The Synapse between Sensory Neurons and Antagonist Motor Neurons in the Patellar Reflex

In the patellar reflex, muscles other than the quadriceps muscle must be taken into account for a more complete description, shown in Figure 9-6. Whereas the quadriceps muscle extends the knee joint, antagonistic muscles at the back of the thigh flex the knee joint. These flexor muscles also have a stretch reflex analogous to that of the quadriceps. That is, stretching the flexor muscle stimulates action potentials in stretch-sensitive sensory neurons, which then make excitatory synapses in the spinal cord onto motor neurons of the flexor muscle. When the patellar tendon is tapped, the quadriceps muscle reflexively contracts, causing the knee joint to extend (the "jerk" of the knee-jerk reflex).

Flexor Quadriceps

Quadriceps (extensor) sensory neuron sensory neuron

Flexor Quadriceps

Quadriceps (extensor) sensory neuron sensory neuron

Figure 9-6 A revised diagram of the circuitry involved in stretch reflexes of thigh muscles.

Extension of the joint stretches the flexor muscles at the back of the thigh, which should then contract because of the action of their own stretch-reflex mechanism. The resulting flexion of the joint should again stretch the quadriceps and elicit reflexive extension, which should elicit another reflexive flexion, and so on. Thus, a single tap to the patellar tendon would send the knee joint into a series of oscillations that would continue until muscle exhaustion sets in.

Instead, tapping the patellar tendon elicits only a single kneejerk. What prevents the oscillatory response described above? The answer lies in the more elaborate neuronal circuitry diagrammed in Figure 9-6. The nerve fibers of the stretch-sensitive sensory neurons from the quadriceps muscle actually branch profusely when they enter the spinal cord and make synaptic connections with many kinds of neurons in addition to the quadriceps motor neurons. Among these other synaptic connections is an excitatory synapse onto neurons that in turn make an inhibitory synapse on the motor neurons of the antagonistic muscles. Thus, action potentials in quadriceps sensory neurons not only excite quadriceps motor neurons but also tend to prevent antagonistic motor neurons from being excited by the antagonistic sensory neurons by indirectly stimulating inhibitory inputs onto the antagonistic motor neurons.

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