Autonomic Nervous System Functions

Overview

• Functions of the autonomic nervous system (ANS) include maintaining homeostasis through neurotransmitters, chemicals, and receptor roles.
• Focus on how receptors trigger cellular changes.

Sympathetic Function (Fight or Flight)

• Evolutionary Role: Ancient part of the brain; related to survival and reproduction.
• Activation: Stressful situations activate sympathetic function.
• Neurotransmitters:
  • Norepinephrine: Released by neurons in the sympathetic nervous system.
  • Epinephrine: Released by the adrenal medulla, similar to norepinephrine.
• Pathways:
  • Preganglionic and postganglionic neurons involved.
  • Synapse at autonomic ganglion (peripheral nervous system).
  • Connects to cardiac muscle, smooth muscle, or glands (e.g., adrenal medulla).

Parasympathetic Function (Rest and Digest)

• Predominantly Active: During rest and non-stressful situations.
• Two-neuron Pathway: Similar to sympathetic, involves acetylcholine.
• Receptors:
  • Nicotinic receptors for initial neurotransmitter reception.
  • Muscarinic receptors for secondary neurotransmitter effects (e.g., M2, M3).

Autonomic Tone

• Continuous Neuronal Firing: Background signaling to effectors.
• Sympathetic Tone: Maintains activity in effectors like blood vessels.
• Parasympathetic Tone: Keeps heart rate below 100 beats/min.

Receptor Types and Functions

• Sympathetic Receptors:
  • Alpha and Beta Receptors: Involved in sympathetic responses.
  • Alpha-1 Receptors: Found on vascular smooth muscle; cause vasoconstriction when activated.
  • Beta-1 Receptors: Found on the heart; increase heart rate.
  • Beta-2 Receptors: Found on bronchial and vascular smooth muscles; cause relaxation and dilation.
• Parasympathetic Receptors:
  • Muscarinic Receptors: Seen on parasympathetic effectors.

Adrenergic and Cholinergic Pathways

• Adrenergic: Involves norepinephrine and epinephrine, activating alpha and beta receptors.
• Cholinergic: Involves acetylcholine, activating nicotinic and muscarinic receptors.
• Enzymes and Second Messengers:
  • Phospholipase C: Activated in alpha-1 pathways.
  • Cyclic AMP: Involved in beta-1 and beta-2 pathways.

Clinical Applications

• Antagonists and Agonists:
  • Antagonists: Block receptor sites preventing usual neurotransmitter effects.
  • Agonists: Mimic neurotransmitter effects.

Important Considerations

• Autonomic Ganglia: Different types for sympathetic and parasympathetic.
• Varicosities in Smooth Muscle: Release neurotransmitters differently from other neurons.
• Exam Focus:
  • Understanding of receptors, neurotransmitters, and autonomic pathways is crucial.
  • Charts and tables provided in exams assist with identifying effects of sympathetic/parasympathetic stimuli.