Peripheral and Autonomic Neuroanatomy

Peripheral Nerves and Neuromuscular Junction

Peripheral Nerves

Peripheral nerves contain axons of lower motor neurons, sensory neurons, and autonomic neurons outside the brain and spinal cord. Each axon is surrounded by endoneurium; bundles of axons form fascicles surrounded by perineurium, the major diffusion barrier; multiple fascicles are enclosed by epineurium, which contains blood vessels. In the peripheral nervous system, Schwann cells myelinate a single axonal segment, unlike oligodendrocytes in the CNS, which myelinate multiple axons. Myelin increases membrane resistance and decreases capacitance, allowing saltatory conduction between nodes of Ranvier.

Fiber type	Function	Myelination	Diameter/conduction	High-yield point
Aα	Skeletal motor; proprioception	Heavily myelinated	Largest; ~80–120 m/s	Lost in large-fiber neuropathies with vibration/proprioception deficits
Aβ	Touch, pressure	Myelinated	~35–75 m/s	Assessed by light touch
Aδ	Fast pain, cold	Thinly myelinated	~5–30 m/s	Sharp, well-localized pain
B	Preganglionic autonomic	Myelinated	~3–15 m/s	Highly sensitive to local anesthetics
C	Slow pain, temperature; postganglionic autonomic	Unmyelinated	~0.5–2 m/s	Dull, burning pain

Axons depend on axonal transport. Anterograde transport uses kinesin to move vesicles and mitochondria from soma to terminal; retrograde transport uses dynein and is exploited by rabies virus, herpes simplex virus, tetanus toxin, and poliovirus. Peripheral nerve injury triggers Wallerian degeneration: the distal axon and myelin degenerate within days, macrophages clear debris, and Schwann cells form regeneration tubes. Axonal regrowth occurs at approximately 1–3 mm/day if the endoneurial tube remains intact.

Injury type	Pathology	Clinical implication
Neuropraxia	Focal conduction block, usually demyelination; axon intact	Recovery in days to weeks; common after compression
Axonotmesis	Axonal disruption with intact connective tissue sheaths	Wallerian degeneration occurs; recovery possible via axonal regrowth
Neurotmesis	Complete nerve transection including connective tissue	Poor spontaneous recovery; misdirected regeneration possible

Classic mononeuropathies are Step 1–relevant because deficits map anatomy to function. Median nerve injury causes carpal tunnel syndrome with thenar weakness and sensory loss over the lateral 3½ digits; proximal injury may cause “ape hand.” Ulnar nerve injury at the medial epicondyle or Guyon canal causes interosseous weakness and sensory loss over the medial 1½ digits; chronic injury produces clawing of digits 4–5. Radial nerve compression in the spiral groove causes wrist drop with preserved triceps; axillary compression injures triceps as well. Common fibular nerve injury at the fibular neck causes foot drop and sensory loss over the dorsum of the foot.

Neuromuscular Junction

The neuromuscular junction is a specialized chemical synapse between an α-motor neuron and skeletal muscle. One motor neuron plus all muscle fibers it innervates is a motor unit; small motor units mediate fine control, whereas large motor units generate force. Acetylcholine is synthesized from choline and acetyl-CoA by choline acetyltransferase, packaged into vesicles by VAChT, and released when presynaptic action potentials open voltage-gated P/Q-type Ca²⁺ channels. Calcium triggers SNARE-mediated vesicle fusion. Each vesicle contains roughly 5,000–10,000 acetylcholine molecules, producing a miniature end-plate potential.

Acetylcholine binds postsynaptic nicotinic Nm receptors, ligand-gated cation channels composed in adults of α₂βδε subunits. Opening permits Na⁺ influx greater than K⁺ efflux, generating an end-plate potential. If threshold is reached, nearby voltage-gated Na⁺ channels trigger a muscle action potential, leading to Ca²⁺ release from the sarcoplasmic reticulum via ryanodine receptors and contraction. Acetylcholine is rapidly hydrolyzed by acetylcholinesterase in the synaptic cleft; choline is recycled.

Disorder/toxin/drug	Mechanism	Key Step 1 association
Myasthenia gravis	IgG autoantibodies against postsynaptic nicotinic ACh receptors, or MuSK/LRP4	Fluctuating weakness, ptosis, diplopia; worsens with use; thymic hyperplasia or thymoma; repetitive stimulation shows >10% decrement
Lambert-Eaton syndrome	Autoantibodies against presynaptic P/Q-type Ca2+ channels	Proximal weakness improves with use; autonomic symptoms; associated with small cell lung carcinoma; high-frequency stimulation may show >100% increment
Botulinum toxin	Cleaves SNARE proteins, preventing ACh release	Descending flaccid paralysis, diplopia, dysphagia; infant botulism from honey
Tetanus toxin	Retrograde transport to spinal cord; cleaves synaptobrevin in inhibitory interneurons	Blocks GABA/glycine release, causing spastic paralysis and trismus
Organophosphates	Irreversible acetylcholinesterase inhibition	Cholinergic excess: DUMBBELSS; treat conceptually with atropine for muscarinic effects and pralidoxime before “aging”
Succinylcholine	Depolarizing Nm agonist; persistent depolarization then desensitization	Fasciculations followed by paralysis; risk of hyperkalemia and malignant hyperthermia; typical intubating dose ~1–1.5 mg/kg IV, duration ~5–10 min
Nondepolarizing blockers	Competitive Nm antagonists, e.g., rocuronium, vecuronium	Paralysis without depolarization; reversed by acetylcholinesterase inhibitors or sugammadex for rocuronium/vecuronium

Local anesthetics such as lidocaine block voltage-gated Na⁺ channels from the intracellular side, stabilizing the inactive state. Blockade typically affects small myelinated autonomic fibers and pain fibers before large motor fibers, explaining loss of pain and temperature before motor function. Clinically and on exams, distinguish nerve conduction failure from NMJ transmission failure: neuropathies often produce sensory abnormalities and reduced reflexes, whereas pure NMJ disease produces fatigable weakness with normal sensation.

Autonomic Nervous System

The autonomic nervous system (ANS) regulates involuntary visceral functions: heart rate, vascular tone, pupillary size, sweating, bronchial caliber, gastrointestinal motility, bladder function, and sexual response. It is a two-neuron efferent system: a preganglionic neuron with its cell body in the CNS synapses in an autonomic ganglion with a postganglionic neuron, which innervates the target organ. The major divisions are sympathetic, parasympathetic, and enteric.

Core Organization

Feature	Sympathetic	Parasympathetic
CNS origin	Thoracolumbar: intermediolateral cell column of spinal cord levels T1–L2	Craniosacral: brainstem nuclei of CN III, VII, IX, X and sacral spinal cord S2–S4
Ganglion location	Paravertebral sympathetic chain or prevertebral ganglia near aorta	In or near target organ
Fiber length	Short preganglionic, long postganglionic	Long preganglionic, short postganglionic
Preganglionic neurotransmitter	Acetylcholine acting on nicotinic neuronal receptors	Acetylcholine acting on nicotinic neuronal receptors
Postganglionic neurotransmitter	Usually norepinephrine; exceptions below	Acetylcholine acting on muscarinic receptors

Preganglionic autonomic fibers are myelinated B fibers with conduction velocities about 3–15 m/s. Postganglionic fibers are mostly unmyelinated C fibers, conducting about 0.5–2 m/s. This slower conduction suits autonomic regulation, which is generally less temporally precise than somatic motor control.

Sympathetic Pathways and Exceptions

Sympathetic preganglionic neurons leave via ventral roots, enter spinal nerves, and pass through white rami communicantes to the sympathetic chain. They may synapse at the same level, ascend or descend before synapsing, or pass through as splanchnic nerves to prevertebral ganglia. Postganglionic fibers return to spinal nerves through gray rami communicantes, which exist at all spinal levels.

• Adrenal medulla: embryologically a modified sympathetic ganglion. Preganglionic sympathetic fibers release acetylcholine onto chromaffin cells, which secrete catecholamines into blood: approximately 80% epinephrine and 20% norepinephrine. Phenylethanolamine N-methyltransferase, induced by cortisol from the adrenal cortex, converts norepinephrine to epinephrine.
• Eccrine sweat glands: sympathetic postganglionic fibers release acetylcholine onto muscarinic receptors, an important exception to “sympathetic = norepinephrine.”
• Renal vasculature: dopamine acting on D1 receptors causes renal vasodilation via increased cAMP.

Parasympathetic Craniosacral Outflow

• CN III: Edinger-Westphal nucleus → ciliary ganglion → sphincter pupillae and ciliary muscle. Lesion causes mydriasis, impaired accommodation, and ptosis due to somatic levator palpebrae involvement.
• CN VII: superior salivatory nucleus → pterygopalatine ganglion for lacrimation; submandibular ganglion for submandibular and sublingual salivation.
• CN IX: inferior salivatory nucleus → otic ganglion → parotid gland.
• CN X: dorsal motor nucleus of vagus and nucleus ambiguus → thoracic and foregut/midgut viscera, including cardiac slowing and increased gastrointestinal motility.
• S2–S4 pelvic splanchnic nerves: distal colon, rectum, bladder detrusor contraction, internal sphincter relaxation, and erection.

Receptors and Second Messengers

Receptor	G protein / mechanism	High-yield effects
α1	Gq → ↑IP3/DAG → ↑Ca2+	Vasoconstriction, mydriasis, urinary sphincter contraction
α2	Gi → ↓cAMP	Decreased norepinephrine release, decreased insulin release
β1	Gs → ↑cAMP	Increased heart rate, contractility, and renin release
β2	Gs → ↑cAMP	Bronchodilation, uterine relaxation, skeletal muscle vasodilation
M2	Gi → ↓cAMP; ↑K+ efflux	Decreased SA and AV nodal activity
M3	Gq → ↑IP3/DAG	Gland secretion, smooth muscle contraction, endothelial NO-mediated vasodilation

A useful Step 1 mnemonic is: “QISS” for α₁ = Gq, α₂ = Gi, β₁/β₂ = Gs. Muscarinic receptors follow: M1, M3, M5 = Gq; M2, M4 = Gi.

Physiology and Reflexes

The baroreceptor reflex maintains short-term blood pressure. Stretch receptors in the carotid sinus, carried by CN IX, and aortic arch, carried by CN X, sense arterial stretch. Increased mean arterial pressure increases baroreceptor firing, causing increased vagal tone and decreased sympathetic tone: heart rate, contractility, and systemic vascular resistance fall. Normal adult resting heart rate is approximately 60–100/min; baroreflex buffering helps prevent abrupt deviations with posture change.

The pupillary light reflex has an afferent limb via CN II and efferent parasympathetic limb via CN III. Light in one eye normally produces both direct and consensual constriction. Sympathetic innervation dilates the pupil via a three-neuron pathway: hypothalamus → ciliospinal center of Budge at C8–T2 → superior cervical ganglion → dilator pupillae. Disruption causes Horner syndrome: ptosis, miosis, anhidrosis, and apparent enophthalmos.

High-Yield Pharmacologic Correlations

Acetylcholinesterase rapidly terminates acetylcholine signaling in the synaptic cleft. Reversible inhibitors such as neostigmine increase muscarinic and nicotinic activity. Organophosphates irreversibly inhibit acetylcholinesterase; “aging” of the phosphorylated enzyme can occur within hours, making early pralidoxime mechanistically important. Classic toxicity is summarized by DUMBBELSS: diarrhea, urination, miosis, bronchospasm/bronchorrhea, bradycardia, emesis, lacrimation, sweating, salivation.

Atropine is a competitive muscarinic antagonist; in severe cholinergic toxicity, adult initial dosing is commonly 1–3 mg IV, repeated until bronchial secretions improve. Pralidoxime reactivates acetylcholinesterase at nicotinic sites if given before aging. Sympathomimetics such as phenylephrine activate α1 receptors, albuterol activates β2 receptors, and dobutamine primarily activates β1 receptors. Understanding receptor localization predicts effects and adverse effects: for example, β2 agonists may cause tremor and hypokalemia due to intracellular K⁺ shift, while nonselective β-blockade can worsen bronchospasm by blocking β2-mediated bronchodilation.