The Autonomic Nervous System (ANS) consists of motor neurons that:
Innervate smooth and cardiac muscle and glands.
Adjust to ensure optimal support for body activities.
Operate via subconscious control.
Have viscera as their primary effectors.
Divisions:
CNS (Central Nervous System)
PNS (Peripheral Nervous System)
PNS Divisions:
Sensory Division
Motor Division: includes Somatic and Autonomic Nervous System (ANS)
Sympathetic Division
Parasympathetic Division
Sympathetic Division:
Mobilizes the body during extreme situations (fight or flight).
Parasympathetic Division:
Performs maintenance activities and conserves energy (rest and digest).
Most visceral organs receive innervation from both divisions, providing a dynamic antagonistic control of visceral activity.
Focuses on conserving body energy and involved in:
D activities: digestion, defecation, and diuresis.
Illustrative Activity:
During relaxation after a meal:
Blood pressure, heart rate, and respiratory rates decrease.
Gastrointestinal activity increases.
Skin remains warm and pupils constrict.
Known as the “fight-or-flight” system:
Nerves emerge from thoracic and upper lumbar regions of spinal cord.
Involves E activities: exercise, excitement, emergency, and embarrassment.
Adjusts blood flow during exercise:
Blood flow to organs is reduced.
Blood flow to muscles is increased.
Illustrative Activity:
In threatening situations:
Heart rate increases.
Breathing becomes rapid and deep.
Skin becomes cold and sweaty; pupils dilate.
Major Neurotransmitters:
Acetylcholine (ACh) and Norepinephrine (NE).
ACh is released by all preganglionic fibers in both divisions.
Fibers:
Cholinergic Fibers: Parasympathetic fibers releasing ACh.
Adrenergic Fibers: Sympathetic fibers releasing NE.
Receptor Types:
Parasympathetic:
Nicotinic: ACh binding is generally excitatory.
Muscarinic: Excitatory or inhibitory effects.
Sympathetic:
Alpha (α)-receptor: Generally excitatory.
Beta (β)-receptor: Generally inhibitory (notably, NE binding at heart's β-receptors is excitatory).
Cardiovascular control primarily under sympathetic tone:
Maintains blood pressure and provides vascular control (partial constriction).
Sympathetic Tone Functions:
Constricts blood vessels through α-receptors.
Prompts dilation of vessels in skeletal muscle.
Pathological Consideration: Orthostatic hypotension - low blood pressure due to delayed sympathetic vasoconstriction.
Characterizes normal bodily functions:
Slows heart rate.
Regulates activities of digestive and urinary systems.
Can be overridden by sympathetic actions during stress.
Influences responses not regulated by the parasympathetic system:
Adrenal medulla activity, sweat glands, arrector pili muscles, kidneys, blood vessels.
Controls:
Heat responses (thermoregulation).
Renin release from kidneys.
Various metabolic responses.
Parasympathetic Division:
Short-lived, localized control.
Sympathetic Division:
Long-lasting, diffuse effects due to:
Slower inactivation of NE compared to ACh.
Indirect actions through second-messenger systems.
Release of epinephrine into the blood for prolonged effects.
Main Integrative Center: Hypothalamus.
Influences from:
Cerebral Cortex: higher conscious control.
Limbic system: emotional influences.
Other brainstem and spinal cord inputs.
Regulates:
Heart activity and blood pressure.
Body temperature, water balance, and endocrine activities.
Emotional states and biological drives (hunger, thirst, sex).
Responses to fear (fight-or-flight system).
Pain from viscera perceived as somatic pain:
Due to shared pathways between visceral and somatic pain fibers.
Youth: ANS impairments typically from injury.
Old Age: Reduced ANS efficiency causes:
Constipation, dry eyes, orthostatic hypotension.