AJ

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.