The Autonomic Nervous System

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 in the Nervous System

ANS Versus Somatic Nervous System (SNS)

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

§     Effectors

§     Efferent pathways

§     Target organ responses


§    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 has a lightly myelinated axon

§     The ganglionic (second) neuron extends to an effector organ

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 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

Comparison of Somatic and Autonomic Systems

Divisions of the ANS

§    ANS divisions: 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

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


Anatomy of ANS


Parasympathetic Division Outflow

Sympathetic Outflow


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 stimuli arising from the viscera are 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 to 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 from the Kidneys

§    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

§     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