Page 572 - Fundamentals of anatomy physiology

Chapter 16 The Autonomic Nervous System and Higher-Order Functions 559
Figure 16–1 The Organization of the Somatic and Autonomic Nervous Systems.

Upper motor Visceral motor
neurons in nuclei in

primary motor hypothalamus
cortex

Somatic motor Brain Brain
nuclei of brain
Spinal cord Preganglionic
stem Somatic motor neuron
nuclei of
spinal cord Visceral Effectors

Skeletal Lower Smooth Autonomic Autonomic
muscle motor muscle ganglia nuclei in
neurons Glands brain stem
Ganglionic
Cardiac neurons Spinal cord
muscle
Autonomic
Adipocytes nuclei in
spinal cord
Preganglionic 16
neuron

Skeletal
muscle

a Somatic nervous system b Autonomic nervous system

in the brain stem and spinal cord are known as preganglionic you and make you jump, but thanks to the ANS, that sound
neurons because they extend to ganglia. These neurons are can also increase your heart rate dramatically and temporar-
part of visceral reflex arcs. Most of their activities represent di- ily stop all digestive gland secretion. These changes in visceral
rect reflex responses, rather than responses to commands from activity take place in response to neurotransmitters released
the hypothalamus. The axons of preganglionic neurons are by postganglionic fibers. As noted in Chapter 12, a specific
called preganglionic fibers. neurotransmitter may stimulate or inhibit activity, depending
on the response of particular plasma membrane receptors. We
Preganglionic fibers leave the CNS and synapse on gangli- consider the major types of receptors later in the chapter. Now
onic neurons—visceral motor neurons in peripheral ganglia. let’s turn to the anatomy and physiology of the ANS.
These ganglia, which contain hundreds to thousands of gan-
glionic neurons, are called autonomic ganglia. Ganglionic &T i p s T r i c k s
neurons innervate visceral effectors such as smooth muscle, Each autonomic ganglion functions somewhat like a baton
glands, cardiac muscle, and adipocytes. The axons of ganglionic handoff in a relay race. Within the ganglion, one runner (the
neurons are called postganglionic fibers, because they begin preganglionic fiber) hands off the baton (a neurotransmitter)
at the autonomic ganglia and extend to the peripheral target to the next runner (the postganglionic fiber), who then con-
organs. tinues on toward the finish line (the target effector).

Somatic or visceral sensory information can trigger vis- Divisions of the ANS
ceral reflexes, and the ANS distributes the motor commands
of those reflexes. Sometimes those motor commands control You are probably already familiar with the names of the two
the activities of target organs. For example, in cold weather, the main subdivisions of the ANS: the sympathetic division and the
ANS stimulates the arrector pili muscles and gives you “goose- parasympathetic division (Spotlight Figure 16-2).
bumps.” p. 189 In other cases, the motor commands may
alter some ongoing activity. A sudden, loud noise can startle