The Autonomic Nervous System and the Adrenal Medulla

The Autonomic Nervous System and the Adrenal Medulla Notes

Nervous System Overview

• Central Nervous System (CNS): Comprises the brain and spinal cord.
• Peripheral Nervous System (PNS): Divided into Somatic and Autonomic systems.
• Somatic Nervous System: Connects skin, muscles, and joints with the CNS.
• Autonomic Nervous System: Controls involuntary actions and connects internal organs with the CNS.

Functions of the Autonomic Nervous System

• Regulates visceral functions, including:
  • Arterial pressure
  • Gastrointestinal motility and secretions
  • Urinary bladder emptying
  • Body temperature regulation
• Control centers located in:
  • Spinal cord
  • Brain stem
  • Hypothalamus
  • Limbic system

Organization of the Autonomic Nervous System

• Divisions:
  • Sympathetic Nervous System:
  • Involved in 'fight or flight' responses.
  • Preganglionic fibers are short, and postganglionic fibers are long.
  • Parasympathetic Nervous System:
  • Involved in 'rest and digest' responses.
  • Preganglionic fibers are long, and postganglionic fibers are short.

Physiologic Anatomy of Sympathetic Nervous System

• Origin: T1-L2 spinal segments.
• Preganglionic cell bodies located in the intermediolateral horn; synapse occurs in ganglia:
  • Sympathetic chain
  • Peripheral ganglia
• Utilizes acetylcholine (ACh) to activate nicotinic receptors on postganglionic neurons.

Special Features of Sympathetic Nervous System

• Adrenal Medulla:
  • Acts as a specialized sympathetic tissue.
  • Releases norepinephrine and epinephrine into the bloodstream.
• Sympathetic nerve fibers affect:
  • Blood vessels, sweat glands, and piloerector muscles.

Neurotransmitter Synthesis and Function

• Norepinephrine:
  • Synthesized from phenylalanine via a series of steps including tyrosine and dopamine.
  • Reuptake and breakdown involve monoamine oxidase and catechol-O-methyl transferase.
• Acetylcholine:
  • Synthesized from acetyl-CoA and choline.
  • Breakdown by acetylcholinesterase, with choline recycled.

Receptor Types in the Autonomic Nervous System

• Adrenergic Receptors: Bind catecholamines (norepinephrine and epinephrine).
  • Alpha (α) receptors:
  • Cause vasoconstriction; stimulated primarily by norepinephrine.
  • Beta (β) receptors:
  • Beta1: Increases heart rate and contractility.
  • Beta2: Causes bronchodilation, vasodilation in skeletal muscles.
• Cholinergic Receptors:
  • Nicotinic receptors: Located at synapses between preganglionic and postganglionic neurons.
  • Muscarinic receptors: Act at target organs stimulated by postganglionic fibers.

Effects of the Autonomic Nervous System on Key Organs

• Eyes:
  • Sympathetic: Pupillary dilation.
  • Parasympathetic: Pupillary constriction and lens accommodation.
• Glands:
  • Sympathetic: Stimulates sweat and apocrine glands.
  • Parasympathetic: Stimulates nasal, lacrimal, and salivary glands and gastrointestinal glands.
• Heart:
  • Sympathetic: Increases heart rate and contractility.
  • Parasympathetic: Decreases heart rate.
• Gastrointestinal Tract:
  • Sympathetic: Minimal effect.
  • Parasympathetic: Stimulates overall activity.

Function of Adrenal Medulla

• Releases 80% epinephrine and 20% norepinephrine when stimulated.
• Prolongs responses seen in sympathetic activation, increasing energy availability and cardiovascular output during stress.

Autonomic Tone

• Sympathetic Tone:
  • Baseline activity proposes ~50% vasoconstriction in vessels.
• Parasympathetic Tone:
  • Promotes background gastrointestinal activity.

Stress Response (Fight or Flight)

• Mass sympathetic discharge: increases heart rate, arterial pressure, metabolism, and muscle strength to prepare for activity under stress.

Autonomic Control Centers in Brain

• Controlled by areas in the hypothalamus and brain stem, managing mechanisms for:
  • Urinary bladder, cardiac function, respiratory control, and stress responses.

Effect of Nicotine

• Stimulates autonomic centers:
  • Causes strong sympathetic reaction (vasoconstriction) and enhanced gastrointestinal activity (increased stomach acid production).
  • Slows heart rate through parasympathetic activation.