Without this reciprocal inhibition, both groups of muscles might contract simultaneously and work against each other.Īutogenic inhibition. Because the interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing, thereby reducing the contraction of the opposing muscle. The other branch innervates the Ia inhibitory interneuron, which in turn innervates the alpha motor neuron that synapses onto the opposing muscle. One branch innervates the alpha motor neuron that causes the homonymous muscle to contract, producing the behavioral reflex. The Ia afferent of the muscle spindle bifurcates in the spinal cord (See Chapter 6 of Section I for review). This inhibition is accomplished by an inhibitory interneuron in the spinal cord. Thus, when a muscle spindle is stretched and the stretch reflex is activated, the opposing muscle group must be inhibited to prevent it from working against the resulting contraction of the homonymous muscle (Figure 2.2). Joints are controlled by two opposing sets of muscles, extensors and flexors, which must work in synchrony. Reciprocal inhibition in the stretch reflex The myotatic reflex is initiated by the muscle spindle, not the Golgi tendon organ.) As discussed below, spinal reflexes can be modulated by higher levels of the hierarchy, and thus a hyperactive or hypoactive stretch reflex is an important clinical sign to localize neurological damage. Do not be confused, however, between this terminology and the Golgi tendon organ. (Because the physician taps the tendon, this reflex is also referred to as the deep tendon reflex. This stretch activates the myotatic reflex, causing an extension of the lower leg. When the physician taps the patellar tendon with a hammer, this action causes the knee extensor muscle to stretch abruptly. It is the same circuit that produces the knee-jerk, or stretch, reflex. The myotatic reflex is an important clinical reflex. The lower levels of the hierarchy implement the command with such mechanisms as the myotatic reflex, freeing the higher levels to perform other tasks such as planning the next sequence of movements. In this way, the higher levels of the motor system are able to send a simple command (“maintain current posture”) and then be uninvolved in its implementation. If one is standing upright and starts to sway to the left, muscles in the legs and torso are stretched, activating the myotatic reflex to counteract the sway. As a result, the stretch of the muscle is quickly counteracted, and the waiter is able to maintain the tray at the same position.Ī major role of the myotatic reflex is the maintenance of posture. Thus, activation of the Ia afferent causes a monosynaptic activation of the alpha motor neuron that causes the muscle to contract. The Ia afferents have their cell bodies in the dorsal root ganglia of the spinal cord, send projections into the spinal cord, and make synapses directly on alpha motor neurons that innervate the same (homonymous) muscle. When the heavy pitcher is placed on the tray, the increased weight stretches the biceps muscle, which results in the activation of the muscle spindle’s Ia afferents. However, a spinal reflex is automatically initiated to keep the tray relatively stable. Because the waiter’s muscles were not prepared to support the increased weight, the tray should fall. A waiter is holding an empty tray, when unexpectedly a pitcher of water is placed on the tray. The myotatic reflex is illustrated in Figure 2.1. Note: Locations of neurons within spinal cord are not meant to be anatomically accurate. This is also known as the stretch reflex, the knee-jerk reflex, and the deep tendon reflex.
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