When this repeated stimulation ceases, synaptic functions regress, and reflex responses return to their original form.

Although a reflex response is said to be rapid and immediate, some reflexes, called recruiting reflexes, can hardly be evoked by a single stimulus. Instead, they require increasing stimulation to induce a response. The reflex contraction of the bladder, for example, requires an increasing amount of urine to stretch the muscle and to obtain muscular contraction.

Reflexes can be altered by impulses from higher levels of the central nervous system. For example, the cough reflex can be suppressed easily, and even the gag reflex (the movements of incipient vomiting resulting from mechanical stimulation of the wall of the pharynx) can be suppressed with training.

The so-called conditioned reflexes are not reflexes at all but complicated acts of learned behaviour. Salivation is one such conditioned reflex; it occurs only when a person is conscious of the presence of food or when one imagines food.

PHYSIOLOGY OF REFLEX
Many reflexes of placental mammals appear to be innate. They are hereditary and are a common feature of the species and often of the genus. Reflexes include not only such simple acts as chewing, swallowing, blinking, the knee jerk, and the scratch reflex, but also stepping, standing, and mating. Built up into complex patterns of many coordinated muscular actions, reflexes form the basis of much instinctive behaviour in animals.

Humans also exhibit a variety of innate reflexes, which are involved with the adjustment of the musculature for optimum performance of the distance receptors (i.e., eye and ear), with the orientation of parts of the body in spatial relation to the head, and with the management of the complicated acts involved in ingesting food. Among the innate reflexes involving just the eyes, for example, are: (1) paired shifting of the eyeballs, often combined with turning of the head, to perceive an object in the field of vision; (2) contraction of the intraocular muscles to adjust the focus of the retina for the viewing of near or far objects; (3) constriction of the pupil to reduce excessive illumination of the retina; and (4) blinking due to intense light or touching of the cornea.

In its simplest form, a reflex is viewed as a function of an idealized mechanism called the reflex arc. The primary components of the reflex arc are the sensory-nerve cells (or receptors) that receive stimulation, in turn connecting to other nerve cells that activate muscle cells (or effectors), which perform the reflex action. In most cases, however, the basic physiological mechanism behind a reflex is more complicated than the reflex arc theory would suggest. Additional nerve cells capable of communicating with other parts of the body (beyond the receptor and effector) are present in reflex circuits. As a result of the integrative action of the nervous system in higher organisms, behaviour is more than the simple sum of their reflexes; it is a unitary whole that exhibits coordination between many individual reflexes and is characterized by flexibility and adaptability to circumstances. Many automatic, unconditioned reflexes can thus be modified by or adapted to new stimuli, making possible the conditioning of reflex responses. The experiments of the Russian physiologist Ivan Petrovich Pavlov, for example, showed that if an animal salivates at the sight of food while another stimulus, such as the sound of a bell, occurs simultaneously, the sound alone can induce salivation after several trials. The animal's behaviour is no longer limited by fixed, inherited reflex arcs but can be modified by experience and exposure to an unlimited number of stimuli.

REFLEX IS AN INSTINCT
A variety of what may be called simple instinctive behaviour has long been known as reflex action. When this term was introduced, it meant the simple and almost invariable response of a simple organ system (e.g., a single muscle) to a simple stimulus, such as a touch or a flash of light. In its most elementary versions, this activity has been seen as the function of an idealized mechanism that has been called the reflex arc. The primary components of the reflex arc have been identified as the sensory-nerve cell (or receptor) that receives the stimulation, in turn connecting (hence the term arc) to another nerve cell that activates the muscle cell (or effector).

Although such a reflex arc might be the simplest imaginable mechanism for inflexibly automatic behaviour, it is, as noted above, a theoretical minimum rather than an actually observed functional arrangement of cells in the body of the animal; nevertheless, a mechanism but little more complicated than this helps to account for the locomotion of such animals as millipedes. In some insects, for example, the stepping movement of one limb or muscle provides stimuli that set off another limb or muscle on a similar course of movement, providing a kind of feedback system or chain of reflex arc activities. In most cases of this sort, however, the basic physiological mechanism is more complicated than the simple arc theory would suggest. Additional nerve cells capable of communicating with other parts of the body (beyond the receptor and effector) are invariably present in reflex circuits. Such connections are what make possible the conditioning of reflex responses.

Among higher animals, and perhaps many others (such as insects), what once were thought to be chain reflexes are not systems simply linked or chained together; they are systems under the precise control of coordinated complexes of nerve cells in particular parts of the nervous system, such as the spinal cord and brain. Even without evidence of a chain of feedback (or reafferent) stimuli, performance may be smoothly integrated. This is well illustrated by the complex movements of swallowing in mammals; in the dog, for example, 11 separate muscles or muscular systems are found to discharge one after the other, precisely timed to a matter of milliseconds, and all under the control of the central nervous system (CNS: brain and spinal cord). Such complexes of precisely controlled movement, known as fixed action patterns (FAP's), are thought to form the hard core of the inborn movement forms of instincts. When such sequences of uniform stereotyped responses seem to constitute an end point or goal-directed climax of some sort, they are known as consummatory acts.

AUTONOMIC NERVOUS SYSTEM

Posted by Prakash
Tuesday, February 9, 2010
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The autonomic nervous system is the part of the peripheral nervous system that regulates the basic visceral processes needed for the maintenance of normal bodily functions. It operates independently of voluntary control, although certain events, such as stress, fear, sexual excitement, and alterations in the sleep-wake cycle, change the level of autonomic activity.

The autonomic system usually is defined as a motor system that innervates three major types of tissue: cardiac muscle, smooth muscle, and glands. However, it also relays visceral sensory information to the central nervous system and processes it so that alterations can be made in the activity of specific autonomic motor outflows, such as those that control the heart, blood vessels, and other visceral organs. It also stimulates the release of certain hormones involved in energy metabolism (e.g., insulin, glucagon, and epinephrine [also called adrenaline]) or cardiovascular functions (e.g., renin and vasopressin). These integrated responses maintain the normal internal environment of the body in an equilibrium state called homeostasis.

The autonomic system consists of two major divisions: the sympathetic nervous system and the parasympathetic nervous system. These often function in antagonistic ways. The motor outflow of both systems is formed by two serially connected sets of neurons. The first set, called preganglionic neurons, originates in the brainstem or the spinal cord, and the second set, called ganglion cells or postganglionic neurons, lies outside the central nervous system in collections of nerve cells called autonomic ganglia. Parasympathetic ganglia tend to lie close to or within the organs or tissues that their neurons innervate, whereas sympathetic ganglia are located at more distant sites from their target organs. Both systems have associated sensory fibres that send feedback into the central nervous system regarding the functional condition of target tissues.

A third division of the autonomic system, the enteric nervous system, consists of a collection of neurons embedded within the wall of the gastrointestinal tract and its derivatives. This system controls gastrointestinal motility and secretion.

SYMPATHETIC NERVOUS SYSTEM

The sympathetic nervous system normally functions to produce localized adjustments (such as sweating as a response to an increase in temperature) and reflex adjustments of the cardiovascular system. Under conditions of stress, however, the entire sympathetic nervous system is activated, producing an immediate, widespread response called the fight-or-flight response. This response is characterized by the release of large quantities of epinephrine from the adrenal gland, an increase in heart rate, an increase in cardiac output, skeletal muscle vasodilation, cutaneous and gastrointestinal vasoconstriction, pupillary dilation, bronchial dilation, and piloerection. The overall effect is to prepare the individual for imminent danger.

Sympathetic preganglionic neurons originate in the lateral horns of the 12 thoracic and the first 2 or 3 lumbar segments of the spinal cord. (For this reason the sympathetic system is sometimes referred to as the thoracolumbar outflow.) The axons of these neurons exit the spinal cord in the ventral roots and then synapse on either sympathetic ganglion cells or specialized cells in the adrenal gland called chromaffin cells.

Sympathetic ganglia
Sympathetic ganglia can be divided into two major groups, paravertebral and prevertebral (or preaortic), on the basis of their location within the body. Paravertebral ganglia generally are located on each side of the vertebrae and are connected to form the sympathetic chain, or trunk. There are usually 21 or 22 pairs of these ganglia—3 in the cervical region, 10 or 11 in the thoracic region, 4 in the lumbar region, and 4 in the sacral region—and a single unpaired ganglion lying in front of the coccyx, called the ganglion impar. The three cervical sympathetic ganglia are the superior cervical ganglion, the middle cervical ganglion, and the cervicothoracic ganglion (also called the stellate ganglion). The superior ganglion innervates viscera of the head, and the middle and stellate ganglia innervate viscera of the neck, thorax (i.e., the bronchi and heart), and upper limbs. The thoracic sympathetic ganglia innervate the trunk region, and the lumbar and sacral sympathetic ganglia innervate the pelvic floor and lower limbs. All the paravertebral ganglia provide sympathetic innervation to blood vessels in muscle and skin, arrector pili muscles attached to hairs, and sweat glands.

The three preaortic ganglia are the celiac, superior mesenteric, and inferior mesenteric. Lying on the anterior surface of the aorta, preaortic ganglia provide axons that are distributed with the three major gastrointestinal arteries arising from the aorta. Thus, the celiac ganglion innervates the stomach, liver, pancreas, and the duodenum, the first part of the small intestine; the superior mesenteric ganglion innervates the small intestine; and the inferior mesenteric ganglion innervates the descending colon, sigmoid colon, rectum, urinary bladder, and sexual organs.

Neurotransmitter and receptors
Upon reaching their target organs by traveling with the blood vessels that supply them, sympathetic fibres terminate as a series of swellings close to the end organ. Because of this anatomical arrangement, autonomic transmission takes place across a junction rather than a synapse. “Presynaptic” sites can be identified because they contain aggregations of synaptic vesicles and membrane thickenings; postjunctional membranes, on the other hand, rarely possess morphological specializations, but they do contain specific receptors for various neurotransmitters. The distance between pre- and postsynaptic elements can be quite large compared with typical synapses. For instance, the gap between cell membranes of a typical chemical synapse is 30–50 nanometres, while in blood vessels the distance is often greater than 100 nanometres or, in some cases, 1–2 micrometres (1,000–2,000 nanometres). Owing to these relatively large gaps between autonomic nerve terminals and their effector cells, neurotransmitters tend to act slowly; they become inactivated rather slowly as well. To compensate for this inefficiency, many effector cells, such as those in smooth and cardiac muscle, are connected by low-resistance pathways that allow for electrotonic coupling of the cells. In this way, if only one cell is activated, multiple cells will respond and work as a group.

At a first approximation, chemical transmission in the sympathetic system appears simple: preganglionic neurons use acetylcholine as a neurotransmitter, whereas most postganglionic neurons utilize norepinephrine (noradrenaline)—with the major exception that postganglionic neurons innervating sweat glands use acetylcholine. On closer inspection, however, neurotransmission is seen to be more complex, because multiple chemicals are released, and each functions as a specific chemical code affecting different receptors on the target cell. In addition, these chemical codes are self-regulatory, in that they act on presynaptic receptors located on their own axon terminals.

The chemical codes are specific to certain tissues. For example, most sympathetic neurons that innervate blood vessels secrete both norepinephrine and neuropeptide Y; sympathetic neurons that innervate the submucosal neural plexus of the gut contain both norepinephrine and somatostatin; and sympathetic neurons that innervate sweat glands contain calcitonin gene-related peptide, vasoactive intestinal polypeptide, and acetylcholine. In addition, other chemicals besides the neuropeptides mentioned above are released from autonomic neurons along with the so-called classical neurotransmitters, norepinephrine and acetylcholine. For instance, some neurons synthesize a gas, nitric oxide, that functions as a neuronal messenger molecule. Thus, neural transmission in the autonomic nervous system involves the release of combinations of different neuroactive agents that affect both pre- and postsynaptic receptors.

Neurotransmitters released from nerve terminals bind to specific receptors, which are specialized macromolecules embedded in the cell membrane. The binding action initiates a series of specific biochemical reactions in the target cell that produce a physiological response. In the sympathetic nervous system, for example, there are five types of adrenergic receptors (receptors binding epinephrine): α1, α2, β1, β2, and β3. These adrenoceptors are found in different combinations in various cells throughout the body. Activation of α1- adrenoceptors in arterioles causes blood-vessel constriction, whereas stimulation of α2 autoreceptors (receptors located in sympathetic presynaptic nerve endings) functions to inhibit the release of norepinephrine. Other types of tissue have unique adrenoceptors. Heart rate and myocardial contractility, for example, are controlled by β1-adrenoceptors; bronchial smooth muscle relaxation is mediated by β2-adrenoceptors; and the breakdown of fat (lipolysis) is controlled by β3-adrenoceptors.

Cholinergic receptors (receptors binding acetylcholine) also are found in the sympathetic system (as well as the parasympathetic system). Nicotinic cholinergic receptors stimulate sympathetic postganglionic neurons, adrenal chromaffin cells, and parasympathetic postganglionic neurons to release their chemicals. Muscarinic receptors are associated mainly with parasympathetic functions and are located in peripheral tissues (e.g., glands and smooth muscle). Peptidergic receptors exist in target cells as well.

The length of time that each type of chemical acts on its target cell is variable. As a rule, peptides cause slowly developing, long-lasting effects (one or more minutes), whereas the classical transmitters produce short-term effects (about 25 milliseconds).

PARASYMPATHETIC NERVOUS SYSTEM

The parasympathetic nervous system primarily modulates visceral organs such as glands. Responses are never activated en masse as in the fight-or-flight sympathetic response. While providing important control of many tissues, the parasympathetic system, unlike the sympathetic system, is not crucial for the maintenance of life.

The parasympathetic nervous system is organized in a manner similar to the sympathetic nervous system. Its motor component consists of preganglionic and postganglionic neurons. The preganglionic neurons are located in specific cell groups (also called nuclei) in the brainstem or in the lateral horns of the spinal cord at sacral levels (segments S2–S4). (Because parasympathetic fibres exit from these two sites, the system is sometimes referred to as the craniosacral outflow.) Preganglionic axons emerging from the brainstem project to parasympathetic ganglia that are located in the head (ciliary, pterygopalatine [also called sphenopalatine], and otic ganglia) or near the heart (cardiac ganglia), embedded in the end organ itself (e.g., the trachea, bronchi, and gastrointestinal tract), or situated a short distance from the urinary bladder (pelvic ganglion). Both pre- and postganglionic neurons secrete acetylcholine as a neurotransmitter, but, like sympathetic ganglion cells, they also contain other neuroactive chemical agents that function as cotransmitters.

The third cranial nerve (oculomotor nerve) contains parasympathetic nerve fibres that regulate the iris and lens of the eye. From their origin in the Edinger-Westphal nucleus of the midbrain, preganglionic axons travel to the orbit and synapse on the ciliary ganglion. The ciliary ganglion contains two types of postganglionic neurons: one innervates smooth muscle of the iris and is responsible for pupillary constriction, and the other innervates ciliary muscle and controls the curvature of the lens.

Various secretory glands located in the head are under parasympathetic control. These include the lacrimal gland, which supplies tears to the cornea of the eye; salivary glands (sublingual, submandibular, and parotid glands), which produce saliva; and nasal mucous glands, which secrete mucus throughout the nasal air passages. The parasympathetic preganglionic neurons that regulate these functions originate in the reticular formation of the medulla oblongata. One group of parasympathetic preganglionic neurons belongs to the superior salivatory nucleus and lies in the rostral part of the medullary reticular formation. These neurons send axons out of the medulla in a separate branch of the seventh cranial nerve (facial nerve) called the intermediate nerve. Some of the axons innervate the pterygopalatine ganglion, and others project to the submandibular ganglion. Pterygopalatine ganglion cells innervate the vasculature of the brain and eye as well as the lacrimal gland, nasal glands, and palatine glands, while neurons of the submandibular ganglion innervate the submandibular and sublingual salivary glands. A second group of parasympathetic preganglionic neurons belongs to the inferior salivatory nucleus, located in the caudal part of the medullary reticular formation. Neurons of this group send axons out of the medulla in the ninth cranial (glossopharyngeal) nerve and to the otic ganglion. From this site, postganglionic fibres travel to and innervate the parotid salivary gland.

Preganglionic parasympathetic fibres of the 10th cranial (vagus) nerve arise from two different sites in the medulla oblongata. Neurons that slow heart rate arise from a part of the ventral medulla called the nucleus ambiguus, while those that control functions of the gastrointestinal tract arise from the dorsal vagal nucleus. After exiting the medulla in the vagus nerve and traveling to their respective organs, the fibres synapse on ganglion cells embedded in the organs themselves. The vagus nerve also contains visceral afferent fibres that carry sensory information from organs of the neck (larynx, pharynx, and trachea), chest (heart and lungs), and gastrointestinal tract into a visceral sensory nucleus located in the medulla called the solitary tract nucleus.

Enteric nervous system
The enteric nervous system is composed of two plexuses, or networks of neurons, embedded in the wall of the gastrointestinal tract. The outermost plexus, located between the inner circular and outer longitudinal smooth-muscle layers of the gut, is called the Auerbach, or myenteric, plexus. Neurons of this plexus regulate peristaltic waves that move digestive products from the oral to the anal opening. In addition, myenteric neurons control local muscular contractions that are responsible for stationary mixing and churning. The innermost group of neurons is called the Meissner, or submucosal, plexus. This plexus regulates the configuration of the luminal surface, controls glandular secretions, alters electrolyte and water transport, and regulates local blood flow.

Three functional classes of intrinsic enteric neurons are recognized: sensory neurons, interneurons, and motor neurons. Sensory neurons, activated by either mechanical or chemical stimulation of the innermost surface of the gut, transmit information to interneurons located within the Auerbach and the Meissner plexi, and the interneurons relay the information to motor neurons. Motor neurons in turn modulate the activity of a variety of target cells, including mucous glands, smooth muscle cells, endocrine cells, epithelial cells, and blood vessels.

Extrinsic neural pathways also are involved in the control of gastrointestinal functions. Three types exist: intestinofugal, sensory, and motor. Intestinofugal neurons reside in the gut wall; their axons travel to the preaortic sympathetic ganglia and control reflex arcs that involve large portions of the gastrointestinal tract. Sensory neurons relay information regarding distention and acidity to the central nervous system. There are two types of sensory neurons: sympathetic neurons, which originate from dorsal-root ganglia found at the thoracic and lumbar levels; and parasympathetic neurons, which originate in the nodose ganglion of the vagus nerve or in dorsal-root ganglia at sacral levels S2–S4. The former innervate the gastrointestinal tract from the pharynx to the left colic flexure, and the latter innervate the distal colon and rectum. Each portion of the gastrointestinal tract receives a dual sensory innervation: pain sensations travel via sympathetic afferent fibres, and sensations that signal information regarding the chemical environment of the gut travel by way of parasympathetic fibres and are not consciously perceived.

The third extrinsic pathway, exercising motor control over the gut, arises from parasympathetic preganglionic neurons found in the dorsal vagal nucleus of the medulla oblongata and from sympathetic preganglionic neurons in the lateral horns of the spinal cord. These pathways provide modulatory commands to the intrinsic enteric motor system and are nonessential in that basic functions can be maintained in their absence.

Through the pathways described above, the parasympathetic system activates digestive processes while the sympathetic system inhibits them. The sympathetic system inhibits digestive processes by two mechanisms: (1) contraction of circular smooth muscle sphincters located in the distal portion of the stomach (pyloric sphincter), small intestine (ileo-cecal sphincter), and rectum (internal anal sphincter), which act as valves to prevent the oral-to-anal passage (as well as reverse passage) of digestive products; and (2) inhibition of motor neurons throughout the length of the gut. In contrast, the parasympathetic system provides messages only to myenteric motor neurons.

PERIPHERAL NERVOUS SYSTEM

Posted by Prakash
Friday, February 5, 2010
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The peripheral nervous system is a channel for the relay of sensory and motor impulses between the central nervous system on one hand and the body surface, skeletal muscles, and internal organs on the other hand. It is composed of (1) spinal nerves, (2) cranial nerves, and (3) certain parts of the autonomic nervous system. As in the central nervous system, peripheral nervous pathways are made up of neurons (that is, nerve cell bodies and their axons and dendrites) and synapses, the points at which one neuron communicates with the next. The structures commonly known as nerves (or by such names as roots, rami, trunks, and branches) are composed of orderly arrangements of the axonal and dendritic processes of many nerve cell bodies.

The cell bodies of peripheral neurons are often found grouped into clusters called ganglia. On the basis of the type of nerve cell bodies found in ganglia, they may be classified as either sensory or motor. Sensory ganglia are oval swellings located on the dorsal roots of spinal nerves and on the roots of certain cranial nerves. The sensory neurons making up these ganglia are unipolar. Shaped much like a golf ball on a tee, they have round or slightly oval cell bodies with concentrically located nuclei, and they give rise to a single fibre that undergoes a T-shaped bifurcation, one branch going to the periphery and the other entering the brain or spinal cord. There are no synaptic contacts between neurons in a sensory ganglion.

Motor ganglia are associated with neurons of the autonomic nervous system, the part of the nervous system that controls and regulates the internal organs. Many motor ganglia are located in the sympathetic trunks, two long chains of ganglia stretching along each side of the vertebral column from the base of the skull to the coccyx; these are referred to as paravertebral ganglia. Prevertebral motor ganglia are located near internal organs innervated by their projecting fibres, while terminal ganglia are found on the surfaces or within the walls of the target organs themselves. Motor ganglia have multipolar cell bodies, which have irregular shapes and eccentrically located nuclei and which project several dendritic and axonal processes. Preganglionic fibres originating from the brain or spinal cord enter motor ganglia, where they synapse on multipolar cell bodies. These postganglionic cells, in turn, send their processes to visceral structures.

SPINAL NERVE
Sensory input from the body surface, from joint, tendon, and muscle receptors, and from internal organs passes centrally through the dorsal roots of the spinal cord. Fibres from motor cells in the spinal cord exit via the ventral roots and course to their peripheral targets (autonomic ganglia or skeletal muscle). Each spinal nerve is formed by the joining of a dorsal root and a ventral root, and it is the basic structural and functional unit of the peripheral nervous system.

STRUCTURAL COMPONENTS OF THE SPINAL NERVE
There are 31 pairs of spinal nerves; in descending order from the most rostral end of the spinal cord, there are 8 cervical (designated C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), and 1 coccygeal (Coc1). Each spinal nerve exits the vertebral canal through an opening called the intervertebral foramen. The first spinal nerve (C1) exits the vertebral canal between the skull and the first cervical vertebra; consequently, spinal nerves C1–C7 exit above the correspondingly numbered vertebrae. Spinal nerve C8, however, exits between the 7th cervical and first thoracic vertebrae, so that, beginning with T1, all other spinal nerves exit below their corresponding vertebrae.

Just outside the intervertebral foramen, two branches, known as the gray and white rami communicantes, connect each spinal nerve with the sympathetic trunk. These rami, along with the sympathetic trunk and more distal ganglia, are involved with the innervation of visceral structures. In addition, small meningeal branches leave each spinal nerve and gray ramus and reenter the vertebral canal, where they innervate the dura mater (the outermost of the meninges) and blood vessels.More peripherally, each spinal nerve divides into ventral and dorsal rami. All dorsal rami (with the exception of those from C1, S4, S5, and Coc1) have medial and lateral branches, which innervate deep back muscles and overlying skin. The medial and lateral branches of the dorsal rami of spinal nerves C2–C8 supply both the muscles and the skin of the neck. Those of T1–T6 are mostly cutaneous (that is, supplying only the skin), while those from T7–T12 are mainly muscular. Dorsal rami from L1–L3 have both sensory and motor fibres, while those from L4–L5 are only muscular. Dorsal rami of S1–S3 may also be divided into medial and lateral branches, serving deep muscles of the lower back as well as cutaneous areas of the lower buttocks and perianal area. Undivided dorsal rami from S4, S5, and Coc1 also send cutaneous branches to the gluteal and perianal regions.

Ventral rami of the spinal nerves carry sensory and motor fibres for the innervation of the muscles, joints, and skin of the lateral and ventral body walls and the extremities. Both dorsal and ventral rami also contain autonomic fibres.

FUNCTIONAL TYPES OF SPINAL NERVES
Because spinal nerves contain both sensory fibres (from the dorsal roots) and motor fibres (from the ventral roots), they are known as mixed nerves. When individual fibres of a spinal nerve are identified by their specific function, they may be categorized as one of four types: (1) general somatic afferent, (2) general visceral afferent, (3) general somatic efferent, and (4) general visceral efferent. The term somatic refers to the body wall (broadly defined to include skeletal muscles as well as the surface of the skin), and visceral refers to structures composed of smooth muscle, cardiac muscle, glandular epithelium, or a combination of these. Efferent fibres carry motor information to skeletal muscle and to autonomic ganglia (and then to visceral structures), and afferent fibres carry sensory information from them.

General somatic afferent receptors are sensitive to pain, thermal sensation, touch and pressure, and changes in the position of the body. (Pain and temperature sensation coming from the surface of the body is called exteroceptive, while sensory information arising from tendons, muscles, or joint capsules is called proprioceptive.) General visceral afferent receptors are found in organs of the thorax, abdomen, and pelvis; their fibres convey, for example, pain information from the digestive tract. Both types of afferent fibre project centrally from cell bodies in dorsal-root ganglia.
General somatic efferent fibres originate from large ventral-horn cells and distribute to skeletal muscles in the body wall and in the extremities. General visceral efferent fibres also arise from cell bodies located within the spinal cord, but they exit only at thoracic and upper lumbar levels or at sacral levels (more specifically, at levels T1–L2 and S2–S4). Fibres from T1–L2 enter the sympathetic trunk, where they either form synaptic contacts within a ganglion, ascend or descend within the trunk, or exit the trunk and proceed to ganglia situated closer to their target organs. Fibres from S2–S4, on the other hand, leave the cord as the pelvic nerve and proceed to terminal ganglia located in the target organs. Postganglionic fibres arising from ganglia in the sympathetic trunk rejoin the spinal nerves and distribute to blood vessels, sweat glands, and the arrector pili muscles of the skin, while postganglionic fibres arising from prevertebral and terminal ganglia innervate viscera of the thorax, abdomen, and pelvis.

CERVICAL PLEXUS
Cervical levels C1–C4 are the main contributors to the group of nerves called the cervical plexus; in addition, small branches of the plexus link C1 and C2 with the vagus nerve, C1 and C2 with the hypoglossal nerve, and C2–C4 with the accessory nerve. Sensory branches of the cervical plexus are the lesser occipital nerve (to the scalp behind the ear), the great auricular nerve (to the ear and to the skin over the mastoid and parotid areas), transverse cervical cutaneous nerves (to the lateral and ventral neck surfaces), and supraclavicular nerves (along the clavicle, shoulder, and upper chest). Motor branches of the plexus serve muscles that stabilize and flex the neck, muscles that stabilize the hyoid bone (to assist in actions like swallowing), and muscles that elevate the upper ribs.
Originating from C4, with small contributions from C3 and C5, are the phrenic nerves, which carry sensory information from parts of the pleura of the lungs and pericardium of the heart as well as motor impulses to muscles of the diaphragm.

BRACHIAL PLEXUS
Cervical levels C5–C8 and thoracic level T1 contribute to the formation of the brachial plexus; small nerve bundles also arrive from C4 and T2. Spinal nerves from these levels converge to form superior (C5 and C6), middle (C7), and inferior (C8 and T1) trunks, which in turn split into anterior and posterior divisions. The divisions then form cords (posterior, lateral, and medial), which provide motor, sensory, and autonomic fibres to the shoulder and upper extremity.
Nerves to shoulder and pectoral muscles include the dorsal scapular (to the rhomboid muscles), suprascapular (to supraspinatus and infraspinatus), medial and lateral pectoral (to pectoralis minor and major), long thoracic (to serratus anterior), thoracodorsal (to latissimus dorsi), and subscapular (to teres major and subscapular). The axillary nerve carries motor fibres to the deltoid and teres minor muscles as well as sensory fibres to the lateral surface of the shoulder and upper arm. The biceps, brachialis, and coracobrachialis muscles, as well as the lateral surface of the forearm, are served by the musculocutaneous nerve.
The three major nerves of the arm, forearm, and hand are the radial, median, and ulnar. The radial nerve innervates the triceps, anconeus, and brachioradialis muscles, eight extensors of the wrist and digits, and one abductor of the hand; it is also sensory to part of the hand. The median nerve branches in the forearm to serve the palmaris longus, two pronator muscles, four flexor muscles, thenar muscles, and lumbrical muscles; most of these serve the wrist and hand. The ulnar nerve serves two flexor muscles and a variety of small muscles of the wrist and hand.
Cutaneous innervation of the upper extremity originates, via the brachial plexus, from spinal cord levels C3–T2. The shoulder is served by supraclavicular branches (C3, C4) of the cervical plexus, while the anterior and lateral aspects of the arm and forearm have sensory innervation via the axillary (C5, C6) nerve as well as the dorsal (C5, C6), lateral (C5, C6), and medial (C8, T1) antebrachial cutaneous nerves. These same nerves have branches that wrap around to serve portions of the posterior and medial surfaces of the extremity. The palm of the hand is served by the median (C6–C8) and ulnar (C8, T1) nerves. The ulnar nerve also wraps around to serve medial areas of the dorsum, or back, of the hand. An imaginary line drawn down the midline of the ring finger represents the junction of the ulnar-radial distribution on the back of the hand and the ulnar-median distribution on the palm. A small part of the thumb and the distal thirds of the index, middle, and lateral surface of the ring finger are served by the median nerve. The inner arm and the armpit is served by the intercostobrachial and the posterior and medial brachial cutaneous nerves (T1–T2).

LUMBAR PLEXUS
Spinal nerves from lumbar levels L1–L4 contribute to the formation of the lumbar plexus, which, along with the sacral plexus, provides motor, sensory, and autonomic fibres to gluteal and inguinal regions and to the lower extremities. Lumbar roots are organized into dorsal and ventral divisions.
Minor cutaneous and muscular branches of the lumbar plexus include the iliohypogastric, genitofemoral, and ilioinguinal (projecting to the lower abdomen and to inguinal and genital regions) and the lateral femoral cutaneous nerve (to skin on the lateral thigh). Two major branches of the lumbar plexus are the obturator and femoral nerves. The obturator enters the thigh through the obturator foramen; motor branches proceed to the obturator internus and gracilis muscles as well as the adductor muscles, while sensory branches supply the articular capsule of the knee joint. An accessory obturator nerve supplies the pectineus muscle of the thigh and is sensory to the hip joint.

The sartorius muscle and medial and anterior surfaces of the thigh are served by branches of the anterior division of the femoral nerve. The posterior division of the femoral nerve provides sensory fibres to the inner surface of the leg (saphenous nerve), to the quadriceps muscles (muscular branches), to the hip and knee joints, and to the articularis genu muscle.

SACRAL PLEXUS
The ventral rami of L5 and S1–S3 form the sacral plexus, with contributions from L4 and S4. Branches from this plexus innervate gluteal muscles, muscles forming the internal surface of the pelvic basin (including those forming the levator ani), and muscles that run between the femur and pelvis to stabilize the hip joint (such as the obturator, piriformis, and quadratus femoris muscles). These muscles lend their names to the nerves that innervate them. Cutaneous branches from the plexus serve the buttocks, perineum, and posterior surface of the thigh.

The major nerve of the sacral plexus, and the largest nerve in the body, is the sciatic. Formed by the joining of ventral and dorsal divisions of the plexus, it passes through the greater sciatic foramen and descends in back of the thigh. There, sciatic branches innervate the biceps femoris, semitendinosus and semimembranosus muscles, and part of the adductor magnus muscle. In the popliteal fossa (just above the knee), the sciatic nerve divides into the tibial nerve and the common fibular (or peroneal) nerve. The tibial nerve (from the dorsal division) continues distally in the calf and innervates the gastrocnemius muscle, deep leg muscles such as the popliteus, soleus, and tibialis posterior, and the flexor muscles, lumbrical muscles, and other muscles of the ankle and plantar aspects of the foot. The peroneal nerve, from the ventral division, travels to the anterior surface of the leg and innervates the tibialis anterior, the fibularis muscles, and extensor muscles that elevate the foot and fan the toes. Cutaneous branches from the tibial and common fibular nerves serve the outer sides of the leg and the top and bottom of the foot and toes.

COCCYGEAL PLEXUS
The ventral rami of S4, S5, and Coc1 form the coccygeal plexus, from which small anococcygeal nerves arise to innervate the skin over the coccyx (tailbone) and around the anus.

CRANIAL NERVES

Cranial nerves can be thought of as modified spinal nerves, since the general functional fibre types found in spinal nerves are also found in cranial nerves but are supplemented by special afferent or efferent fibres. Fibres conveying olfaction (in cranial nerve I) and taste (in cranial nerves VII, IX, and X) are classified as special visceral afferent, while the designation of special somatic afferent is applied to fibres conveying vision (cranial nerve II) and equilibrium and hearing (cranial nerve VIII). Skeletal muscles that arise from the branchial arches are innervated by fibres of cranial nerves V, VII, IX, and X; these are classified as special visceral efferent fibres.

The 12 pairs of cranial nerves are identified either by name or by Roman or Arabic numeral.
Olfactory nerve (CN I or 1): Bipolar cells in the nasal mucosa give rise to axons that enter the cranial cavity through foramina in the cribriform plate of the ethmoid bone. These cells and their axons, totaling about 20 to 24 in number, make up the olfactory nerve. Once in the cranial cavity, the fibres terminate in a small oval structure resting on the cribriform plate called the olfactory bulb. As stated above, the functional component of olfactory fibres is special visceral afferent. Injury or disease of the olfactory nerve may result in anosmia, an inability to detect odours; it may also dull the sense of taste.
Optic nerve (CN II or 2): Rods and cones in the retina of the eye receive information from the visual fields and, through intermediary cells, convey this input to retinal ganglion cells. Ganglion cell axons converge at the optic disc, pass through the sclera, and form the optic nerve. A branch from each eye enters the skull via the optic foramen, and they join to form the optic chiasm. At the chiasm, fibres from the nasal halves of each retina cross, while those from the temporal halves remain uncrossed. In this way the optic tracts, which extend from the chiasm to the thalamus, contain fibres conveying information from both eyes. Injury to one optic nerve therefore results in total blindness of that eye, while damage to the optic tract on one side results in partial blindness in both eyes.

Optic fibres also participate in accommodation of the lens and in the pupillary light reflex. Since the subarachnoid space around the brain is continuous with that around the optic nerve, increases in intracranial pressure can result in papilledema, or damage to the optic nerve, as it exits the bulb of the eye.
Oculomotor nerve (CN III or 3): The oculomotor nerve arises from two nuclei in the rostral midbrain. These are (1) the oculomotor nucleus, the source of general somatic efferent fibres to superior, medial, and inferior recti muscles, to the inferior oblique muscle, and to the levator palpebrae superious muscle, and (2) the Edinger-Westphal nucleus, which projects general visceral efferent preganglionic fibres to the ciliary ganglion.

The oculomotor nerve exits the ventral midbrain, pierces the dura mater, courses through the lateral wall of the cavernous sinus, and exits the cranial cavity via the superior orbital fissure. Within the orbit it branches into a superior ramus (to the superior rectus and levator muscles) and an inferior ramus (to the medial and inferior rectus muscles, the inferior oblique muscles, and the ciliary ganglion). Postganglionic fibres from the ciliary ganglion innervate the sphincter pupillae muscle of the iris as well as the ciliary muscle.

Oculomotor neurons project primarily to orbital muscles on the same side of the head. A lesion of the oculomotor nerve will result in paralysis of the three rectus muscles and the inferior oblique muscle (causing the eye to rotate downward and slightly outward), paralysis of the levator palpebrae superious muscle (drooping of the eyelids), and paralysis of the sphincter pupillae and ciliary muscles (so that the iris will remain dilated and the lens will not accommodate).
Trochlear nerve (CN IV or 4): The fourth cranial nerve is unique for three reasons. First, it is the only cranial nerve to exit the dorsal side of the brainstem. Second, fibres from the trochlear nucleus cross in the midbrain before they exit, so that trochlear neurons innervate the contralateral (opposite side) superior oblique muscle of the eye. Third, trochlear fibres have a long intracranial course before piercing the dura mater.

The trochlear nucleus is located in the caudal midbrain; the functional component of these cells is general somatic efferent. After exiting at the dorsal side of the midbrain, the trochlear nerve loops around the midbrain, pierces the dura mater, and passes through the lateral wall of the cavernous sinus. It then enters the orbit through the superior orbital fissure and innervates only the superior oblique muscle, which rotates the eye downward and slightly outward. Damage to the trochlear nerve will result in a loss of this eye movement and may produce double vision (diplopia).
Trigeminal nerve (CN V or 5): The trigeminal nerve is the largest of the cranial nerves. It has both motor and sensory components, the sensory fibres being general somatic afferent and the motor fibres being special visceral efferent. Most of the cell bodies of sensory fibres are located in the trigeminal ganglion, which is attached to the pons by the trigeminal root. These fibres convey pain and thermal sensations from the face, oral and nasal cavities, and parts of the dura mater and nasal sinuses, sensations of deep pressure, and information from sensory endings in muscles. Trigeminal motor fibres, projecting from nuclei in the pons, serve the muscles of mastication (chewing). Lesions of the trigeminal nerve result in sensory losses over the face or in the oral cavity. Damage to the motor fibres results in paralysis of the masticatory muscles; as a result, the jaw may hang open or deviate toward the injured side when opened. Trigeminal neuralgia, or tic douloureux, is an intense pain originating mainly from areas supplied by sensory fibres of the maxillary and mandibular branches of this nerve.

The trigeminal ganglion gives rise to three large nerves: the ophthalmic, maxillary, and mandibular.
1.Ophthalmic nerve: The ophthalmic nerve passes through the wall of the cavernous sinus and enters the orbit via the superior orbital fissure. Branches in the orbit are (1) the lacrimal nerve, serving the lacrimal gland, part of the upper eyelid, and the conjunctiva, (2) the nasociliary nerve, serving the mucosal lining of part of the nasal cavity, the tentorium cerebelli and some of the dura mater of the anterior cranial fossa, and skin on the dorsum and tip of the nose, and (3) the frontal nerve, serving the skin on the upper eyelid, the forehead, and the scalp above the eyes up to the v ertex of the head.
2.Maxillary nerve: The maxillary nerve courses through the cavernous sinus below the ophthalmic nerve and passes through the foramen rotundum into the orbital cavity. Branches of the maxillary nerve are (1) the meningeal branches, which serve the dura mater of the middle cranial fossa, (2) the alveolar nerves, serving the upper teeth and gingiva and the lining of the maxillary sinus, (3) the nasal and palatine nerves, which serve portions of the nasal cavity and the mucosa of the hard and soft palate, and (4) the infraorbital, zygomaticotemporal, and zygomaticofacial nerves, serving the upper lip, the lateral surfaces of the nose, the lower eyelid and conjunctiva, and the skin on the cheek and the side of the head behind the eye.
3.Mandibular nerve: The mandibular nerve exits the cranial cavity via the foramen ovale and serves (1) the meninges and parts of the anterior cranial fossae (meningeal branches), (2) the temporomandibular joint, skin over part of the ear, and skin over the sides of the head above the ears (auriculotemporal nerve), (3) oral mucosa, the anterior two-thirds of the tongue, gingiva adjacent to the tongue, and the floor of the mouth (lingual nerve), and (4) the mandibular teeth (inferior alveolar nerve). Skin over the lateral and anterior surfaces of the mandible and the lower lip is served by cutaneous branches of the mandibular nerve.

Trigeminal motor fibres exit the cranial cavity via the foramen ovale along with the mandibular nerve. They serve the muscles of mastication (temporalis, masseter, medial and lateral pterygoid), three muscles involved in swallowing (anterior portions of the digastric muscle, the mylohyoid muscle, and the tensor veli palatini), and the tensor tympani, a muscle that has a damping effect on loud noises by stabilizing the tympanic membrane.
Abducens nerve (CN VI or 6): From its nucleus in the caudal pons, the abducens nerve exits the brainstem at the pons-medulla junction, pierces the dura mater, passes through the cavernous sinus close to the internal carotid artery, and exits the cranial vault via the superior orbital fissure. In the orbit the abducens nerve innervates the lateral rectus muscle, which turns the eye outward. Damage to the abducens nerve results in a tendency for the eye to deviate medially, or cross. Double vision may result on attempted lateral gaze. The nerve often is affected by increased intracranial pressure.
Facial nerve (CN VII or 7): The facial nerve is composed of a large root that innervates facial muscles and a small root (known as the intermediate nerve) that contains sensory and autonomic fibres.

From the facial nucleus in the pons, facial motor fibres enter the internal auditory meatus, pass through the temporal bone, exit the skull via the stylomastoid foramen, and fan out over each side of the face in front of the ear. Fibres of the facial nerve are special visceral efferent; they innervate the small muscles of the external ear, the superficial muscles of the face, neck, and scalp, and the muscles of facial expression.

The intermediate nerve contains autonomic (parasympathetic) as well as general and special sensory fibres. Preganglionic autonomic fibres, classified as general visceral efferent, project from the superior salivatory nucleus in the pons. Exiting with the facial nerve, they pass to the pterygopalatine ganglion via the greater petrosal nerve (a branch of the facial nerve) and to the submandibular ganglion by way of the chorda tympani nerve (another branch of the facial nerve, which joins the lingual branch of the mandibular nerve). Postganglionic fibres from the pterygopalatine ganglion innervate the nasal and palatine glands and the lacrimal gland, while those from the submandibular ganglion serve the submandibular and sublingual salivary glands. Among the sensory components of the intermediate nerve, general somatic afferent fibres relay sensation from the caudal surface of the ear, while special visceral afferent fibres originate from taste buds in the anterior two-thirds of the tongue, course in the lingual branch of the mandibular nerve, and then join the facial nerve via the chorda tympani branch. Both somatic and visceral afferent fibres have cell bodies in the geniculate ganglion, which is located on the facial nerve as it passes through the facial canal in the temporal bone.

Injury to the facial nerve at the brainstem produces a paralysis of facial muscles known as Bell palsy as well as a loss of taste sensation from the anterior two-thirds of the tongue. If damage occurs at the stylomastoid foramen, facial muscles will be paralyzed but taste will be intact.
Vestibulocochlear nerve or Auditory nerve (CN VIII or 8): This cranial nerve has a vestibular part, which functions in balance, equilibrium, and orientation in three-dimensional space, and a cochlear part, which functions in hearing. The functional component of these fibres is special somatic afferent; they originate from receptors located in the temporal bone.

Vestibular receptors are located in the semicircular canals of the ear, which provide input on rotatory movements (angular acceleration), and in the utricle and saccule, which generate information on linear acceleration and the influence of gravitational pull. This information is relayed by the vestibular fibres, whose bipolar cell bodies are located in the vestibular (Scarpa) ganglion. The central processes of these neurons exit the temporal bone via the internal acoustic meatus and enter the brainstem alongside the facial nerve.

Auditory receptors of the cochlear division are located in the organ of Corti and follow the spiral shape (about 2.5 turns) of the cochlea. Air movement against the eardrum initiates action of the ossicles of the ear, which, in turn, causes movement of fluid in the spiral cochlea. This fluid movement is converted by the organ of Corti into nerve impulses that are interpreted as auditory information. The bipolar cells of the spiral, or Corti, ganglion branch into central processes that course with the vestibular nerve. At the brainstem, cochlear fibres separate from vestibular fibres to end in the dorsal and ventral cochlear nuclei.

Lesions of the vestibular root result in eye movement disorders (nystagmus), unsteady gait with a tendency to fall toward the side of the lesion, nausea, and vertigo. Damage to the cochlea or cochlear nerve results in complete deafness, ringing in the ear (tinnitus), or both.
Glossopharyngeal nerve (CN IX or 9): The ninth cranial nerve, which exits the skull through the jugular foramen, has both motor and sensory components. Cell bodies of motor neurons, located in the nucleus ambiguus in the medulla oblongata, project as special visceral efferent fibres to the stylopharyngeal muscle. The action of the stylopharyngeus is to elevate the pharynx, as in gagging or swallowing. In addition, the inferior salivatory nucleus of the medulla sends general visceral efferent fibres to the otic ganglion via the lesser petrosal branch of the ninth nerve; postganglionic otic fibres innervate the parotid salivary gland.

Among the sensory components of the glossopharyngeal nerve, special visceral afferent fibres convey taste sensation from the back third of the tongue via lingual branches of the nerve. General visceral afferent fibres from the pharynx, the back of the tongue, parts of the soft palate and eustachian tube, and the carotid body and carotid sinus have their cell bodies in the superior and inferior ganglia, which are situated, respectively, within the jugular foramen and just outside the cranium. Sensory fibres in the carotid branch detect increased blood pressure in the carotid sinus and send impulses into the medulla that ultimately reduce heart rate and arterial pressure; this is known as the carotid sinus reflex.
Vagus nerve (CN X or 10): The vagus nerve has the most extensive distribution in the body of all the cranial nerves, innervating structures as diverse as the external surface of the eardrum and internal abdominal organs. The root of the nerve exits the cranial cavity via the jugular foramen. Within the foramen is the superior ganglion, containing cell bodies of general somatic afferent fibres, and just external to the foramen is the inferior ganglion, containing visceral afferent cells.

Pain and temperature sensations from the eardrum and external auditory canal and pain fibres from the dura mater of the posterior cranial fossa are conveyed on general somatic afferent fibres in the auricular and meningeal branches of the nerve. Taste buds on the root of the tongue and on the epiglottis contribute special visceral afferent fibres to the superior laryngeal branch. General visceral afferent fibres conveying sensation from the lower pharynx, larynx, trachea, esophagus, and organs of the thorax and abdomen to the left (splenic) flexure of the colon converge to form the posterior (right) and anterior (left) vagal nerves. Right and left vagal nerves are joined in the thorax by cardiac, pulmonary, and esophageal branches. In addition, general visceral afferent fibres from the larynx below the vocal folds join the vagus via the recurrent laryngeal nerves, while comparable input from the upper larynx and pharynx is relayed by the superior laryngeal nerves and by pharyngeal branches of the vagus. A vagal branch to the carotid body usually arises from the inferior ganglion.

Motor fibres of the vagus nerve include special visceral efferent fibres arising from the nucleus ambiguus of the medulla oblongata and innervating pharyngeal constrictor muscles and palatine muscles via pharyngeal branches of the vagus as well as the superior laryngeal nerve. All laryngeal musculature (excluding the cricothyroid but including the muscles of the vocal folds) are innervated by fibres arising in the nucleus ambiguus. Cells of the dorsal motor nucleus in the medulla distribute general visceral efferent fibres to plexuses or ganglia serving the pharynx, larynx, esophagus, and lungs. In addition, cardiac branches arise from plexuses in the lower neck and upper thorax, and, once in the abdomen, the vagus gives rise to gastric, celiac, hepatic, renal, intestinal, and splenic branches or plexuses.

Damage to one vagus nerve results in hoarseness and difficulty in swallowing or speaking. Injury to both nerves results in increased heart rate, paralysis of pharyngeal and laryngeal musculature, atonia of the esophagus and intestinal musculature, vomiting, and loss of visceral reflexes. Such a lesion is usually life-threatening, as paralysis of laryngeal muscles may result in asphyxiation.
Accessory nerve (CN XI or 11): The accessory nerve is formed by fibres from the medulla oblongata (known as the cranial root) and by fibres from cervical levels C1–C4 (known as the spinal root). The cranial root originates from the nucleus ambiguus and exits the medulla below the vagus nerve. Its fibres join the vagus and distribute to some muscles of the pharynx and larynx via pharyngeal and recurrent laryngeal branches of that nerve. For this reason, the cranial part of the accessory nerve is, for all practical purposes, part of the vagus nerve.

Fibres that arise from spinal levels exit the cord, coalesce and ascend as the spinal root of the accessory nerve, enter the cranial cavity through the foramen magnum, and then immediately leave through the jugular foramen. The accessory nerve then branches into the sternocleidomastoid muscle, which tilts the head toward one shoulder with an upward rotation of the face to the opposite side, and the trapezius muscle, which stabilizes and shrugs the shoulder.
Hypoglossal nerve (CN XII or 12): The hypoglossal nerve innervates certain muscles that control movement of the tongue. From the hypoglossal nucleus in the medulla oblongata, general somatic efferent fibres exit the cranial cavity through the hypoglossal canal and enter the neck in close proximity to the accessory and vagus nerves and the internal carotid artery. The nerve then loops down and forward into the floor of the mouth and branches into the tongue musculature from underneath. Hypoglossal fibres end in intrinsic tongue muscles, which modify the shape of the tongue (as in rolling the edges), as well as in extrinsic muscles that are responsible for changing its position in the mouth.

A lesion of the hypoglossal nerve on the same side of the head results in paralysis of the intrinsic and extrinsic musculature on the same side. The tongue atrophies and, on attempted protrusion, deviates toward the side of the lesion.

ANATOMY:CNS-SPINAL CORD

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The spinal cord is an elongated cylindrical structure, about 45 cm (18 inches) long, that extends from the medulla oblongata to a level between the first and second lumbar vertebrae of the backbone. The terminal part of the spinal cord is called the conus medullaris. The spinal cord is composed of long tracts of myelinated nerve fibres (known as white matter) arranged around the periphery of a symmetrical butterfly-shaped cellular matrix of gray matter. The gray matter contains cell bodies, unmyelinated motor neuron fibres, and interneurons connecting either the two sides of the cord or the dorsal and ventral ganglia. Many interneurons have short axons distributed locally, but some have axons that extend for several spinal segments. Some interneurons may modulate or change the character of signals, while others play key roles in transmission and in patterned reflexes. The gray matter forms three pairs of horns throughout most of the spinal cord: (1) the dorsal horns, composed of sensory neurons, (2) the lateral horns, well defined in thoracic segments and composed of visceral neurons, and (3) the ventral horns, composed of motor neurons. The white matter forming the ascending and descending spinal tracts is grouped in three paired funiculi, or sectors: the dorsal or posterior funiculi, lying between the dorsal horns; the lateral funiculi, lying on each side of the spinal cord between the dorsal-root entry zones and the emergence of the ventral nerve roots; and the ventral funiculi, lying between the ventral median sulcus and each ventral-root zone.

Associated with local regions of the spinal cord and imposing on it an external segmentation are 31 pairs of spinal nerves, each of which receives and furnishes one dorsal and one ventral root. On this basis the spinal cord is divided into the following segments: 8 cervical (C), 12 thoracic (T), 5 lumbar (L), 5 sacral (S), and 1 coccygeal (Coc). Spinal nerve roots emerge via intervertebral foramina; lumbar and sacral spinal roots, descending for some distance within the subarachnoid space before reaching the appropriate foramina, produce a group of nerve roots at the conus medullaris known as the cauda equina. Two enlargements of the spinal cord are evident: (1) a cervical enlargement (C5 through T1), which provides innervation for the upper extremities, and (2) a lumbosacral enlargement (L1 through S2), which innervates the lower extremities. (The spinal nerves and the area that they innervate are described in the section The peripheral nervous system: Spinal nerves.)

CELLULAR LAMINAE
The gray matter of the spinal cord is composed of nine distinct cellular layers, or laminae, traditionally indicated by Roman numerals. Laminae I to V, forming the dorsal horns, receive sensory input. Lamina VII forms the intermediate zone at the base of all horns. Lamina IX is composed of clusters of large alpha motor neurons, which innervate striated muscle, and small gamma motor neurons, which innervate contractile elements of the muscle spindle. Axons of both alpha and gamma motor neurons emerge via the ventral roots. Laminae VII and VIII have variable configurations, and lamina VI is present only in the cervical and lumbosacral enlargements. In addition, cells surrounding the central canal of the spinal cord form an area often referred to as lamina X.

All primary sensory neurons that enter the spinal cord originate in ganglia that are located in openings in the vertebral column called the intervertebral foramina. Peripheral processes of the nerve cells in these ganglia convey sensation from various receptors, and central processes of the same cells enter the spinal cord as bundles of nerve filaments. Fibres conveying specific forms of sensation follow separate pathways. Impulses involved with pain and noxious stimuli largely end in laminae I and II, while impulses associated with tactile sense end in lamina IV or on processes of cells in that lamina. Signals from stretch receptors (i.e., muscle spindles and tendon organs) end in parts of laminae V, VI, and VII; collaterals of these fibres associated with the stretch reflex project into lamina IX.

Virtually all parts of the spinal gray matter contain interneurons, which connect various cell groups. Many interneurons have short axons distributed locally, but some have axons that extend for several spinal segments. Some interneurons may modulate or change the character of signals, while others play key roles in transmission and in patterned reflexes.

ASCENDING SPINAL TRACT
Sensory tracts ascending in the white matter of the spinal cord arise either from cells of spinal ganglia or from intrinsic neurons within the gray matter that receive primary sensory input.
Dorsal column: The largest ascending tracts, the fasciculi gracilis and cuneatus, arise from spinal ganglion cells and ascend in the dorsal funiculus to the medulla oblongata. The fasciculus gracilis receives fibres from ganglia below thoracic 6, while spinal ganglia from higher segments of the spinal cord project fibres into the fasciculus cuneatus. The fasciculi terminate upon the nuclei gracilis and cuneatus, large nuclear masses in the medulla. Cells of these nuclei give rise to fibres that cross completely and form the medial lemniscus; the medial lemniscus in turn projects to the ventrobasal nuclear complex of the thalamus. By this pathway, the medial lemniscal system conveys signals associated with tactile, pressure, and kinesthetic (or positional) sense to sensory areas of the cerebral cortex.

Spinothalamic tracts: Fibres concerned with pain, thermal sense, and light touch enter the lateral-root entry zone and then ascend or descend near the periphery of the spinal cord before entering superficial laminae of the dorsal horn—largely parts of laminae I, IV, and V. Cells in these laminae then give rise to fibres of the two spinothalamic tracts. Those fibres crossing in the ventral white commissure (ventral to the central canal) form the lateral spinothalamic tract, which, ascending in the ventral part of the lateral funiculus, conveys signals related to pain and thermal sense. The anterior spinothalamic tract arises from fibres that cross the midline in the same fashion but ascend more anteriorly in the spinal cord; these fibres convey impulses related to light touch. At medullary levels the two spinothalamic tracts merge and cannot be distinguished as separate entities. Many of the fibres, or collaterals, of the spinothalamic tracts terminate upon cell groups in the reticular formation, while the principal tracts convey sensory impulses to relay nuclei in the thalamus.
Spinocerebellar tracts: Impulses from stretch receptors are carried by fibres that synapse upon cells in deep laminae of the dorsal horn or in lamina VII. The posterior spinocerebellar tract arises from the dorsal nucleus of Clarke and ascends peripherally in the dorsal part of the lateral funiculus. The anterior spinocerebellar tract ascends on the ventral margin of the lateral funiculus. Both tracts transmit signals to portions of the anterior lobe of the cerebellum and are involved in mechanisms that automatically regulate muscle tone without reaching consciousness.
DESCENDING SPINAL TRACTS
Tracts descending to the spinal cord are involved with voluntary motor function, muscle tone, reflexes and equilibrium, visceral innervation, and modulation of ascending sensory signals. The largest, the corticospinal tract, originates in broad regions of the cerebral cortex. Smaller descending tracts, which include the rubrospinal tract, the vestibulospinal tract, and the reticulospinal tract, originate in nuclei in the midbrain, pons, and medulla oblongata. Most of these brainstem nuclei themselves receive input from the cerebral cortex, the cerebellar cortex, deep nuclei of the cerebellum, or some combination of these.

In addition, autonomic tracts, which descend from various nuclei in the brainstem to preganglionic sympathetic and parasympathetic neurons in the spinal cord, constitute a vital link between the centres that regulate visceral functions and the nerve cells that actually effect changes.
Corticospinal tract: The corticospinal tract originates from pyramid-shaped cells in the premotor, primary motor, and primary sensory cortex and is involved in skilled voluntary activity. Containing about one million fibres, it forms a significant part of the posterior limb of the internal capsule and is a major constituent of the crus cerebri in the midbrain. As the fibres emerge from the pons, they form compact bundles on the ventral surface of the medulla, known as the medullary pyramids. In the lower medulla about 90 percent of the fibres of the corticospinal tract decussate and descend in the dorsolateral funiculus of the spinal cord. Of the fibres that do not cross in the medulla, approximately 8 percent cross in cervical spinal segments. As the tract descends, fibres and collaterals branch off at all segmental levels, synapsing upon interneurons in lamina VII and upon motor neurons in lamina IX. Approximately 50 percent of the corticospinal fibres terminate within cervical segments.

At birth, few of the fibres of the corticospinal tract are myelinated; myelination takes place during the first year after birth, along with the acquisition of motor skills. Because the tract passes through, or close to, nearly every major division of the neuraxis, it is vulnerable to vascular and other kinds of lesions. A relatively small lesion in the posterior limb of the internal capsule, for example, may result in contralateral hemiparesis, which is characterized by weakness, spasticity, greatly increased deep tendon reflexes, and certain abnormal reflexes.
Rubrospinal tract: The rubrospinal tract arises from cells in the caudal part of the red nucleus, an encapsulated cell group in the midbrain tegmentum. Fibres of this tract decussate at midbrain levels, descend in the lateral funiculus of the spinal cord (overlapping ventral parts of the corticospinal tract), enter the spinal gray matter, and terminate on interneurons in lamina VII. Through these crossed rubrospinal projections, the red nucleus exerts a facilitating influence on flexor alpha motor neurons and a reciprocal inhibiting influence on extensor alpha motor neurons. Because cells of the red nucleus receive input from the motor cortex (via corticorubral projections) and from globose and emboliform nuclei of the cerebellum (via the superior cerebellar peduncle), the rubrospinal tract effectively brings flexor muscle tone under the control of these two regions of the brain.
Vestibulospinal tract: The vestibulospinal tract originates from cells of the lateral vestibular nucleus, which lies in the floor of the fourth ventricle. Fibres of this tract descend the length of the spinal cord in the ventral and lateral funiculi without crossing, enter laminae VIII and IX of the anterior horn, and terminate upon both alpha and gamma motor neurons, which innervate ordinary muscle fibres and fibres of the muscle spindle (see below Functions of the human nervous system: Movement). Cells of the lateral vestibular nucleus receive facilitating impulses from labyrinthine receptors in the utricle of the inner ear and from fastigial nuclei in the cerebellum. In addition, inhibitory influences upon these cells are conveyed by direct projections from Purkinje cells in the anterior lobe of the cerebellum. Thus, the vestibulospinal tract mediates the influences of the vestibular end organ and the cerebellum upon extensor muscle tone.

A smaller number of vestibular projections, originating from the medial and inferior vestibular nuclei, descend ipsilaterally in the medial longitudinal fasciculus only to cervical levels. These fibres exert excitatory and inhibitory effects upon cervical motor neurons.
Reticulospinal tract: The reticulospinal tracts arise from relatively large but restricted regions of the reticular formation of the pons and medulla oblongata—the same cells that project ascending processes to intralaminar thalamic nuclei and are important in the maintenance of alertness and the conscious state. The pontine reticulospinal tract arises from groups of cells in the pontine reticular formation, descends ipsilaterally as the largest component of the medial longitudinal fasciculus, and terminates among cells in laminae VII and VIII. Fibres of this tract exert facilitating influences upon voluntary movements, muscle tone, and a variety of spinal reflexes. The medullary reticulospinal tract, originating from reticular neurons on both sides of the median raphe, descends in the ventral part of the lateral funiculus and terminates at all spinal levels upon cells in laminae VII and IX. The medullary reticulospinal tract inhibits the same motor activities that are facilitated by the pontine reticulospinal tract. Both tracts receive input from regions of the motor cortex.
Autonomic tracts: Descending fibres involved with visceral and autonomic activities emanate from groups of cells at various levels of the brainstem. For example, hypothalamic nuclei project to visceral nuclei in both the medulla oblongata and the spinal cord; in the spinal cord these projections terminate upon cells of the intermediolateral cell column in thoracic, lumbar, and sacral segments. Preganglionic parasympathetic neurons originating in the oculomotor nuclear complex in the midbrain project not only to the ciliary ganglion but also directly to spinal levels. Some of these fibres reach lumbar segments of the spinal cord, most of them terminating in parts of laminae I and V. Pigmented cells in the isthmus, an area of the rostral pons, form a blackish-blue region known as the locus ceruleus; these cells distribute the neurotransmitter norepinephrine to the brain and spinal cord. Fibres from the locus ceruleus descend to spinal levels without crossing and are distributed to terminals in the anterior horn, the intermediate zone, and the dorsal horn. Other noradrenergic cell groups in the pons, near the motor nucleus of the facial nerve, project uncrossed noradrenergic fibres that terminate in the intermediolateral cell column (that is, lamina VII of the lateral horn). Postganglionic sympathetic neurons associated with this system have direct effects upon the cardiovascular system. Cells in the nucleus of the solitary tract project crossed fibres to the phrenic nerve nucleus (in cervical segments three through five), the intermediate zone, and the anterior horn at thoracic levels; these innervate respiratory muscles.

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