151 ANS

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Autonomic Nervous System

(ANS)• The ANS consists of motor neurons that:

– Innervate smooth and cardiac muscle and

glands

– Make adjustments to ensure optimal support for

body activities

– Operate via subconscious control – Have viscera as most of their effectors

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ANS Versus Somatic Nervous

System (SNS)• The ANS differs from the SNS in the

following three areas

– Effectors

– Efferent pathways

– Target organ responses

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Effectors

• The effectors of the SNS are skeletal

muscles

• The effectors of the ANS are cardiac

muscle, smooth muscle, and glands

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Efferent Pathways

• Heavily myelinated axons of the somatic

motor neurons extend from the CNS to the

effector

• Axons of the ANS are a two-neuron chain

– The preganglionic (first) neuron has a lightly

myelinated axon – The ganglionic (second) neuron extends to an

effector organ

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Neurotransmitter Effects

• All somatic motor neurons release Acetylcholine(ACh), which has an excitatory effect

• In the ANS: – Preganglionic fibers release ACh

– Postganglionic fibers release norepinephrine or AChand the effect is either stimulatory or inhibitory

– ANS effect on the target organ is dependent upon theneurotransmitter released and the receptor type of theeffector

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Comparison of Somatic and

Autonomic Systems

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Interactions of the Autonomic Divisions

• Most visceral organs are innervated by both

sympathetic and parasympathetic fibers

• This results in dynamic antagonisms that

precisely control visceral activity

• Sympathetic fibers increase heart and respiratory

rates, and inhibit digestion and elimination

• Parasympathetic fibers decrease heart and

respiratory rates, and allow for digestion and the

discarding of wastes

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Role of the Parasympathetic

Division• Concerned with keeping body energy use low

• Involves the D activities – digestion, defecation,

and diuresis• Its activity is illustrated in a person who relaxes

after a meal

– Blood pressure, heart rate, and respiratory rates are low

– Gastrointestinal tract activity is high

– The skin is warm and the pupils are constricted

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Role of the Sympathetic Division

• The sympathetic division is the “fight-or-flight”system

• Involves E activities – exercise, excitement,emergency, and embarrassment

• Promotes adjustments during exercise – blood flowto organs is reduced, flow to muscles is increased

• Its activity is illustrated by a person who isthreatened

– Heart rate increases, and breathing is rapid and deep

– The skin is cold and sweaty, and the pupils dilate

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Visceral Reflexes

Figure 14.7

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Cholinergic Receptors

• The two types of receptors that bind ACh

are nicotinic and muscarinic

• These are named after drugs that bind to

them and mimic ACh effects

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Nicotinic Receptors

• Nicotinic receptors are found on:

– Motor end plates (somatic targets)

– All ganglionic neurons of both sympathetic and

parasympathetic divisions

– The hormone-producing cells of the adrenal

medulla• The effect of ACh binding to nicotinic

receptors is always stimulatory

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Muscarinic Receptors

• Muscarinic receptors occur on all effector

cells stimulated by postganglionic

cholinergic fibers

• The effect of ACh binding:

– Can be either inhibitory or excitatory

– Depends on the receptor type of the targetorgan

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Adrenergic Receptors

• The two types of adrenergic receptors are alphaand beta

• Each type has two or three subclasses

(1, 2, 1, 2 , 3)• Effects of NE binding to:

– receptors is generally stimulatory

– receptors is generally inhibitory

• A notable exception – NE binding to receptorsof the heart is stimulatory

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Dual Innervation

• Most of viscera receive nerve fibers from both

parasympathetic and sympathetic divisions• Both divisions do not normally innervate an organ

equally

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Dual Innervation

• Antagonistic effects

– oppose each other

– exerted through dual innervation of same effector • heart rate decreases (parasympathetic)

• heart rate increases (sympathetic)

– exerted because each division innervates different cells

• pupillary dilator muscle (sympathetic) dilates pupil

• constrictor pupillae (parasympathetic) constricts pupil

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Dual Innervation

• Cooperative effects seen when 2 divisions act ondifferent effectors to produce a unified effect

– parasympathetics increase salivary serous cell secretion

– sympathetics increase salivary mucous cell secretion

• ANS cooperation is best seen in control of theexternal genitalia

– Parasympathetic fibers cause vasodilation and are

responsible for erection of the penis and clitoris – Sympathetic fibers cause ejaculation of semen in males

and reflex peristalsis in females

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Dual Innervation of the Iris

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Without Dual Innervation

• Some effectors receive only sympathetic – adrenal medulla, arrector pili muscles, sweat glands and

many blood vessels

• Sympathetic tone – a baseline firing frequency

– vasomotor tone provides partial constriction

• increase in firing frequency = vasoconstriction

• decrease in firing frequency = vasodilation

• can shift blood flow from one organ to another as needed

– sympathetic stimulation increases blood to skeletal and cardiac

muscles -- reduced blood to skin

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Sympathetic and Vasomotor Tone

Sympathetic divisionprioritizes blood vesselsto skeletal muscles andheart in times ofemergency.

Blood vessels to skin

vasoconstrict tominimize bleeding ifinjury occurs duringstress or exercise.

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Regulation of ANS

• Autonomic reflexes control most of activity of visceral

organs, glands, and blood vessels.• Autonomic reflex activity influenced by hypothalamus

and higher brain centers, but it is the hypothalamus thathas overall control of the ANS.

• Sympathetic and parasympathetic divisions influenceactivities of enteric (gut) nervous system throughautonomic reflexes. These involve the CNS. But, theenteric nervous system can function independently of CNS through local reflexes. E.g., when wall of

digestive tract is stretched, sensory neurons sendinformation to enteric plexus and then motor responsessent to smooth muscle of gut wall and the musclecontracts.

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Levels of ANS Control

• The hypothalamus is the main integrationcenter of ANS activity

• Subconscious cerebral input via limbic lobeconnections influences hypothalamicfunction

• Other controls come from the cerebralcortex, the reticular formation, and thespinal cord

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Hypothalamic Control

• Centers of the hypothalamus control:

– Heart activity and blood pressure

– Body temperature, water balance, and

endocrine activity

– Emotional stages (rage, pleasure) and biological

drives (hunger, thirst, sex) – Reactions to fear and the “fight-or-flight”

system

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Levels of ANS Control

Figure 14.9

151 ANS

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