UNIT 12 - Motor Control Mechanism
In this chapter, we will cover those mechanisms governing the four components of the Systems Model of motor control with emphasis on the role of the muscle spindle.
The four major components of this model consist of corticospinal, corticoreticular, planning and postural control mechanisms. Emphasis is placed on the muscle spindle as all components of the system rely heavily on this information as the muscle spindle provides exact information about the status of our skeletal musculature and can be integrated more rapidly than other incoming sensory input. Therefore, in order to have a comprehensive understanding of motor control both from a functional and postural standpoint, one must understand the major role the muscle spindle plays in both these aspects of motor function. First we will examine the role the muscle spindle plays in the stretch reflex which is an important component of postural control being our first line of defense to a loss of balance.
1. The Stretch Reflex
The stretch reflex is based upon the function of the muscle spindle. As previously discussed, the muscle spindle, which contains intrafusal fiber lies in parallel with the extrafusal muscle fibers and is controlled by the gamma motor neurons. The role of the gamma motor neuron is to control the contraction of the intrafusal fibers in the polar region of the spindle so that the spindle always maintains the same length as the muscle in which it lies. Muscle fibers called extrafusal fibers are controlled by the alpha motor neurons. The two types of stretch reflexes that can be elicited are the myotatic, or monosynaptic stretch reflex (dynamic) and the postural, or polysynaptic stretch reflex (static).
a. Myotatic or Monosynaptic Stretch Reflex
This monosynaptic reflex is a simple involuntary response of a muscle to stretch. Although present in all musculature, the myotatic stretch mechanism is normally strongest in antigravity muscles. Its strength is related directly to gamma motor neuron activity and can therefore be altered from other levels. This reflex is non-functional and is seen at rest. The only example of a myotatic stretch reflex is the tendon tap. The following describes the mechanism of the myotatic stretch reflex.
There is always a low level of constant gamma motor activity in the muscle spindle which keeps the spindle biased to the length of the muscle fiber even when at rest. Thus, when a muscle is stretched, the muscle spindle is also stretched no matter the state of activity for that muscle. This sudden stretching of the muscle causes the annulospiral endings or IA fibers on the spindle to send action potentials to the cerebellum (for monitoring purposes) and to the lateral portion of lamina IX in the spinal cord via a synapse within the internuncial pool. This portion of lamina IX controls alpha motor neuron activity, therefore when stimulated by this input causes contraction of the extrafusal muscle fibers. This resultant contraction puts the spindle on slack (out of bias) and the annulospiral endings, or IA fibers, are no longer stimulated, thus extrafusal contraction ceases.
b. Postural/Polysynaptic Stretch Reflex
This reflex occurs by a mechanism similar to that of the myotatic/monosynaptic stretch reflex. It is a functional reflex and is seen when movement is occuring or a static position is being maintained. As movement occurs or a posture is maintained both the alpha and gamma motor neurons are facilitated. The alpha motor neurons cause contraction of the extrafusal fibers, and the gamma motor neurons cause contraction of the intrafusal fibers which allows the spindle to maintain the same length as the extrafusal muscle fibers. Thus, if a muscle is suddenly stretched the IA fiber on the equatorial region of the nuclear bag will fire, sending an unsuspected burst of impulses to the cerebellum (for monitoring purposes) and to the internuncial pool of the spinal cord. When this unsuspected input reaches the internuncial pool, a polysynaptic reflex is set off facilitating both the alpha (lateral portion of lamina IX) and gamma motor neurons (medial portion of lamina IX) to fire. The alpha motor neurons will cause the muscle to contract, while the gamma motor neurons will cause the muscle spindle to contract appropriately with the muscle fiber. This mechanism allows the contraction to be sustained until altered by input from the cerebellum, as well as to allow that muscle to instantaneously respond if again stretched.
It is important to remember that the resting tension of the muscle spindle can be altered (increased) to place our body musculature in a state of readiness to respond to a loss of balance or to prepare the musculature for movement. The altering of muscle tone via the muscle spindle increases or decreases depending on the situation and is achieved by output from the reticular formation via medial and lateral reticulospinal tracts, which in turn is controlled by the cerebellum. The vestibular nuclei are primarily concerned with altering tone within the antigravity muscles during high risk activities based on vestibular input as does the archi/vestibulocerebellum for our non-antigravity muscles. Total postural control involves much more than the above outlined stretch mechanism which is only our initial response to a loss of balance. Also to be considered is the response of the cerebellum and vestibular nuclei along with the importance of visual and vestibular input.
Fig. 1 –
Fig. 2 –
The monosynaptic stretch reflex. (b) A useful function, Carlson's Movement, Reflexive Control of Movement, http://nawrot.psych.ndsu.nodak.edu/Courses/
Psych465.S.02/Movement/Reflex.html
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