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PART 3 Integration and Control Systems

Learn to Predict

From page 404

Answer

toy car with his right foot, he transferred nearly all of his weightto his right leg before the withdrawal reflex was activated. Thepainful stimulus of stepping on the toy car activated the with-drawal reflex, causing Javier to pick up his right leg. At the sametime, Javier extended his left leg because of the crossed exten-sor reflex. However, as Javier extended his left leg, he steppedon another toy car, activating the withdrawal reflex in his left legand the crossed extensor reflex in his right leg. The repeated se-quence of withdrawing one leg and stepping down on the otherfoot prevented Javier from falling down.

Javier reacted to the pain of stepping on the toy cars not once buttwice. We know that each time he stepped on a toy he switchedthe leg that supported his weight, first from the right to the leftand then from the left to the right. This chapter described thewithdrawal reflex that moves a limb away from a painful stimu-lus. Javier demonstrated this reflex when he stepped on the toys.Javier displayed the withdrawal reflex and the crossedextensor reflex. Both of these reflexes are polysynaptic spinalreflexes, involving sensory neurons, interneurons, and motor neu-rons. Chapter 11 explained the functions of each of these neurontypes. The sensory neurons delivered the painful stimulus to thecentral nervous system; the interneurons relayed the informationbetween sensory and motor neurons; and the motor neuronsregulated skeletal muscle activity. When Javier stepped on the

Answers to the rest of this chapter’s Predict questions are inappendix G.

12.1 Spinal Cord (p. 405)

General Structure

1. The spinal cord gives rise to 31 pairs of spinal nerves. Nerves of thelimbs enter and leave the spinal cord at the cervical and lumbosacralenlargements.2. The spinal cord is shorter than the vertebral column. Nerves from theend of the spinal cord form the cauda equina.

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Summary

Interneurons in the CNS synapse with sensory neurons and withmotor neurons. ■ Motor neurons carry action potentials from the CNS to effectororgans. ■ Effector organs, such as muscles or glands, respond to the actionpotentials.2. Reflexes do not require conscious thought, and they produce a con-sistent and predictable result.3. Reflexes are homeostatic.4. Reflexes are integrated within the brain and spinal cord. Higher braincenters can suppress or exaggerate reflexes.

Meninges of the Spinal Cord

Three meningeal layers surround the spinal cord: the dura mater, arach-noid mater, and pia mater.

Cross Section of the Spinal Cord

1. The spinal cord consists of peripheral white matter and central graymatter.2. White matter is organized into columns, which are subdivided intonerve tracts, or fascicles, which carry action potentials to and fromthe brain.3. Gray matter is divided into horns. ■ The dorsal horns contain sensory axons that synapse with inter-neurons. The ventral horns contain the neuron cell bodies of somaticmotor neurons, and the lateral horns contain the neuron cell bodiesof autonomic motor neurons. ■ The gray and white commissures connect each half of the spinal cord.4. The dorsal root conveys sensory input into the spinal cord, and theventral root conveys motor output away from the spinal cord.

Stretch Reflex

Muscle spindles detect the stretch of skeletal muscles and cause the muscleto shorten reflexively.

Golgi Tendon Reflex

Golgi tendon organs respond to increased tension within tendons andcause skeletal muscles to relax.

Withdrawal Reflex

1. Activation of pain receptors causes muscles to contract and movesome part of the body away from a painful stimulus.2. Reciprocal innervation causes muscles that would oppose withdrawalto relax.3. In the crossed extensor reflex, flexion of one limb caused by thewithdrawal reflex stimulates the opposite limb to extend.

12.2 Reflexes (p. 408)

1. A reflex arc is the functional unit of the nervous system. ■ Sensory receptors respond to stimuli and produce action potentialsin sensory neurons. ■ Sensory neurons propagate action potentials to the CNS.

Interactions with Spinal Cord Reflexes

Convergent and divergent pathways interact with reflexes.