Spinal Reflexes (continued)
Spinal reflexes can be initiated by nonproprioceptive receptors as well as by proprioceptors. An important reflex initiated by cutaneous receptors and pain receptors is the flexor reflex. We have all experienced this reflex after accidentally touching a hot stove or a sharp object, as we withdraw our hand even before we consciously experience the sensation of pain. This quick reflex removes the limb from the damaging stimulus more quickly than if the pain signal had to travel up to the brain, be brought to conscious awareness, and then trigger a decision to withdraw the limb. The reflex circuit is illustrated in Figure 2.5. A sharp object touching the foot causes the activation of Group III afferents of pain receptors. These afferents enter the spinal cord and then travel up the cord. A branch of the afferent innervates an excitatory interneuron in the lumbar region of the spinal cord, which then excites an alpha motor neuron that causes contraction of the thigh flexor muscle. The Group III afferent also continues upward to the L2 vertebra, where another branch innervates an excitatory interneuron at this level. This interneuron excites the alpha motor neurons that excite the hip flexor muscle, allowing the coordinated activity of two muscle groups to withdraw the whole leg away from the painful stimulus. Thus, spinal reflexes work not only at a single joint; they can also coordinate the activity of multiple joints simultaneously.
Figure 2.5 |
Flexor reflex. (Nolte, 2002) Note: Locations of neurons within spinal cord are not meant to be anatomically accurate. |
Reciprocal inhibition in the flexor reflex
When the knee joints and the hip joints are flexed, the antagonist extensor muscles must be inhibited (just as in the stretch reflex). Thus, the Group III afferents innervate inhibitory interneurons that in turn innervate the alpha motor neurons controlling the antagonist muscle.
Further circuitry is needed to make the flexor reflex adaptive. Because the weight of the body is supported by both legs, the flexor reflex must coordinate the activity not only of the leg being withdrawn but also of the opposite leg (Figure 2.6). Imagine stepping on a tack, and having the flexor reflex withdraw your right leg immediately. The left leg must simultaneously extend in order to support the body weight that would have been supported by the right leg. Without this coordination of the two legs, the shift in body mass would cause a loss of balance. Thus, the flexor reflex incorporates a crossed extension reflex. A branch of the Group III afferent innervates an excitatory interneuron that sends its axon across the midline into the contralateral spinal cord. There it excites the alpha motor neurons that innervate the extensor muscles of the opposite leg, allowing balance and body posture to be maintained.
Figure 2.6 |
Crossed extension reflex. (Nolte, 2002) Note: Locations of neurons within spinal cord are not meant to be anatomically accurate. |
Recurrent inhibition of motor neurons (Renshaw cells)
Axons of alpha motor neurons bifurcate in the spinal cord and innervate a special inhibitory interneuron called the Renshaw cell (Figure 2.7). This interneuron innervates and inhibits the very same motor neuron that caused it to fire. Thus, a motor neuron regulates its own activity by inhibiting itself when it fires. This negative feedback loop is thought to stabilize the firing rate of motor neurons.
Figure 2.7 Renshaw cell. |
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