Description
• Explain how various regions of the central nervous system regulate
autonomic nervous system function
• Explain how autonomic reflexes contribute to homeostasis
• Describe how the neuroeffector junction in the autonomic nervous
system differs from that of a neuron-to-neuron synapse
• Compare and contrast anatomical features of the sympathetic and
parasympathetic systems
• For each neurotransmitter in the autonomic nervous system, list the
neurons that release it and the type and location of receptors that
bind with it
• Describe the mechanisms by which neurotransmitters are removed
• Distinguish between cholinergic and adrenergic receptors
• Describe the overall and specific functions of the sympathetic system
• Describe the overall and specific functions of the parasympathetic
system
• Explain how effects of the catecholamines differ from those of direct
sympathetic stimulation
Parasympathetic division
Preganglionic neurons of the parasympathetic system arise from several
nuclei of the brainstem and from the sacral region of the spinal cord (seg-
ments S2 to S4; see Figure 9.1). The axons of the preganglionic neurons are
quite long compared to those of the sympathetic system and synapse with
postganglionic neurons within terminal ganglia that are close to or embedded
within the effector tissues. The very short axons of the postganglionic neu-
rons then provide input to the cells of that effector tissue.
The preganglionic neurons that arise from the brainstem exit the CNS
through cranial nerves. The occulomotor nerve (III) innervates the eyes; the
facial nerve (VII) innervates the lacrimal gland, salivary glands, and mucus
membranes of the nasal cavity; the glossopharyngeal nerve (IX) innervates
the parotid (salivary) gland; and the vagus nerve (X) innervates the viscera
of the thorax and abdomen (e.g., heart, lungs, stomach, intestines, and pan-
creas). The physiological significance of this latter nerve in terms of influence
of the parasympathetic system is clearly illustrated by its widespread dis-
tribution and the fact that 75% of all parasympathetic fibers are in the vagus
nerve. Preganglionic neurons that arise from the sacral region of the spinal
cord exit the CNS and join together to form the pelvic nerves. These nerves
innervate the viscera of the pelvic cavity (e.g., urinary bladder, colon).
Because the terminal ganglia are located within the innervated tissue,
there is typically little divergence in the parasympathetic system compared
to the sympathetic system. In many organs, the ratio of preganglionic fibers
to postganglionic fibers is 1:1. Therefore, the effects of the parasympathetic
system tend to be more discrete and localized, with only specific tissues
stimulated at any given moment, compared to the sympathetic system in
which a more diffuse discharge is possible.
9.7 Neurotransmitters of autonomic nervous system
The two most common neurotransmitters released by neurons of the ANS are
acetylcholine (Ach) and norepinephrine (NE). Several distinguishing features of
these neurotransmitters are summarized in Table 9.3. Nerve fibers that release
acetylcholine are referred to as cholinergic fibers and include all preganglionic
fibers of the ANS — sympathetic and parasympathetic systems; all postgan-
glionic fibers of the parasympathetic system; and sympathetic postganglionic
Endocrine system
Classification of hormones, mechanism of hormone action, structure and
functions of pituitary gland, thyroid gland, parathyroid gland, adrenal
gland, pancreas, pineal gland, thymus and their disorders
