Autonomic Nervous System Overview

Oct 6, 2024

Lecture on Autonomic Nervous System Functions

Overview

  • Main Topic: Functions of the autonomic nervous system (ANS)
  • Focus: Neurotransmitters and receptor roles in maintaining homeostasis
  • Key Parts: Sympathetic and Parasympathetic Systems

Sympathetic Nervous System (SNS)

  • Function: Fight or flight response
    • Evolutionary aspect: One of the oldest parts of the brain
    • Purpose: Survival and gene propagation
  • Activation:
    • Neural Pathway:
      • Neurons release norepinephrine
      • Adrenal medulla releases epinephrine (75%) and norepinephrine (25%)
    • Autonomic Pathway:
      • Involves two neurons (preganglionic and postganglionic)
      • Synapses occur at autonomic ganglia
      • Effectors include cardiac muscle, smooth muscle, and glands
  • Role of Neurons and Endocrine System:
    • Adrenal Medulla: Part of endocrine, releasing epinephrine
    • Neuronal Pathway: Involves release of norepinephrine
  • Effectors: Cardiac muscle, digestive glands, endocrine glands

Parasympathetic Nervous System (PNS)

  • Function: Resting and digesting
    • Active at rest: Opposite of fight or flight
  • Neurotransmitters:
    • First neuron releases acetylcholine
    • Acetylcholine affects nicotinic receptors (ligand-gated)
  • Receptor Types:
    • Muscarinic Receptors: M2 and M3
      • M2: Inhibitory
      • M3: Excitatory
  • Pathways:
    • Two-neuron pathways (preganglionic and postganglionic)
    • Released acetylcholine binds to muscarinic receptors on effectors

Autonomic Tone

  • Definition: Continuous firing of autonomic neurons
  • Sympathetic Tone:
    • Continuous sympathetic stimuli to effectors
    • Maintains vascular tone (blood vessel diameter)
    • Loss of tone leads to vasodilation and hypotension
  • Parasympathetic Tone:
    • Vagus nerve reduces heart rate below 100 BPM

Neurotransmitter and Receptor Dynamics

  • Acetylcholine: Binds to nicotinic and muscarinic receptors
  • Norepinephrine/Epinephrine:
    • Binds to alpha and beta receptors
    • Alpha 1 Receptors: Contribute to smooth muscle contraction
    • Beta Receptors:
      • Beta 1: Increases heart rate (excitatory)
      • Beta 2: Inhibits smooth muscle contraction (inhibitory)

Receptor Mechanisms

  • Alpha 1 Receptors:
    • Trigger release of calcium into cells, causing contraction
  • Beta 1 Receptors:
    • Increase heart activity through cyclic AMP pathway
  • Beta 2 Receptors:
    • Lead to relaxation of smooth muscles in blood vessels and bronchioles

Important Concepts for Understanding

  • Receptor Types Influence:
    • Alpha and beta receptors associated with sympathetic effects
    • Muscarinic receptors associated with parasympathetic effects
  • Chemical Agonists and Antagonists:
    • Agonists mimic natural neurotransmitters
    • Antagonists block receptor sites, preventing natural actions

Key Takeaways

  • Understand roles of different neurotransmitters and receptors in ANS
  • Autonomic tone is crucial for maintaining homeostasis
  • Different receptors mediate specific physiological responses in the body
  • Knowledge of receptor pathways assists in understanding drug effects and treatments