Functions of the Nervous System

Movement

Every movement of the body has to be correct for force, speed, and position. These aspects of movement are continuously reported to the central nervous system by receptors sensitive to the position, posture, equilibrium, and internal conditions of the body. These receptors are called proprioceptors, and those proprioceptors that keep a continuous report on the position of the limbs are the muscle spindles and tendon organs. (Proprioceptors reporting acceleration and direction of movement are located in the vestibular apparatus of the inner ear. For detailed discussion, see below The vestibular system.)

Forty-five percent of the weight of an adult male consists of muscle. The muscles of the trunk and limbs are known as skeletal muscles; they are attached to bones and produce movement at the joints. Skeletal muscles are innervated by efferent motor nerves and sometimes by efferent sympathetic and parasympathetic nerves. Muscles of the viscera (alimentary canal and bladder, for example) are called smooth muscle; they are innervated by efferent sympathetic and parasympathetic nerves.

The movements of the body are brought about by the harmonious contraction and relaxation of selected muscles. Contraction occurs when nerve impulses are transmitted across the neuromuscular junctions to the membrane covering each muscle fiber. Most of the muscles of the body are not continually contracting but are kept in a state ready to contract. The slightest movement or even the intention to move results in widespread activity of the muscles of the trunk and limbs.

Movements may be intrinsic to the body itself and carried out by muscles of the trunk and body cavity. Examples are those involved in breathing, swallowing, laughing, sneezing, urinating, and defecating. Other movements relate the body to the environment, either for moving or for signaling to other individuals. These are carried out by skeletal muscle.

Movements can be organized at several levels of the nervous system. At the lowest level are movements of the viscera, some of which do not involve the central nervous system, being controlled by neurons of the autonomic nervous system within the viscera themselves.

At the next level is the spinal cord. If the spinal cord is cut across so that no nerve impulses reach it from the brain, certain movements of the trunk and limbs below the level of the injury can still occur. This state is called paraplegia; if the damage to the cord is above the upper limbs, it is called quadriplegia. Immediately after the cord has been divided, there is a state called spinal shock, when no movements of the skeletal muscles can be induced. Weeks or months later the period of shock passes and movements return. They are not commanded by the brain, however, and they are greatly augmented—that is, uncontrolled reflex movements may continue for minutes or hours, one reflex following another.

At a higher level, respiratory movements are controlled by the lower brain stem. The upper brain stem controls muscles of the eye, the bladder, and basic movements of walking and running. At the next level is the hypothalamus. It commands certain totalities of movement, such as those of vomiting, urinating and defecating, and curling up and going to sleep.

At the highest level is gray matter of the cerebral hemispheres, both the cortex and the subcortical basal ganglia. This is the level of conscious control of movements.

The natural phenomena included in movement cannot be neatly classified. The categories given above do not include some essential contributions, such as the influence of the cerebellum, occurring at every level.

The regulation of muscular contraction

Only a minority of the nerve fibers supplying a muscle are the ordinary motor fibers that actually make it contract. The rest are either afferent sensory fibers telling the central nervous system what the muscle is doing, or they are specialized motor fibers regulating the behaviour of the sensory nerve endings. If the constant feedback of proprioceptive information from the muscles, tendons, and joints is cut off, movements can still occur, but they cannot be adjusted in the face of external disturbances or readily modified to suit changing conditions; nor can new motor skills be developed. As stated above, the sensory receptors chiefly concerned with body movement are the muscle spindles and the tendon organs. The muscle spindle is vastly more complicated than the tendon organ, so that although it has been much more intensively studied, it is less well understood.

The muscle spindle

The familiar knee jerk, tested routinely by physicians, is a spinal reflex in which a brief, rapid stretch excites muscle spindle afferent neurons, which then excite the motor neurons of the stretched muscle via a single synapse in the spinal cord. In this simplest of reflexes, which is not transmitted through interneurons of the spinal cord, virtually all the delay (approximately 0.02 second) occurs in the conduction of impulses to and from the spinal cord.

Information provided by muscle spindles is also put to elaborate use by the cerebellum and the cerebral cortex in ways that continue to elude detailed analysis. One example is kinesthesia, or the subjective sensory awareness of the position of limbs in space. It might be supposed (as it long was) that sensory receptors in joints, not the muscles, provide kinesthetic signals, since people are very aware of joint angle and not at all of the length of the various muscles involved. In fact, kinesthesia depends largely upon the integration within the cerebral cortex of signals from the muscle spindles.

The tendon organ consists simply of an afferent nerve fiber that terminates in a number of branches upon slips of tendon where the tendons join onto muscle fibers. By lying in series with muscle, the tendon organ is well placed to signal muscular tension. In fact, the afferent fiber of the tendon organ has proved to be sufficiently sensitive to give a useful signal on the contraction of a single muscle fiber. In this way tendon organs provide a continuous flow of information on the level of muscular contraction.

The muscle spindle is much larger and more complicated; moreover, new features of its structure and function continue to be discovered. Within it are several specialized muscle fibers, known as intrafusal muscle fibers (from Latin fusus, “spindle”). The spindle is several millimetres long, and the approximately five intrafusal muscle fibers run throughout its length. They are considerably thinner and shorter than the ordinary skeletal muscle fibers, though they show similar contractions and have the same histological appearance. The characteristic central swelling of the spindle (giving it a shape reminiscent of the spindle of a spinning wheel) is produced by fluid contained in a capsule surrounding the central millimetre of the intrafusal fibers.

Classically, the nerve terminals are considered to be of three kinds, namely primary sensory endings, secondary sensory endings, and plate motor endings. There are approximately equal numbers of primary and secondary sensory endings, so that they may be considered equally important. However, the primary, or annulo-spiral, ending has traditionally attracted the most attention, largely through its prominent appearance and the simplicity of its chief reflex action, the tendon jerk. It consists of a large axon, which branches to wind spirals around the equatorial region of every intrafusal fiber. The secondary ending is supplied by a smaller axon. It has less dramatic “flower spray” terminals lying mostly upon the smaller intrafusal fibers to one side of the primary ending. The reflex actions of the secondary ending remain incompletely understood. The plate motor endings lie toward the ends of the intrafusal fibers. They are fairly similar to the ordinary motor end plates of the skeletal, or extrafusal, muscle fibers.

Two separate types of intrafusal muscle fiber are distinguished, both histologically and on their contractile properties, as nuclear-bag and nuclear-chain intrafusal fibers. These have since been further subdivided and the complexities multiply, but there is general agreement that just as the spindle has a dual afferent innervation, it also has a dual motor supply.

The working of this elaborate piece of biologic machinery is not yet fully understood. The muscle spindle lies “in parallel” with the main muscle fibers and so sends a signal whenever the muscle changes its length. Both types of sensory endings increase their discharge of impulses when the muscle is stretched and reduce their firing when the muscle is slackened. The primary ending differs from the secondary ending in two important respects: first, it is much more sensitive to changing length of the muscle; second, it is much more sensitive to small stimuli than large ones. Together, these properties explain the exquisite sensitivity of the primary sensory ending to the stimulus of a tendon tap, which has little effect on the secondary ending or on the tendon organ. The essential principle is that the ability of the muscle spindle to signal a wide range of movement is increased by its having two separate output channels of different sensitivity.

The motor supply of the muscle spindle is more complex. Most of the intrafusal fibers receive specialized fusimotor nerve fibers. These are much smaller than the motor axons innervating extrafusal muscle fibers and are given the name gamma ( g ) efferents. As their sole function is to regulate the behaviour of the muscle spindles, their stimulation produces no significant contraction of the muscle as a whole. The g efferents are of two functionally distinct kinds with different effects on the afferent fibers—especially on the primary ending. One type, the dynamic fusimotor axon, increases the normal sensitivity of the primary ending to movement; the other type, the static fusimotor axon, decreases its sensitivity, making it behave much more like a secondary ending. Thus, the two types of efferent fiber provide a means whereby the sensitivity of the muscle spindle to external stimuli may be regulated over a very wide range. Stimulation of both types also increases the rate of firing of the afferent fibers when the length of the muscle is constant; this is called a biasing action. It is believed that they produce these different effects by supplying different types of intrafusal fiber.

In addition to receiving specialized fusimotor fibers, the muscle spindle may also receive, though on a less regular basis, branches of the ordinary extrafusal motor axons. Called alpha ( a ) efferents, these fibers have either a static or dynamic effect. The physiologically important point is that most of the motor supply to the muscle spindles is largely independent of that of the ordinary muscle fibers, and only a small part is obligatorily coupled with them. The specific mechanisms by which the sensitivity of the spindle is regulated remain obscure; they may differ from muscle to muscle and for movements of different kinds.

Stretch reflexes

The primary afferents are responsible for the stretch reflex, in which pulling the tendon of a muscle causes the muscle to contract. As noted above, the basis for this simple spinal reflex is a monosynaptic excitation of the motor neurons of the stretched muscle. At the same time, however, motor neurons of the antagonist muscle (the muscle that moves the limb in the opposite direction) are inhibited. This action is mediated by an inhibitory interneuron interposed between the afferent neuron and the motor neuron. These reflexes have a transitory, or phasic, action, even though the afferent impulses continue unabated; this is probably because they become submerged in more complex delayed reflex responses elicited by the same and other afferent inputs.

Traditionally, it was thought that the stretch reflex provided uniquely for the automatic reflex control of standing, so that if the body swayed, then the stretched muscle would automatically take up the load and the antagonist would be switched off. This is now recognized to be only part of the process, since more powerful, slightly delayed reflex responses occur not only in the stretched muscle but also in others that help restore balance but have not themselves been stretched. Some of these late responses seem to be spinal reflexes, but in humans, with their huge brains, there is evidence that certain others are transcortical reflexes, in which the afferent impulse is transmitted at high speed up to the motor areas of the cerebral cortex to influence the level of ongoing voluntary motor impulses.

The basic organization of movement

Reciprocal innervation

Any cold, hot, or noxious stimulus striking the skin of the foot contracts the flexor muscle of that limb, relaxes the extensor muscles of the same limb, and extends the opposite limb. The purpose of these movements is to remove one limb from harm while shifting weight to the opposite limb. That is the first and immediate response, but a slower and longer-lasting reflex response is also possible. For example, noxious stimulation of the deep tissues of the limb can cause a prolonged discharge of impulses conducted by nonmyelinated afferent fibers to the spinal cord. The result is prolonged flexion of the damaged limb or at least a pattern of posture and movement favouring flexion. These effects last far longer than the original discharges from the afferent neurons of the damaged region—often continuing not for minutes but for weeks or months.

The flexor and extensor reflexes are only two examples of the sequential ordering of muscular contraction and relaxation. Underlying this basic organization is the principle of reciprocal innervation—the contraction of one muscle or group of muscles with the relaxation of muscles that have the opposite function. In reciprocal innervation, afferent nerve fibers from the contracting muscle excite inhibitory interneurons in the spinal cord; the interneurons, by inhibiting certain motor neurons, cause an antagonist muscle to relax.

Reciprocal innervation can be seen in eye movements. On looking to the right, the right lateral rectus and left medial rectus muscles contract, while the antagonist left lateral rectus and right medial rectus muscles relax. One eye cannot be turned without turning the other eye in the same direction (except in the movement of convergence, when both eyes turn medially toward the nose in looking at a near object).

Reciprocal innervation does not underlie all movement. For example, in order to fix the knee joint, antagonist muscles must contract simultaneously. Indeed, in the movement of walking, there are both reciprocal innervation and simultaneous contraction of different sets of muscle. Because this basic organization of movement takes place at lower levels of the nervous system, the training of skilled movements such as walking requires the suppression of some lower-level reflexes as well as a proper arrangement of the reciprocal inhibition and simultaneous contraction of antagonist muscles.

The basic organization of movement

Posture

Posture is defined as the position and carriage of the limbs and the body as a whole. Except when lying down, the first postural requirement is to counteract the pull of gravity. This is done by the stretch reflexes. Gravity pulls the body—every part of the body—toward the ground. This force induces stretch reflexes to keep the lower limbs extended and the back upright. The muscles are not kept contracting all the time, however. As the posture alters and the centre of gravity changes, different muscles are stretched and contracted. Another important reflex is the extensor thrust reflex of the lower limb. Pressure on the foot stretches the ligaments of the sole, which causes reflex contraction of both flexor and extensor muscles, making the leg into a rigid pillar. As soon as the sole of the foot leaves the ground, the reflex response ceases, and the limb is free to move again.

The body is balanced when the centre of gravity is above the base formed by the feet. When the centre of gravity moves outside this base, the body starts to fall and has to bring the centre back to the base. Striding forward in walking depends on leaning forward so that the centre of gravity moves in front of the feet. When a baby is learning to walk, he must either take a step forward or fall down. Both happen; eventually the former happens more frequently than the latter.

In addition to continuous postural adjustment for the changing centre of gravity, all movements require that certain parts of the body be fixed so that other parts can be supported as they move. For instance, when manipulating objects with the fingers, the forearm and arm are fixed. This does not mean that they do not move; they move so as to support the fine movements of the fingers. This changing postural fixation is carried out automatically and unconsciously. Before any movement is carried out, the essential posture is arranged and continues to be adjusted throughout the movement.

Cerebral hemispheres

Basic organizations of movement, such as reciprocal innervation, are organized at levels of the central nervous system lower than the cerebral hemispheres—both at the spinal and brain-stem level. Examples of brain-stem reflexes are turning of the eyes and head toward a light or sound. The same movements, of course, can also be organized consciously when one decides to turn the head and eyes to look. The cerebral hemispheres themselves can organize certain series of movements that are often referred to as programmed movements. These are movements that need to be performed so rapidly that there is no time for correction of error by local feedback. For this reason the program is arranged before the movements begin. Examples of such movements are those of a pianist performing a trill or of an athlete hitting a ball.

Most of the movements organized by the cerebral cortex are carried out automatically. But when a new series of movements is being learned, or when a movement is difficult, the attributes usually associated with the higher levels of the brain—such as planning, internal speech, remembering, and learning—are used.

In the 19th century the first motor area of the cerebral cortex was discovered by electrical stimulation. Successive areas responding to electrical stimulation were numbered in the order of their discovery. Therefore, although motor and sensory areas are named numerically, no hierarchical organization is implied.

The first motor area to be discovered was the motor strip of the precentral gyrus; it became known as the primary motor area. Immediately behind it is the postcentral gyrus, found on electrical stimulation to be sensory; it was named the primary sensory area. Each of these areas displays a maplike correspondence with various body parts, the legs represented near the top of the hemispheres and the arms and face lower on the cortical surface. Each of these areas is to some extent both motor and sensory. The motor region, for example, receives inputs from the skin, joints, and muscles via the postcentral gyrus behind and the thalamus below.

Experiments in monkeys have shown that the motor strip is able to arrange activity of muscles to produce the correct force for the loading conditions of the limbs. To do this, it continually receives information from the primary sensory area both before and during the movement. Cutaneous areas having the greatest tactile acuity have the largest representative in the primary sensory area; these areas are connected to equally large areas in the primary motor area.

In front of the motor strip is an area known as the premotor cortex or area. When this area is stimulated in a monkey, the animal turns its head and eyes as though it is looking in a particular direction. This cortical area, then, organizes the guiding of movements by vision and hearing.

An area labeled the secondary motor area is at the lower end of the precentral gyrus. It is secondary not only because it was discovered after the primary motor area but also because it does not function in a discrete manner like the primary area. Stimulation of this small area produces movements of large parts of the body. It is also a sensory area, as sensations in the parts of the body being moved are felt during stimulation.

On the medial surface of the hemisphere, in front of the motor strip, is the supplementary motor area. Stimulation there can produce vocalization or can interrupt speech. There also may be large movements of both sides of the body—often symmetrical movements of the two limbs. Stimulation also produces movements of the opposite side, such as raising the upper limb and turning the head and eyes as if looking at something opposite. In experiments on monkeys, when the animal chooses to respond to one kind of sensation rather than to another, it is the supplementary area that is active rather than the precentral area. In this animal—it is unknown for humans—the fibers descending from the supplementary motor area run to the spinal cord and terminate throughout its whole length. fibers are also sent to the precentral gyri of both hemispheres, the reticular formation of the pons, the hypothalamus, the midbrain, and many other masses of cerebral gray matter such as the caudate nucleus and the globus pallidus. The supplementary motor area is upstream from the primary motor area; it initiates movements, whereas the motor strip of the precentral gyrus is part of the apparatus for carrying them out.

Other regions of the cerebral hemisphere from which movements are produced on electrical stimulation are the insula and the surface of the temporal lobe. Stimulation of the anterior end of one temporal lobe causes movements of the head and body toward the other side. The insula is a region below the frontal and temporal lobes hidden from the surface; stimulation there causes movements of the face, larynx, and neck.

fibers from the anterior part of the cingulate gyrus are involved in the control of urination and defecation. The organization of these functions also depends on regions anterior to the cingulate gyrus in the medial wall of the frontal lobe. These regions form a part of the limbic lobe, which is responsible for some emotional states with their autonomic components.

Movements closely guided by vision have their own pathways. Occipital visual areas send fibers to the pons and from there to the cerebellum. Also just in front of the visual cortex in the parietal lobe are neurons organizing certain types of eye movement. In the monkey, these neurons are at rest during steady gaze, becoming active when the animal turns its eyes to look at something. That the movements constitute a high level of motor behaviour is shown by the activation of these neurons only when the animal is attempting to satisfy an appetite by using its upper limbs and hands; using the limbs for other purposes does not activate them. The neurons are also active when the animal is carrying out the movements of grooming, which also satisfies an innate drive.

One of the main pathways for cortically directed movement of the limbs is the corticospinal tract. This tract developed among animals that used their forelimbs for exploring and affecting the environment as well as for locomotion. It is largest in man. fibers of the tract go to various regions of the brain stem and the spinal cord that organize movement. Excitation via the corticospinal tract is then brought to many muscles, all of them presumably working together in a coordinated manner. This is achieved by the anatomical arrangement of the motor neurons and by the termination of the corticospinal tract on interneurons, which convey a coordinated pattern of stimulation to the motor neurons.

The corticospinal tract is not merely a straight pathway to medullary and spinal motor neurons. Activity in this tract can suppress the input from cutaneous areas while facilitating proprioceptive input. This is probably a mechanism important in the organization of movement. The corticospinal neurons themselves receive constant input needed for internal feedback. This input comes from the cerebellum, much of it having originated in the muscles, joints, and skin of the body parts being moved.

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