Spinal Reflexes

As noted in the previous chapter, a sense of body position is necessary for adaptive motor control.  In order to move a limb toward a particular location, it is imperative to know the initial starting position of the limb, as well as any force applied to the limb.  Muscle spindles and Golgi tendon organs provide this type of information.  In addition, these receptors are components of certain spinal reflexes that are important for both clinical diagnosis as well as for a basic understanding of the principles of motor control.

Myotatic reflex (stretch reflex)

The myotatic reflex is illustrated in Figure 2.1.  A waiter is holding an empty tray, when unexpectedly a pitcher of water is placed on the tray.  Because the waiter’s muscles were not prepared to support the increased weight, the tray should fall.  However, a spinal reflex is automatically initiated to keep the tray relatively stable.  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.  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.  Thus, activation of the Ia afferent causes a monosynaptic activation of the alpha motor neuron that causes the muscle to contract.  As a result, the stretch of the muscle is quickly counteracted, and the waiter is able to maintain the tray at the same position.

A major role of the myotatic reflex is the maintenance of posture.  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.  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.  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.

The myotatic reflex is an important clinical reflex.  It is the same circuit that produces the knee-jerk, or stretch, reflex.    When the physician taps the patellar tendon with a hammer, this action causes the knee extensor muscle to stretch abruptly.  This stretch activates the myotatic reflex, causing an extension of the lower leg.  (Because the physician taps the tendon, this reflex is also referred to as the deep tendon reflex.  Do not be confused, however, between this terminology and the Golgi tendon organ.  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.

Reciprocal inhibition in the stretch reflex

Joints are controlled by two opposing sets of muscles, extensors and flexors, which must work in synchrony.  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).  This inhibition is accomplished by an inhibitory interneuron in the spinal cord.  The Ia afferent of the muscle spindle bifurcates in the spinal cord (See Chapter 6 of Section I for review).  One branch innervates the alpha motor neuron that causes the homonymous muscle to contract, producing the behavioral reflex.  The other branch innervates the Ia inhibitory interneuron, which in turn innervates the alpha motor neuron that synapses onto the opposing muscle.  Because the interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing, thereby reducing the contraction of the opposing muscle.  Without this reciprocal inhibition, both groups of muscles might contract simultaneously and work against each other.

Autogenic inhibition reflex

The Golgi tendon organ is involved in a spinal reflex known as the autogenic inhibition reflex (Figure 2.3). When tension is applied to a muscle, the Group Ib fibers that innervate the Golgi tendon organ are activated.  These afferents have their cell bodies in the dorsal root ganglia, and they project into the spinal cord and synapse onto an interneuron called the Ib inhibitory interneuron. This interneuron makes an inhibitory synapse onto the alpha motor neuron that innervates the same muscle that caused the Ib afferent to fire.

As a result of this reflex, activation of the Ib afferent causes the muscle to cease contraction, as the alpha motor neuron becomes inhibited.  Because this reflex contains an interneuron between the sensory afferent and the motor neuron, it is an example of a disynaptic reflex.

For many years, it was thought that the function of the autogenic inhibition circuit was to protect the muscle from excessive amounts of force that might damage it.  A classic example is that of the weightlifter straining to raise a heavy load, when suddenly the autogenic inhibition reflex is activated and the muscle loses power, causing the weight to fall to the ground.  This function was ascribed to the reflex because early work suggested that the Golgi tendon organ was only activated when large amounts of force were applied to it. More recent evidence indicates, however, that the Golgi tendon organ is sensitive to much lower levels of force than previously believed.  Thus, the autogenic inhibition reflex may be more extensively involved in motor control under normal conditions.  One possibility is that this reflex helps to spread the amount of work evenly across the entire muscle, so that all motor units are working efficiently.  That is, if some muscle fibers are bearing more of the load than others, their Golgi tendon organs will be more active, which will tend to inhibit the contraction of those fibers.  As a result, other muscle fibers that are less active will have to contract more to pick up the slack, thereby sharing the work load more efficiently. 

Reciprocal excitation in the autogenic inhibition reflex

Just as in the stretch reflex, the autogenic inhibition reflex must coordinate the activity of the extensor and flexor muscle groups (Figure 2.4).  The Ib afferent fiber bifurcates in the spinal cord.  One branch innervates the Ib inhibitory interneuron.  The other branch innervates an excitatory interneuron that, in turn, innervates the alpha motor neuron that controls the antagonist muscle.  Thus, when the homonymous muscle is inhibited from contracting, the antagonist muscle is caused to contract, allowing the opposing muscle groups to work in synchrony.

Contact the author(s) at nba_course@uth.tmc.edu
© 2003-present, All Rights Reserved
The University of Texas Health Science Center at Houston
Created through the Multimedia Scriptorium in Academic Technology