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Chapter: Medical Physiology: Motor Functions of the Spinal Cord; the Cord Reflexes

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Crossed Extensor Reflex

About 0.2 to 0.5 second after a stimulus elicits a flexor reflex in one limb, the opposite limb begins to extend.

Crossed Extensor Reflex

About 0.2 to 0.5 second after a stimulus elicits a flexor reflex in one limb, the opposite limb begins to extend.

This is called the crossed extensor reflex. Extension of the opposite limb can push the entire body away from the object causing the painful stimulus in the with-drawn limb.

Neuronal Mechanism of the Crossed Extensor Reflex. Theright-hand portion of Figure 54–8 shows the neuronal circuit responsible for the crossed extensor reflex, demonstrating that signals from sensory nerves cross to the opposite side of the cord to excite extensor muscles. Because the crossed extensor reflex usually does not begin until 200 to 500 milliseconds after onset of the initial pain stimulus, it is certain that many interneurons are involved in the circuit between the incoming sensory neuron and the motor neurons of the opposite side of the cord responsible for the crossed extension. After the painful stimulus is removed, the crossed extensor reflex has an even longer period of afterdischarge than does the flexor reflex. Again, it is presumed that this prolonged after-discharge results from reverberating circuits among the interneuronal cells.


Figure 54–10 shows a typical myogram recorded from a muscle involved in a crossed extensor reflex. This demonstrates the relatively long latency before the reflex begins and the long afterdischarge at the end of the stimulus. The prolonged afterdischarge is of benefit in holding the pained area of the body away from the painful object until other nervous reactions cause the entire body to move away.




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