The human nervous system differs from that of other mammals chiefly in the great enlargement and elaboration of the cerebral hemispheres. Much of what is known of the function of the brain is derived from observations of the effects of disease or by analogy with the results of experimentation on animals, particularly the monkey. Such sources of information are clearly inadequate for the elucidation of the nervous activity underlying many properties of the human brain—particularly speech and mental processes. It is not surprising, therefore, that knowledge of the functions of this uniquely complex system, although rapidly expanding, is far from complete.
Receptors convertthe various kinds of energy they receive from the external and internal environments into electrical impulses. They are of many kinds and are classified in many ways. Steady-state receptors, for example, generate impulses as long as a particular state such as temperature remains constant. Changing-state receptors, on the other hand, respond to variation in the intensity or position of the stimulus. Receptors are also classified as exteroceptive (reporting the external environment), interoceptive (sampling the environment of the body itself), and proprioceptive (sensing the posture and movements of the body). Exteroceptors are used for looking, listening, smelling, tasting, touching, and feeling. Interoceptors report the state of the bladder, the alimentary canal, the blood pressure, and the osmotic pressure of the blood plasma. Proprioceptors report the position and movements of parts of the body and the position of the body in space.
Receptors are also classified according to the kinds of stimulus to which they are sensitive. Chemical receptors, or chemoreceptors, are sensitive to substances taken into the mouth (taste or gustatory receptors), inhaled through the nose (smell or olfactory receptors), or found in the body itself (detectors of glucose or of acid–base balance in the blood). Receptors of the skin are classified as thermoreceptors, mechanoreceptors, and nociceptors—the last being sensitive to stimulation that is noxious, or likely to damage the tissues of the body.
Thermoreceptors of the skin are of two kinds, warmth and cold. Warmth fibers are excited by rising temperature and inhibited by falling temperature, and cold fibers respond in the opposite manner.
Mechanoreceptors in the skin are also of several different types. Sensory nerve terminals around the base of hairs are activated by very slight movement of the hair, but they rapidly adapt to continued stimulation and stop firing. In hairless skin there are both rapidly and slowly adapting receptors. These can provide information about the force of mechanical stimulation. The Pacinian corpuscles, elaborate structures found in the skin of the fingers but also in other organs, are layers of fluid-filled membranes forming structures just visible to the naked eye at the terminals of axons. The precise function of the corpuscles is not fully known, but they are probably activated by rapidly changing or alternating stimuli such as vibration.
In some places receptors are massed together to form a sense organ, such as the eye or ear. At other places they are scattered, as are those of the skin and viscera. Receptors are connected to the central nervous system by afferent nerve fibers. The region or area in the periphery from which a neuron within the central nervous system receives input is called its receptive field. Receptive fields are changing and not fixed entities.
All receptors report two features of stimulation, its intensity and its site. Intensity is signaled by the frequency of nerve impulse discharge in a neuron and also by the number of afferent nerves reporting the stimulation. As the strength of a stimulus increases, the change in electrical potential of the receptor increases, and the frequency of nerve impulse generation likewise increases.
The location of a stimulus, whether in the environment or in the body, is readily resolved by the nervous system. Localization of stimuli in the environment depends to a great extent on having pairs of receptors, one on each side of the body. For example, people in early childhood learn that a loud sound is probably coming from a nearer source than a weak sound. They localize the sound by noticing the difference in intensity and the minimal difference in time of arrival at the ears, increasing these differences by turning the head.
Localization of a stimulus on the skin depends upon the arrangement of nerve fibers in the skin and in the deep tissues beneath the skin, as well as upon the overlap of receptive fields. Most mechanical stimuli indent the skin, stimulating nerve fibers in the connective tissue below the skin. Any point on the skin is supplied by at least three, and sometimes up to 40, nerve fibers, and no two points are supplied by precisely the same pattern of fibers.
Finer localization is achieved by what is called surround inhibition. In the retina, for example, there is an inhibitory area around the excited area. This mechanism accentuates the excited area, making it stand out. There can also be a mechanism called surround excitation, in which a central area is inhibited and the surrounding area excited. In both cases contrast is brought out and discrimination sharpened.
In seeking information about the environment, the nervous system presents the most sensitive receptors to a stimulating object. At its simplest, this action is reflex. In the retina is a small region about the size of a pinhead, called the fovea, which is particularly sensitive to colour. When a part of the periphery of the visual field is excited, a reflex movement of the head and eyes focuses the light rays upon that part on the fovea. A similar reflex turns the head and eyes in the direction of a noise. As the English physiologist Charles Sherrington said in 1900, “In the limbs and mobile parts, when a spot of less discriminative sensitivity is touched, instinct moves the member, so that it brings to the object the part where its own sensitivity is delicate. . . .”
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 was introduced into biology by a 19th-century neurologist, Marshall Hall, who fashioned the word from Latin reflexus (“reflection”) 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 word is now used to mean a certain kind of inborn central nervous activity, not involving consciousness, in which a particular stimulus, by exciting an afferent nerve, produces a stereotyped, immediate response of muscle or gland. If the same activity involves consciousness, it is not categorized as a reflex.
The anatomical pathway used in 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. (For detailed description of the structures making up the reflex arc, see the anatomy of the central nervous system.)
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 fiber and the motor neuron (see below Movement: The regulation of muscular contraction). 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 ocular reflex is the lacrimal reflex. When something irritates the conjunctiva or cornea of the eye, nerve impulses pass along the trigeminal nerve and reach the midbrain. The efferent limb of this reflex arc is autonomic and mainly parasympathetic. These nerve fibers 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 set off 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 diversion of respiration from its usual biochemically ordered function 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 some kind of lasting change. When this repeated stimulation is stopped, the reflex responses return to their original form, for there is a regression of synaptic function with disuse.
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 the 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 changed 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 the person is conscious of the presence of food or when he imagines food. 
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