Autonomic Nervous System (ANS) – Lecture Vocabulary

High-Level Orientation

• Autonomic Nervous System (ANS) = 1 of the 2 branches of the motor (efferent) nervous system.
  • Other branch = Somatic Nervous System (SNS) → skeletal muscle.
• ANS subdivides into:
  • Sympathetic division (“fight or flight”).
  • Parasympathetic division (“rest & digest”).
  • Enteric division (largely independent network for GI tract).
• All ANS activity is involuntary / automatic (no conscious control).

Overall Nervous-System Map (review)

• Sensory (afferent) division
  • Somatic senses & special senses → CNS (brain + spinal cord).
• Motor (efferent) division
  • Somatic motor → skeletal muscle (voluntary).
  • Autonomic motor → smooth & cardiac muscle, glands, viscera (involuntary).

Somatic vs Autonomic Side-by-Side

• Somatic
  • 1 motor neuron from spinal cord → skeletal muscle.
  • Axon myelinated; neurotransmitter = acetylcholine (ACh) at neuromuscular junction.
  • Receptors serviced = exteroceptors (external env.) + some proprioceptors.
• Autonomic
  • Typical pathway has 2 motor neurons:
  • Preganglionic: cell body in CNS, axon exits via ventral root.
  • Postganglionic: cell body in an autonomic ganglion outside CNS; axon → effector organ.
  • Receptors serviced = interoceptors (organs, vessels, muscles).
  • Neurotransmitters vary (ACh, norepinephrine).

Anatomy of ANS Neurons & Ganglia

• Ganglion = collection of neuronal cell bodies outside CNS.
• Examples (all parasympathetic):
  • Ciliary ganglion → iris muscle → pupil constrict/dilate.
  • Pterygopalatine ganglion → palate & upper oral cavity glands.
  • Otic ganglion → parotid salivary gland.
• Sympathetic pathways can also synapse directly in adrenal medulla → systemic release of epinephrine & norepinephrine.

Neurotransmitters & Receptors

• Major transmitters
  • Acetylcholine (ACh)
  • ALL preganglionic (symp. & parasymp.).
  • Postganglionic parasymp..
  • Some specialized sympathetic (sweat glands).
  • Norepinephrine (NE)
  • Most postganglionic sympathetic.
• Receptor classes
  • Cholinergic (bind ACh)
  • Nicotinic (stimulated by nicotine; on all postganglionic neurons & adrenal medulla).
  • Muscarinic (on effectors innervated by parasymp. & selected symp.).
  • Adrenergic (bind NE/epinephrine)
  • Multiple sub-types (α₁, α₂, β₁, β₂, etc.)—details not required in lecture.

Sympathetic Division (Thoracolumbar)

• Preganglionic cell bodies: lateral horns of spinal cord segments T{1} through L{2} ("thoracolumbar").
• Sympathetic trunk (chain) ganglia: paired vertical chains lateral to vertebral column.
• Prevertebral ganglia: anterior to vertebral column, near abdominal arteries.
• Splanchnic nerves: preganglionic fibers that pass through trunk & synapse in prevertebral ganglia (esp. sacral region).
• Physiologic Theme: Maximize oxygen-rich blood delivery to skeletal muscle for immediate survival.
  • ↑ Alertness & metabolic rate.
  • Eyes: pupil dilation, ↓ lacrimation.
  • Cardiovascular: ↑ heart rate & contractility.
  • Respiratory: bronchial dilation, ↑ rate.
  • GI & Urinary: ↓ motility, ↓ secretion (except rectum & bladder may reflexively evacuate to "lighten the load").
  • Liver: glycogenolysis → ↑ blood glucose.

Parasympathetic Division (Craniosacral)

• Preganglionic cell bodies in:
  • Brainstem CN III, VII, IX, X.
  • Spinal cord S2–S4 segments.
• Ganglia often intramural (in or near organ wall) → very short postganglionic fibers.
• Physiologic Theme: Promote maintenance, conservation, & restoration.
  • Eyes: pupil constriction.
  • Salivary/Lacrimal: ↑ secretion.
  • Cardiovascular: ↓ heart rate.
  • Respiratory: bronchial constriction, ↓ rate.
  • GI: ↑ motility & enzyme/acid/bile release.
  • Urinary: bladder contraction, sphincter relaxation; kidney perfusion normal.

Major Autonomic Plexuses (Sympathetic Focus)

• Cardiac plexus – around heart base.
• Pulmonary plexus – at lung hila.
• Celiac (solar) plexus – below diaphragm; receives shots in martial arts.
• Superior & Inferior Mesenteric plexuses – follow mesenteric arteries to intestines.
• Renal plexus – kidneys.
• Hypogastric plexus – pelvic organs/genitals.

Autonomic Reflex Arcs

• Components identical to somatic reflex arcs:
  1. Receptor (interoceptor).
  2. Sensory neuron → CNS.
  3. Integrating center (brainstem or spinal cord).
  4. Preganglionic → Postganglionic motor neurons.
  5. Effector (smooth/cardiac muscle, gland).

Clinical Correlates: Autonomic Dysreflexia

• General Definition: inappropriate, exaggerated, or absent autonomic reflex → disturbed homeostasis.
• Asthma
  • Expected sympathetic response = bronchial dilation.
  • Dysreflexic response = bronchial constriction → airflow obstruction & exercise intolerance.
• Raynaud’s Phenomenon
  • Dysregulated vasomotor tone in digits.
  • Cold or stress → extreme vasoconstriction → white/blue fingertips; can also produce syncope on standing due to insufficient cerebral perfusion.

Quick Reference Table

• Origin
  • Symp.: T{1}–L{2}.
  • Parasymp.: CN III, VII, IX, X & S{2}–S{4}.
• Ganglia Location
  • Symp.: trunk & prevertebral, distant from target.
  • Parasymp.: intramural or very near target.
• Primary Transmitters
  • Symp.: \text{ACh} \to \text{NE} \,(\pm ACh).
  • Parasymp.: \text{ACh} \to \text{ACh}.
• Global Effect
  • Symp.: catabolic, emergency prep.
  • Parasymp.: anabolic, housekeeping.

Memory Aids & Connections

• Think “E” situations (exercise, excitement, emergency, embarrassment) → Sympathetic.
• Think “SLUDD” (salivation, lacrimation, urination, digestion, defecation) → Parasympathetic.
• Nicotine addiction exploits nicotinic ACh receptors (both somatic NMJ & autonomic ganglia).
• Adrenal medulla acts like a giant sympathetic ganglion → endocrine amplification of fight/flight.

Ethical / Practical Implications

• Understanding ANS pharmacology = basis for treating hypertension (β-blockers), asthma (β₂-agonists), GI motility disorders (muscarinic agonists/antagonists), etc.
• Recognition of autonomic dysreflexia critical in spinal-cord-injured patients (can cause life-threatening hypertensive crises).

Study Tips

• Always connect a sympathetic action to “Will this move more O₂-rich blood to skeletal muscle?” If yes, that’s the likely effect.
• Map cranial parasymp. outflow: CN III (eye), VII (tears/saliva), IX (parotid), X (thorax/abdomen).
• Drill receptor → transmitter pairs: Nicotinic = ganglia; Muscarinic = parasymp. effector; Adrenergic = most symp. effector.