Week Ten Lecture on the Autonomic Nervous System

Overview

  • Lecture Recap
    • This is week ten's lectures focused on the autonomic nervous system (ANS).
    • Discussion of content covered includes autonomic sensory receptors, the roles of various neuron types, and the pathways of motor output in the ANS.

Functional Organization of the Nervous System

  • Components of the Nervous System
    • The discussion focuses on the functional organization of the ANS.
    • Key divisions within the ANS include autonomic sensory receptors (GVAs and SVAs).

Sensory Afferent Neurons

  • General Visceral Afferents (GVAs): Carry information from the visceral organs to the central nervous system (CNS).
  • Special Visceral Afferents (SVAs): Associated with sensory neurons that relay information related to taste and smell via cranial nerves.
  • Ascending fiber tracts in spinal nerves convey sensory information to the spinal cord managing sensory perception.
    • Key fiber tracts include the anterolateral tracts.

Integration Centers

  • Hypothalamus: Critical area for integration and motor output in the ANS.
    • Connects with upper motor neurons (CNS) and lower motor neurons (peripheral nervous system).

Peripheral Nervous System (PNS) Motor Output

  • Lower Motor Neurons: This section recaps the output pathways from the central nervous system to target organs via cranial and spinal nerves.
  • General Visceral Efferents: Divided into sympathetic and parasympathetic fibers.

Sympathetic and Parasympathetic Pathways

Sympathetic Division

  • The sympathetic division, described as the fight or flight system, originates from the thoracolumbar region of the spinal cord (T1 to L3).
    • Lower motor neurons (preganglionics) emerge from these regions and connect to sympathetic ganglia via splanchnic nerves.

Parasympathetic Division

  • Craniosacral Division: The parasympathetic division operates primarily from the cranial and sacral regions (Cranial nerves III, VII, IX, X - with ganglia near effector organs).
    • Preganglionic fibers are long, reaching the target organ, where they synapse with short postganglionic fibers.
  • Sacral Origin: Preganglionic nerve cell bodies found in S2, S3, S4, located in the intermediate gray matter.

Anatomy of Autonomic Output

  • Pelvic Splanchnic Nerves: A connection between the sacral spinal nerves and pelvic viscera different from the lumbar and thoracic nerves relevant to sympathetic functions.

Examination of Motor Functions of the ANS

Functions of the Sympathetic Nervous System

  • The sympathetic branch is responsible for activating bodily functions during stressful or emergency situations.
    • Dilates pupils for improved vision in stressful situations.
    • Increased heart rate and blood pressure to enhance blood flow to muscles and vital organs.
    • Suppresses digestive functions, shifting resources towards survival functions (fight or flight).
  • Key Physiological Changes Induced:
    • Vasoconstriction in non-vital areas, pale skin, and blood shifting towards skeletal muscles and the heart.
    • Increased metabolism (glycogenolysis and lipolysis) to supply energy.
    • Sympathetic effects on sexual function include ejaculation controlled by sympathetic inputs.

Functions of the Parasympathetic Nervous System

  • Known simplistically as "rest and digest," its role is misunderstood in terms of digestive timing, as digestion can be active during alert states.
    • Promotes maximum digestive function through increased secretion and intestinal motility.
    • Engages in functions of bladder and bowel control, particularly during hydration and regular urination.
  • Functions Include:
    • Decreased heart rate and end activity in response to less stressful environments.
    • Control of sexual arousal (erection) through parasympathetic pathways.

Complexity of Autonomic Control

Interplay between Sympathetic and Parasympathetic Systems

  • Both systems can simultaneously manage functions unrelated purely to stress or relaxation.
    • E.g., bladder control involves both systems, where sympathetic inhibition allows storage and parasympathetic stimulation allows urination.

Mechanism of Bladder Control

  • Micturition Reflex (Parasympathetic):
    • Initiated when bladder fills and stretch receptors activate, sending signals that lead to contraction of the detrusor muscle.
    • This simple reflex pathway involves sensory afferent neurons, interneurons, and motor efferent connections.
  • Bladder Storage (Sympathetic):
    • In contrast, storage of urine involves sympathetic inhibition, allowing bladder filling without micturition.
    • Control comes from higher centers in the nervous system, requiring coordination of voluntary control via somatic inputs to the external sphincter.

Key Concepts of Reflex Pathways

Bladder Filling and Coordination

  • The reflex starts in the bladder, where stretch activates sensory neurons that transmit signals to the spinal cord.
  • Signals descend to coordinate micturition response through integrated brain centers to control bladder function voluntarily or involuntarily.

Summary of Autonomic Pathways

  • Summarizing brings understanding of how the sympathetic and parasympathetic systems communicate and interact.
  • Highlighting the musculoskeletal aspects, voluntary control, and integrations of functions leads to conscious awareness of physiological states, showcasing the intricacies of bodily control.
  • This is especially apparent in control mechanisms governing micturition and responses to hydration and emergencies.