CHAPTER 12 Spinal Cord and Spinal Nerves

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contract and reciprocal innervation causes the extensor muscles torelax. This reduces the resistance to movement that the extensormuscles would otherwise generate.Reciprocal innervation is also involved in the stretch reflex.When the stretch reflex causes a muscle to contract, reciprocalinnervation causes opposing muscles to relax. In the patellar reflex,for example, the quadriceps femoris muscle contracts and thehamstring muscles relax.

Crossed Extensor Reflex

The crossed extensor reflex is another reflex associated with thewithdrawal reflex (figure 12.10). The crossed extensor reflex occurson the opposite side of the body from the stimulus. When the with-drawal reflex occurs in one limb, the crossed extensor reflex causesthe opposite response to occur in the other limb. This mechanism ispossible because the interneurons that stimulate alpha motor neu-rons to cause withdrawal of a limb also have collateral axons that

Crossed extensor reflex

1 During the withdrawal reflex, sensory neuronsfrom pain receptors conduct action potentialsto the spinal cord.

2 Sensory neurons synapse with excitatoryinterneurons that are part of thewithdrawal reflex.

3 The excitatory interneurons that are part of thewithdrawal reflex stimulate alpha motor neuronsthat innervate flexor muscles, causing withdrawalof the limb from the painful stimulus.

4 Collateral branches of the sensory neuronsalso synapse with excitatory interneuronsthat cross to the opposite side of the spinalcord as part of the crossed extensor reflex.

5 The excitatory interneurons that cross the spinalcord stimulate alpha motor neurons supplyingextensor muscles in the opposite limb, causingthem to contract and support body weight duringthe withdrawal reflex.

Sensoryneuron

extend through the white commissure to the opposite side of thespinal cord. These interneurons synapse with alpha motor neuronsthat innervate extensor muscles in the opposite side of the body.When a withdrawal reflex is initiated in one lower limb, the crossedextensor reflex causes extension of the opposite lower limb.The crossed extensor reflex is adaptive in that it helps preventfalls by shifting the weight of the body from the affected to theunaffected limb. For example, when you step on a sharp object withyour right foot, you withdraw your right lower limb from thestimulus (withdrawal reflex) while extending your left lower limb(crossed extensor reflex). Therefore, your body weight shifts fromthe right to the left lower limb. Initiating a withdrawal reflex inboth legs at the same time would cause you to fall.

Interactions with Spinal Cord Reflexes

Reflexes do not operate as isolated entities. Rather, because ofdivergent and convergent pathways (see chapter 11), their activi-ties are integrated with the functions of the nervous system as awhole. Diverging branches of the sensory neurons or interneurons ina reflex arc send action potentials along ascending nerve tracts tothe brain (figure 12.11). A pain stimulus, for example, not only initi-ates a withdrawal reflex, causing you to remove the affected bodypart from the painful stimulus, but also enables you to perceive thepain as a result of action potentials sent to your brain.Axons within descending tracts from the brain carry actionpotentials to motor neurons in the anterior horn of the spinal cord,

To brain

5

3

Alpha motorneuron

Excitatoryinterneuron

Crossedextensorreflex

PROCESS FIGURE 12.10 Withdrawal Reflex with Crossed Extensor Reflex