AUTONOMIC
NERVOUS SYSTEM
The autonomic system controls the
visceral functions of the body: arterial pressure,
gastrointestinal motility and secretion, urinary
bladder emptying, sweating, body
temperature, and many other activities.
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There are two major subdivisions in the
autonomic nervous system:
a) sympathetic nervous system, and
b) parasympathetic nervous system.
General scheme of the reflex arc of the
autonomic nervous system
receptor – afferent nerve
fiber – nerve center –
preganglionic efferent
nerve fiber – peripheral
ganglion – postganglionic
efferent nerve fiber –
visceral organ.
Effectors

Somatic nervous system
– Skeletal muscles

ANS
– Cardiac muscle
– Smooth muscle
– Glands
Efferent Pathways
 Somatic
nervous system
– A, thick, heavily myelinated somatic motor fiber
makes up each pathway from the CNS to the
muscle
 ANS
pathway is a two-neuron chain
1. Preganglionic neuron (in CNS) has a thin, lightly
myelinated preganglionic axon
2. Ganglionic neuron in autonomic ganglion has an
unmyelinated postganglionic axon that extends
to the effector organ
Neurotransmitter Effects

Somatic nervous system
– All somatic motor neurons release acetylcholine
(ACh)
– Effects are always stimulatory

ANS
– Preganglionic fibers release ACh
– Postganglionic fibers release norepinephrine or
ACh at effectors
– Effect is either stimulatory or inhibitory,
depending on type of receptors
Divisions of the ANS
Sympathetic division
2. Parasympathetic division
 Dual innervations
–Almost all visceral organs are served
by both divisions, but they cause
opposite effects
1.
SYMPATHETIC NERVOUS SYSTEM
Preganglionic neurons (sympathetic nerve
centers) lie in the spinal cord from T-1 to L-2
segments (in the intermediolateral horn).

Postganglionic neurons lie either in the two
paravertebral sympathetic chains of ganglia or
two prevertebral ganglia (the celiac and
gipogastric). The postganglionic nerve fibers
travel to their destinations in the various
organs.

Approximate distribution of the sympathetic
innervation:

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From T-1
to the head
From T-2
into the neck
From T-3, 4, 5, 6 into the thorax
From T-7, 8, 9, 10, 11 into the
abdomen
From T-12, L-1, 2 into the legs.
Note: some sympathetic
preganglionic nerve fibers pass all
the way (without synapsing) from
the spinal cord into the adrenal
medullae. They end directly on
modified neuronal cells that secrete
epinerphine and norepinerphine
into the blood stream.
Role of the Sympathetic Division


Mobilizes the body during activity; is the
“fight-or-flight” system
Promotes adjustments during exercise, or
when threatened
– Blood flow is shunted to skeletal muscles and
heart
– Bronchioles dilate
– Liver releases glucose
PARASYMPATHETIC NERVOUS SYSTEM
a)
b)
Preganglionic neurons lie
into the brain stem (nuclei
of the cranial nerves III,
VII, IX, and X) and
into the sacral segments
of the spinal cord.
Postganglionic neurons lie
in
the wall of the innervated
organs
(except a few parasympathetic
nerves). Postganglionic nerve
fibers are short (from a few
mm to cm).
Role of the Parasympathetic Division

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Promotes maintenance activities and
conserves body energy
Its activity is illustrated in a person who
relaxes, reading, after a meal
– Blood pressure, heart rate, and respiratory
rates are low
– Gastrointestinal tract activity is high
– Pupils are constricted and lenses are
accommodated for close vision
Distribution of parasympathetic innervation:

III cranial nerve – ciliary ganglion – ciliary muscle of eye;
papillary sphincter;
VII cranial nerve –
a) sphenopalatine ganglion – lacrimal glands; nasal glands;
b) submandibular ganglion – submandibular glands;

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IX cranial nerve – optic ganglion – parotid gland;

X cranial nerve – into the thorax and abdomen;

Sacral segments of the spinal cord – into the pelvis
(rectum, bladder).
NEUROTRANSMITTERS:
1.
All preganglionic neurons are cholinergic in both
sympathetic and parasympathetic nervous systems;
2.
All postganglionic neurons of the parasympathetic
nervous system are cholinergic;
3.
Most of the postganglionic neurons of the
sympathetic nervous system are adrenergic;
NOTE: postganglionic sympathetic nerve fibers
to the sweat glands, the piloerector muscles and a
few blood vessels are cholinergic as an exception.
RECEPTORS FOR
NEUROTRANSMITTERS
There are two principle types of acetylcholine
receptors in the postsynaptic membrane:
1.

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nicotinic receptors – in the synapses between preganglionic
and postganglionic neurons of both the sympathetic and
parasympathetic nervous systems;
muscarinic receptors – between postganglionic
parasympathetic nerve firbers and innervated cells.
NOTE: specific drugs are used to stimulate or block one or
the other of the two types of receptors
2.

There are two major types of adrenergic receptors
in the postsynaptic membrane of innervated cells:
alpha-receptors (alpha-1 and alpha-2) and
beta-receptors (beta-1 and beta-2).
Norepinerphine and epinerphine have somewhat
different effects in exciting the alpha- and betareceptors:
= norepinerphine excites mainly alpha-receptors (and
much less beta-receptors);
= epinerphine excites both types of receptors equally.
Effects are determined by the types of receptors in
the organs.
AUTONOMIC EFFECTS ON VARIOUS ORGANS OF THE BODY
CONTROL OF BRAIN STEM AUTONOMIC
CENTERS BY THE HYPOTHALAMUS
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Control of medullary
cardiovascular centers;
Controls body temperature
(heat control);
Feeding control;
Control of salt and water
balance.
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The hypothalamus sends output signals in three direction:
– downwards to the brain stem, mainly into the reticular areas and
then into the autonomic nervous system;
– upward toward many higher areas of the diencephalons and
cerebellum, especially to the anterior thalamus and limbic cortex;
– into the infundibulum to control most of the secretary functions
of both the posterior and anterior pituitary glands.

The hypothalamus controls most of the vegetative and
endocrine functions of the body as well as many aspects
of emotional behavior.
Developmental Aspects of the ANS

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During youth, ANS impairments are
usually due to injury
In old age, ANS efficiency declines,
partially due to structural changes at
preganglionic axon terminals
Developmental Aspects of the ANS
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Effects of age on ANS
– Constipation
– Dry eyes
– Frequent eye infections
– Orthostatic hypotension
 Low blood pressure occurs because aging
pressure receptors respond less to
changes in blood pressure with changes in
body position and because of slowed
responses by sympathetic vasoconstrictor
centers
Role of the Autonomic nervous System in Exercise