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

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)

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)

Cardiac muscle (excitation; increased force and rate of contraction)

posterior pituitary (secretion of antidiuretic hormones)

adipose cells (breakdown of triglycerides)

Smooth muscle in airways (relaxation; bronchodilation)

blood vessels (relaxation; vasodilation)

walls of internal organs (relaxation)

cillary muscles (inhibition; relaxation)

hepatocytes (glycogenolysis)

3 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 Paravertebral Contraction
Sweat Glands Paravertebral Sweat production
Eyes Paravertebral Dilates pupils and relaxes ciliary muscles for far
vision
Lungs Paravertebral Dilates bronchi
Heart Paravertebral Increased rate and force of contraction
Liver Prevertebral Breakdown of glycogen to glucose and triglycerides
to fatty acids

Small and Large Intestines Paravertebral Slows digestion, stops secretions and contracts
sphincters
Urinary Bladder Paravertebral Contracts sphincter
Adrenal Medulla Stimulates release of norepinephrine and
epinephrine
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 Stimulation of exocrine secretions and release

of insulin
Small and Large Cranial Nerve X and Increases motility and secretions and relaxes
Intestines Sacral Nerves sphincters for increase defecation
Urinary Bladder Sacral Nerves Contracts bladder (micturition) and relaxes
sphincter
Genitals Sacral Nerves Erection

Drugs that Effect the Autonomic Nervous System

Drug Mechanism Action Uses
Drugs that Stimulate the Sympathetic Nervous System (Sympathomimetic)
Phenylephrine Stimulates 1 Elevates blood Nasal decongestant
receptors pressure; stimulates and to elevate blood
smooth muscle pressure
Clonidine Stimulates 2 Lowers blood Treatment of high
receptors pressure blood pressure
Isoproterenol Stimulates receptors Stimulates heart rate; Treatment of
dilates respiratory respiratory disorders
passages and as a cardiac
stimulant during
cardiac resuscitation
Albuteral, terbutaline Stimulates 2 Dilates respiratory Treatment of asthma,
receptors passages severe allergies and
other respiratory
disorders
Ephedrine, Stimulates NE release Similar to epinephrine Nasal decongestant
psuedoephedrine at synapses and to elevate blood
pressure or dilate
respiratory passages
MAO Inhibitors Increases NE levels in Boosts mood in the Antidepressant
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) Blocks 1 receptors Lowers blood Treatment of high
pressure blood pressure
Tamsulosin (Flomax) Blocks 1a receptors Relaxes smooth Treatment of benign
muscle surrounding prostrate hyperplasia
the urethra
Yohimbine (herbal Blocks 2 receptors Dilates blood vessels Aphrodisiac
supplement) and relaxes smooth
muscles
Propranolol (Inderal) Blocks 1 and 2 Reduces metabolic Treatment of high
receptors activity in cardiac blood pressure: used
muscle but may to reduce heart rate
constrict respiratory and force of
passageways: slows contraction in heart
heart rate disease
Metoprolol Blocks 1 receptors Reduces metabolic Similar to those of
(Lopressor), atenolol activity in cardiac Inderal but has less of
muscle an effect on
respiratory muscles
Drugs that Block the Parasympathetic Nervous System
Atropine, related Block muscarinic Inhibit Treating diarrhea;
drugs receptors parasympathetic dilating pupils; raising
activity 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