Principles of Human Anatomy and Physiology, 11e 1 Chapter 15 The Autonomic Nervous System Lecture Outline

Principles of Human Anatomy and Physiology, 11e

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Chapter 15

The Autonomic Nervous System

Lecture Outline


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INTRODUCTION The autonomic nervous system (ANS) operates via

reflex arcs. Operation of the ANS 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.

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

Functionally, the ANS usually operates without conscious control.

The ANS is regulated by the hypothalamus and brain stem.


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The Autonomic Nervous System

Regulate activity of smooth muscle, cardiac muscle & certain glands

Structures involved general visceral afferent neurons general visceral efferent neurons integration center within the brain

Receives input from limbic system and other regions of the cerebrum


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SOMATIC AND AUTONOMIC NERVOUS SYSTEMS The somatic nervous system contains both

sensory and motor neurons. The somatic sensory neurons receive input from

receptors of the special and somatic senses. These sensations are consciously perceived. Somatic motor neurons innervate skeletal

muscle to produce conscious, voluntary movements.

The effect of a motor neuron is always excitation.


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SOMATIC AND AUTONOMIC NERVOUS SYSTEMS

The autonomic nervous system contains both autonomic sensory and motor neurons. Autonomic sensory neurons are associated with

interoceptors. Autonomic sensory input is not consciously perceived.

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

The autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities of cardiac muscle, smooth muscle, and glands. Most autonomic responses can not be consciously

altered or suppressed.


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SOMATIC vs AUTONOMIC NERVOUS SYSTEMS All somatic motor pathways consist of a single

motor neuron Autonomic motor pathways consists of two motor

neurons in series The first autonomic neuron motor has its cell body in

the CNS and its myelinated axon extends to an autonomic ganglion.

It may extend to the adrenal medullae rather than an autonomic ganglion

The second autonomic motor neuron has its cell body in an autonomic ganglion; its nonmyelinated axon extends to an effector.


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Somatic versus Autonomic NS


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Basic Anatomy of ANS

Preganglionic neuron cell body in brain or spinal cord axon is myelinated type B fiber that extends to

autonomic ganglion Postganglionic neuron

cell body lies outside the CNS in an autonomic ganglion axon is unmyelinated type C fiber that terminates in a

visceral effector


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Sympathetic vs. Parasympathetic NS


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AUTONOMIC NERVOUS SYSTEM The output (efferent) part of the ANS is

divided into two principal parts: the sympathetic division the parasympathetic division Organs that receive impulses from both

sympathetic and parasympathetic fibers are said to have dual innervation.

Table 15.1 summarizes the similarities and differences between the somatic and autonomic nervous systems.


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Sympathetic ANS vs. Parasympathetic ANS


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Divisions of the ANS

2 major divisions parasympathetic sympathetic

Dual innervation one speeds up organ one slows down

organ Sympathetic NS

increases heart rate Parasympathetic NS

decreases heart rate


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Divisions of the ANS

2 major divisions parasympathetic sympathetic

Dual innervation one speeds up organ one slows down

organ Sympathetic NS

increases heart rate Parasympathetic NS

decreases heart rate


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Autonomic Ganglia


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Sympathetic Ganglia

These ganglia include the sympathetic trunk or vertebral chain or paravertebral ganglia that lie in a vertical row on either side of the vertebral column (Figures 15.2).

Other sympathetic ganglia are the prevertebral or collateral ganglia that lie anterior to the spinal column and close to large abdominal arteries. celiac superior mesenteric inferior mesenteric ganglia (Figures 15.2 and 15.4).


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Parasympathetic Ganglia

Parasympathetic ganglia are the terminal or intramural ganglia that are located very close to or actually within the wall of a visceral organ.

Examples of terminal ganglia include (Figure 15.3) ciliary, pterygopalatine, submandibular, otic ganglia


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Sympathetic ANS vs. Parasympathetic ANS


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Dual Innervation, Autonomic Ganglia

Sympathetic (thoracolumbar) division preganglionic cell bodies

in thoracic and first 2 lumbar segments of spinal cord

Ganglia trunk (chain) ganglia near

vertebral bodies prevertebral ganglia near

large blood vessel in gut (celiac, superior mesenteric, inferior mesenteric)

Parasympathetic (craniosacral) division preganglionic cell

bodies in nuclei of 4 cranial nerves and the sacral spinal cord

Ganglia terminal ganglia in

wall of organ


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Autonomic Plexuses

These are tangled networks of sympathetic and parasympathetic neurons (Figure 15.4) which lie along major arteries.

Major autonomic plexuses include cardiac, pulmonary, celiac, superior mesenteric, inferior mesenteric, renal and hypogastric


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Autonomic Plexuses

Cardiac plexus Pulmonary

plexus Celiac (solar)

plexus Superior

mesenteric Inferior

mesenteric Hypogastric


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Autonomic Plexuses

Cardiac plexus Pulmonary

plexus Celiac (solar)

plexus Superior

mesenteric Inferior

mesenteric Hypogastric


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Structures of Sympathetic NS Preganglionic cell bodies at T1 to L2 Rami communicantes

white ramus = myelinated = preganglionic fibers gray ramus = unmyelinated = postganglionic fibers

Postganglionic cell bodies sympathetic chain ganglia along the spinal column prevertebral ganglia at a distance from spinal cord

celiac ganglion superior mesenteric ganglion inferior mesenteric ganglion


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Postganglionic Neurons: Sympathetic vs. Parasympathetic

Sympathetic preganglionic neurons pass to the sympathetic trunk. They may connect to postganglionic neurons in the following ways. May synapse with postganglionic neurons in the

ganglion it first reaches. May ascend or descend to a higher of lower ganglion

before synapsing with postganglionic neurons. May continue, without synapsing, through the

sympathetic trunk ganglion to a prevertebral ganglion where it synapses with the postganglionic neuron.

Parasympathetic preganglionic neurons synapse with postganglionic neurons in terminal ganglia


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Pathways of Sympathetic Fibers

Spinal nerve route out same level

Sympathetic chain route up chain & out spinal

nerve Collateral ganglion route

out splanchnic nerve to collateral ganglion


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Organs Innervated by Sympathetic NS

Structures innervated by each spinal nerve sweat glands, arrector pili mm., blood vessels to

skin & skeletal mm. Thoracic & cranial plexuses supply:

heart, lungs, esophagus & thoracic blood vessels plexus around carotid artery to head structures

Splanchnic nerves to prevertebral ganglia supply: GI tract from stomach to rectum, urinary &

reproductive organs


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Ganglia & Plexuses of Sympathetic NS


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Circuitry of Sympathetic NS

Divergence = each preganglionic cell synapses on many postganglionic cells

Mass activation due to divergence multiple target organs fight or flight response explained

Adrenal gland modified cluster of postganglionic cell bodies

that release epinephrine & norepinephrine into blood


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Application

In Horner’s syndrome, the sympathetic innervation to one side of the face is lost.


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Structure of the Parasympathetic Division The cranial outflow consists of preganglionic

axons that extend from the brain stem in four cranial nerves. (Figure 15.3). The cranial outflow consists of four pairs of

ganglia and the plexuses associated with the vagus (X) nerve.

The sacral parasympathetic outflow consists of preganglionic axons in the anterior roots of the second through fourth sacral nerves and they form the pelvic splanchnic nerve. (Figure15.3)


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Anatomy of Parasympathetic NS

Preganglionic cell bodies found in 4 cranial nerve nuclei

in brainstem S2 to S4 spinal cord

Postganglionic cell bodies very near or in the wall of the target organ in a terminal ganglia


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Parasympathetic Cranial Nerves

Oculomotor nerve ciliary ganglion in orbit ciliary muscle & pupillary constrictor muscle inside

eyeball Facial nerve

pterygopalatine and submandibular ganglions supply tears, salivary & nasal secretions

Glossopharyngeal otic ganglion supplies parotid salivary gland

Vagus nerve many brs supply heart, pulmonary and GI tract as far as

the midpoint of the colon


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Parasympathetic Sacral Nerve Fibers

Form pelvic splanchnic nerves

Preganglionic fibers end on terminal ganglia in walls of target organs

Innervate smooth muscle and glands in colon, ureters, bladder & reproductive organs


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ANS NEUROTRANSMITTERS AND RECEPTORS


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ANS Neurotransmitters

Classified as either cholinergic or adrenergic neurons based upon the neurotransmitter released

Adrenergic

Cholinergic


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Cholinergic Neurons and Receptors

Cholinergic neurons release acetylcholine all preganglionic neurons all parasympathetic

postganglionic neurons few sympathetic

postganglionic neurons (to most sweat glands)

Excitation or inhibition depending upon receptor subtype and organ involved.


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Cholinergic Neurons and Receptors

Cholinergic receptors are integral membrane proteins in the postsynaptic plasma membrane.

The two types of cholinergic receptors are nicotinic and muscarinic receptors (Fig 15.6 a , b). Activation of nicotinic receptors causes excitation of the

postsynaptic cell. Nicotinic receptors are found on dendrites & cell

bodies of autonomic NS cells (and at NMJ.) Activation of muscarinic receptors can cause either

excitation or inhibition depending on the cell that bears the receptors.

Muscarinic receptors are found on plasma membranes of all parasympathetic effectors


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Adrenergic Neurons and Receptors

Adrenergic neurons release norepinephrine (NE) ) from postganglionic

sympathetic neurons only Excites or inhibits organs

depending on receptors NE lingers at the synapse

until enzymatically inactivated by monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT)


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Adrenergic Neurons and Receptors

The main types of adrenergic receptors are alpha and beta receptors. These receptors are further classified into subtypes. Alpha1 and Beta1 receptors produce excitation Alpha2 and Beta2 receptors cause inhibition Beta3 receptors (brown fat) increase thermogenesis

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

Table 15.2 describes the location of the subtypes of cholinergic and adrenergic receptors and summarizes the responses that occur when each type of receptor is activated.


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Receptor Agonists and Antagonists

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.

Drugs can serve as agonists or antagonists to selectively activate or block ANS receptors.


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Physiological Effects of the ANS

Most body organs receive dual innervation innervation by both sympathetic & parasympathetic

Hypothalamus regulates balance (tone) between sympathetic and parasympathetic activity levels

Some organs have only sympathetic innervation sweat glands, adrenal medulla, arrector pili mm &

many blood vessels controlled by regulation of the “tone” of the

sympathetic system


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Sympathetic Responses

Dominance by the sympathetic system is caused by physical or emotional stress -- “E situations” emergency, embarrassment, excitement, exercise

Alarm reaction = flight or fight response dilation of pupils increase of heart rate, force of contraction & BP decrease in blood flow to nonessential organs increase in blood flow to skeletal & cardiac muscle airways dilate & respiratory rate increases blood glucose level increase

Long lasting due to lingering of NE in synaptic gap and release of norepinephrine by the adrenal gland


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Parasympathetic Responses

Enhance “rest-and-digest” activities Mechanisms that help conserve and restore body

energy during times of rest Normally dominate over sympathetic impulses SLUDD type responses = salivation, lacrimation,

urination, digestion & defecation and 3 “decreases”--- decreased HR, diameter of airways and diameter of pupil

Paradoxical fear when there is no escape route or no way to win causes massive activation of parasympathetic division loss of control over urination and defecation


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PHYSIOLOGICAL EFFECTS OF THE ANS - Summary The sympathetic responses prepare the body

for emergency situations (the fight-or-flight responses).

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

Table 15.4 summarizes the responses of glands, cardiac muscle, and smooth muscle to stimulation by the ANS.


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INTEGRATION AND CONTROL OF AUTONOMIC FUNCTIONS


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Autonomic or Visceral Reflexes

A visceral autonomic reflex adjusts the activity of a visceral effector, often unconsciously. changes in blood pressure, digestive functions etc filling & emptying of bladder or defecation

Autonomic reflexes occur over autonomic reflex arcs. Components of that reflex arc: sensory receptor sensory neuron integrating center pre & postganglionic motor neurons visceral effectors


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Control of Autonomic NS

Not aware of autonomic responses because control center is in lower regions of the brain

Hypothalamus is major control center input: emotions and visceral sensory information

smell, taste, temperature, osmolarity of blood, etc output: to nuclei in brainstem and spinal cord posterior & lateral portions control sympathetic NS

increase heart rate, inhibition GI tract, increase temperature

anterior & medial portions control parasympathetic NS decrease in heart rate, lower blood pressure, increased

GI tract secretion and mobility


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Autonomic versus Somatic NS - Review Somatic nervous system

consciously perceived sensations excitation of skeletal muscle one neuron connects CNS to organ

Autonomic nervous system unconsciously perceived visceral sensations involuntary inhibition or excitation of smooth

muscle, cardiac muscle or glandular secretion two neurons needed to connect CNS to organ

preganglionic and postganglionic neurons


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DISORDERS

Raynaud’s phenomenon is due to excessive sympathetic stimulation of smooth muscle in the arterioles of the digits as a result the digits become ischemic after exposure to cold or with emotional stress.


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Autonomic Dysreflexia

Exaggerated response of sympathetic NS in cases of spinal cord injury above T6

Certain sensory impulses trigger mass stimulation of sympathetic nerves below the injury

Result vasoconstriction which elevates blood pressure parasympathetic NS tries to compensate by

slowing heart rate & dilating blood vessels above the injury

pounding headaches, sweating warm skin above the injury and cool dry skin below

can cause seizures, strokes & heart attacks


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Principles of Human Anatomy and Physiology, 11e 1 Chapter 15 The Autonomic Nervous System Lecture Outline