HUMAN BIOLOGY

250 Chapter 13

autonomic nerves

Nerves that service

internal organs.

somatic nerves Nerves

that carry signals related

to head, trunk, and limb

movements.

Major expressways: peripheral nerves

and the spinal Cord

Unlike somatic neurons, single autonomic neurons do

not extend the entire distance between muscles or glands

and the central nervous system. Instead, preganglionic

(“before a ganglion”) neurons have cell bodies inside the

spinal cord or brain stem, but their axons travel through

nerves to autonomic system ganglia outside the CNS.

There, the axons synapse with post ganglionic (“after a gan-

glion”) neurons, which make the actual connection with

effectors—the body’s muscles and glands.

autonomic nerves are divided into

parasympathetic and sympathetic groups

Autonomic nerves are divided further into parasympathetic

and sympathetic groups. Normally these two sets of nerves

work antagonistically—the signals from one oppose those

of the other. However, both these groups of nerves carry

exciting and inhibiting signals to internal organs. Often

their signals arrive at the same time at muscle or gland

cells and compete for control. When that situation arises,

synaptic integration leads to minor adjustments in an

organ’s activity.

n    Peripheral nerves and the spinal cord carry signals to and

from the brain.

the peripheral nervous system consists

of somatic and autonomic nerves

Nerves of the PNS are grouped by function. To begin with,

the cranial and spinal nerves are subdivided into somatic

and autonomic categories. Somatic

nerves carry signals related to

movements of the head, trunk, and

limbs. autonomic nerves carry

signals beween internal organs and

other structures.

In somatic nerves, sensory axons

carry information from receptors in

skin, skeletal muscles, and tendons

to the central nervous system. Their motor axons deliver

commands from the brain and spinal cord to skeletal

muscles. In the autonomic category, motor axons of spi-

nal and cranial nerves carry messages to smooth muscle,

cardiac (heart) muscle, and glands (Figure 13.13).

Spinal nerves

Optic nerve

Cranial nerves

Parasympathetic Division Sympathetic Division

Vagus

nerve

Chain of

sympathetic

ganglia

Genitals

Bladder

Intestines

Liver

Stomach

Heart

Salivary glands

Eyes

Lungs

Genitals

Bladder

Intestines

Liver

Stomach

Heart

Salivary glands

Eyes

Lungs

Dilates pupil; adjusts

eye for far vision

Constricts pupil; adjusts
eye for near vision

Stimulates secretion

Decreases heart rate

Constricts

bronchioles (airways)

Stimulates stomach

activity

Inhibits glucose

release

Stimulates activity

Stimulates contraction

(emptying)

Stimulates penile or

clitoral arousal

Inhibits stomach

activity

Stimulates glucose

release

Inhibits activity

Relaxes bladder

muscles

Inhibits penile or

clitoral arousal

Increases heart rate

Dilates bronchioles

Inhibits secretion

Figure 13.13 Animated! autonomic nerves serve internal organs. This is a diagram of the major sympathetic and parasympathetic

nerves leading out from the central nervous system to some major organs. There are pairs of both kinds of nerves, servicing the right

and left halves of the body. The ganglia are clusters of cell bodies of the neurons that are bundled together in nerves. (© Cengage Learning)

13.6

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