Ch. 14 Autonomic Nervous System(1)

Autonomic Nervous System Overview

• 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.

Nervous System Structure

• 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

Divisions of the ANS

• 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.

Role of the Parasympathetic Division

• 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.

Role of the Sympathetic Division

• 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.

Neurotransmitters and Receptors

• 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).

Sympathetic Tone

• 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.

Parasympathetic Tone

• Characterizes normal bodily functions:

  • Slows heart rate.

  • Regulates activities of digestive and urinary systems.

  • Can be overridden by sympathetic actions during stress.

Unique Roles of the Sympathetic Division

• 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.

Localized vs. Diffuse Effects

• 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.

Levels of ANS Control

• Main Integrative Center: Hypothalamus.

• Influences from:

  • Cerebral Cortex: higher conscious control.

  • Limbic system: emotional influences.

  • Other brainstem and spinal cord inputs.

Hypothalamic Control Functions

• 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).

Referred Pain

• Pain from viscera perceived as somatic pain:

  • Due to shared pathways between visceral and somatic pain fibers.

Developmental Aspects of the ANS

• Youth: ANS impairments typically from injury.

• Old Age: Reduced ANS efficiency causes:

  • Constipation, dry eyes, orthostatic hypotension.