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