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.