Chapter 4. The Neuromuscular Basis of Human Motion

At the conclusion of this chapter, the student should be able to:

• 1. Name and describe the functions of the basic structures of the nervous system.
• 2. Explain how gradations in strength of muscle contraction and precision of movements occur.
• 3. Name and define the receptors important in musculoskeletal movement.
• 4. Explain how the various receptors function, and describe the effect each has on musculoskeletal movement.
• 5. Describe reflex action, and enumerate and differentiate among the reflexes that affect musculoskeletal action.
• 6. Demonstrate a basic understanding of volitional movement by describing the nature of the participation of the anatomical structures and mechanisms involved.
• 7. Perform an analysis of the neuromuscular factors influencing the performance of a variety of motor skills.

The role of the bones, joints, and muscles in human movement were presented in the previous two chapters. The appropriate function of these systems requires an intact nervous system. Therefore, this chapter takes up the role of the nervous system in initiating, modifying, and coordinating muscular action.

Loofbourrow (1973) presented this topic so succinctly and vividly that his classic and still applicable introductory paragraph is quoted here in full as an introduction to this chapter:

The forces which move the supporting framework of the body are unleashed within skeletal muscles on receipt of signals by way of their motor nerves. In the absence of such signals, the muscles normally are relaxed. Movement is almost always the result of the combined action of a group of muscles which pull in somewhat different directions, so the control of movement involves a distribution of signals within the central nervous system (CNS) to appropriate motor nerves with precise timing and in appropriate number. In order for movements to be useful in making adjustments to external situations, it is necessary for the central nervous system to be appraised of these situations, which are continually changing. A means of providing this information promptly exists in a variety of receptors sensitive to changes in temperature, light, pressure, etc. These receptors are signal generators which dispatch signals (nerve impulses) to the CNS over afferent nerve fibers. The CNS receives these signals together with identical ones from within the muscles, joints, tendons, and other body structures and is led thereby to generate and distribute in fantastically orderly array myriads of signals to various muscles. This, despite the enormous complexity of the machinery involved, enables the individual to do one main thing at a time. This is integration. It is what Sir Charles Sherrington meant by “the integrative action of the nervous system.”

The following discussion does not presume to be an exhaustive treatise on neuromuscular mechanisms. It attempts rather to present as simply as possible those mechanisms that are pertinent to the study of kinesiology. Because of techniques made possible by electronic sensing and imaging devices, great strides have been made in acquiring more accurate information concerning the intricacies of neuromuscular function.

It is assumed that the kinesiology student is already familiar with the general plan of the nervous system; hence, it will not be described in full here. Only a brief outline of the major divisions will be presented to give the reader a framework for the topics selected for discussion:

• I. Central nervous system
  • A. Brain
  • B. Spinal cord
• II. Peripheral nervous system
  • A. Cranial nerves (12 pairs)
  • B. Spinal nerves (31 pairs)
• III. Autonomic nervous system (sympathetic and parasympathetic)

The autonomic nervous system is not a distinct system based on structure and geographic location, as are the central and peripheral systems, but is rather a functional division that overlaps with those in specific areas. It includes those portions of the brain, spinal cord, and peripheral nervous system that supply cardiac muscle, smooth muscle, and gland cells.

Neurons

A neuron, the structural unit of the nervous system, is a single nerve cell consisting of a cell body and one or more projections. Three kinds of neurons propagate impulses. Two kinds of neurons whose long fibers constitute the peripheral nervous system are the sensory (or afferent) and motor (or efferent) neurons (Figure 4.1). The primary motor neurons are also referred to as alpha motor neurons. There are also numerous connector neurons located only within the central nervous system.

The cell bodies of the majority of efferent, or motor, neurons are situated within the anterior horns of the spinal cord. (Some also exist in the brainstem and sympathetic ganglia.) Many short, threadlike extensions of the cell body, known as dendrites, synapse with the axons of other cells, the latter being either sensory or connector neurons.

Each motor and connector neuron has a specialized process termed an axon. The axon of the motor neuron emerges from the spinal cord in a ventral root. It then travels by way of a peripheral nerve to the muscle that it helps activate. There it divides and subdivides into smaller and smaller branches, the most distal being known as the terminal branches. Each terminal branch ends within a single muscle fiber in the structure called the motor endplate or neuromuscular junction.

The cell body of a spinal afferent, or sensory, neuron, unlike that of a motor neuron, is situated in a dorsal root ganglion just outside the spinal cord. (The cell bodies of cranial sensory neurons are in cranial nerve ganglia.) The neuron has a single short process that projects from the cell body and then bifurcates into two branches that go in opposite directions. One, the so-called central fiber, travels in the dorsal root of the nerve to the posterior horn of the spinal cord, where it divides into numerous branches. It may terminate in the cord, or it may ascend in the cord to the brain and terminate there. The other branch of the afferent neuron is the long peripheral fiber, which comes from a receptor. It travels in a nerve trunk (peripheral nerve) to the vicinity of the appropriate dorsal root ganglion, where it unites with the cell body via the short stemlike process mentioned earlier (Figure 4.1b).

An axon is the fiber over which impulses are conducted away from the cell body, as opposed to dendrites, which convey impulses toward the cell body. When referring to a sensory neuron, the term dendrite is applied not to the long fiber that conveys impulses from peripheral regions to the cell body but rather to its branches. The long sensory fiber itself is known simply as the peripheral fiber.

It was noted earlier that the dendrites of efferent (motor) neurons make contact within the spinal cord, either with the terminal branches of afferent (sensory) neurons or with connector neurons. Connector neurons, also known as interneurons, are a third type of nerve cell. They exist completely within the central nervous system and serve as connecting links. They may vary from a single small neuron, connecting a sensory neuron with a motor neuron, to an intricate system of neurons whereby a sensory impulse may be relayed to many motor cell bodies. We know from common experience that a complex motor act may result from a single sensory impulse. For instance, a sudden loud noise may cause us to jump, turn around, and tense nearly every muscle in our body. The connector neurons are responsible for this widespread response to the single sensory impulse. Thus only one connector neuron may be participating in a movement, or there may be an intricate network making possible an almost limitless number of connections with other neurons.

Nerves

Just as an electric cable is an insulated bundle of wires for the transmission of electric currents, so a nerve is a bundle of fibers, enclosed within a connective tissue sheath, for the transmission of impulses from one part of the body to another. A nerve, or nerve trunk as it is frequently called, may consist entirely of efferent fibers from the central nervous system to the muscles and other tissues, or it may consist only of afferent fibers from the sensory organs to the central nervous system. The typical spinal (peripheral) nerve, however, is mixed; that is, it contains both efferent and afferent fibers. Each spinal nerve is attached to the spinal cord by an anterior (motor) root and a posterior (sensory) root (Figure 4.2). The posterior root bears a ganglion (a collection of cell bodies), and it is just beyond the ganglion that the two roots unite to form the spinal nerve. Once outside the vertebral canal, each spinal nerve divides into an anterior and a posterior branch, each of which contains both motor and sensory fibers. The anterior branches supply the trunk and limbs, the posterior branches the back. Motor nerves that activate skeletal muscle fibers are referred to as the alpha motor system.

The 31 pairs of mixed peripheral nerves exit from both sides of the vertebral column between every two vertebrae, with the first spinal nerve exiting from between the skull and the first cervical vertebra. Therefore, there are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccyx spinal nerves. Specific areas of the body are activated by fibers from specific peripheral nerves. Table 4.1 outlines the spinal nerve innervation patterns.

The Synapse

Synapses are connections between and among neurons and occur only within the central nervous system (Figure 4.3). A synapse, and there may be thousands between any two neurons, is a proximity of the membrane of an axon and the membrane of a dendrite or a cell body. No physical union occurs between or among neurons. Conduction of impulses takes place in one direction only, from the axon of one neuron to the dendrites, cell body, or, rarely, to the axon of another. Synapses are influenced by use and disuse. The more often one is crossed, the easier it becomes for signals to pass through it. Conduction velocity is very rapid along the nerve fiber. Whenever there is a delay, it occurs at the synapses. The greater the number of synapses from receptor to effector, the longer the time from stimulus to response.

The transmission of an impulse across a synapse depends on the release of a chemical transmitter substance by the axon. This substance diffuses throughout the membranes, and an action potential is created in the postsynaptic neuron. The action potential may be excitatory or inhibitory. Threshold level is the minimum level of stimulus (chemical transmitter) necessary to result in the initiation or propagation of a signal. Too little transmitter results in no impulse being carried to the next neuron, but an increase in this transmitter beyond the threshold level produces a response equal to but not greater than that needed for the threshold level. It must be remembered that it is at the synapse where facilitation (making it easier) or inhibition (making it more difficult) occurs. If the threshold level is altered so that only a small amount of additional neurotransmitter needs to be released for the threshold level to be achieved, facilitation will occur. Conversely, if a larger quantity of the chemical is needed before threshold level is reached, inhibition will occur. The student is reminded that more than one motor neuron may attach to a single muscle fiber, and therefore neurotransmitters released from several neurons simultaneously may have an additive or inhibitory effect.

The structural units of the nervous and motor systems are, respectively, the neuron and the muscle fiber. Functionally, the two systems combine to form the neuromuscular system. The functional unit of the neuromuscular system is the motor unit, and it consists of a single motor neuron (see Figure 4.1a), together with all of the muscle fibers that its axon supplies (Figure 4.4). All muscle fibers activated by the same motor neuron are of the same muscle fiber type.

Motor units vary widely in the number of muscle fibers supplied by one motor neuron. Some motor units may have as many as 2000 or more muscle fibers (gastrocnemius); others may have fewer than 10 (eye muscles). A muscle that has a small ratio of muscle fibers to alpha motor neurons is capable of more precise movements than is the muscle with a small number of motor neurons for the same number of muscle fibers. Hence, the ratio of muscle fibers to motor neurons has a direct bearing on the precision of the movements executed by the muscle. For example, the small muscles of the thumb and index finger are capable of movements of great precision because their motor units have such a low ratio of muscle fibers to motor neurons or, to state it differently, such a large number of motor neurons per muscle. By way of contrast, the gluteus maximus has a relatively small number of motor neurons for its size, hence a large number of muscle fibers per neuron. It does not need to be pointed out that the movements for which the gluteus maximus is responsible can scarcely be described as precise.

Gradations in the Strength of Muscular Contractions

Common experience indicates that the same muscles contract with various gradations of strength according to the requirements of the task. The elbow flexors, for example, are able to contract just enough to enable the hand to lift a piece of paper from the desk; they can also contract forcefully enough to lift a 15-pound briefcase. How do they adjust to such extremes? Two major factors influence the gradation of contraction. These are (1) the number of motor units that participate in the act and (2) the frequency of stimulation. If the stimulus is of threshold value, all the muscle fibers in the motor unit will contract maximally. If the stimulus is subliminal, in other words, below threshold value, none of the muscle fibers in the unit will contract at all. This characteristic is known as the all-or-none principle of muscular contraction. It must be emphasized that the principle applies only to individual motor units, not to entire muscles. To reiterate, if the stimulus is of threshold value, each muscle fiber in the participating unit whose threshold level has been reached will contract. Hence, it follows that, other things being equal, the more motor units that contract, the greater will be the total strength developed.

There is an orderly sequence to the recruitment of motor units. Innervation begins with the recruitment of the smaller slow-twitch fiber motor units, which have lower thresholds. They are followed by the larger fast-twitch motor units, which have high threshold levels. The sequence of recruitment is reversed with the release of tension—that is, “last on” is “first off.”

If stimuli are discharged at low frequency, the muscle fibers will partially relax between impulses, but if the stimuli are discharged at high frequency, the fibers will have insufficient time to relax and the result is summation or maximal contraction. If these two factors are combined—that is, if the maximum number of fibers are stimulated with the impulses being discharged at high frequency—the resulting contraction is of maximal strength.

It should be noted that motor unit areas overlap, with a single muscle fiber often belonging to more than one motor unit. It takes a number of motor units firing to result in joint angle changes. The ability of a single muscle fiber to respond to more than one motor unit also becomes important with aging; there is evidence to suggest that with age, the number of active motor units decreases (Deschenes 2004). With appropriate equipment (electromyography and audio feedback), it is also possible to learn how to control single motor unit firing. This phenomenon has played an important role in research, biofeedback, and control of some prosthetic devices.

Most activities of the nervous system begin with stimuli that activate sensory nerve terminals. Whereas motor nerves end on muscle fibers, forming motor units, sensory nerves are attached to specialized endings referred to as receptors. These receptors are specialized cells or organs that are selective in their response to different types of stimuli, although they often respond to more than one type of stimulus. There are two major classifications: exteroceptors and interoceptors.

Exteroceptors are located at or near a body surface and receive and transmit stimuli that come from outside the body, including the receptors of the familiar five senses: sight, hearing, smell, taste, and touch.

Interoceptors include the receptors for other cutaneous sensations, such as heat, cold, pain, and pressure (Figure 4.5). The interoceptors may be subdivided into receptors that receive impulses from the viscera and those that receive impulses from the tissues directly concerned with musculoskeletal movements and positions. The former are known as visceroceptors, and the latter are called proprioceptors.

Proprioceptors

Students of motor activity and posture find the proprioceptors of special interest, for these are the receptors that receive impulses that occur because of body movements or positions. They are located in the muscles, tendons, and joints, including the surrounding and protective tissues such as capsules, ligaments, and other fibrous membranes, and in the labyrinth of the inner ear. The proprioceptors are stimulated by motions of the body and in turn are responsible for transmitting a constant flow of information from these structures to the central nervous system. This information involves the appropriateness of the response in regard to the degree, direction, and rate of change of body movements. Without these sensory reports, effective coordination in motor patterns would not occur. Information from the receptors is directed both to the conscious and unconscious levels and, in addition to giving us a sense of awareness of body and limb positions, provides us with automatic reflexes. Proprioceptors are classified as muscle proprioceptors (muscle spindles and Golgi tendon organs), joint and skin proprioceptors (Ruffini endings and pacinian corpuscles), and labyrinthine and neck proprioceptors (Figures 4.5 and 4.6).

Muscle Proprioceptors

An abundance of two types of receptors are located within muscles and tendons. Both are responsive to stretch tension. The muscle spindle detects relative muscle length and the Golgi tendon organ (GTO) detects muscle tension and active contraction through tension in the tendon.

Muscle Spindles

The muscle spindles are scattered throughout the muscle but predominate in the belly of the muscle, lying between the muscle fibers and parallel with them. More spindles are located in muscles controlling precise movements than are located in postural muscles. When the spindle is stretched, a sensory nerve located in its center sends impulses to the CNS, which in turn activates the motor neurons innervating the muscle, thus causing it to contract. The muscle spindle is responsive to both length (tonic response) and rate of change in length (phasic response). A single spindle is a tiny capsule (about 1 millimeter [mm] long), filled with fluid and containing some specialized muscle fibers known as intrafusal (fusimotor) fibers, to distinguish them from the extrafusal (skeletomotor) or “regular” muscle fibers (Figure 4.7). There are two kinds of intrafusal fibers, nuclear bag fibers and nuclear chain fibers. They are similar in that they both have central noncontractile areas in which the nuclei are situated, and both have polar ends that are contractile. They differ in size and in the arrangement of the nuclei. The nuclear bag fiber is the larger of the two, and the nuclei appear crowded into the baglike central area, which gives the fiber its name. The small nuclear chain fiber is named for the single line, chainlike arrangement of the nuclei in its slim, noncontractile central portion. There are differences also in the intricate system of innervation.

Each spindle is supplied with a type Ia afferent neuron that has a characteristic ending known as the primary, or annulospiral, ending. This ending is divided into as many branches as there are intrafusal fibers, and each branch is coiled around the noncontractile midsection of the intrafusal fiber. The annulospiral (AS) ending is highly sensitive to the velocity of changes in fiber length (phasic response), but only while the change is occurring. It also reacts to static (tonic) or length responses but with a sharp decline in frequency of impulse.

Most muscle spindles also have from one to five sensory endings that, because of their appearance, are given the picturesque name of flower-spray (FS) endings. Each FS ending has its own sensory (type II) fiber. The FS endings (also called secondary endings) are found at either end of the noncontractile midsection of the intrafusal fibers. They are believed to register static muscular length only (tonic response). The impulses transmitted by the FS endings increase almost directly in proportion to the amount of stretch and continue for a prolonged period of time. The FS endings are less sensitive to muscle stretch than the AS endings and therefore require a greater stimulus before responding. Because both the AS and FS endings activate the nuclear chain fibers, it is assumed that these fibers are responsible for the static response of both AS and FS endings. In contrast, only the AS endings activate the nuclear bag fibers. These fibers, then, must be responsible for the strong phasic response of the AS endings (Gowitzke and Milner 1988; Park et al. 1999).

Muscle spindles are also supplied with their own small efferent fibers. To differentiate these fibers from the motor neurons of the “regular” muscle fibers, they are termed gamma fibers, in contrast to the “regular” motor neurons whose axons are termed alpha fibers (see Figure 4.6). As a group, the gamma fibers form a gamma fiber system. Approximately one-third of the fibers in the peripheral nerve are gamma fibers. Impulses conveyed by gamma fibers (also called gamma efferents) cause the intrafusal muscle fibers to contract. This shortening of the spindle muscle fibers stretches their central noncontractile region where the AS endings are situated, and this stimulates them, causing their rate of firing to increase. Hence the effect of the gamma system is to control the sensitivity of the spindle afferents (gamma bias). The AS endings can be caused to fire, not only by passive stretch of the muscle as a whole, but also in the absence of such stretch by the function of the gamma system.

An exaggerated but familiar example of a simple adjustment of this nature is seen when picking up an object that is thought to be heavy. When the opposite proves to be the case, the correction in muscular effort is almost instantaneous. Less spectacular examples of this kind of coordination occur in all movements all the time. The gamma system provides a means of maintaining a position regardless of the tension put on it and enables smooth rather than jerky muscle response.

In summary, a muscle spindle can be stimulated in two different ways: (1) by stretching the whole muscle, causing the spindle to stretch; and (2) by contracting the ends of the intrafusal fibers via the gamma efferent system, thus stretching the center receptor portion of the spindle. The muscle spindle response is tonic in reacting to a static length and phasic in reacting to the rate at which length changes. Primary (AS) endings are sensitive to both tonic and phasic stretches, but secondary (FS) endings respond only to tonic stretch. Muscle spindles are very complex organs that are responsible for controlling the coordination of our muscular behavior. The feedback, which they supply continually, ensures a constant adjustment of muscular contraction.

Golgi Tendon Organ

In contrast to the muscle spindle, the Golgi tendon organ, when stretched, sends signals to the central nervous system and causes the muscle to relax rather than contract. It consists of a mass of nerve endings, which are enclosed within a connective tissue capsule and embedded in a muscle tendon (Figure 4.8). It is situated close to the junction of the tendon with the fleshy part of the muscle in such a way that it has an end-to-end relationship with the muscle fibers. It is said to be “in series” with the muscle fibers. As the muscle shortens in contraction, the tension in the tendon increases and the Golgi tendon organs are stretched and activated. They are much less sensitive to stretch than spindles and require a stronger stretch to be activated. When the stress is greater than the Golgi tendon organ stretch threshold, the reflex contraction that is due to spindle stimulation is overridden and the muscle relaxes. The Golgi tendon organ provides instantaneous information about the degree of tension on each small segment of muscle. It is, therefore, a protective mechanism, and when the tension is extreme, its inhibitory effect can be great enough to effect a whole muscle relaxation.

Joint and Skin Proprioceptors

Two important receptors located in the joints or skin are the pacinian corpuscles and the Ruffini endings. Pacinian corpuscles are found beneath the skin, concentrated in regions around the joint capsules, ligaments, and tendon sheaths. They are large end organs consisting of a tip of nerve fiber surrounded by many concentric layers of capsule. They are activated by rapid joint angle changes and by pressure that compresses and distorts the capsule, but only for a very brief period of time. Consequently, they are important for detecting rapid changes in pressure but useless for constant pressure awareness. In running, the information provided by the pacinian corpuscles allows the nervous system to predict where the feet will be at any time so that appropriate adjustments in limb position can be anticipated and effected as needed. Ruffini endings are also activated by mechanical deformation and, in contrast to the pacinian corpuscles, are important for signaling continuous states of pressure. They adapt slowly at first but then transmit a steady signal thereafter. Ruffini endings are located in the deep layers of the skin and are scattered throughout the collagenous fibers of the joint capsules. They are stimulated strongly by sudden joint movements and are thus important in sensing joint position and changes in joint angle of as little as 2 degrees. Each joint receptor monitors a specific section of the total range of motion of the joint. By knowing which receptor is stimulated, the brain can tell how far the joint is bent. The sensing of the complete movement by the central nervous system is the result of the integration of the stimuli received from the individual receptors.

Although cutaneous receptors are fundamentally exteroceptors, the ones that receive stimuli from touch (Meissner’s corpuscles), pressure (pacinian corpuscles), and pain (free nerve endings) serve as proprioceptors when they show sensitivity to texture, hardness, softness, and shape and when they participate in the extensor thrust reflex, pain, or flexion withdrawal reflex.

Labyrinthine and Neck Proprioceptors

These proprioceptors detect sensations concerned with determination of body position and changes in position as they relate to equilibrium. The labyrinths detect orientation and movements only in the head, whereas the neck proprioceptors inform the nervous system of the orientation of the head in relation to the body.

The labyrinths of the inner ear consist of the cochlea, the three semicircular canals, and the utricle and saccule (Figure 4.9). The cochlea is concerned with hearing, but the rest of the labyrinth is concerned with the sense of balance, or equilibrium. Each of the canals contains a membranous tube, and the bony spaces for the macule of the utricle and the saccule contain membranous sacs correspondingly named. The entire membranous labyrinth is filled with fluid called endolymph and is lined with hair cells that are sensitive to the movement of the fluid as the head moves. These hair cells are intimately related to branches of the eighth cranial nerve. Thus, movement of the head is translated into nerve impulses that reach the brain.

In addition to hair cells located throughout the labyrinth, there is a gelatinous substance in which otoliths (small carbonate of lime crystals) are embedded. The otoliths accentuate the effects of gravity on the hair cells and thus are able to detect changes in position that upset static equilibrium. They are also sensitive to linear acceleration by the head.

The semicircular canals are each in a different plane at right angles to each other. When the head turns, the canals move with it, but at first the fluid in them tends to remain stationary owing to inertia. When the head stops, the canals do also, but again, because of inertia, the fluid does not stop moving immediately. Relative motion in the endolymph produces motion in the hair cells, triggering impulses to the cranial nerve. This process enables the canals to detect any changes in angular velocity of the head. The anatomical arrangement of the entire labyrinth is such that some part of it is especially sensitive to any position or direction of movement of the head with respect to gravity.

The most important proprioceptive information needed for the maintenance of equilibrium is that provided by the joint (ligament) receptors of the neck (C1–C3) because they are sensitive to the angle between the body and the head. When the head is bent in one direction, the impulses from the neck prevent the labyrinthine proprioceptors from producing a feeling of imbalance. They do this by sending signals exactly opposite to those transmitted by the labyrinth. If the entire body’s orientation with respect to gravity is altered, however, the neck receptors do not counteract the labyrinthine receptors, and change in equilibrium is sensed.

Reflexes are integrated at various levels of the nervous system. A reflex movement is a specific pattern of response that occurs without volition and without the need of direction from the cerebrum. The anatomical basis for a reflex act is the reflex arc (Figure 4.10). This consists of an afferent neuron that comes from a receptor organ, enters the spinal cord, and there makes a synaptic connection either directly with the dendrites and the cell body of an efferent neuron or indirectly through one or more connector neurons. The axon of the efferent neuron extends from the cord to the muscle, where its distal branches terminate in muscle fibers. The point of contact between an axon and a muscle fiber is known as a myoneural junction, or motor endplate. The number of reflex arcs and the number of motor units involved depend both on the nature of the reflex and on the extent of muscular activity needed. Automatic reflex motions accompany all normal voluntary motion. Indeed, very few muscles in most movement patterns are under conscious control.

Reflex action, by its very nature, is complex and highly integrated with other control mechanisms. The complicated relationship between propioceptive afferents, exteroceptive afferents, and subconscious levels of control is even now not well understood. What is presented here is a highly simplified discussion of reflex activity and is intended as a basic introduction to the concepts of reflex activity. A simplistic but useful list of the reflexes covered is presented in Table 4.2.

Exteroceptive Reflexes

Although some overlap occurs, as could be inferred from the discussion of receptors, two main classes of reflexes are related to skeletal movements—namely, exteroceptive and proprioceptive. Many of the exteroceptor reflexes exhibited by animals and human beings are familiar to us. A horse will twitch its skin when flies alight on it; a dog will scratch when its skin is irritated by a flea, or perhaps tickled by a person. A human jumps upon hearing a sudden loud noise. A person also blinks when a foreign body strikes the eyeball, or even threatens to strike it. Three exteroceptive reflexes that may be of special interest are the extensor thrust, flexor, and crossed extensor reflexes.

Extensor Thrust Reflex

Pressure against the sole of the foot stimulates the pacinian corpuscles in the subcutaneous tissue and elicits the reflex contraction of the extensor muscles of the lower extremity. When the weight is supported by the feet, the pressure of the floor is sufficient to bring about this reaction. As the weight is shifted to the balls of the foot in preparation for a jump, or to the palms of the hand in preparation for a handspring, the pressure results in the extensor thrust reflex facilitating contractions of the extensor muscles of the legs or arms, respectively, thus assisting the push-off from the floor (Figure 4.11). Similarly in archery, the pacinian corpuscles in the bow hand are stimulated, and facilitation of the extensor muscles of the bow arm occurs. However, in this instance, the archer must counteract this reflex action and prevent full extension at the elbow or suffer a painful lesson of being whipped by the released strings. The extensor thrust reflex is often included as a proprioceptive reflex in some classifications.

Flexor Reflex

The flexor reflex most frequently operates in response to pain and is a device for self-protection. Because of the flexor reflex, we quickly withdraw a part of the body the instant it is hurt. If a finger is pricked by a pin or if it inadvertently touches a hot pan, we do not have to decide to remove our hand from the source of pain; we jerk it back even before we realize what happened to it. Furthermore, all the necessary muscles for withdrawing it are activated promptly, not just those in the immediate vicinity of the injury. Although we are all too aware of the pain, this awareness occurs after the withdrawal and plays no part in the reflex action. The value of the awareness is rather in teaching us to avoid repeating the act that caused the pain. This reflex is also called the nociceptive withdrawal reflex.

Crossed Extensor Reflex

This reflex functions cooperatively with the flexor reflex in response to pain in a weight-bearing limb. For instance, when an animal injures its paw, the flexor reflex causes it to withdraw the paw. Simultaneously, owing to the crossed extensor reflex, the extensor muscles of the opposite limb contract to support the additional weight thrust upon it. Similarly, if a person who is barefoot happens to step on a tack with the right foot, the body weight quickly shifts to the left foot and the right foot is withdrawn from the floor. As the flexors of the right limb contract to enable the lifting of the right foot, the extensors of the left limb contract more strongly to support the weight of the entire body.

Applications of the crossed extensor reflex have also been made to a non-weight-bearing limb. When a pain stimulus is applied to one hand, at the same moment that the hand is withdrawn, the opposite arm will extend as though to push the body away. Some individuals have also suggested that walking may reflect an application of the crossed extensor reflex. Release of pressure from the sole of the back foot results in facilitation of the extensor muscles of the front leg to accommodate the shift in weight over that foot.

Proprioceptive Reflexes

Earlier in this chapter receptors were classified as exteroceptors and interoceptors, the latter being subdivided into visceral receptors and proprioceptors. Proprioceptive reflexes are generally described as those reflexes that occur in response to stimulation of receptors located in the skeletal muscles, tendons, joints, and labyrinths of the inner ear. According to this interpretation, the proprioceptive reflexes are those interoceptors related to motions and positions of the body. The stretch, or myotatic, reflex is always included among these. Some classifications also include the extensor thrust, the labyrinth and neck, and the tendon organ reflex.

Stretch Reflex

The stretch reflex is so named because stretch on the muscle stimulates the muscle spindle, resulting in the reflex contraction of the stretched muscle and its synergists and relaxation of its antagonists. The muscle spindle picks up the stretch stimulus and transmits it by way of the afferent neuron to the spinal cord. There the central terminal branches of the sensory neuron synapse directly with the dendrites of the motor neuron, which activates the same muscle fibers that were stretched. These fibers then contract. This two-neuron reflex arc is the simplest type of reflex arc, consisting of a single sensory neuron and single motor neuron, which may involve one or more synapses. The important characteristic of this type of reflex arc is that it does not make use of connector neurons in the spinal cord.

There are two responses to the stretch reflex—phasic or tonic—depending on the velocity at which the stretch occurs. The frequency of discharge from the primary endings of the muscle spindle is directly related to the velocity at which the muscle fibers are being stretched. The greater the frequency of these impulses, the greater will be the resulting muscle fiber contraction.

The phasic type includes familiar clinical tests, such as the knee jerk. Reflexes of this type are extremely rapid and the contraction is of brief duration. The word “jerk” gives an accurate picture. Although it is true that the cause of the stimulus in this instance is exteroceptive in nature (a rubber-headed hammer or the edge of the hand), it is nevertheless classed as a proprioceptive reflex. If the stretch is sudden and sufficiently strong, the stretched muscle contracts quickly and forcefully because of the response of the primary endings of the muscle spindles.

A slow stretch, such as is elicited in postural sway, will result in a tonic response (Figure 4.12). Another demonstration of this reflex may be observed when a weight is placed in the hand, with the forearm in 90 degrees of flexion at the elbow joint. The result is likely to be a subtle depression of the hand and forearm (slight extension at the elbow) immediately followed by a return movement to compensate.

Movements that put muscles on a stretch in the backswing or preparatory phase can take advantage of the stretch reflex. If the desired outcome is a strong application of force, the preparatory movement should be rapid, to benefit from the phasic increase in spindle discharge, and long, to increase the tonic response. The backswing in forceful striking and throwing patterns, the crouch before a vertical jump, and the stretch before a tuck dive are all examples (Figure 4.13a). If, however, the desired outcome is accuracy, as in a badminton low serve or a golf putt, the backswing should be short and slow, with a pause before the force application (Figure 4.13b). This approach allows the phasic frequencies of the primary endings to slow down to tonic level.

In the static type of stretch reflex, the muscle is stretched slowly. This causes primary and secondary endings of several spindles to be stimulated and results in a more sustained muscular contraction. The importance of the static stretch is that it causes muscle contraction as long as a muscle is put on excessive stretch. When such a reflex is elicited by the stretch caused by the tendency of weight-bearing joints to flex, the response of the extensor muscles is commonly referred to as the antigravity reflex. Some also use this term to include the response of lower extremity and trunk muscles to the involuntary forward-backward swaying that usually occurs when a person stands in one position for a long time.

The gamma efferent system functions to adjust the muscle spindle length by causing the intrafusal fibers to contract (see Figure 4.7). This mechanism receives signals whenever the alpha motor system stimulates the extrafusal (skeletal muscle) fibers. Thus, a balance between the length of the skeletal and spindle fibers is maintained. In addition, the gamma efferent system is under voluntary control (gamma bias). For example, this mechanism is at work when a subject anticipates receiving a weight. Impulses are sent along the gamma system, adjusting the threshold level of the muscle spindle so that as soon as any change occurs, a reflex contraction results. Remember what happens when lifting an empty box after thinking it would be weighted.

The flower-spray endings, or secondary afferents, are less sensitive to stretch and only result in the tonic response. Therefore, a greater or prolonged stretch is necessary to stimulate the secondary endings. Whereas the primary endings always facilitate the contraction of the muscle in which they are located, the influence of the secondary afferents depends on the type of muscle in which it is located. Secondary afferents facilitate flexor muscles and inhibit extensor muscle contraction. Therefore, the secondary afferent response would reinforce the primary afferent response if the stimulated spindle was located in a flexor muscle but would result in a cocontraction if the stimulated spindle were in an extensor muscle. This would be due to the reflex contraction facilitated by the primary endings in the extensor muscle and the facilitation of the flexor muscle by the secondary ending. Two-joint muscles respond to secondary afferent stimulation as though they are flexor muscles.

Spindles may also be stimulated by pressure and vibration. This fact is important whenever pressure must be placed on another individual. In gymnastics, if pressure is placed on flexor muscles, the gymnast may fall rather than be helped to hold a position. Similarly, pressure on the extensor muscles when working in rehabilitation may inhibit elbow flexion. Use of this technique may provide additional variations to the rehabilitation setting.

In some instances it would be desirable to minimize the effect of the stretch reflex, as in flexibility exercises. This can be done if the stretch is done slowly and is held, as in static stretching. A quick stretch or bounce will evoke a stronger, phasic, reflex contraction.

Tendon Reflex

The second muscle proprioceptor, the Golgi tendon organ (GTO), is located in series at the musculotendinous junction. It too is stimulated by tension, but because of its location, the stretch may be induced on the tendon when the muscle contracts as well as when the muscle is stretched. The threshold of the GTO to muscle stretch is higher than that of the muscle spindle. That is, if the stimulus is strong enough to stimulate the GTO, the muscle spindle response is overridden. On the other hand, the GTO is very sensitive to tension when it is produced by tendon stretch resulting from muscle contraction. In either case, the reflex effect is to inhibit impulses from the motor nerve to the muscle and its synergists, thus causing the muscle to relax. Simultaneously, the antagonist contracts from being facilitated.

When tension on the muscle is extreme, the Golgi tendon organs’ inhibiting action results in sudden relaxation of the muscle. This effect is called the lengthening reaction and undoubtedly serves as protection for muscles or tendons that could be torn or ruptured by the strong contractile force. Guyton (2000) has suggested that the tendon reflex serves as a feedback mechanism to control the tension in the muscle. If the tension in the muscle became too great, the inhibiting action of the receptors would cause relaxation. If the muscle tension became too little, the receptors would stop firing, and the muscle tension would be able to increase.

The behavior of the tendon reflex in the performance of some skills by beginners is also a matter of interest. It may be that the reason beginners do not follow through has to do with the fact that the tendon reflex causes relaxation of vigorously contracting muscle. It is suggested that until an increase in the Golgi tendon organ threshold develops with learning, beginners may need voluntary effort to counteract the inhibitory effect of the tendon reflex.

Labyrinth and Neck Reflexes

Newborn infants start out with very simple reflexes, which in the process of normal motor development are suppressed or modified. This phenomenon is especially evident in the labyrinth and neck reflexes. The tonic labyrinth reflexes emanate from utricle and semicircular canal receptors, which are sensitive to changes in position of the head with respect to gravity. Because of the primitive tonic labyrinth reflex present in the newborn child, a supine position of the head facilitates extension of the extremities, whereas a prone position inhibits extension and facilitates flexion. At a few months of age or more, this primitive reflex is suppressed, and the labyrinth righting reflex is evident. In cooperation with other righting reflexes of the neck and eyes, it evokes muscular responses to restore the body to a normal upright position.

Righting reflexes can be demonstrated in adults. They respond to any body-tilting action by attempting to restore balance through facilitating limb actions, such as an arm or leg being thrust out. In spinning, the limbs facilitate restoration of the normal head position by actions that inhibit the rotation. The arm on the same side as the direction of the spin is thrust out during the spin, and the opposite arm is thrust out at the termination of the spin. This latter action is probably a response to the imbalance caused by the dizziness experienced at the conclusion of the spin because of the continuation of movement (inertia) of the fluid and resulting stimulation of the hair cells.

Tonic neck reflexes are evident when the joint receptors in the neck are stimulated by any movement in the neck. They are also present at birth, and even though they are masked by further development, they remain in adults. Like the tonic labyrinth reflex, they can also be demonstrated in adults. Actions due to tonic neck reflexes in the primitive infant form are predictable. If the head is flexed, flexion will occur in the upper extremities and extension in the lower ones. With head extension, the opposite occurs—extension in the upper extremities and flexion in the lower ones. Rotation of the head to the right is accompanied by extension and abduction of the limbs on the chin side, and flexion and adduction on the opposite side.

Tonic neck and tonic labyrinth reflexes become most apparent in adults in stressful situations. They are hard to distinguish from each other because movements of the head and neck occur together and labyrinth and neck receptors are stimulated simultaneously. In some instances they reinforce each other in their effect on joint actions of the extremities, and in some instances they oppose each other. There are times when we should take advantage of them to facilitate actions and other times when it would be beneficial to suppress them.

The neck reflexes are more effective in modifying upper extremity actions and the labyrinthine in modifying those of the lower extremities, and these results are still accepted. There is no question that head position influences the actions of the arms. When a strong pulling action is required, flexion of the head will facilitate it to some extent. On the other hand, the neck reflex facilitation suggests that the head should be extended for strong pushing actions. For facilitating a one-armed pull, the head should be turned away in addition to being flexed and, in a one-armed push, the head should be turned toward the pushing arm. Other examples of reflex facilitation include extension of the head in a handstand to reinforce extension in the arms, flexion of the head to reinforce trunk, arms, and leg flexion in forward or backward rolls, and head rotation in archery to facilitate bow arm pushing on the chin side and bowstring arm pulling by the opposite side (Deutsch et al. 1987; Gowitzke and Milner 1988; Le Pellec and Maton 1993).

Undoubtedly one of the difficulties in learning new motor skills is the failure to suppress reflex responses. In falling backward, our natural reaction is to extend the arms and throw the head forward. If a beginner did this in attempting a back dive, the entry into the water would be uncomfortable, to say the least. The labyrinth righting reflex must be suppressed consciously so that the head may be extended back and the body follow. Belly flops in front dives can also be attributed to the labyrinth righting reflex.

Posture and Locomotor Mechanisms

Whether because of reflex action or to some other mechanism, it seems apparent that the human body has certain provisions for remaining more or less erect and for engaging in locomotion and that these follow the general pattern of reflex behavior. The coordinated efforts of the body to resist the downward pull of gravity include the extensor thrust reflex, the static type of stretch reflex in response to gravitational pull, the muscular action evoked by forward-backward swaying, and the various mechanisms for preserving equilibrium, including visual orientation and labyrinthine reflexes.

In regard to locomotion, the action of the legs of a four-footed animal has been attributed to reflex action. Some classify this reflex as a division of the crossed extensor reflex. Because research in this area has been done primarily on dogs and cats, it has been suggested that this reflex exists only in quadrupeds. Nevertheless, it is logical to assume that it exists also in humans inasmuch as the early forms of locomotion—creeping and crawling—resemble the locomotion of quadrupeds. Even after the erect position is assumed, the swing of the arms in opposition to the lower extremities reflects the earlier four-footed gait.

This topic involves such an extensive and complex body of knowledge that only the bare essentials and a few concepts can be touched on here. The chief anatomical structures concerned with volitional movement, in addition to those mentioned earlier (skeletal muscle, basic nerve structures, motor units, and sensory receptors), are the cerebral cortex, the cerebellum, the brainstem, spinal cord, and the numerous motor pathways, both pyramidal (corticospinal tracts) and extrapyramidal tracts.

Volitional movement is goal directed where the higher centers of the brain have the most general control, with each succeeding level having more specific control. The central nervous system is often divided into five levels in hierarchical order.

Central Nervous System: Levels of Control

The cerebral cortex (Figure 4.14) is the first and highest level of control. It is the level at which consciousness occurs and the level responsible for the initiation of voluntary movement. The cerebral cortex contains a motor area situated in front of the transverse central fissure known as the precentral gyrus. Within this area, all body parts are represented by an area whose size correlates with the complexity or precision of the body part. Therefore, the area representing the thumb and fingers requires a larger area than that representing the foot. Movements, not muscles, are represented here. The premotor area located in front of the motor cortex is responsible for learning complex acts.

The basal ganglia area is the second level of control. This area is located at the thalamus level and is often identified as containing the subcortical nuclei. The basal ganglia area is responsible for homeostasis; that is, for maintaining a level of constancy within prescribed ranges before adjustments are made. It is responsible for the coordination and control of some learned acts involving posture and equilibrium, including the sensory integration for the righting reflexes.

Level three is in the cerebellum (Figure 4.14). The cerebellum is often referred to as the “little brain.” It plays a key role in sensory integration, receiving diverse inputs and acting as a feedback center comparing what should have been with the motion that occurred. The cerebellum is a vital center for regulating the timing and intensity of muscle activity to produce smooth, precise movement.

The pons–medulla area, known as the brain-stem (Figure 4.14), is level four of the central nervous system. This area contains the reticular formation, which encompasses numerous neurons and nuclei and houses important centers for arousal and monitoring of physiological parameters from the cardiorespiratory mechanisms. The brainstem also contains key facilitory and inhibitory centers, thereby having a direct effect on muscle sensitivity.

The lowest level of control of the central nervous system, level five, is the spinal cord (Figure 4.14), containing cell bodies of the lower motor neurons that control the skeletal muscles. It has the most specific control and is the common pathway between the central and peripheral nervous systems. Because signal integration occurs at synapses, and all synaptic connections between and among neurons occur within the central nervous system, the spinal cord is the final point for nerve impulse integration and control.

Although reported as discrete levels and areas with specific functions, functions of the five levels and areas of the central nervous system overlap depending on the classification scheme used. It is true, however, that higher centers may override lower centers, but the reverse is not true. For accurate volitional movement to occur, sensory information is imperative, and continuous sensory stimulation is necessary for accurate volitional action. Moreover, the response of the central nervous system depends on the initial position of the segment or limb (joint angle) for the intensity of muscle contraction.

Pyramidal and Extrapyramidal Tracts

Tracts are bundles of fibers. The pyramidal and extrapyramidal tracts are two tracts originating from the cerebral cortex and ending at the spinal cord. Together, these two tracts play a vital role in a person’s ability to initiate fine coordinations representative of skilled movement. The corticospinal, or pyramidal, tract is predominantly comprised of axons from motor neurons, which originate in the cerebral cortex and end in the spinal cord on other motor neurons. Because there is only one synapse, this is a very fast system. During the descent, the majority of the fibers cross on a level with the medulla. The fibers pass in the anterior portion of the medulla, forming the pyramids, and thus this tract is referred to as the pyramidal tract. Because the majority of its fibers cross, fibers originating on the right side control the limb muscles on the left side. Specifically, this tract controls muscles for precision. Because myelination of the axons occurs with maturity, it is reasonable that young children would have difficulty with skills requiring fine coordination. Disruption of this system results in weakness and difficulty in initiating voluntary movements.

Structurally, the extrapyramidal system also originates at the cerebral cortex and ends at the spinal cord, but during its descent the axons have many important synaptic connections with all levels of the central nervous system. These communication links are named for their connections. There are, for instance, the vestibulospinal, reticulospinal, and rubrospinal pathways. There are also many significant connections with the cerebellum. Fibers from the extrapyramidal tract cross just above the medulla and travel outside the pyramids. This system predominates in young children. Its function is one of stabilization and control of posture as well as general movement patterning.

Kinesthesis

The conscious awareness of position of body parts and the amount and rate of joint movement is known as kinesthesis. Kinesthetic sensations originate primarily in the sensory receptors of the joint capsules and ligaments and include the Ruffini endings, pacinian corpuscles, and Golgi tendon organs. Signals from these receptors are transmitted rapidly to the cord and brain in beta type A nerve fibers so that the central nervous system is instantaneously aware (at the cerebral cortex level of conscious awareness) of the exact position of the body parts. Without this rapid transmission and processing of information, accurately controlled movements could not proceed. Kinesthetic perception and memory are the basis of voluntary movement and motor learning. This perception and memory enable the performer to initiate a whole movement pattern or modify a part of it, such as the elimination or addition of a joint action, or a change in timing or speed of an action.

Reciprocal Inhibition and Coactivation

One of the mechanisms that provides for economical and coordinated movement is known as reciprocal inhibition. According to this concept, when motor neurons are transmitting impulses to muscles, causing them to contract, the motor neurons that supply their antagonists are simultaneously and reciprocally inhibited. The antagonistic muscles, therefore, remain relaxed and the movers, or agonists, contract without opposition. The relaxation occurs to the degree to which the agonists are activated. Reciprocal inhibition operates automatically in movements elicited by the stretch reflex and also in familiar volitional movements. In more complicated and in less familiar coordinations, its operation depends on the degree of skill developed by the performer.

Coactivation, or reciprocal activation, most frequently appears in movement when there is uncertainty about the movement task. As practice or training increases familiarity with the movement, coactivation decreases in favor of reciprocal inhibition. In this way efficiency of movement also increases. Further evidence suggests that coactivation also occurs in order to maintain joint stiffness, especially when faced with an unstable or perturbed posture.

The muscle-response patterns of well-learned motor skills involve the integrated action of many reflexes and the inhibition of others. After repeated viewing of the performance “live” or on film, the student should name and discuss the reflexes that could be acting at various points in each phase. The reflexes that should be considered are spindle reflexes (stretch reflex), the Golgi tendon organ reflex, joint reflexes, cutaneous responses, labyrinthine reflexes, neck reflexes, and visual righting reflexes. For each reflex, the receptors involved should be identified, the expected action facilitated by the reflex described, and the actual results explained. To return to the standing long jump in Figure 1.3, the following reflex actions are plausible examples of reflex action:

• 1. Reflex: Labyrinthine head righting reflex is present.
  • Receptor: Labyrinthine system—inner ear.
  • Time: Preparation for takeoff.
  • Evidence: As the trunk leans farther forward, the head and neck become more hyperextended.
• 2. Reflex: Stretch reflexes in extensors of hip, knee, and ankle are present.
  • Receptor: Muscle spindle.
  • Time: Early preparation for takeoff (crouch).
  • Evidence: The stretch reflexes are activated in the extensors as flexion occurs in the hips, knees, and ankles. The result is facilitation of contraction of the extensor muscles.
• 3. Reflex: Extensor thrust reflexes.
  • Receptor: Pacinian corpuscles (soles of the feet).
  • Time: As force is applied to the floor for takeoff.
  • Evidence: Pressure facilitates extensor muscle contraction of the lower extremities.

• 1. Engage a partner in arm wrestling until one of you loses. Explain the sudden cessation of muscle tension in the loser that resulted in the end of the contest.
• 2. Record and explain what happens neurologically when an individual is upset from a blow in front of the knees as opposed to behind them.
• 3. Explain what happens neurologically when an individual pushes a door at the same time a person on the other side pulls it open.
• 4. Ask your partner to stand facing you with eyes closed. By moving body segments, place your partner in some novel pose. Ask your partner, with eyes still closed, to describe the position exactly. After once again assuming the original standing position, ask your partner to reproduce the novel position. Explain the neuromuscular mechanisms that enable your partner to sense and reproduce the movement.
• 5. Record the distance of your best of three standing broad jumps. Repeat the jumps, but this time flex the head and neck vigorously as the legs are extended. After a few practice trials record the distance of the three jumps, which incorporate the reflex pattern. Compare your results with those of others in the class. What effect did the reflex pattern have? Explain.
• 6. Stand in a doorway with your arms at your side. Now press the backs of your hands strongly against the door jambs. Hold this position for at least 30 seconds. Step clear of the doorway and turn your head to the right. What happens to your arms? Explain.
• 7. Look at the sequence of film tracings for the backward somersault depicted in Exercise 5.6 in Appendix G. Which proprioceptive reflexes could be present in the position represented in frame (a)? For each reflex named, state the evidence that would suggest the presence of the reflex, the expected effect the reflex could have on the movement, and the verification of the effect as shown by the action in the next frames. Were there any expected reflex actions that apparently did not occur? Explain.