Chapter 15: The Autonomic Nervous System
Chapter Objectives
COMPARISON OF SOMATIC AND AUTONOMIC NERVOUS SYSTEMS
1. List the structural and functional characteristics of the autonomic nervous system.
2. Distinguish between the pre- and postganglionic neurons, in terms of location.
3. Specify the different origins and destination ganglia for the sympathetic (thoracolumbar)
and parasympathetic (craniosacral) preganglionic neurons.
4. Describe the differing locations of the ganglia of the sympathetic and parasympathetic
systems.
PHYSIOLOGICAL EFFECTS OF THE ANS
5. Discuss the primary purpose of the sympathetic division and the general body functions it
directs.
6. Describe specific responses of effectors due to increased sympathetic stimulation.
7. Discuss the primary purpose of the parasympathetic division and the general body
functions it directs.
8. Describe specific responses of effectors due to increased parasympathetic stimulation.
ANS NEUROTRANSMITTERS AND RECEPTORS
9. Identify the cholinergic neurons, receptors, and neurotransmitters.
10. Identify the adrenergic neurons, receptors, and neurotransmitters.
Chapter Lecture Notes
Characteristics of the autonomic nervous system (Table 15.1)
Involuntary control
Sensory input mostly from interoceptors
Motor pathways divided into sympathetic and parasympathetic divisions
Two-neuron pathway (Fig 15.1)
Preganglionic
Postganglionic
Neurotransmitters
Preganglionic – acetylcholine
Postganglionic – acetylcholine (parasympathetic and sympathetic to sweat glands (except on
palms and soles), blood vessels in skeletal muscle and arrector pili muscles) or
norepinephrine (remainder of sympathetic)
Effects smooth muscle, cardiac muscle and glands
Physiological effects of the autonomic nervous system (Table 15.4)
Sympathetic – “E” situations (exercise, emergency, excitement and embarrassment) - fight or
flight response
Pupils dilate
Heart rate, force of contraction and blood pressure increase
Airways dilate
Blood vessels to kidneys and gastrointestinal tract constrict
Blood vessels to skeletal muscles, cardiac muscle, liver and adipose tissue dilate
Liver cells perform glycogenolysis and lipid cell perform lipolysis
Release of glucose by the liver
Parasympathetic – rest and digest response
Increased salivation, lacrimation, urination, digestion and defecation
Decreased heart rate, diameter of airways and diameter of pupils (constriction)
Anatomical Differences between the Parasympathetic and Sympathetic Divisions (Table 15.3)
Characteristics
Parasympathetic
Sympathetic
Origin (Fig 15.2 & 15.3)
Craniosacral outflow:
Thoracolumnar outflow:
brainstem nuclei of cranial
lateral horn of gray matter of
nerves III, VII, IX and X;
spinal cord segments T1-L2
spinal cord segments S2-S4
Location of ganglia
Ganglia in (intramural=
Ganglia within a few cm of
terminal) or close to visceral
CNS: alongside vertebral
organ served
column (paravertebral
ganglia=chain) and anterior to
vertebral column (prevertebral
ganglia)
Relative length of pre- and
Long preganglionic; short
Short preganglionic; long
postganglionic fibers
postganglionic
postganglionic
Sympathetic ganglia (Fig 15.5)
Paravertebral ganglia = chain ganglia
Innervate organs above the diaphragm like the heart and lungs
Prevertebral ganglia = collateral ganglia
Prevertebral ganglia surround the following arteries that branch from the aorta
Celiac artery (celiac ganglia forms the solar plexus)
Superior mesenteric artery
Inferior mesenteric artery
Neurotransmitters and receptors (Table 15.2 & Fig 15.7)
Cholinergic neurons – release acetylcholine (all preganglionic neurons, select sympathetic
postganglionic neurons and all parasympathetic postganglionic neurons)
Cholinergic receptors – receptors on the postsynaptic membrane that bind acetylcholine
Nicotinic receptor
Found in the dendrites and cell bodies of sympathetic and parasympathetic
postganglionic neurons, the motor end plate in the neuromuscular junction and
adrenal medullary cells (excitatory)
Is mimicked by nicotine
Muscarinic receptor
Found in all parasympathetic target organs (excitatory in all but cardiac muscle where it
is inhibitory), sweat glands (activation) and some blood vessels in skeletal muscle
(inhibition; vasodilation) and arrector pili muscle (excitatory)
Is mimicked by muscarine (a mushroom poison)
Adrenergic neurons – release norepinephrine (most sympathetic postganglionic neurons)
Adrenergic receptors – bind norepinephrine and epinephrine
α1
Smooth muscle of blood vessels (excitation; vasoconstriction)
radial muscles of eye (dilation of pupil)
sphincter muscles of stomach and urinary bladder (closing)
salivary gland cells (decreased salivation)
sweat glands on palms and soles (increased sweating)
α2
Smooth muscle in some blood vessels (inhibition; vasodilation)
beta cells in pancreatic islets (decrease insulin secretion)
pancreatic acinar cells (inhibition of digestive enzyme secretion)
blood platelets (aggregation)
β1
Cardiac muscle (excitation; increased force and rate of contraction)
posterior pituitary (secretion of antidiuretic hormones)
adipose cells (breakdown of triglycerides)
β2
Smooth muscle in airways (relaxation; bronchodilation)
blood vessels (relaxation; vasodilation)
walls of internal organs (relaxation)
cillary muscles (inhibition; relaxation)
hepatocytes (glycogenolysis)
β3 – Brown adipose tissue (thermogenesis)
Sympathetic Responses
Target Organ
Ganglia
Effect
Location
Blood vessels of skeletal and Paravertebral Dilation
cardiac muscle
Blood vessels of skin, gut
Paravertebral Constriction
and kidneys
Arrector pili muscle
Sweat Glands
Eyes
Paravertebral Contraction
Paravertebral Sweat production
Paravertebral Dilates pupils and relaxes ciliary muscles for far
vision
Lungs
Heart
Paravertebral Dilates bronchi
Paravertebral Increased rate and force of contraction
Liver
Prevertebral
Small and Large Intestines
Paravertebral Slows digestion, stops secretions and contracts
sphincters
Paravertebral Contracts sphincter
Stimulates release of norepinephrine and
epinephrine
Urinary Bladder
Adrenal Medulla
Breakdown of glycogen to glucose and
triglycerides to fatty acids
Parasympathetic Responses
Target Organ
Origin
Effect
Eyes
Cranial Nerve III
Constricts pupils and ciliary muscles for near
vision and stimulates tears
Salivary Glands
Cranial Nerve VII&IX Stimulate salivation
Lungs
Cranial Nerve X
Constricts bronchi and stimulates secretion
Heart
Cranial Nerve X
Decreased rate and force of contraction
Gall Bladder
Cranial Nerve X
Constriction
Pancreas
Cranial Nerve X
Small and Large
Intestines
Urinary Bladder
Cranial Nerve X and
Sacral Nerves
Sacral Nerves
Genitals
Sacral Nerves
Stimulation of exocrine secretions and release
of insulin
Increases motility and secretions and relaxes
sphincters for increase defecation
Contracts bladder (micturition) and relaxes
sphincter
Erection
Drugs that Effect the Autonomic Nervous System
Drug
Mechanism
Action
Uses
Drugs that Stimulate the Sympathetic Nervous System (Sympathomimetic)
Phenylephrine
Stimulates α1
receptors
Clonidine
Stimulates α2
receptors
Stimulates β receptors
Elevates blood
pressure; stimulates
smooth muscle
Lowers blood
pressure
Stimulates heart rate;
dilates respiratory
passages
Albuteral, terbutaline
Stimulates β2
receptors
Dilates respiratory
passages
Ephedrine,
psuedoephedrine
Stimulates NE release
at synapses
Similar to epinephrine
Isoproterenol
MAO Inhibitors
Nasal decongestant
and to elevate blood
pressure
Treatment of high
blood pressure
Treatment of
respiratory disorders
and as a cardiac
stimulant during
cardiac resuscitation
Treatment of asthma,
severe allergies and
other respiratory
disorders
Nasal decongestant
and to elevate blood
pressure or dilate
respiratory passages
Antidepressant
Increases NE levels in Boosts mood in the
the synapse by
brain
inhibiting the enzyme
that breaks it down
Tricyclic
Increases NE levels in Boosts mood in the
Antidepressant
antidepressants
the synapse by
brain
(Moxadil, Endep)
blocking reuptake
Drugs that Stimulate the Parasympathetic Nervous System (Parasympathomimetic)
Muscarine,
Stimulates muscarinic Similar to effects of
Applied topically to
pilocarpine,
receptors
Ach
cornea of eye to cause
methacholine
pupil contraction
Physostigmine,
Block action of
Increase Ach
Stimulate digestive
neostigmine,
acetylcholinesterase
concentrations at
tract and smooth
malathion, nerve
synapses
muscles of urinary
gases
bladder
Drugs that Block the Sympathetic Nervous System
Prazosin (Minipress)
Lowers blood
Treatment of high
Blocks 1 receptors
pressure
blood pressure
Tamsulosin (Flomax) Blocks 1a receptors
Relaxes smooth
Treatment of benign
muscle surrounding
prostrate hyperplasia
the urethra
Yohimbine (herbal
Dilates blood vessels
Aphrodisiac
Blocks 2 receptors
supplement)
and relaxes smooth
muscles
Propranolol (Inderal)
Reduces metabolic
Treatment of high
Blocks 1 and 2
activity
in
cardiac
blood pressure: used
receptors
muscle but may
to reduce heart rate
constrict respiratory
and force of
passageways: slows
contraction in heart
heart rate
disease
Metoprolol
(Lopressor), atenolol
Blocks 1 receptors
Reduces metabolic
activity in cardiac
muscle
Drugs that Block the Parasympathetic Nervous System
Atropine, related
Block muscarinic
Inhibit
drugs
receptors
parasympathetic
activity
Similar to those of
Inderal but has less of
an effect on
respiratory muscles
Treating diarrhea;
dilating pupils; raising
heart rate; blocking
secretions of digestive
and respiratory tracts
prior to surgery; used
to treat accidental
exposure to
anticholinesterase
drugs, such as
pesticides or military
nerve gases