(figures relate to Tortora/Grabowski 9^th edition of Principles of Anatomy and Physiology)

17-1

Chapter 17 - THE AUTONOMIC NERVOUS SYSTEM.

I. INTRODUCTION

A. The autonomic nervous system (ANS) regulates the activity of smooth muscle,
cardiac muscle, and certain glands.

B. Operation of the ANS is to maintain homeostasis, however, depends on a
continual flow of sensory afferent input, from receptors in organs, and efferent
motor output to the same effector organs.

C. Structurally, the ANS includes autonomic sensory neurons, integrating
centers in the CNS, and autonomic motor neurons.

D. Functionally, the ANS usually operates without conscious control.

E. The ANS is regulated by centers in the brain, mainly the hypothalamus and
medulla oblongata, which receive input from the limbic system and other regions
of the cerebrum.

D. COMPARISON OF SOMATIC AND AUTONOMIC NERVOUS SYSTEMS

A. The somatic nervous system contains both somatic sensory and motor neurons.

1. The somatic sensory neurons receive input from receptors of the special and somatic senses.

2. These sensations are consciously perceived.

3. Somatic motor neurons innervate skeletal muscle to produce
conscious, voluntary movements.

4. The effect of a somatic motor neuron is always excitation. (P.549 Ach & nicotinic receptor)

B. The autonomic nervous system contains both autonomic sensory and motor neurons.

1. Autonomic sensory neurons are associated with interoceptors, (chemo-
and mechano-receptors)

a. Autonomic sensory input is not consciously perceived.

b. The ANS also receives sensory input from somatic senses and
special sensory neurons.

2. The autonomic motor neurons regulate visceral activities by either
increasing (exciting) or decreasing (inhibiting) ongoing activities of
cardiac muscle, smooth muscle, and glands.

17-2

3. Most autonomic responses can not be consciously altered or suppressed. (What about bio-feedback??)

C. All autonomic motor pathways consists of two motor neurons in series (Figure 17.1).

1. The axon of the first motor neuron of the ANS extends from the CNS
and synapses in a ganglion with the second neuron.

2. The second neuron synapses on an effector. Preganglionic fibers
release acetylcholine and postganglionic fibers release acetylcholine or
norepinephrine.

D. The output (efferent) part of the ANS is divided into two principal parts: the
sympathetic and the parasympathetic divisions. Organs that receive impulses
from both sympathetic and parasympathetic fibers are said to have dual
innervation. (Figure 17.2)

E. Table 17.1 summarizes the similarities and differences between the somatic
and autonomic nervous systems.

III. ANATOMY OF AUTONOMIC MOTOR PATHWAYS

A. Overview

1. The first of two autonomic motor neurons is called a preganglionic
neuron (Figure 17.1)

a. Its cell body is in the brain or spinal cord.

b. Its myelinated axon, called a preganglionic fiber, passes out of
the CNS and extending to an autonomic ganglion where it
synapses with the postganglionic neuron.

2. The postganglionic neuron, the second neuron in the autonomic motor
pathway, lies entirely outside the CNS.

a. The axon of a postganglionic neuron, the postganglionic fiber, is unmyelinated and terminates in a visceral effector.

B. Autonomic Plexuses

1. These are tangled networks of sympathetic and parasympathetic
neurons (Figure 17.3 )..

2. Major autonomic plexuses include the cardiac, pulmonary, celiac, and
hypogastric.

17-3

C. Anatomical features of the sympathetic and parasympathetic divisions are

compared in Table 17.2.

IV. ANS NEUROTRANSMITTERS AND RECEPTORS

A. Cholinergic Neurons and Cholinergic Receptors

1. Cholinergic neurons release the neurotransmitter acetylcholine and
include all sympathetic and parasympathetic preganglionic neurons, all
parasympathetic postganglionic neurons, and few sympathetic
postganglionic neurons (sweat glands) (Figure 17.5).

2. The two types of cholinergic receptors are nicotinic and muscarinic
receptors.

a. Activation of nicotinic receptors (always)causes excitation, of the postsynaptic cell.

b. Activation of muscarinic receptors can cause either excitation
or inhibition depending on the cell that bears the receptors.

B. Adrenergic Neurons and Adrenergic Receptors (Epi/NorEpi)

1. The adrenergic neurons release norepinephrine (Figure 17.5) and
include most sympathetic postganglionic neurons.

2. The main types of adrenergic receptors are alpha and beta receptors.

a. Depending on the subtype, activation of the receptor can result in either excitation or inhibition.

3. Effects triggered by adrenergic neurons typically are longer lasting
than those triggered by cholinergic neurons.

C. Receptor Agonists and Antagonists

1. An agonist is a substance that binds to and activates a receptor,
mimicking the effect of a natural neurotransmitter or hormone; an
antagonist is a substance that binds to and blocks a receptor,
preventing a natural neurotransmitter or hormone from exerting its
effect.

2. Different drugs can serve as agonists or antagonists to selectively
activate or block ANS receptors.

17-4

V. PHYSIOLOGICAL EFFECTS OF THE ANS

A. Most body structures receive dual innervation, that is, fibers from both the
sympathetic and parasympathetic divisions. Usually one division causes
excitation and one causes inhibition.

B. The sympathetic responses prepare the body for emergency situations (the
fight-or-flight responses). (p.559)

1. The effects of sympathetic stimulation are longer-lasting and more
widespread than those of the parasympathetic stimulation.

2. Raynaud's disease is due to excessive sympathetic stimulation of
arterioles within the fingers and toes resulting in diminished blood
flow to the digits. (Clinical Application, p.560)

C. The parasympathetic division regulates activities that conserve and restore
body energy (energy conservation-restorative system).

1. Parasympathetic responses stimulate salivation, lacrimation, urination,
digestion, and defecation ("SLUDD").

2. Parasympathetic responses also decrease heart rate, airway diameter,
and pupil diameter.

D. Table 17.4 summarizes the responses of glands, cardiac muscle, and smooth
muscle to stimulation by the ANS.

VI. INTEGRATION AND CONTROL OF AUTONOMIC FUNCTIONS
A. Control by Higher Centers

1. The hypothalamus controls and integrates the autonomic nervous
system. It is connected to both the sympathetic and the
parasympathetic divisions.

2. Control of the ANS by the cerebral cortex occurs primarily during
emotional stress.

B. Autonomic dysreflexia is an exaggerated response of the sympathetic division of the ANS that occurs in about 86 % of individuals with spinal cord injury at or above the level of T6. It occurs due to interruption of the control of the ANS by higher centers. (Clinical Application p562)