Autonomic Nervous System - Chapter 14

•     Efferent division

–   Carries motor commands through motor neurons

–   Innervate smooth and cardiac muscle and glands (General visceral motor system)

•     Controls routine homeostatic adjustments

•     Does so without input from the conscious mind

–   Operate via subconscious control (Involuntary nervous system)

ANS in the Nervous System

Efferent Pathways of ANS

•     Two-neuron chain

–   Preganglionic fibers (first)

•   Found in brain or spinal cord

•   Lightly, thinner myelinated axons

•   Release ACh

•   Excitatory

–   Postganglionic (second)

•   Extends to an effector organ

•   More thinner, unmyelinated axons

•   Release Ach or norepinephrine (NE)

•   Excitatory or inhibitory

Comparison of Somatic and Autonomic Systems

Divisions of ANS

•     Sympathetic

–   Fight or flight

–   Stimulates metabolism and alertness

•     Parasympathetic

–   Rest and digest

–   Conserves energy

•     Dual innervation

•     Table 14.5  (Summary of effects of each division)

Sympathetic Division

•      “Fight or flight” system

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

•      Affects peripheral tissues and CNS activity

–   Increased alertness (dilation of pupils)

–   Euphoria

–   Increased cardiovascular activity (increased heart rate)

–   Increased breathing

–   Elevated muscle tone

–   Release of energy reserves

Parasympathetic Division

•     Concerned with keeping body energy use low

•     Involves D activities – digestion, defecation, and diuresis

•     Gastrointestinal activity is high

–   Secretion by digestive glands and increase in digestive activity

•     Reduction of heart rate and blood pressure

•     Respiratory rates are low

•     Pupils are constricted

Anatomy of ANS

Visceral Reflexes

•     Automated motor responses

•     Have the same elements as somatic reflexes

•     Always polysynaptic pathways

•     Visceral reflex arc

–   Receptor

–   Sensory neuron

–   Processing center

–   2 visceral neurons

Visceral Reflexes

ANS Neurotransmitters

•     Acetylcholine and Norepinephrine

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

•     Cholinergic

–   ACh-releasing fibers

–   All preganglionic axons and all parasympathetic postganglionic axons

–   Cholinergic receptors

•   Nicotinic

•   Muscarinic

•     Adrenergic

–   Release NE

–   Sympathetic postganglionic fibers

–   Adrenergic receptors

•   Alpha receptors

•   Beta receptors (β₁ and β₂)

Effects of Drugs

•     Atropine – blocks parasympathetic effects

•     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

•     Albuterol

Drugs that Influence the ANS

INTERACTION

•     Dual innervation

•     Especially in heart, digestive tract, and lungs

–   Sympathetic fibers increase heart and respiratory rates, and inhibit digestion and elimination

–   Parasympathetic fibers decrease heart and respiratory rates, and allow for digestion and discarding of wastes

Autonomic Tone

•     Resting level of spontaneous activity

–   Allows increase or decrease of activity

•     Especially important in areas without dual innervation

•     Heart

Sympathetic Tone

•     Blood vessel diameter

–   Keeps blood vessels in a continual state of partial constriction (1/2 their max diameter)

–   Alpha blockers

•   Used to treat hypertension

–  What is hypertension?

Parasympathetic Tone

•     Slows heart

•     Dictates normal activity levels of the digestive and urinary systems

•     Sympathetic division can override

•     So, what does the drugs that block the parasympathetic responses do?