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In-depth Notes on Autonomic Nervous System and Related Concepts

  • Autonomic Nervous System Overview

    • Divided into two main parts: Parasympathetic and Sympathetic systems.
    • Function to regulate involuntary body functions such as heart rate, digestion, respiratory rate, and reflex actions.

  • Differences Between Parasympathetic and Sympathetic Nervous Systems

    • Neurons:
    • Sympathetic: Short preganglionic and long postganglionic neurons.
    • Parasympathetic: Long preganglionic and short postganglionic neurons.
    • Locations of Origin:
    • Sympathetic: Thoracolumbar (T1-L2).
    • Parasympathetic: Craniosacral (brainstem and sacral spinal cord).
    • Neurotransmitters:
    • Sympathetic: Generally uses norepinephrine at the postganglionic level.
    • Parasympathetic: Uses acetylcholine exclusively.

  • Key Functions

    • Sympathetic Functions:
    • Activates 'fight or flight' responses:
      • Increases heart rate, blood pressure, and blood flow to muscles.
      • Activates processes for quick energy use (glycogenolysis and lipolysis).
    • Parasympathetic Functions:
    • Promotes 'rest and digest' responses:
      • Stimulates digestion, promotes bladder contraction, and promotes relaxation and energy conservation.

  • Neurotransmitter Receptors

    • Cholinergic Receptors:
    • Bind acetylcholine (ACh).
    • Subtypes:
      • Nicotinic receptors: Always stimulatory.
      • Muscarinic receptors: Can be either stimulatory or inhibitory.
    • Adrenergic Receptors:
    • Bind norepinephrine (NE) and epinephrine.
    • Subtypes:
      • Alpha receptors: Generally stimulatory.
      • Beta receptors: Usually inhibitory (exception on heart, where they can stimulate).

  • Autonomic Reflexes

    • Sympathetic Reflexes:
    • Involvement of two neurons (preganglionic to postganglionic).
    • Involves sensory neurons relaying information about organ function (e.g., pain signals).
    • Parasympathetic Reflexes:
    • Longer pathways directly to organs without large ganglia.

  • Impact of Stress and Medications

    • Chronic stress activates the sympathetic system, leading to health issues (hypertension, heart disease).
    • Beta Blockers:
    • Block effects of norepinephrine on beta-adrenergic receptors, lowering heart rate and blood pressure.
    • Atropine:
    • Blocks the effects of ACh, leading to increased heart rate.

  • Homeostatic Imbalances

    • Hypertension:
    • Caused by prolonged activation of the sympathetic nervous system; dietary, lifestyle factors also contribute.
    • Raynaud's Disease:
    • Abnormal vasoconstriction leading to decreased blood flow in extremities.

  • Referred Pain and Shared Nerve Pathways

    • Pain perceived in areas outside the injury due to shared spinal pathways.
    • Example: Heart attacks often manifest pain in the left arm due to shared nerve signal pathways.

  • Interplay Between Parasympathetic and Sympathetic Systems

    • Generally, they have opposing effects, but may need to work together in certain physiological responses (e.g., sexual arousal).
    • The autonomic nervous system is also influenced by psychological factors via the limbic system, which connects to emotional responses.