THE AUTONOMIC NERVOUS
SYSTEM (ANS)
What is the autonomic nervous system?
The
autonomic nervous system
(ANS) is the motor division of the
peripheral nervous system that
controls visceral activities, with the
goal of maintaining internal
homeostasis.
What does the autonomic nervous system do?
 It
provides motor fibers to smooth and
cardiac muscles and glands
 It operates subconsciously
 It causes excitation and inhibition
 It makes adjustments to support body
activities
 It has an efferent pathway made up of a
two-neuron chain: preganglionic and
ganglionic
Two divisions of the ANS
 1)
Parasympathetic division
 2)
Sympathetic
Although they have different roles, they
have effects on many of the same organs of
the body
Organs affected by both divisions
Roles of the two different divisions
1. The parasympathetic division…
_conserves body energy and maintains body
activities at basal levels
_is involved in digestion, diuresis, and
defecation
_causes heart rate and blood pressure to be
low, the skin to be warm and the pupils to
be constricted
2. The sympathetic division…
_activates the body under conditions of emergency
which is why it is often called the “fight or flight
system”
_is involved in emergency, exercise, and excitement
_causes blood flow to organs to reduce and blood flow
to muscles to increase
_also causes skin to be cold, heart rate to increase, and
rapid breathing
Parasympathetic division outflow
Neurons
of the cranial and sacral
outflows of the parasympathetic
division are located in the following
nerves and create an effect on the
organs mentioned in the table below.
Table of parasympathetic outflow
Cranial Outflow
Sacral Outflow
Cranial Nerve
Occulomotor (III)
Effector Organ(s)
Eye
Facial (VII)
Salivary, nasal, and
lacrimal glands
Glosso-pharyngeal
(IX)
Parotid salivary glands
Vagus (X)
Heart, lungs, and most
visceral organs
S2-S4
Large intestine, urinary
bladder, ureters, and
reproductive organs
Parasympathetic division outflow
Sympathetic division outflow
Sympathetic division outflow is different from parasympathetic
division outflow because…

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It arises from spinal cord segments T1 through L2
Preganglionic fibers pass through the white rami
communicantes and synapse in the chain (paravertebral)
ganglia
Fibers from T5-L2 form splanchnic nerves and synapse with
collateral ganglia
Postganglionic fibers innervate the numerous organs of the
body
Sympathetic neurons produce the lateral horns of the spinal
cord
Sympathetic division outflow
ANS Physiology
Neurotransmitters and Receptors
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The two major neurotransmitter involved in the ANS are:
 Acetylcholine (ACh)
 Norepinephrine (NE)
ACh is the same neurotransmitter that is found in the somatic
motor neurons and is released in the ANS:
 All ANS preganglionic axons
 All parasympathetic postganglionic axons at synapses
with their effectors
ACh releasing fibers are called Cholinergic Fibers
NE is released by most sympathetic postganglionic axons
NE releasing fibers are called Adrenergic Fibers
The effects of ACh and NE are either excitation or inhibition
which is dependant on the receptor type allowing them to exert
these different effects at different areas in the body
Receptors

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ACh binds to two types of receptors:
 Nicotinic
 Muscarine
Nicotinic Receptors are located on:
 Motor end plates of skeletal muscles (somatic targets)
 All ganglionic neurons (sypathetic and parasympathetic)
 The hormone-producing cells of the adrenal medulla
ACh always produces a stimulatory effect when it binds with
nicotinic receptors
Muscarine Receptors occur on all effector cells stimulated by
postganglionic cholinergic fibers (parasympathetic targets like
the eccrine sweat glands and some blood vessels of skeletal
muscle)
ACh binding produces a inhibitory or excitatory depending on
the receptor type of the target organ
Receptors
There are also receptors know as Adrenergic
Receptors (two types):
 Alpha with two subtypes (A1,A2)
 Beta with three subtypes (B1,B2,B3)
 The general effects of NE binding are:
 Alpha receptors are mostly stimulatory
 Beta receptors are mostly inhibitory
 However, an exception to this is when NE binds to
the Beta receptors of cardiac muscle and the result is
stimulatory

Overview of the location and effects of receptor
subclasses
Drugs and the ANS

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Knowing the locations of the cholinergic and adrenergic
receptors subtypes allows specific drugs to be prescribed
to obtain the desired inhibitory or stimulatory effect on
selected organs
An example of this is Atropine (blocks parasympathetic
effects):
administered during surgery to prevent salivation
and dry up respiratory secretions
Ophthalmologists use it to dilate the pupils for an
eye exam
There are several examples of drugs in Table 14.4 that
influence the Activity of the ANS on page 544
Interactions of the Autonomic Divisions
 Most
visceral organs receive innervation by both
sympathetic and parasympathtic fibers
 This dual innervation produces a dynamic
antagonism that allows visceral activity to be
precisely controlled
 Sympathetic fibers increase heart and respiratory
rates. They also inhibit digestion and elimination
 Parasympathetic fibers allow for digestion and
elimination and decrease respiratory and heart
rates
Parasympathetic & Sympathetic

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The parasympathetic division is know as the “resting and
digesting” division
The sympathetic division controls blood pressure and
keeps the blood vessels in a continual state of partial
constriction
Flight or Fright response – the sympathetic can override
the parasympathetic in an emergency to increase heart
and respiratory rates while inhibiting digestion.
Parasympathetic division restores the heart and
respiratory rates back to resting levels when the
emergency is over and then returns back to it’s functions
of digestion and elimination
Sympathetic Tone
& Parasympathetic Tone

The Sympathetic Tone:
 Constricts
blood vessels & controls blood pressure to
rise in response to the body’s needs
 Prompts vessels to dilate if blood pressure needs to
be decreased

The Parasympathetic Tone:
 Slows
the heart
 Controls Digestion and Elimination

ANS control is best seen in the external genitalia:
 Parasympathetic
fibers cause vasodilation resulting in
the erection of the penis and clitoris
 Sympathetic fibers cause ejaculation of sperm and
reflex peristalsis in females
Sympathetic Division
 The
adrenal medulla, sweat glands and the
arrector pili muscles of the skin, the kidneys, and
most blood vessels receive only sympathetic
fibers.
 The Sympathetic Division controls:
Thermoregulatory Responses to heat
Release of Renin from the kidneys
Metabolic Effects
Control of Autonomic Functions
Autonomic functions are controlled by several factors:
1.
Reflex Activities mediated by the Spinal Cord and
Brain Stem (Medullary Center)
2.
Hypothalamic integration centers interact with both
higher and lower centers of autonomic, somatic,
and endocrine response
3.
Cortical centers influence autonomic functioning
via connections with the limbic system; conscious
controls are rare but maybe learned via biofeedback
training