Lecture-13- The Autonomic Nervous System- Organization -Nov 2009

8/14/2019 Lecture-13- The Autonomic Nervous System- Organization -Nov 2009

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THE AUTONOMIC NERVOUS STSTEM

INTRODUCTION

Human Nervous System consists of:

1. Central Nervous Systema)Brain

b)Spinal cord

2. Peripheral Nervous System:a)Cranial nerves (12 pair) & their branches

b)Spinal nerves (31 pair) & their branches
http://faculty.washington.edu/chudler/flash/brainfly.htmlhttp://faculty.washington.edu/chudler/cranial.htmlhttp://bio.winona.msus.edu/berg/IMAGES/sciaticX.jpghttp://bio.winona.msus.edu/berg/IMAGES/sciaticX.jpghttp://faculty.washington.edu/chudler/cranial.htmlhttp://faculty.washington.edu/chudler/cranial.htmlhttp://faculty.washington.edu/chudler/flash/brainfly.html


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Peripheral Nervous System

2 - Spinal nerves (31 pair) & their

branches

31 pairs of mixed peripheral nerves (8

cervical, 12 thoracic, 5 lumbar, 5 sacral,

and 1 coccygeal) that are connected

segmentally with the spinal cord,

dorsal sensory trunk, and ventral motorroot.
http://bio.winona.msus.edu/berg/IMAGES/sciaticX.jpghttp://bio.winona.msus.edu/berg/IMAGES/sciaticX.jpg


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Divisions of Peripheral Nervous1. Somatic neurons - supply & receive fibers to

& from:the skin, skeletal muscles, joints, & tendons.

2. Visceral neurons - supply & receive fibers to& from:

smooth muscle, cardiac muscle, and glands.

- visceral motor fibers- those supplyingsmooth muscle, cardiac muscle, & glands-make up the autonomic nervous system(ANS)


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AUTONOMIC NERVOUS SYSTEM (ANS)The ANS (visceral or involuntary nervous system):

functions without conscious, voluntary control

Innervates cardiac muscle, smooth muscle, and variousendocrine and exocrine glands

This nervous system influences the activity of mosttissues and organ systems in the body

Therefore, the ANS makes a significant contribution to

homeostasis

e.g. regulation of blood pressure, gastrointestinal responsesto food, contraction of the urinary bladder, focusing of the

eyes, and thermoregulation

S O


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ILLUSTRATION

A story about Peter and his next door neighbor,

Joe Peters skeleton, Matilda. Joe leaves for work at 5:00 am when it is still

quite dark outside. On the night before, Peter

placed Matilda in the driver's seat of Joe'spickup truck. The following morning Peter arose at 4:45 am,

poured coffee and waited patiently by thewindow located nearest to Joe's truck.

Completely unsuspecting, Joe came walkingdown the driveway at his usual time. When heopened the truck door, he was scared stiff andscreamed! Poor Joe stood by his truck wide-

eyed and clutching his chest.


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ILLUSTRATION What happened to Joe?

Several events occurred in his body at

once.

His heart began racing, his blood pressureincreased, his pupils dilated, he began

sweating, the hair on his arms and at the

back of his neck stood on end

he felt a surge of adrenaline.

These are some of the effects of

sympathetic nervous activity in Joe's

body.


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ILLUSTRATION

Meanwhile, as Peter waited for Joe's early

morning arrival, the events occurring in

his body were quite different

His heart rate was comparatively slowerand his digestive system was processing

the cream and sugar in his coffee.

These are some of the effects ofparasympathetic nervous activity in

Peters body.


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SYMPATHETIC NERVOUS SYSTEM

prepares the body for emergencies

Mediates the "fight-or-flight" response by:

increasing blood pressure and directing blood to skeletal musclesand other tissues that must work hardest in the face of impendingdanger.

Adrenaline is the NT of the sympathetic nervous system.

Thus the fibers of the sympathetic nervous system are said to beadrenergic.

The actions of adrenaline in the bloodstream complement those ofnoradrenaline released by sympathetic nerves.

These two agents work on the heart and blood vessels to raiseblood pressure in stressful situations.


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PARASYMPATHETIC NERVOUS SYSTEM

The parasympathetic nervous system is

involved in the conservation of body energy

Its activities stimulate digestion and slow heart

rate, both effects that help the body to eithergather or conserve energy

Acetylcholine is the neurotransmitter of the

parasympathetic nervous system

Thus, the fibers of the parasympathetic system

are said to be cholinergic.


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FIG 3: SUMMARY OF THEFEATURES OF THE ANS

DISTINCTIONS BETWEEN

THE SYMPATHETIC &PARASYMPATHETIC

DIVISIONS


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Subdivision NervesEmployed

LocationofGanglia

ChemicalMessenger

GeneralFunction

Sympathetic Thoracolumbar Alongsidevertebralcolumn

Norepinephrine Fight or flight

Parasympathetic Craniosacral On or nearan effector

organ

Acetylcholine Conservationof body energy


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REVIEW OF NEURONS

There are three functional classes of neurons:

1. Sensory neurons are afferent: i.e they transmit a

continuous stream of information about the thermal,physical, and chemical status of the body to the CNS.

The coordination of different physiological functions

depends on continual surveillance of the body tissues,which is provided by specialized sensory receptorsdistributed throughout the body.

More than 20 different kinds of sensory receptorsconstantly monitor conditions in the body.

Each type generates an electrical signal in the form ofan action potential that is transmitted by afferent nerve

fibers to a specific site in the CNS.


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REVIEW OF NEURONS

2. Motor neurons are efferent: i.e. they

transmit instructions from the CNS to

effector body tissues, such as the heart or

skeletal muscles.

These signals adjust the tissue's activities

from moment to moment.


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REVIEW OF NEURONS

3. Inter neurons - a mix of both a sensory

neuron and a motor neuron.

These neurons usually only found in theCNS.

These neurons send messages betweenboth motor and sensory neurons.


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ANS


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ANS The efferent pathways of the ANS each contain two neurons

They communicate with each other in one of the autonomicganglia outside of the CNS (a ganglion is a cluster of nervecell bodies):

1. Preganglionic neuron - the first autonomic neuron has its

cell body in the CNS and sends a fiber out to a ganglion.

The preganglionic neuron originates in the CNS with its cellbody in the lateral horn of the gray matter of the spinal cordor in the brainstem.

The axon of this neuron travels to an autonomic ganglionlocated outside the CNS

where it synapses with a postganglionic neuron.


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ANS

2. Postganglionic neuron -The second autonomicneuron has its cell body in a ganglion

and its fiber completes the efferent circuit byinnervating a particular effector tissue

The Postganglionic neuron innervates the

effector tissue.


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The Sympathetic and Parasympathetic

Divisions of the Nervous System

The autonomic nervous system directs all activities of the body that occur without aperson's conscious control, such as breathing and food digestion. It has two parts: thesympathetic division, which is most active in times of stress, and the parasympatheticdivision, which controls maintenance activities and helps conserve the body's energy.
http://en.wikipedia.org/wiki/Skin


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ANS

Synapses between the autonomic

postganglionic neuron and effector tissue

differ greatly from neuron-to-neuron synapses.

The postganglionic fibers in the ANS do notterminate in a single swelling like the synaptic

knob,

nor do they synapse directly with the cells of a

tissue


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ANS Instead, where the axons of these fibers enter a given tissue, they

contain multiple swellings called varicosities.

When the neuron is stimulated, these varicosities releaseneurotransmitters along a significant length of the axon and,therefore, over a large surface area of the effector tissue.

The neurotransmitter diffuses through the interstitial fluid to

wherever its receptors are located in the tissue. This diffuserelease of the neurotransmitter affects many tissue cellssimultaneously.

Furthermore, cardiac muscle and most smooth muscle have gapjunctions between cells. These specialized intercellular

communications allow for the spread of electrical activity from onecell to the next.

As a result, the discharge of a single autonomic nerve fiber to aneffector tissue may alter the activity of the entire tissue.


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Regulation of ANS Activity Reflex- rapid unconscious response to changes in the

internal or external environment needed to maintainhomeostasis.

Reflex arc - the neural pathway over which impulsestravel during a reflex.

The components of a reflex arc include:

receptor - responds to the stimulus afferent pathway (sensory neuron) - transmits impulse

into the spinal cord

CNS - where spinal cord then processes information

efferent pathway (motor neuron) - transmits impulse outof spinal cord

effector - a muscle or gland that receives the impulsefrom the motor neuron & carries out the desiredresponse.

R l i f ANS A i i
http://www.dushkin.com/connectext/psy/ch02/spinal.mhtmlhttp://www.dushkin.com/connectext/psy/ch02/spinal.mhtml


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Regulation of ANS Activity

Visceral afferent neurons are sensory neurons that

conduct impulses initiated in receptors in smoothmuscle & cardiac muscle.

These neurons are collectively referred to asenteroceptors or visceroceptors.

Visceral afferent neurons are unipolar neurons

that enter the spinal cord through the dorsal root

their cell bodies are located in the dorsal root

ganglia.


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N t itt f th ANS


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Neurotransmitters of the ANS The 2 most common neurotransmitters released by neurons of the

ANS are acetylcholine and norepinephrine.

Neurotransmitters are synthesized in the axon varicosities andstored in vesicles for subsequent release.

Cholinergic neurons that use acetylcholine as a neurotransmitterinclude:

all preganglionic neurons (sympathetic & parasympathetic)

all parasympathetic postganglionic neurons

the sympathetic postganglionic neurons that supply the sweatglands

Adrenergic neurons that use adrenaline include all postganglionicsympathetic neurons (except those that go to the sweat glands).

Receptors for Autonomic Neurotransmitters


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The effect caused by neurotransmitters or hormones is

determined by the:

receptor distribution in a particular tissue

the biochemical properties of the cells in that tissue,specifically, the second messenger and enzyme systemspresent within the cell

The neurotransmitters of the ANS and the circulatingcatecholamines bind to specific receptors on the cellmembranes of the effector tissue:

Acetylcholine binds to 2 types of cholinergic receptors


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1. Nicotinic receptors are found on the cell bodies of all

postganglionic neurons, both sympathetic and parasympathetic,in the ganglia of the ANS.

They bind acetylcholine and other nicotine-like agents on autonomicganglia, adrenal medulla, and the motor end-plate of striated

muscle.

Acetylcholine released from the preganglionic neurons binds tothese nicotinic receptors and causes a rapid increase in thecellular permeability to Na+,K+ and Ca++ ions.

The resulting influx of these 2 cations causes depolarization andexcitation of the postganglionic neurons of the ANS pathways.

R t f A t i N t itt


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2. Muscarinic receptors are found on the cell membranes of the effectortissues and are linked to G proteins and second messenger systemswhich carry out the intracellular effects.

Acetylcholine released from all parasympathetic postganglionicneurons and some sympathetic postganglionic neurons traveling tosweat glands binds to these receptors.

Muscarinic receptors may be either inhibitory or excitatory, dependingon the tissue upon which they are found.

Eg muscarinic receptor stimulation in the myocardium is inhibitory and

decreases heart rate

while stimulation of these receptors in the lungs is excitatory, causingcontraction of airway smooth muscle and bronchoconstriction.


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Receptors for Autonomic


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Receptors for AutonomicNeurotransmitters

There are 2 classes of adrenergic receptors for norepinephrine andepinephrine, alpha () and beta ().

there are at least 2 subtypes of receptors in each class: 1, 2, 1 and2.

All of these receptors are linked to G proteins and second messenger

systems which carry out the intracellular effects.

Alpha receptors are the more abundant of the adrenergic receptors.The 1 receptors are more widely distributed on the effector tissues.

1 receptor stimulation leads to an increase in intracellular calcium. Asa result, these receptors tend to be excitatory.

stimulation of 1 receptors causes contraction of vascular smoothmuscle resulting in vasoconstriction and increased glandularsecretion by way of exocytosis.



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Neurotransmitters

All adrenergic receptors and muscarinic receptors are coupled to G

proteins which are also embedded within the plasma membrane.

Receptor stimulation causes activation of the G protein and theformation of a the second messenger.

The function of the intracellular second messenger molecules is toelicit tissue-specific biochemical events within the cell which alter thecell's activity.

In this way, a given neurotransmitter may stimulate the same type ofreceptor on 2 different types of tissue and cause 2 different responses

This is due to the presence of different biochemical pathways withineach tissue.

Sympathetic stimulation of 3 receptors in adipose tissue causeslipolysis

Termination of Neurotransmitter Activity


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Termination of Neurotransmitter Activity

For any substance to serve effectively as a neurotransmitter, it must be rapidlyinactivated or removed from the synapse or, in this case, the neuroeffectorjunction.

This is necessary in order to allow new signals to get through and influenceeffector tissue function.

The primary mechanism used by cholinergic synapses is enzymaticdegradation.

Acetylcholinesterase hydrolyzes acetylcholine to its component choline andacetate. It is one of the fastest acting enzymes in the body and acetylcholineremoval occurs in less than 1 msec.

The most important mechanism for the removal of norepinephrine from theneuroeffector junction is the reuptake of this neurotransmitter into thesympathetic nerve that released it.

Norepinephrine may then be metabolized intraneuronally by monoamineoxidase (MAO).

The circulating catecholamines, epinephrine and norepinephrine, areinactivated by catechol-O-methyltransferase (COMT) in the liver.


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N A (t itti ) t B ( i i )


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Neuron A (transmitting) to neuron B (receiving)

1. Mitochondrion

2. synaptic vesicle with neurotransmitters

3. Autoreceptor

4. Synapse with NT released (serotonin)

5. Postsynaptic receptors activated by NT (induction of apostsynaptic potential)

6. Calcium channel

7. Exocytosis of a vesicle

8. Recaptured neurotransmitter
http://en.wikipedia.org/wiki/Mitochondriahttp://en.wikipedia.org/wiki/Synaptic_vesiclehttp://en.wikipedia.org/wiki/Neurotransmittershttp://en.wikipedia.org/wiki/Synapsehttp://en.wikipedia.org/wiki/Serotoninhttp://en.wikipedia.org/wiki/Calcium_channelhttp://en.wikipedia.org/wiki/Calcium_channelhttp://en.wikipedia.org/wiki/Serotoninhttp://en.wikipedia.org/wiki/Synapsehttp://en.wikipedia.org/wiki/Neurotransmittershttp://en.wikipedia.org/wiki/Synaptic_vesiclehttp://en.wikipedia.org/wiki/Mitochondria


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Lecture-13- The Autonomic Nervous System- Organization -Nov 2009