The Human Body

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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.

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.

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.


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