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Chapter 15
The Autonomic
Nervous System
Introduction to the ANS In this chapter, we examine the structural and functional
features of the autonomic nervous system (ANS) and compare the organization and actions of its two major parts, the sympathetic and parasympathetic divisions. The autonomic nervous system contributes to homeostasis
by responding to subconscious visceral sensations and exciting or inhibiting smooth muscle, cardiac muscle, and many glands.
Structurally, the ANS includes autonomic sensory neurons, integrating centers in the CNS, and autonomic motor neurons.
The enteric division is a specialized network of nerves and ganglia forming an independent nerve network within the wall of the gastrointestinal (GI) tract.
Introduction to the ANS ANS has many distinctive features which set it apart from
SNS. Perhaps the biggest difference between these two
systems is the involvement of conscious control. The ANS usually operates without conscious control,
though centers in the hypothalamus and brain stem do provide regulation for ANS reflexes. Sensory receptors called interoceptors located in blood
vessels, visceral organs, muscles, and the nervous system monitor conditions in the internal environment.
Examples of interoceptors are chemoreceptors that monitor blood CO2 level and mechanoreceptors that detect the degree of stretch in the walls of organs or blood vessels.
Introduction to the ANS The anatomy of all autonomic pathways can best be understood
by picturing a double-barrelled neuronal construct consisting of
a preganglionic neuron leading to an intermediate ganglion that
contains the cell bodies of post-ganglionic neurons (that
innervate an effector).
Introduction to the ANS
Most body organs have dual ANS innervation; that is, they receive impulses from both sympathetic and parasympathetic neurons. Usually the nerve impulses from one division
stimulate an organ, while impulses from the other division decrease activity.
Furthermore, the responses of the various organs to
ANS stimulation neatly group into two functional
categories :
Like children on a teeter-totter, the sympathetic
divisions “fight or flight” response is balanced
against the “rest and relax” (or rest and digest)
activities of the parasympathetic division.
Divisions of the ANS
Divisions of the ANS
The Sympathetic Division • The cell bodies of neurons which participate in motor
responses of the sympathetic nervous system are located in
the lateral horns of the gray matter in the 12 thoracic
segments and the first two lumbar segments of the cord.
– Sympathetic preganglionic neurons exit the spinal cord
only between levels T1-L2 (hence the name thoracolumbar
division), though sympathetic ganglia extend in the vicinity
of the cord from the cervical to the sacral region.
Some of the major groups of sympathetic
ganglia include:
The sympathetic trunk
(vertebral chain) ganglia
Prevertebral ganglia
The celiac, superior
mesenteric, inferior
mesenteric, aorticorenal
and renal ganglia
The Sympathetic Division
The Sympathetic Division Axons leave the sympathetic trunk in four
possible ways:
They can enter and travel with spinal nerves.
They can form fine networks of periarterial
preganglionic traveling cephalad to synapse in the
cervical ganglia.
Postganglionic axons exiting the sympathetic trunk
can form sympathetic nerves to the heart and lungs.
Preganglionic axons can leave the sympathetic trunk
without synapsing and form splanchnic nerves.
The Sympathetic Division
Major groups of sympathetic ganglia.
The Parasympathetic Division The cell bodies of preganglionic neurons which participate in
motor responses of the parasympathetic nervous system are located in nuclei of 4 cranial nerves in the brainstem (III, VII, IX and X) and in the lateral gray matter of sacral areas of the spinal cord (S2-S4). Four pairs of cranial parasympathetic ganglia innervate
structures in the head: The ciliary, pterygopalatine, submandibular, and otic ganglia. • The cranial-sacral division also has the ganglia
associated with the vagus (X) nerve and the sacral nerves.
The vagus nerve (CN X) carries nearly 80% of the total parasympathetic flow to the organs of the thorax and upper abdomen. Lower abdominal and pelvic organs are innervated by the sacral output.
• The sacral preganglionic axons branch off of sacral spinal nerves to form pelvic splanchnic nerves which synapse with parasympathetic postganglionic neurons located in terminal ganglia in the walls of the innervated viscera.
• From the terminal ganglia, postganglionic axons innervate smooth muscle and glands in the walls of the colon, ureters, urinary bladder, and reproductive organs
The Parasympathetic Division
The Parasympathetic Division In contrast to the sympathetic system, the parasympathetic response is more controlled.
Presynaptic parasympathetic neurons
usually synapse with only 4–5 postsynaptic neurons, all of which supply a single visceral effector. Parasympathetic stimulation leads to a narrow, focused action on specific organs.
The division of the sympathetic and parasympathetic
divisions of the ANS are compared in Table 15.3
ANS Neurotransmitters The total number of neurotransmitters used in the entire nervous
system is not known, but is well over 100. Despite the variety of possible chemicals that could be used to
transmit chemical messages in the ANS, only 2, acetylcholine and norepinephrine, are used to any great degree. • Synapses at which ACh is used are termed cholinergic. • Synapses at which norepinephrine or epinephrine are used
are termed adrenergic. The neurotransmitter used in all of the synapses of sympathetic
and parasympathetic ganglia (between the synapses of the preganglionic and postganglionic fibers) is acetylcholine. Receptors that respond to Ach released by these cholinergic
neurons are called cholinergic receptors and there are 2 subtypes: nicotinic receptors (found in the ganglia) and muscarinic receptors (found in the synapses with the effector organs).
ANS Neurotransmitters
Acetylcholine acts on a sub-
type of cholinergic receptor
(called nicotinic receptors) at
ganglia of the ANS.
ANS Neurotransmitters
The neurotransmitter used at
most sympathetic postganglionic
synapses is norepinephrine.
The exception to this rule is
that ACh is used at sympathetic
postganglionic synapses for
sweat glands.
ANS Neurotransmitters
The neurotransmitter used at all parasympathetic postganglionic synapses is Ach. These are all a variety
of cholinergic receptors called muscarinic.
ANS Neurotransmitters
Neurons and Neurotransmitters of the Parasympathetic Nervous System
Preganglionic Postganglionic
Cell body in brain or spinal cord
Cell body in intramural ganglion
Acetylcholine (ACh) Acetylcholine (ACh)
ANS Neurotransmitters
Neurons and Neurotransmitters of the Sympathetic Nervous System
Preganglionic Postganglionic
oCell body in lateral horn of ospinal cord
Cell body in sympathetico chain gangliono
oAcetylcholine (ACh) (norepinephrine, NE) ol
except sweat glands (Ach) o
Physiology of the ANS Sympathetic stimulation leads to secretion of norepinephrine by the adrenal
glands, an increase in the rate and strength of the heartbeat, constriction of blood vessels of non-essential organs, dilation of vessels of essential organs (skeletal muscle and the cerebral cortex), an increase in the rate and depth of breathing, hepatic conversion of glycogen to glucose, and decrease in GI activity.
SLUDD is as an acronym used to describe the responses of the
parasympathetic nervous system:
Salivation (increased)
Lacrimation (increased)
Urination (increased)
Digestion (increased)
Defecation (increased)
• … and 3 decreases (in the rate and force of the heart beat, airway
size and rate of breathing, and pupil size)
Physiology of the ANS
Physiology of the ANS
Physiology of the ANS