Autonomic Nervous System – Comprehensive Study Notes

Overview of the Autonomic Nervous System (ANS)

• Subdivision of the Peripheral Nervous System (PNS)
  • Functions involuntarily; not under conscious control
  • Coordinates “Fight, Flight, or Freeze” vs. “Rest and Digest” responses to maintain homeostasis
• Controlled primarily by the medulla oblongata and hypothalamus
• Dual innervation of most visceral organs (both sympathetic & parasympathetic fibers reach the same target) enables fine-tuned balance rather than all-or-nothing responses

Gross Organization of the Nervous System

• CNS ➔ brain & spinal cord
• PNS ➔ cranial nerves & spinal nerves
  • Sensory (afferent) division
  • Somatic sensory: touch, pain, pressure, vibration, temperature, proprioception in skin, body wall & limbs
  • Special somatic senses: hearing, equilibrium, vision
  • Visceral sensory: stretch, pain, temperature, chemical changes, irritation in viscera; nausea & hunger
  • Special visceral senses: taste, smell
  • Motor (efferent) division
  • Somatic motor: skeletal muscle (voluntary)
  • Visceral motor (ANS): smooth muscle, cardiac muscle, glands
    • Sympathetic division
    • Parasympathetic division

Ganglia: Definition & Variations

• Ganglion = cluster of neuronal cell bodies outside the CNS
• Sympathetic pathways show 3 stereotypical arrangements:
  1. Preganglionic fiber synapses in sympathetic chain ganglion at the same spinal level
  2. Preganglionic fiber ascends/descends the chain before synapsing at a different level
  3. Preganglionic fiber passes through chain (via white ramus) without synapsing, continues as a splanchnic nerve to a pre-vertebral (collateral) ganglion
• Terms to know
  • Sympathetic chain (paravertebral) ganglia: base of skull ➔ coccyx, flanking vertebral column
  • Pre-vertebral ganglia: anterior to vertebral bodies, innervate organs below diaphragm
  • Terminal (intramural) ganglia: embedded in or near target organ wall (parasympathetic)

Divisions of the ANS

Sympathetic (“Thoracolumbar”) Division

• Preganglionic neurons originate in spinal cord segments T1–L2 (lateral horn)
• Motto: “E = exercise, excitement, emergency, embarrassment”
• Widespread effect due to:
  • Short preganglionic, long highly-branched postganglionic axons
  • Divergence ratio ≈ 1:20 (one preganglionic fiber can activate many postganglionic neurons)
• Neurotransmitters
  • Preganglionic: ACh → nicotinic receptors on ganglionic neurons
  • Postganglionic: usually norepinephrine (NE) → \alpha or \beta adrenergic receptors
  • Exception: sympathetic fibers to sweat glands & some skeletal-muscle vessels release ACh → muscarinic receptors

Parasympathetic (“Craniosacral”) Division

• Preganglionic neurons arise from:
  • Brainstem nuclei of cranial nerves III, VII, IX, X
  • Sacral spinal cord segments S2–S4
• Motto: “D = digestion, defecation, diuresis”
• Localized effect due to:
  • Long preganglionic, short postganglionic axons with minimal branching
• Neurotransmitters
  • Preganglionic: ACh → nicotinic receptors
  • Postganglionic: ACh → muscarinic receptors

Comparative Anatomy & Fiber Characteristics

Signaling Molecules & Receptors Summary

• Nicotinic receptors (ligand-gated \text{Na}^+ channels)
  • Found on all postganglionic neurons (both divisions) & adrenal medulla chromaffin cells
• Muscarinic receptors (G-protein–coupled)
  • All parasympathetic target cells + sweat glands & select blood vessels (sympathetic exception)
• Adrenergic receptors (G-protein–coupled)
  • \alpha_1: vasoconstriction, pupil dilation, sphincter contraction
  • \alpha_2: inhibitory feedback on NE release
  • \beta_1: increased heart rate & contractility
  • \beta_2: bronchodilation, vasodilation in skeletal muscle
  • \beta_3: lipolysis in adipose tissue

Classic Pathway Schematics (Text Description)

Sympathetic Chain Path

1. Preganglionic fiber (myelinated, in white ramus communicants) → sympathetic chain ganglion
2. Synapse (ACh-nicotinic)
3. Postganglionic fiber (unmyelinated, gray ramus) → effector; releases NE

Sympathetic Splanchnic Path

1. Preganglionic fiber bypasses chain ganglion → thoracic, lumbar, or sacral splanchnic nerve
2. Synapse in pre-vertebral ganglion (ACh-nicotinic)
3. Postganglionic fiber travels on arteries to abdominal viscera → releases NE

Adrenal Medulla Path (Modified Sympathetic)

1. Preganglionic fiber releases ACh onto chromaffin cells (nicotinic)
2. Chromaffin cells secrete \approx 80\% epinephrine & 20\% norepinephrine into bloodstream (systemic sympathetic surge)

Parasympathetic Cranial Path (e.g., CN X – Vagus)

1. Long preganglionic fiber (ACh-nicotinic) reaches terminal ganglion on or near target organ
2. Short postganglionic fiber releases ACh → muscarinic receptors locally

Functional Effects

Sympathetic (Fight/Flight)

• ↑ Heart rate & contractility; ↑ BP
• Vasodilation of skeletal muscle vessels; vasoconstriction of skin & visceral vessels
• Bronchodilation
• Pupil dilation (mydriasis)
• ↓ GI motility & urinary bladder tone
• Secretion of viscous saliva
• Sweating (cholinergic sympathetic)
• Mobilization of energy: liver glycogenolysis, adipose lipolysis → ↑ blood glucose & free fatty acids
• Lower reticular formation threshold (alertness)

Parasympathetic (Rest/Digest)

• ↓ Heart rate (vagal tone maintains 60–80\,\text{bpm} vs the intrinsic \approx 100\,\text{bpm})
• Bronchoconstriction
• Pupil constriction (miosis)
• ↑ GI peristalsis & secretions; watery saliva
• Contraction of urinary bladder (micturition)
• ↑ Pancreatic insulin & glucagon secretion

Autonomic vs. Somatic Nervous Systems

• Somatic: one-neuron efferent pathway; myelinated; always excitatory (ACh at neuromuscular junction)
• Autonomic: two-neuron chain (pre & post); preganglionic myelinated, postganglionic unmyelinated; can be excitatory or inhibitory; multiple neurotransmitters; varicosities create diffuse release

Autonomic Reflex Arc (Visceral Reflex)

1. Receptor (e.g., baroreceptor in carotid sinus)
2. Visceral sensory neuron → CNS (afferent limb has no part in ANS “motor” definition)
3. Integration center (hypothalamus, brainstem, or spinal cord)
4. Preganglionic motor neuron (lateral horn) → ganglion (ACh, nicotinic)
5. Postganglionic motor neuron → effector organ (NE or ACh)

• Example: Baroreceptor reflex adjusts HR & vessel diameter to maintain BP

Heart-Rate Control Example

• Sympathetic postganglionic fiber releases NE → \beta_1 receptors on pacemaker cells → faster depolarization → HR ↑ to 120–140\,\text{bpm} during exercise
• Parasympathetic vagal fiber releases ACh → muscarinic receptors → hyperpolarization → resting HR 60–80\,\text{bpm}

Autonomic Varicosities

• Postganglionic axons form swellings (varicosities) along their length
  • Contain synaptic vesicles; permit “spray-and-pray” neurotransmitter release over broad smooth-muscle surfaces

Pupillary Control & Reflex

• Light intensity detected by retinal ganglion cells ➔ pretectal nucleus ➔ bilateral Edinger-Westphal nuclei
• Parasympathetic CN III → ciliary ganglion → sphincter pupillae (ACh, muscarinic) → constriction
• Sympathetic pathway (hypothalamus ➔ spinal cord ➔ superior cervical ganglion) → dilator pupillae (NE, \alpha_1) → dilation

Central Control of ANS

• Hypothalamus: major integrative “headquarters”
  • Regulates autonomic & endocrine function; maintains homeostasis
  • Communicates via medial forebrain bundle & dorsal longitudinal fasciculus
• Brainstem nuclei: Edinger-Westphal, salivatory nuclei, dorsal motor nucleus of vagus, nucleus ambiguus
• Limbic system (amygdala, hippocampus, cingulate gyrus) links emotion & memory to autonomic output

Referred Pain Concept

• Visceral afferents share spinal pathways with somatic afferents at the same segmental level
• CNS misinterprets visceral pain as originating from somatic region (e.g., cardiac ischemia ➔ left arm pain)

Pharmacology of the ANS

Sympathetic vs Parasympathetic Ganglia Recap

• Sympathetic ganglia
  • Sympathetic trunk ganglia (paravertebral)
  • Prevertebral (collateral) ganglia: celiac, superior mesenteric, inferior mesenteric, aorticorenal, etc.
• Parasympathetic ganglia (terminal/intramural)
  • Ciliary, pterygopalatine, otic, submandibular (cranial)
  • Numerous minute ganglia embedded in thoracic & abdominal organs

Quick Self-Check (Exam Style)

• Can you list the ANS components and their central origins?
• Distinguish sympathetic vs parasympathetic fiber lengths, branching, and neurotransmitters.
• Contrast a somatic reflex arc with an autonomic (visceral) reflex arc.
• Given a receptor type (\alpha1, \beta2, muscarinic), predict the effector response.
• Describe how hypothalamus, brainstem nuclei, and limbic structures coordinate autonomic output.