Chapter 15 – Autonomic Nervous System (Comprehensive Study Notes)

Organization of the Nervous System

• CNS: brain & spinal cord
• PNS: all nervous tissue outside CNS
  • Somatic Nervous System (SNS)
    • Receptors: somatic & special senses (touch, pain, temperature, proprioception, vision, hearing, taste, smell, equilibrium)
    • Efferent fibers: single, myelinated motor neuron → skeletal muscle (voluntary)
  • Autonomic Nervous System (ANS)
    • Receptors: interoceptors in visceral organs, blood vessels, muscles, nervous tissue
    • Motor: 2-neuron chain (preganglionic + postganglionic) → smooth & cardiac muscle, glands (involuntary)
    • Divisions: sympathetic (thoracolumbar), parasympathetic (craniosacral)
  • Enteric Nervous System (ENS)
    • Independent network (≈ $100 \text{ million}$ neurons) in GI tract wall, pancreas, gallbladder
    • Can function without CNS input

Somatic vs Autonomic Overview

• Sensory input
  • SNS: consciously perceived
  • ANS: usually subconscious; intense activation can reach consciousness (e.g., angina pectoris)
• Motor control
  • SNS: precise, voluntarily regulated
  • ANS: automatic; organs often continue if nerves severed (heart transplant)
• Neurotransmitters
  • SNS: $\text{ACh}$ only
  • ANS: $\text{ACh}$ or $\text{NE}$

Interoceptors (ANS Sensory Receptors)

• Chemoreceptors → monitor $\text{CO}_2$ in blood
• Mechanoreceptors → sense stretch in organ / vessel walls

Autonomic Motor Pathway Architecture

• Preganglionic neuron
  • Cell body in CNS
  • Myelinated axon → autonomic ganglion / adrenal medulla
• Postganglionic neuron
  • Cell body in ganglion
  • Unmyelinated axon → effector
• Exception: chromaffin cells of adrenal medullae act as post-synaptic cells; secrete $\text{EPI}$ & $\text{NE}$ to blood

Sympathetic vs Parasympathetic Divisions

• Sympathetic (fight-or-flight)
  • Origin: lateral gray horns $T<em>{1-12},\;L</em>{1-2/3}$
  • Ganglia near spinal cord → short preganglionic, long postganglionic fibers
  • Global, divergent (one preganglionic fiber → $\ge 20$ postganglionic)
• Parasympathetic (rest-and-digest)
  • Origin: cranial nerves III, VII, IX, X + sacral $S_{2-4}$
  • Terminal ganglia in/near organ wall → long preganglionic, short postganglionic fibers
  • More localized responses
• Dual innervation: most organs receive both; heart rate modulated by relative impulse frequency (↑ symp → ↑HR; ↑ parasymp → ↓HR)

Sympathetic Ganglia Types

• Sympathetic trunk (paravertebral) ganglia
  • Vertical rows beside vertebral column
  • Postganglionic fibers → organs above diaphragm (head, neck, shoulders, heart)
• Prevertebral (collateral) ganglia
  • Anterior to vertebral column (e.g., celiac, superior mesenteric)
  • Postganglionic fibers → organs below diaphragm (liver, stomach, kidneys, intestine, colon)

Parasympathetic Terminal Ganglia

• Located within or close to visceral organ wall → long preganglionic axons

Four Routes for Sympathetic Preganglionic Axons

1. Synapse in first trunk ganglion encountered
2. Ascend/descend trunk, synapse elsewhere
3. Pass through trunk → prevertebral ganglion via splanchnic nerves
4. Pass through both ganglion sets → adrenal medullae (chromaffin cells)

Autonomic Plexuses (Mixed Symp & Parasymp)

• Thorax:
  • Cardiac plexus → heart
  • Pulmonary plexus → bronchial tree
• Abdomen/pelvis: celiac, esophageal, renal, hypogastric, etc.

Enteric Nervous System Details

• Sensory neurons → monitor GI wall tension & lumen chemistry
• Interneurons → integrative network within plexuses
• Motor neurons → control GI motility & secretion
• Major plexuses: myenteric (Auerbach), submucosal (Meissner)

ANS Neurotransmitters

• Cholinergic neurons (release $\text{ACh}$)
  • All preganglionic (symp & parasymp)
  • Parasympathetic postganglionic
  • Sympathetic postganglionic to most sweat glands
• Adrenergic neurons (release $\text{NE}$)
  • Most sympathetic postganglionic neurons

Cholinergic Receptors

1. Nicotinic (ionotropic)
   • On postganglionic neurons (symp & parasymp), chromaffin cells, neuromuscular junction
   • $\text{ACh}$ → depolarization (excitation)
2. Muscarinic (metabotropic)
   • On all parasympathetic effectors & sympathetic-innervated sweat glands
   • $\text{ACh}$ → excitation OR inhibition depending on cell type

Adrenergic Receptors

• Two families: $\alpha$ and $\beta$
• Subtypes: $\alpha<em>1,\;\alpha</em>2,\;\beta<em>1,\;\beta</em>2,\;\beta<em>3$ • $\alpha</em>1,\;\beta<em>1$ → generally excitatory • $\alpha</em>2,\;\beta<em>2$ → generally inhibitory • $\beta</em>3$ (brown adipose) → thermogenesis
• Drugs: propranolol blocks $\beta$ receptors (e.g., $\beta_{1}$ on heart)

Functional Effects of Neurotransmitters (Selected Examples)

• Eye pupil: Symp → dilation; Parasymp → constriction
• Heart rate: Symp ↑; Parasymp ↓
• Bronchioles: Symp dilation; Parasymp constriction
• GI motility: Symp slows; Parasymp speeds
• Blood distribution: Symp → shift to skeletal muscle; Parasymp → shift to GI tract
• Glycemia: Symp ↑ blood glucose; Parasymp ↓

Autonomic Tone

• Continuous background activity of both divisions
  • Regulated by hypothalamus
  • Reciprocal control: ↑ symp tone ↔ ↓ parasymp tone
• Structures with only sympathetic innervation: sweat glands, arrector pili, kidneys, most blood vessels, adrenal medullae
  • Sympathetic vasomotor tone sets baseline vascular diameter
  – Strong tone → vasoconstriction
  – Weak tone → vasodilation

Sympathetic (Fight-or-Flight) Responses

• Triggered during "E situations": exercise, emergency, excitement, embarrassment
• Effects:
  • ↑ HR & contractility
  • ↑ breathing rate, bronchodilation
  • Pupil dilation
  • Vasodilation to skeletal/cardiac muscle; vasoconstriction to non-critical organs
  • Sweaty but cool skin
  • Mobilization of glucose (glycogenolysis)
• Longer-lasting & widespread because:
  1. High divergence
  2. $\text{NE}$ persists longer than $\text{ACh}$
  3. Adrenal medulla releases $\text{NE}$/EPI into blood

Parasympathetic (Rest-and-Digest) Responses

• Conserves/restores energy
• SLUDD mnemonic: Salivation, Lacrimation, Urination, Digestion, Defecation
• Three decreases: heart rate, airway diameter (bronchoconstriction), pupil diameter (constriction)

Autonomic Reflexes

• Components:
  1. Receptor (interoceptor)
  2. Sensory neuron
  3. Integrating center (hypothalamus & brain stem; spinal cord for urination/defecation)
  4. Motor neurons (preganglionic & postganglionic)
  5. Effector (smooth/cardiac muscle, glands)
• Example: Baroreflex
  • ↑ BP → baroreceptors → glossopharyngeal nerve → medulla → vagus nerve → ↓ SA-node firing → ↓ HR → BP normalized

Higher-Level Control

• Hypothalamus: principal ANS integration center
  • Receives visceral sensory, olfactory, gustatory, limbic, temperature, osmolarity, blood-chemistry inputs
  • Posterior & lateral nuclei → sympathetic outflow
  • Anterior & medial nuclei → parasympathetic outflow
• Descending pathways to autonomic centers in brain stem (cardiovascular, salivation, swallowing, vomiting) & spinal cord (defecation, urination)

End of comprehensive notes for Chapter 15: The Autonomic Nervous System.