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

Organ

Effect of sympathetic division

Effect of parasympathetic division

Eyes

Pupils dilate

Pupils constrict

Heart

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

Intestines

Gastric secretions decrease

Gastric secretions increase

Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Post a comment