78 Steps Health Journal

Figure 125

(a) Passive stretch of the muscle activates the spindle stretch receptors and causes an increased rate of action potentials in the afferent nerve. (b) Contraction of the extrafusal fibers removes tension on the stretch receptors and lowers the rate of action potential firing. Blue arrows indicate direction of force on the muscle spindles.

magnitude of the stretch and the speed with which it occurs. Although the two kinds of stretch receptors are separate entities, they will be referred to collectively as the muscle-spindle stretch receptors.

The muscle spindles are parallel to the extrafusal fibers such that stretch of the muscle by an external force pulls on the intrafusal fibers, stretching them and activating their receptor endings (Figure 12-5a). The more the muscle is stretched or the faster it is stretched, the greater the rate of receptor firing. In contrast, contraction of the extrafusal fibers and the resultant shortening of the muscle remove tension on the spindle and slow the rate of firing of the stretch receptor (Figure 12-5b).

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Control of Body Movement CHAPTER TWELVE

Control of Body Movement CHAPTER TWELVE

When the afferent fibers from the muscle spindle enter the central nervous system, they divide into branches that take several different paths. In Figure 12-6, path A directly stimulates motor neurons that go back to the muscle that was stretched, thereby completing a reflex arc known as the stretch reflex.

This reflex is probably most familiar in the form of the knee jerk, part of a routine medical examination. The examiner taps the patellar tendon (Figure 12-6), which passes over the knee and connects extensor muscles in the thigh to the tibia in the lower leg. As the tendon is pushed in and thereby stretched by

Neurons ending with:

Neurons ending with:

To central nervous system

Motor neuron to extensor muscle originally stretched

Flexor muscle physician's tap on knee

Tibia

Patellar tendon

FIGURE 12-6

Neural pathways involved in the knee jerk reflex. Start with the begin logo. Tapping the patellar tendon stretches the extensor muscle, causing (paths A and C) compensatory contraction of this and other extensor muscles, (path B) relaxation of flexor muscles, and (path D) information about muscle length to be sent to the brain. Arrows indicate direction of action-potential propagation.

To central nervous system

-Spinal cord

— Motor neuron to flexor muscles

Motor neuron to other extensor muscles

Motor neuron to extensor muscle originally stretched

Flexor muscle physician's tap on knee

Tibia

Patellar tendon

FIGURE 12-6

Neural pathways involved in the knee jerk reflex. Start with the begin logo. Tapping the patellar tendon stretches the extensor muscle, causing (paths A and C) compensatory contraction of this and other extensor muscles, (path B) relaxation of flexor muscles, and (path D) information about muscle length to be sent to the brain. Arrows indicate direction of action-potential propagation.

tapping, the thigh muscles to which it is attached are stretched, and all the stretch receptors within these muscles are activated. More action potentials are generated in the afferent nerve fibers from the stretch receptors and are transmitted to the motor neurons that control these same muscles. The motor units are stimulated, the thigh muscles contract, and the patient's lower leg is extended to give the knee jerk. The proper performance of the knee jerk tells the physician that the afferent fibers, the balance of synaptic input to the motor neurons, the motor neurons themselves, the neuromuscular junctions, and the muscles are all functioning normally.

During normal movement, in contrast to the knee jerk reflex, the stretch receptors in the various muscles are rarely all activated at the same time.

Because the afferent nerve fibers mediating the stretch reflex synapse directly on the motor neurons without the interposition of any interneurons, the stretch reflex is called monosynaptic. Stretch reflexes are the only known monosynaptic reflex arcs. All other reflex arcs—including nonmuscular reflexes—are polysynaptic, having at least one interneuron, and usually many, between the afferent and efferent neurons.

In path B of Figure 12-6, the branches of the afferent nerve fibers from stretch receptors end on interneu-rons that, when activated, inhibit the motor neurons controlling antagonistic muscles; these are muscles whose contraction would interfere with the reflex response (in the knee jerk, for example, the flexor muscles of the knee are inhibited). The activation of one muscle with the simultaneous inhibition of its antagonistic muscle is called reciprocal innervation and is characteristic of many movements, not just the stretch reflex.

Path C in Figure 12-6 activates motor neurons of synergistic muscles—that is, muscles whose contraction assists the intended motion (in our example, other leg extensor muscles). The muscles activated are on the same side of the body as the receptors, and the response is therefore ipsilateral; a response on the opposite side of the body is contralateral.

In path D in Figure 12-6, the axon of the afferent neuron continues to the brainstem and synapses there with interneurons that form the next link in the pathway that conveys information about the muscle length to areas of the brain dealing with motor control. This information is especially important, as we have mentioned, during slow, controlled movements such as the performance of an unfamiliar action. Ascending paths also provide information that contributes to the conscious perception of the position of a limb.

Alpha-Gamma Coactivation As indicated in Figure 12-5b, stretch on the intrafusal fibers decreases when the muscle shortens. In this example, the spindle

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems stretch receptors go completely slack at this time, and they stop firing action potentials. In this situation, there can be no indication of any further changes in muscle length the whole time the muscle is shortening. Physiologically, to prevent this loss of information, the two ends of each intrafusal muscle fiber are stimulated to contract during the shortening of the extra-fusal fibers, thus maintaining tension in the central region of the intrafusal fiber, where the stretch receptors are located (Figure 12-7). It is important to recognize that the intrafusal fibers are not large enough or strong enough to shorten a whole muscle and move joints; their sole job is to maintain tension on the spindle stretch receptors.

The intrafusal fibers contract in response to activation by motor neurons, but the motor neurons supplying them are usually not those that activate the ex-trafusal muscle fibers. The motor neurons controlling the extrafusal muscle fibers are larger and are classified as alpha motor neurons, whereas the smaller motor neurons whose axons innervate the intrafusal fibers are known as gamma motor neurons (Figure 12-7).

Afferent nerve fiber from stretch receptor

Axon from -

gamma motor neuron to intrafusal muscle fiber

Afferent nerve fiber from stretch receptor

Axon from -

gamma motor neuron to intrafusal muscle fiber

Intrafusal muscle fiber

FIGURE 12-7

As the ends of the intrafusal fibers contract in response to gamma motor neuron activation, they pull on the center of the fiber and stretch the receptor. The black arrows indicate direction of action-potential propagation.

Direction of force of contraction for this end

Intrafusal muscle fiber

Direction of force of contraction for this end

FIGURE 12-7

As the ends of the intrafusal fibers contract in response to gamma motor neuron activation, they pull on the center of the fiber and stretch the receptor. The black arrows indicate direction of action-potential propagation.

Both alpha and gamma motor neurons are activated by interneurons in their immediate vicinity and directly by neurons of the descending pathways. In fact, as described above, during many voluntary and involuntary movements they are coactivated—that is, excited at almost the same time. Coactivation ensures that information about muscle length will be continuously available to provide for adjustment during ongoing actions and to plan and program future movements.

Tension-Monitoring Systems Any given set of inputs to a given set of motor neurons can lead to various degrees of tension in the muscles they innervate, the tension depending on muscle length, the load on the muscles, and the degree of muscle fatigue. Therefore, feedback is necessary to inform the motor control systems of the tension actually achieved.

Some of this feedback is provided by vision as well as afferent input from skin, muscle, and joint receptors, but an additional receptor type specifically monitors how much tension is being exerted by the contracting motor units (or imposed on the muscle by external forces if the muscle is being stretched).

The receptors employed in this tensionmonitoring system are the Golgi tendon organs, which are located in the tendons near their junction with the muscle (see Figure 12-4). Endings of afferent nerve fibers are wrapped around collagen bundles in the tendon, bundles that are slightly bowed in the resting state. When the attached extrafusal muscle fibers contract, they pull on the tendon, which straightens the collagen bundles and distorts the receptor endings, activating them. Thus, the Golgi tendon organs discharge in response to the tension generated by the contracting muscle and initiate action potentials that are transmitted to the central nervous system.

Branches of the afferent neuron from the Golgi tendon organ cause widespread inhibition, via interneurons, of the motor neurons to the contracting muscle (A in Figure 12-8) and its synergists. They also stimulate the motor neurons of the antagonistic muscles (B in Figure 12-8). Note that this reciprocal innervation is the opposite of that produced by the muscle-spindle afferents.

To summarize, the activity of afferent fibers from the Golgi tendon organ supplies the motor control systems (both locally and in the brain) with continuous information about the muscle's tension. In contrast, the spindle afferent fibers provide information about the muscle's length.

The Withdrawal Reflex In addition to the afferent information from the spindle stretch receptors and Golgi tendon organs of the activated muscle, other input is fed into the local motor control systems. For example, painful stimulation of the skin activates the

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Control of Body Movement CHAPTER TWELVE

Control of Body Movement CHAPTER TWELVE

Neurons ending with:

> > Excitatory synapse

Inhibitory synapse

> > Excitatory synapse

Inhibitory synapse

Motor neuron to flexor muscles

Motor neuron to extensor muscles

Flexor muscle

Kneecap (bone)

FIGURE 12-8

Neural pathways underlying the Golgi tendon organ component of the local control system. In this diagram, contraction of the extensor muscles causes tension in the Golgi tendon organ and increases the rate of action-potential firing in the afferent nerve fiber. By way of interneurons, this increased activity results in (path A) inhibition of the motor neuron of the extensor muscle and its synergists and (path B) excitation of flexor muscles' motor neurons. Arrows indicate direction of action-potential propagation. % %

Spinal cord

Motor neuron to flexor muscles

Motor neuron to extensor muscles

Flexor muscle

Extensor muscle tendon with

Golgi tendon organ

Kneecap (bone)

FIGURE 12-8

Neural pathways underlying the Golgi tendon organ component of the local control system. In this diagram, contraction of the extensor muscles causes tension in the Golgi tendon organ and increases the rate of action-potential firing in the afferent nerve fiber. By way of interneurons, this increased activity results in (path A) inhibition of the motor neuron of the extensor muscle and its synergists and (path B) excitation of flexor muscles' motor neurons. Arrows indicate direction of action-potential propagation. % %

Neurons ending with:

y 0 Excitatory synapse synapse

To central nervous system synapse

To central nervous system

Spinal cord

FIGURE 12-9

In response to pain, the ipsilateral flexor muscle's motor neuron is stimulated (withdrawal reflex). In the case illustrated, the opposite limb is extended (crossed-extensor reflex) to support the body's weight. Arrows indicate direction of action-potential propagation.

ipsilateral flexor motor neurons and inhibits the ipsi-lateral extensor motor neurons, moving the body part away from the stimulus. This is called the withdrawal reflex (Figure 12-9). The same stimulus causes just the opposite response on the contralateral side of the body—activation of the extensor motor neurons and inhibition of the flexor motor neurons (the crossed-extensor reflex). In the example in Figure 12-9, the strengthened extension of the contralateral leg means that this leg can support more of the body's weight as the hurt foot is raised from the ground by flexion.

FIGURE 12-9

In response to pain, the ipsilateral flexor muscle's motor neuron is stimulated (withdrawal reflex). In the case illustrated, the opposite limb is extended (crossed-extensor reflex) to support the body's weight. Arrows indicate direction of action-potential propagation.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems