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Chapter 16
Overview of the ANS
▪In the SNS, motor neurons of the CNS exert direct control over skeletal muscles, in the ANS,
motor neurons of the CNS synapse on visceral motor neurons in autonomic ganglia.
▪Visceral motor neurons in the CNS are called preganglionic neurons, their axons are called
preganglionic fibers.
▪Visceral motor neurons in autonomic ganglia are called ganglionic neurons, their axons are
called postganglionic fibers.
▪The ANS contains two major subdivisions: the sympathetic and parasympathetic divisions.
▫Sympathetic division- The “fight or flight” response
-preganglionic fibers from the spinal cord synapse in ganglia near the spinal
cord.
-increased sympathetic activity generally stimulates tissue metabolism and
increases alertness.
-also decreases digestive and urinary activities.
▫Parasympathetic division- The “rest and digest” response
-preganglionic fibers from the brain and spinal cord synapse in ganglia very
close to (or within) the target organs.
-stimulates visceral activity: digestion and urination for example
▫Enteric Nervous System- lesser known division, and will be discussed in greater detail in
CH 24
The Sympathetic Division
▪Preganglionic neurons are located between segments T1 and L2 of the spinal cord
▪Cell bodies of preganglionic neurons are situated in the lateral gray horns, and their axons enter
the ventral roots of these segments.
▪Ganglionic neurons are located in ganglia near the vertebral column
The ganglionic neurons occur in three locations:
1. sympathetic chain ganglia
2. collateral ganglia
3. suprarenal medullae
▪Sympathetic Chain Ganglia
▫Postganglionic fibers that control visceral effectors in the body wall, head, neck, or limbs
enter the gray ramus and return to the spinal nerve for subsequent distribution. (right
side of fig. 16-3 (a)).
▫Postganglionic fibers innervating structures in the thoracic cavity form bundles known as
sympathetic nerves (left side of fig. 16-3 (a).
▫Each sympathetic chain ganglion contains 3 cervical, 10-12 thoracic, 4-5 lumbar, and 45 sacral ganglia, plus 1 coccygeal ganglion.
▫Preganglionic neurons are limited to spinal cord segments T1-L2, and have both white
and gray rami.
▫Every spinal nerve has a gray ramus that carries sympathetic postganglionic fibers for
distribution in the body wall.
▪Collateral Ganglia
▫Preganglionic fibers pass through the sympathetic chain without synapsing.
▫Preganglionic fibers that innervate the collateral ganglia form the splanchnic nerves.
▫Celiac ganglion
▫Superior mesenteric ganglion
▫Inferior mesenteric ganglion
▪Adrenal Medulla- modified sympathetic ganglion
▫preganglionic fibers synapse on neuroendocrine cells, specialized neurons that secrete
hormones (epinephrine and norepinephrine) into the blood stream.
▫cells not innervated by postganglionic fibers are affected
▫effects last much longer than direct innervation.
▪Sympathetic Activation- The major effects include:
▫Increased alertness via RAS
▫Feeling of energy or euphoria
▫Increased activity in the cardiovascular and respiratory centers of the pons and medulla
oblongata.
▫General elevation in muscle tone
▫Mobilization of energy reserves.
▪Neurotransmitters and Sympathetic Function
▫Stimulation of sympathetic preganglionic neurons lead to the release of ACh at synapses
with ganglionic neurons.
▫Telodendria form branching networks with swollen segments called varicosities.
▫Most sympathetic ganglionic neurons release norepinephrine (NE)
▫Effects of NE last a few seconds
▫The effects of NE and E released from the adrenal medulla last even longer.
▪Receptors (1, 2,  1, 2, 3)
▫Alpha and beta receptors are G proteins, results vary on secondary messengers
produced.
▫1 receptors
▫2 receptors
▫1 receptors
▫2 receptors
▫3 receptors
▪Sympathetic release of ACh and NO
Sympathetic postganglionic neurons that release ACh innervate:
Sweat glands of skin
Smooth muscles of blood vessels to skeletal muscles and brain
Sympathetic postganglionic neurons that release NO innervate:
Smooth muscles of blood vessels to skeletal muscles and brain
Parasympathetic Division
▪Preganglionic neurons are located in the brain stem and in the sacral segments of the spinal
cord
▪Ganglionic neurons are in peripheral ganglia within or adjacent to the target organs.
▪Parasympathetic Activation- The major effects include:
▫Constriction of pupils
▫Secretion of digestive glands; salivary, gastric, duodenal, intestinal, pancreatic, and
livers
▫Secretion of hormones that promote the absorption and utilization of nutrients by
peripheral cells.
▫Changes in blood flow and glandular activity associated with sexual arousal
▫Increase in smooth muscle activity along digestive tract
▫The stimulation and coordination of defecation
▫Contraction of the urinary bladder during urination
▫Constriction of respiratory passageways
▫Reduction of heart rate and in force of contraction
▪Neurotransmitters and Parasympathetic Function
▫All parasympathetic neurons release ACh as a neurotransmitter
▫The overall effect is short-lived, due to the break down of ACh by enzymes
▫Effects on postsynaptic cell can vary widely due to variations in second messengers.
▫Two types of ACh receptors: nicotinic and muscarinic
-Nicotinic- located on both sympathetic and parasympathetic ganglion cells, as
well as neuromuscular junctions of the somatic nervous system
-Muscarinic- located at cholinergic neuromuscular or neuroglandular junctions in
the parasympathetic division as well as a few cholinergic junctions in the
sympathetic division.
Interactions Between the Sympathetic and Parasympathetic Divisions
▪Most vital organs receive innervation by both divisions, dual innervation.
▪At dual innervation sites, the effects are usually opposing (heart, digestive tract, and lungs),
other sites may see separate or complimentary responses.
▪Anatomy of Dual Innervation
▫Head and neck innervations
▫Thoracic innervations
▫Abdominopelvic innervations
▫Nerves leaving these networks travel with the blood vessels and lymph vessels that
supply visceral organs.
▪Autonomic Tone- autonomic motor neurons show a resting level of spontaneous activity, even in
the absence of stimuli.
▫Allows for an increase or decrease in activity, providing a wide range of control options.
Integration and Control of Autonomic Functions
▪Visceral reflexes▫Visceral reflex arc- consists of a receptor, a sensory neuron, a processing center, and
two visceral motor neurons.
▫All visceral reflexes are polysynaptic, and they are either long or
short.
-long reflex-short reflex▫Short reflexes may control patterns of activity in one small part of an organ, where long
reflexes may coordinate the activities of the entire organ.
▪Higher levels of Autonomic Control
▫Sympathetic and parasympathetic processes are subject to regulation by the
hypothalamus
▫Because the hypothalamus interacts with all portions of the brain, activity in the limbic
system, thalamus, and cerebral cortex can have a dramatic effect on autonomic function.
Higher Order Functions (memory and consciousness)
▪Higher order functions share three characteristics
1.
2.
3.
▪Memory- stored bits of information gathered through experience
▫Fact memories▫Skill memories▫Short-term memories (primary memories)▫Long-term memories-
-secondary memories-tertiary memories▫Memory consolidation▫The amygdaloid body and hippocampus are essential to memory consolidation
▫Tracts leading from the amygdaloid body to the hypothalamus may link memories to
certain emotions.
▫Most long-term memories are stored in the cerebral cortex.
▫Cellular mechanisms of memory formation and storage include the following:
1.
2.
3.
▫Memory engram▪States of Consciousness- conscious individuals are awake and alert, unconscious individuals are
not. There are many gradation levels of both states.
▫Sleep- unconscious but can still be awakened by normal sensory stimuli, two general
levels of sleep are recognized; deep sleep and REM sleep.
-Periods of REM and deep sleep alternate throughout the night.
-Roughly 25% of the US population has a sleep disorder.
▫Arousal- awakening from sleep, one of the functions of the RAS.
-the center of the RAS is located in the mesencephalon
-stimulation of the RAS will result in activity of the cerebral cortex and other
brain areas.
-inhibition of the RAS will result in lethargy and sleep.