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Chapter 14

Autonomic Nervous System (ANS)

 

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

ANS Versus Somatic Nervous System (SNS)

•      The ANS differs from the SNS in the following three areas

•    Effectors

•    Efferent pathways

•    Target organ responses

Effectors

•      The effectors of the SNS are skeletal muscles

•      The effectors of the ANS are cardiac muscle, smooth muscle, and glands

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 with a lightly myelinated axon

•    The gangionic (second) neuron that extends to an effector organ

Neurotransmitter Effects

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

•      In the ANS:

•    Preganglionic fibers release ACh

•    Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory

•    ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type of the effector

Divisions of the ANS

•      The two divisions of the ANS are the sympathetic and parasympathetic

•      The sympathetic mobilizes the body during extreme situations

•      The parasympathetic performs maintenance activities and conserves body energy

•      The two divisions counterbalance each other’s activity

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

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 flow to organs is reduced, flow to muscles is increased

•      Its activity is illustrated by a person who is threatened

•    Heart rate increases, and breathing is rapid and deep

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

Sympathetic Outflow

•      Is from nerves T1 through L2

•      Sympathetic neurons produce the lateral horns of the spinal cord

•      Preganglionic fibers pass through the white rami communicantes and synapse in the paravertebral ganglia

•      Fibers from T5-L2 form splanchnic nerves and synapse in collateral ganglia

•      Postganglionic fibers innervate the numerous organs of the body

Sympathetic Trunks and Pathways

•      Preganglionic fibers pass through white rami communicantes and enter paravertebral ganglia

•      The paravertebral ganglia form part of the sympathetic chain

•      Typically there are 23 ganglia – 3 cervical,
11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal

•      A pregangiolonic fiber follows one of three pathways upon entering the paravertebral ganglia:

•    Synapse with the ganglionic neuron within the same ganglion

•    Ascend or descend the
sympathetic chain to
synapse in another
chain ganglion

•    Pass through the chain
ganglion and emerge
without synapsing

Pathways with Synapses in a Chain Ganglion

•      Postganglionic axons enter the ventral rami via the gray rami communicantes

•      These fibers innervate sweat glands and arrector pili muscles

•      Rami communicantes are associated only with the sympathetic division

Pathways to the Head

•      Preganglionic fibers emerge from T1-T4 and synapse in the superior cervical ganglion

•      These fibers:

•    Serve the skin and blood vessels of the head

•    Stimulate dilator muscles of the iris

•    Inhibit nasal and salivary glands

Pathways to the Thorax

•      Preganglionic fibers emerge from T1-T6 and synapse in the cervical chain ganglia

•      Postganglionic fibers emerge from the middle and inferior cervical ganglia and enter nerves C4-C8

•      These fibers innervate the heart via the cardiac plexus, as well as innervating, the thyroid and the skin

•      Other T1-T6 preganglionic fibers synapse in the nearest chain ganglia

•      Postganglionic fibers directly serve the heart, aorta, lungs, and esophagus

Pathways with Synapses in a Collateral Ganglion

•      These fibers (T5-L2) leave the sympathetic chain without synapsing

•      They form thoracic, lumbar, and sacral splanchnic nerves

•      Their ganglia include the celiac, the superior and inferior mesenterics, and the hypogastric

Pathways to the Abdomen

•      Sympathetic nerves innervating the abdomen have preganglionic fibers from T5-L2

•      They travel through the thoracic splanchnic nerves and synapse at the celiac and superior mesenteric ganglia

•      Postganglionic fibers serve the stomach, intestines, liver, spleen, and kidneys

Pathways to the Pelvis

•      Preganglionic fibers originate from T10-L2

•      Most travel via the lumbar and sacral splanchnic nerves to the inferior mesenteric and hypogastric ganglia

•      Postganglionic fibers serve the distal half of the large intestine, the urinary bladder, and the reproductive organs

Pathways with Synapses in the Adrenal Medulla

•      Fibers of the thoracic splanchnic nerve pass directly to the adrenal medulla

•      Upon stimulation, medullary cells secrete norepinephrine and epinephrine into the blood

Visceral Reflexes

•      Visceral reflexes have the same elements as somatic reflexes

•      They are always polysynaptic pathways

•      Afferent fibers are found in spinal and autonomic nerves

Referred Pain

•      Pain arising from the viscera but is perceived as somatic in origin

•      This may be due to the fact that visceral pain afferents travel along the same pathways as somatic pain fibers

Neurotransmitters and Receptors

•      Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters of the ANS

•      ACh is released by all preganglionic axons and all parasympathetic postganglionic axons

•      Cholinergic fibers – ACh-releasing fibers

•      Adrenergic fibers – sympathetic postganglionic axons that release NE

•      Neurotransmitter effects can be excitatory or inhibitory depending upon the receptor type

Cholinergic Receptors

•      The two types of receptors that bind ACh are nicotinic and muscarinic

•      These are named after drugs that bind them and mimic ACh effects

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

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 target organ

Adrenergic Receptors

•      The two types of adrenergic receptors are alpha and beta

•      Each type has two or three subclasses (a1, a2,  b1, b2 , b3)

•      Effects of NE binding to:

•    a receptors is generally stimulatory

•    b receptors is generally inhibitory

•      A notable exception – NE binding to b receptors of the heart is stimulatory

Effects of Drugs

•      Atropine – blocks parasympathetic effects

•      Neostigmine – inhibits acetylcholinesterase and is used to treat myasthenia gravis

•      Tricyclic antidepressants – prolong the activity of NE on postsynaptic membranes

•      Over-the-counter drugs for colds, allergies, and nasal congestion – stimulate a-adrenergic receptors

•      Beta-blockers – attach mainly to b1 receptors and reduce heart rate and prevent arrhythmias

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

Sympathetic Tone

•      The sympathetic division controls blood pressure and keeps the blood vessels in a continual state of partial constriction

•      This sympathetic tone (vasomotor tone):

•    Constricts blood vessels and causes blood pressure to rise as needed

•    Prompts vessels to dilate if blood pressure is to be decreased

•      Alpha-blocker drugs interfere with vasomotor fibers and are used to treat hypertension

Parasympathetic Tone

•      Parasympathetic tone:

•    Slows the heart

•    Dictates normal activity levels of the digestive and urinary systems

•      The sympathetic division can override these effects during times of stress

•      Drugs that block parasympathetic responses increase heart rate and block fecal and urinary retention

Cooperative Effects

•      ANS cooperation is best seen in control of the external 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

Unique Roles of the Sympathetic Division

•      Regulates many functions not subject to parasympathetic influence

•      These include the activity of the adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels

•      The sympathetic division controls:

•    Thermoregulatory responses to heat

•    Release of renin from the kidneys

•    Metabolic effects

Thermoregulatory Responses to Heat

•      Applying heat to the skin causes reflex dilation of blood vessels

•      Systemic body temperature elevation results in widespread dilation of blood vessels

•      This dilation brings warm blood to the surface and activates sweat glands to cool the body

•      When temperature falls, blood vessels constrict and blood is retained in deeper vital organs

Release of Renin

•      Sympathetic impulses activate the kidneys to release renin

•      Renin is an enzyme that promotes increased blood pressure

Metabolic Effects

•      The sympathetic division promotes metabolic effects that are not reversed by the parasympathetic division

•    Increases the metabolic rate of body cells

•    Raises blood glucose levels

•    Mobilizes fat as a food source

•    Stimulates the reticular activating system (RAS) of the brain, increasing mental alertness

Localized Versus Diffuse Effects

•      The parasympathetic division exerts short-lived, highly localized control

•      The sympathetic division exerts long-lasting, diffuse effects

Effects of Sympathetic Activation

•      Sympathetic activation is long-lasting because NE:

•    Is inactivated more slowly than ACh

•    Is an indirectly acting neurotransmitter, using a second-messenger system

•    NE and epinephrine are released into the blood and remain there until destroyed by the liver

Levels of ANS Control

•      The hypothalamus is the main integration center of ANS activity

•      Subconscious cerebral input via limbic lobe connections influences hypothalamic function

•      Other controls come from the cerebral cortex, the reticular formation, and the spinal cord

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

Embryonic Development of the ANS

•      Preganglionic neurons are derived from the embryonic neural tube

•      ANS structures in the PNS–ganglionic neurons, the adrenal medulla, and all autonomic ganglia–derive from the neural crest

•      Nerve growth factor (NGF) is a protein secreted by target cells that aids in the development of ANS pathways

Developmental Aspects of the ANS

•      During youth, ANS impairments are usually due to injury

•      In old age, ANS efficiency decreases, resulting in constipation, dry eyes, and orthostatic hypotension

•    Orthostatic hypotension is a form of low blood pressure that occurs when sympathetic vasoconstriction centers respond slowly to positional changes