Copyright 2010, John Wiley & Sons, Inc. Chapter 11 Autonomic Nervous System (ANS)

Copyright 2010, John Wiley & Sons, Inc.

Chapter 11

Autonomic Nervous System (ANS)


Introduction to the ANS Somatic nervous system (SNS) + ANS

peripheral nervous system (PNS) ANS

Not under conscious control Is regulated by hypothalamus, brainstem

The ANS supplies nerves to viscera Smooth muscle (stomach, blood vessels) Cardiac muscle (heart) Glands (sweat and digestive glands)


Comparison: SNS vs ANS SNS

Controls skeletal muscle Conscious, voluntary

control Motor pathway: one neuron

from CNS to effector Does include sensory

neurons (from skin, skeletal muscles, and special sense organs)

All release the neurotransmitter ACh

ANS Controls viscera: smooth

and cardiac muscle, and glands

Unconscious, involuntary Motor pathway: series of

two neurons from CNS to effector

Does include sensory neurons (monitors viscera)

Two divisions: sympathetic, parasympathetic

Release either ACh or NE


Somatic Nervous System


ANS Motor Pathways Autonomic motor pathway includes two motor

neurons Preganglionic neuron from CNS to neuron in

autonomic ganglion Postganglionic neuron from cell body in ganglion

to effector


ANS Motor Pathways


Divisions of the ANS Sympathetic (S) division + parasympathetic

(P) division Most viscera supplied with nerves of both S

and P divisions: dual innervation S and P have opposite (antagonistic) effects

Heart rate: S stimulates, P inhibits Digestive organs: S inhibit, P stimulate

S: “flight or flight,” P: “rest and digest” Some viscera receive only S (not P) nerves:

Sweat glands, many blood vessels, hair muscles


Sympathetic (S) Division Sympathetic preganglionic neurons

Have cell bodies located in lateral gray of spinal cord segments T1-T12 + L1-L2

So S division is called “thoracolumbar” Axons pass through ventral roots of spinal nerves

May branch many times May ascend or descend to many levels of S trunk

ganglia (from cervical to sacral) Can synapse with 20 or more postganglionic neuron cell

bodies Results: widespread S effects (viscera respond “in

sympathy with one another”)


Sympathetic (S) Division Sympathetic postganglionic neurons

S postganglionic neurons cell bodies located In S “trunk ganglia” (2 long chains lateral to vertebrae)

From cervical to sacral regions widespread S effects Many axons from these cell bodies pass back into

spinal nerves to reach viscera in skin (sweat glands, hair muscles, blood vessels)

In S “prevertebral ganglia” anterior to 3 large abdominal arteries

Named celiac, superior and inferior mesenteric ganglia Supply abdominal viscera: stomach, intestine, kidneys,

liver, spleen Axons pass from ganglia to viscera in S nerves


Sympathetic (S) Division


Parasympathetic (P) Division P preganglionic neurons

Cell bodies located in brainstem + in spinal cord segments S2-S4 Therefore P division is called “craniosacral”

Axons in cranial nerves III, VII, IX and X and in pelvic nerves from S2-S4 Vagus nerves (cranial nerves X) carry 80% of all P

nerve impulses. Vagus nerves carry both motor and sensory neurons

to/from viscera within the thorax and most of the abdominal cavity.

P preganglionic axons do not branch or pass though S trunk ganglia but pass directly almost to viscera


Parasympathetic (P) Division P postganglionic neurons

Cell bodies lie in terminal ganglia Located within or near the innervated organ So P nerves cause precise, localized (not

widespread) effects Because of anatomical arrangement, S nerves supply

all viscera but P nerves do not reach some viscera. These include sweat glands, arrector pili muscles of hairs in skin, kidneys, spleen, adrenal medullae, and the walls of most blood vessels.

Axons pass from ganglia to viscera in P nerves


Parasympathetic (P) Division


ANS Neurotransmitters: Comparison Acetylcholine (ACh) ACh more common;

released by: All S and P preganglionic

axons All P postganglionic

axons Some S postganglionic

axons (to sweat glands) ACh destroyed by

enzyme ACh-ase so short-lived response

Norepinephrine (NE) NE less common;

released by: Almost all S

postganglionic axons NE has longer lasting

effects enhanced by epinephrine + NE from adrenal medullae


Sympathetic Effects Fight-or-flight activities

Increase heart rate and contraction, and blood pressure (BP)

Dilate pupils Dilate airways Dilate vessels to skeletal muscles, heart, liver and

adipose tissue Constrict blood vessels to nonessential organs:

skin, GI tract, kidneys Mobilize nutrients for energy: glucose and fats


Parasympathetic Effects Rest-and-digest activities

SLUDD Salivation Lacrimation Urination Digestion Defecation

Decrease heart rate, airway diameter, pupil diameter


End of Chapter 11

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Copyright 2010, John Wiley & Sons, Inc. Chapter 11 Autonomic Nervous System (ANS)