Ch14ANS

THE AUTONOMIC NERVOUS SYSTEM

Overview of ANS

  • The Autonomic Nervous System (ANS) consists of motor neurons that innervate smooth and cardiac muscle, and glands.

  • It makes adjustments to ensure optimal support for body activities and operates via subconscious control.

  • Also referred to as the involuntary nervous system or general visceral motor system.

Structural Organization

  • Structural framework of the ANS:

    • Central Nervous System (CNS)

    • Peripheral Nervous System (PNS)

      • Sensory (Afferent) Division

      • Motor (Efferent) Division

        • Somatic Nervous System

        • Autonomic Nervous System (ANS)

          • Sympathetic Division

          • Parasympathetic Division

Somatic vs Autonomic Nervous Systems

Common Characteristics

  • Both somatic and autonomic systems contain motor fibers.

Differences

  • Effectors:

    • Somatic nervous system targets skeletal muscles.

    • ANS targets cardiac muscle, smooth muscle, and glands.

  • Efferent Pathways and Ganglia:

    • Somatic: Cell body in CNS, myelinated fibers directly to muscles.

    • ANS: Two-neuron chain, including a preganglionic neuron (CNS) and a postganglionic neuron (PNS).

  • Target Organ Responses:

    • Somatic responses to acetylcholine (ACh) are always stimulatory.

    • ANS responses vary; can be stimulatory or inhibitory based on receptor type.

Efferent Pathways and Ganglia

  • Somatic Nervous System

    • Features a single, thick, myelinated axon extending from the CNS to skeletal muscle.

  • Autonomic Nervous System

    • Utilizes a two-neuron chain:

      • Preganglionic Neuron: Thin, lightly myelinated axon from CNS to autonomic ganglion.

      • Postganglionic Neuron: Nonmyelinated axon extending from ganglion to effector organ.

Neurotransmitter Effects

  • Somatic Nervous System:

    • All motor neurons release ACh, always leading to stimulation.

  • Autonomic Nervous System:

    • Preganglionic fibers release ACh.

    • Postganglionic fibers may release norepinephrine (NE) or ACh, resulting in varied responses depending on receptor type.

Divisions of the ANS

  • Sympathetic Division: Mobilizes body during activity (fight-or-flight).

  • Parasympathetic Division: Promotes maintenance activities and conserves body energy.

    • Dynamic antagonism between both divisions helps maintain homeostasis.

Roles of the Parasympathetic Division

  • Direct involvement in digestion, diuresis (urine production), and defecation.

  • Characteristics during activity:

    • Low blood pressure, heart rate, and respiratory rates.

    • Increased activity in the gastrointestinal tract and constricted pupils.

Roles of the Sympathetic Division

  • Activated during exercise, excitement, emergencies, and embarrassment.

  • Enhancements include:

    • Increased heart rate, dry mouth, cold and sweaty skin, and dilated pupils.

    • During vigorous activity, blood is redirected to skeletal muscles and heart, bronchioles dilate, and liver releases glucose.

Control of ANS Functions

  • Hypothalamus: Main integrative center of ANS, regulating various autonomic responses.

  • Other controls: Involvement of the limbic system, cerebral cortex, reticular formation, and spinal cord.

Homeostatic Imbalances

  1. Hypertension

    • Caused by overactive sympathetic response: treated with adrenergic receptor-blocking drugs.

  2. Raynaud's Disease

    • Exaggerated vasoconstriction causing pain and color changes in fingers/toes: treated with vasodilators.

  3. Autonomic Dysreflexia

    • Life-threatening condition in quadriplegics due to uncontrolled autonomic activation leading to skyrocketing blood pressure.

Unique Roles of the Sympathetic Division

  • Areas solely receiving sympathetic fibers: adrenal medulla, sweat glands, kidneys, and most blood vessels.

  • Functions include thermoregulation, metabolic rate increase, and regulation of blood glucose levels.