The Motor Division

How do muscles and glands receive commands from the central nervous system?

The nervous system plays a key role in maintaining homeostasis by coordinating the activities of other systems and organs. Once it has gathered and processed sensory information, the nervous system sends commands to the rest of the body. The motor division of the peripheral nervous system transmits impulses from the central nervous system to muscles or glands. These messages are relayed through one of two divisions, the somatic nervous system or the autonomic nervous system.

Somatic Nervous System The somatic nervous system regulates body activities that are under conscious control, such as the movement of skeletal muscles. Most of the time you have control over skeletal muscle movement, but when your body is in danger the central nervous system may take over.

▸ Voluntary Control Every time you lift your finger or wiggle your toes, you are using motor neurons of the somatic nervous system. Impulses originating in the brain are carried through the spinal cord where they synapse with the dendrites of motor neurons. The axons from these motor neurons extend from the spinal cord carrying impulses directly to muscles, causing the contractions that produce voluntary movements.

▸ Reflex Arcs Although the somatic nervous system is generally considered to be under conscious control, some actions of the system occur automatically. If you accidentally step on a tack with your bare foot, your leg may recoil before you are even aware of the pain.

FIGURE 31–11 Reflex Arc When you step on a tack, sensory receptors stimulate a sensory neuron, which relays the signal to an interneuron within the spinal cord. The signal is then sent to a motor neuron, which in turn stimulates a muscle that lifts your leg.

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This rapid response (a reflex) is caused by impulses that travel a pathway known as a reflex arc, as shown in Figure 31–11. In this example, sensory receptors react to the sensation of the tack and send an impulse to sensory neurons. Sensory neurons relay the information to the spinal cord. An interneuron in the spinal cord processes the information and forms a response. A motor neuron carries impulses to its effector, a muscle that it stimulates. The muscle contracts and your leg moves. Meanwhile, impulses carrying information about the injury are sent to your brain. By the time your brain interprets the pain, however, your leg and foot have already moved. The spinal cord does not control all reflexes. Many reflexes that involve structures in your head, such as blinking or sneezing, are controlled by the brain.

In Your Notebook In your own words, describe how a reflex arc works. Include the role of the three types of neurons in your description.

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