Chapter 20 The Autonomic Nervous System - Autonomic NS.pdf11/27/2012 10 Sacral Outflow Sacral...

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Copyright 2009 John Wiley & Sons

Chapter 20 The Autonomic Nervous System


Introduction� Autonomic nervous system (ANS)- division of the peripheral

nervous system that consists of:

� Autonomic sensory neurons in visceral organs and in blood vessels

� convey information (interoceptors)

� Integrating centers in the central nervous system (CNS)

� Autonomic motor neurons- propagate from the CNS to various effectors

� regulate the activity of smooth muscle, cardiac muscle, and many glands.

� The enteric division, a specialized network of nerves and ganglia forming an independent nerve network within the wall of the gastrointestinal (GI) tract.


Anatomy of Autonomic Motor Pathway

� Preganglionic neuron- first of the two motor neurons in any

autonomic motor pathway

� Cell body is in the brain or spinal cord, the axon exits the CNS as a small-diameter, myelinated fiber extending to an autonomic

ganglion.

� Postganglionic neuron lies outside the CNS, the cell body and

dendrites are in an autonomic ganglion

� Forms one or more synapses with preganglionic neurons

� Axon of a postganglionic neuron is a small-diameter, unmyelinated Type C fiber that terminates in a visceral effector.

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Motor Neuron Pathways in the

Somatic Nervous System


Motor Neuron Pathways in the ANS


Preganglionic Neurons

� Sympathetic division- thoracolumbar division-

� Preganglionic neurons have cell bodies in the lateral horns of the

gray matter in the12 thoracic segments and the lumbar 1 and 2 segments of the spinal cord.

� Sympathetic preganglionic axons are relatively short

� Parasympathetic division- craniosacral division

� Cell bodies of preganglionic neurons in the nuclei of 4 cranial nerves in the brain stem (III, VII, IX, and X) and in the lateral gray

horns of the sacral segments 2-4 of the spinal cord.

� Parasympathetic preganglionic axons are relatively long

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3 General Groups of Autonomic Ganglia

� Sympathetic Ganglia- sites of synapses between sympathetic preganglionic and postganglionic neurons.

� Sympathetic trunk ganglia- lie in a vertical row on either side of the vertebral column, skull to coccyx

� Postganglionic axons innervate areas above the diaphragm

� Neck innervations called: Superior, Middle, and Inferior Cervical Ganglia

� Prevertebral (collateral) ganglia- lies anterior to the vertebral column and close to the large abdominal arteries.

� Postganglionic axons innervate organs below the diaphragm.


Autonomic Ganglia

4 major Prevertebral Ganglia:

1. Celiac ganglion- on either side of the celiac artery just inferior to the

diaphragm.

2. Superior mesenteric ganglion- near the beginning of the superiormesenteric artery in the upper abdomen.

3. Inferior mesenteric ganglion- near the beginning of the inferior

mesenteric artery in the middle of the abdomen.

4. Aorticorenal ganglion- near the renal artery as it branches from the

aorta.


Sympathetic Division of the ANS

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Parasympathetic ganglia

� Preganglionic axons of the parasympathetic division synapse with

postganglionic neurons in terminal ganglia.

� Most are located close or within the wall of a visceral organ

� They are longer than axons of sympathetic preganglionicneurons.

� Terminal ganglia in the head have specific names:

� ciliary ganglion

� pterygopalatine ganglion

� submandibular ganglion

� otic ganglion


Parasympathetic Division of the ANS


Autonomic Plexuses

� Autonomic plexuses- in the thorax, abdomen, and pelvis

� axons of both sympathetic and parasympathetic

preganglionic neurons forming tangled networks

� Many of which lie along major arteries.

� May contain sympathetic ganglia and axons of autonomic

sensory neurons.

� Major plexuses in the thorax:

� Cardiac plexus- supplies the heart

� Pulmonary plexus- supplies the bronchial tree

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Autonomic Plexuses

� Abdominal and Pelvic Plexuses

� Celiac Plexus- supplies the liver, gall bladder, stomach, pancreas, spleen, kidneys, adrenal medulla, testes, ovaries

� Superior Mesenteric Plexus- supplies the small intestine to the large intestine

� Inferior Mesenteric Plexus- supplies the large intestine

� Hypogastric Plexus- supplies the pelvic organs

� Renal Plexuses- supplies the kidneys and ureters



Autonomic Plexus in the Thorax,

Abdomen, and Pelvis


Postganglionic Neurons

� Postganglionic autonomic fibers do not end in a single terminal

swelling like a synaptic knob or end plate.

� Terminal branches of autonomic fibers contain swellings, called varicosities

� Simultaneously release neurotransmitter over a large area of the organ

rather than on single cells.

� This extensive release of neurotransmitter and the greater number

of postganglionic neurons means that entire organs, rather than

discrete cells, are typically influenced by autonomic activity.

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Sympathetic Postganglionic Neurons

Sympathetic preganglionic neurons pass to sympathetic trunk ganglia,

they may connect with postganglionic neurons in one of the following ways:➊ Axon may synapse with postganglionic neurons in the first ganglion it reaches.➋ Axon may ascend or descend to a higher or lower ganglion before synapsing with postganglionic neurons.➌ Axon may continue through the sympathetic trunk ganglion to end at a prevertebral ganglion and synapse with postganglionic neurons.


Ganglia and Postglanglionic Neurons

Process Diagram Step-by-Step


Posterior ramusof spinal nerve

Anterior ramusof spinal nerve

Gray ramuscommunicans

White ramuscommunicans

To visceral effectors:smooth muscle ofblood vessels,arrector pili muscles,sweat glands of skin

Posterior rootPosterior horn

Posteriorrootganglion

Anterior root

Prevertebralganglion(celiac ganglion)

Spinalnerve

Lateralhorn

Anteriorhorn

Spinal cord

Visceral effector: intestine

Anterior viewPreganglionic neuron

Sympathetictrunk ganglion

Postganglionic neurons

Skin

1








Anterior root


Spinalnerve

Lateralhorn

Anteriorhorn

Spinal cord





Skin

1

2

2








Anterior root


Spinalnerve

Lateralhorn

Anteriorhorn

Spinal cord





Skin

1

3

2

2

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Parasympathetic postganglionic

neurons

� Parasympathetic division neurons pass to terminal ganglia near or within a visceral effector.

� In the ganglion, presynaptic neuron usually synapses with only four or five postsynaptic neurons, all of which supply a single visceral effector.

� Thus, parasympathetic responses can be localized to a single effector.

� Parasympathetic postganglionic axons are relatively short.


Structure of the Sympathetic Division

� Cell bodies of sympathetic preganglionic neurons- part of the lateral

horns of all thoracic segments and of the lumbar 1 and 2 segments of the spinal cord.

� Preganglionic axons leave the spinal cord along with the somatic

motor neurons at the same segmental level.

� Myelinated preganglionic sympathetic axons pass into the anterior

root of a spinal nerve and enter a white ramus before passing to the nearest sympathetic trunk ganglion on the same side.


Cervical Ganglion of the Sympathetic

Trunk

� Superior cervical ganglion serve the head and heart.

� Distributed to sweat glands, smooth muscle of the eye, blood

vessels of the face, lacrimal glands, nasal mucosa, salivary glands, and the heart.

� Gray rami communicantes pass to the upper two to four

cervical spinal nerves.

� Structures containing the postganglionic axons that connect the ganglia of the various portions of the sympathetic trunk ganglion to

spinal nerves.

� Postganglionic neurons leaving the middle cervical ganglion and inferior cervical ganglion innervate the heart.

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Thoracic Ganglion of the Sympathetic

Trunk

� Thoracic portion of each sympathetic trunk ganglion lies anterior to

the necks of the corresponding ribs.

� Receives most of the sympathetic preganglionic axons, and its postganglionic neurons

� Innervate the heart, lungs, bronchi, and other thoracic viscera.

� In the skin, these neurons also innervate sweat glands, blood

vessels, and arrector pili muscles of hair follicles.



Lumbar region of Sympathetic truck

� Unmyelinated postganglionic axons from the lumbar and sacral sympathetic trunk ganglia enter a gray ramus

� Then merge with a spinal nerve or join the hypogastric plexus via direct visceral branches.

� Lead to 31 pairs of spinal nerves producing sympathetic output to smooth muscle and glands

Prevertebral Ganglia

� Preganglionic axons extend from a white ramus communicans into the sympathetic trunk ganglion, they give off several axon collaterals (branches).

� Splanchnic nerves extend to and terminate in the outlying prevertebral ganglia.

� Splanchnic nerves from Thoracic region become the

Greater, Lesser, Least (Lowest) Splanchnic nerves, and the Lumbar Splanchnic nerve from the Lumbar region


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Structure of the Parasympathetic Division

� Parasympathetic preganglionic neuron cell bodies are found in

nuclei in the brain stem and in the lateral horns of sacral segments 2-4 of the spinal cord.

� Axons emerge as part of a cranial nerve or anterior spinal nerve root

� Cranial parasympathetic outflow consists of preganglionic axons

extending from the brain stem in 4 cranial nerves.

� Sacral parasympathetic outflow consists of preganglionic axons in

anterior roots of the sacral nerves 2-4.

� Both preganglionic axons outflows end in terminal ganglia, where they synapse with postganglionic neurons.


The Cranial Outflow

1. Ciliary ganglia- lateral to each optic (II) nerve near the

posterior aspect of the orbit.

Preganglionic axons pass with the oculomotor (III) nerves to the ciliary ganglia.

Postganglionic axons from the ganglia innervate smooth muscle fibers in the eyeball.

2. Pterygopalatine ganglia- lateral to the sphenopalatine foramen between the sphenoid and palatine bones.

Preganglionic axons from the facial nerve (VII).

Postganglionic axons innervate the nasal mucosa, palate, pharynx, and lacrimal glands.

The Cranial Outflow

3. Submandibular ganglia- near the ducts of the submandibular salivary glands.

Receive preganglionic axons from the facial nerves

Postganglionic axons innervate the submandibular and sublingual salivary glands.

4. Otic ganglia- just inferior to each foramen ovale.

Receive preganglionic axons from the glossopharyngeal (IX) nerves

Postganglionic axons innervate the parotid salivary glands.


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Sacral Outflow

� Sacral parasympathetic outflowReceive preganglionic axons from the anterior nerve root from S2-S4 forming the Pelvic Splanchnic nerve

Postganglionic axons innervate the smooth muscle and glands of the colon, ureters, bladder, and reproductive organs



Structure of the Enteric Division

� Enteric division of the autonomic nervous system forms

an extensive area of contact with the environment.

� Specialized network of nerves and ganglia forming an

integrated neuronal network.

� System of nerves makes possible the normal motility and

secretory functions of the gastrointestinal tract

� Myenteric plexus- between the outer longitudinal and

circular muscle layers from the upper esophagus to the anus.

� Submucous plexus- occupies the gut wall between the

circular muscle layer and the muscularis mucosae and runs from the stomach to the anus.


Cholinergic Neurons and Receptors� Release the neurotransmitter acetylcholine.

� In the ANS, the cholinergic neurons include

� (1) all sympathetic and parasympathetic preganglionic neurons

� (2) sympathetic postganglionic neurons that innervate sweat glands

� (3) all parasympathetic postganglionic neurons

� ACh is stored in synaptic vesicles and released by exocytosis.

� Nicotinic receptors- present in plasma membranes of dendrites and cell

bodies of both sympathetic and parasympathetic postganglionic neurons and in the motor end plate at the neuromuscular junction.

� Muscarinic receptors- present in plasma membranes of all effectors

innervated by parasympathetic postganglionic axons. Most sweat glands, which receive innervation from cholinergic sympathetic postganglionic

neurons, possess muscarinic receptors

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Adrenergic Neurons and Receptors

� In the ANS, adrenergic neurons release norepinephrine or NE,

also known as noradrenalin.

� Most sympathetic postganglionic neurons are adrenergic.

� NE is synthesized and stored in synaptic vesicles and released by exocytosis.

� Adrenergic receptors bind both NE and epinephrine, a hormone.

� The main types of adrenergic receptors are alpha (a) receptors

and beta (b) receptors, which are found on visceral effectors

innervated by most sympathetic postganglionic axons.


Cholinergic Neurons and Adrenergic

Neurons (Fig. 20.6)


Fight or Flight Response

1. The pupils of the eyes dilate.

2. Heart rate, force of heart contraction, and blood pressure

increase.

3. The airways dilate, allowing faster movement of air into and out of the lungs.

4. Blood vessels that supply organs that are involved in exercise

or fighting off danger dilate.

5. Liver cells perform glycogenolysis and adipose tissue cells

perform lipolysis

6. Release of glucose by the liver increases blood glucose level.

7. Processes that are not essential for meeting the stressful

situation are inhibited.

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Effects of Sympathetic Stimulation

� The effects of sympathetic stimulation are longer lasting and

more widespread than of parasympathetic stimulation for 3

reasons:

� (1) Sympathetic postganglionic axons diverge more

extensively so many tissues are activated simultaneously.

� (2) AChE quickly inactivates ACh, but NE lingers in the

synaptic cleft for a longer period.

� (3) Epinephrine and NE secreted as hormones into the blood from the adrenal medulla intensify and prolong the

responses caused by NE released as a neurotransmitter

from sympathetic postganglionic axons.

� In time, blood-borne NE and epinephrine are inactivated by

enzymatic destruction in the liver.


Parasympathetic Responses

� The parasympathetic division enhances “rest-and-digest”

activities.

� Parasympathetic responses support body functions that conserve

and restore body energy during times of rest and recovery.

� In the quiet intervals between periods of exercise, parasympathetic impulses to the digestive glands and the smooth muscle of the

gastrointestinal tract predominate over sympathetic impulses.

� SLUDD-Salivation, Lacrimation, Urination, Digestion, Defecation


Autonomic Reflexes

� Autonomic reflexes- responses that occur when nerve

impulses pass over an autonomic reflex arc.

� Key role in regulating controlled conditions in the body such as:

� blood pressure, by adjusting heart rate, force of ventricular

contraction, and blood vessel diameter

� digestion, by adjusting the motility (movement) and muscle

tone of the gastrointestinal tract

� defecation and urination, by regulating the opening and

closing of sphincters.

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Autonomic Reflex Components

� Receptor: Distal end of a sensory neuron (interoceptors), which responds to a stimulus and produces a change that will ultimately trigger nerve impulses

� Sensory neuron: Conducts nerve impulses from receptors to the CNS.

� Integrating center: Interneurons within the CNS relay signals from sensory neurons to motor neurons. The main integrating centers for most autonomic

reflexes are located in the hypothalamus and brain stem.

� Motor neurons: Two motor neurons connect the CNS to an effector:

� The preganglionic neuron conducts motor impulses from the CNS to an autonomic ganglion

� The postganglionic neuron conducts motor impulses from an autonomic ganglion to an effector.

� Effector: Smooth muscle, cardiac muscle, and glands

� The reflex is called an autonomic reflex.

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