Somatic Nervous System

Within the peripheral nervous system's motor division, the somatic nervous system contains motor neurons that control the movement of skeletal muscles. The somatic system is voluntary— that is, skeletal muscles can be moved at will. The somatic system can also operate without conscious control, as it does when helping to maintain balance.

The peripheral nervous system relays signals in reflexes, which are involuntary and often self-protective movements. The patellar reflex is shown in Figure 49-9. A tap on the tendon below the patella stimulates sensory receptors in the quadriceps muscle. The receptors send impulses to the spinal cord. These impulses activate motor neurons that lead back to the quadriceps. As a result, the quadriceps contracts. The impulses also activate interneurons that have an inhibitory, or calming, effect on the motor neurons of the hamstring muscles in the rear thigh. The contraction of the quadriceps coupled with the relaxation of the hamstrings extends the lower leg. This type of reflex is a spinal reflex. It involves only neurons in the body and bypasses the brain.

figure 49-9

In the patellar, or knee-jerk, reflex, a sensory receptor (red) that detects stretch in the quadriceps muscle sends signals to the spinal cord. The sensory

TABLE 49-1 Effects of Sympathetic and Parasympathetic Divisions on Various Organs


Effect of sympathetic division

Effect of parasympathetic division


Pupils dilate

Pupils constrict


Heart rate increases

Heart rate decreases

Blood vessels

Blood vessels to skeletal muscles dilate

Little or no effect

Adrenal glands

Glands secrete hormones

Glands inactive


Gastric secretions decrease

Gastric secretions increase

Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

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