Chapter 15

The Autonomic Nervous System

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INTRODUCTION

• The

autonomic nervous system

arcs.

(

ANS

) operates via reflex • 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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Chapter 15 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

motor neurons.

contains both sensory and • 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

sensory and motor neurons.

contains both autonomic – Autonomic sensory input is not consciously perceived.

• 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 fiber that extends to autonomic ganglion • Postganglionic neuron – cell body lies outside the CNS in an autonomic ganglion – axon is unmyelinated 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

12

Principles of Human Anatomy and Physiology, 11e

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

13

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,

– –

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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Structures of Sympathetic NS

• Preganglionic cell bodies at T1 to L2 • 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. (Figure 17.5).

– 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 (Figure 17.3).

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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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Structure of the Parasympathetic Division

• Preganglionic axons 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 –

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

• 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

• The two types of cholinergic receptors are

nicotinic

and

muscarinic receptors

(Figure 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

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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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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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