Peripheral nervous system Study Guide

📖 Core Concepts Peripheral Nervous System (PNS) – nerves & ganglia outside the brain & spinal cord; links CNS to limbs & organs. Somatic vs. Autonomic Division – Somatic = voluntary control of skeletal muscle; Autonomic = involuntary control of smooth muscle, cardiac muscle, glands. Sensory vs. Motor Sectors – Each division carries afferent (sensory) information to CNS and efferent (motor) signals away from CNS. Cranial Nerves & PNS – All cranial nerves are PNS except olfactory (I) & optic (II); their ganglia are peripheral structures. Spinal Nerve Organization – 31 pairs (C8‑T12‑L5‑S5‑Co1); exit pattern differs: cervical roots exit above vertebrae, thoracic‑coccygeal exit below. Plexuses – Networks of spinal nerves: brachial (C5‑T1) for upper limb; lumbosacral (L1‑Co1) for lower limb & pelvic region. Autonomic Sub‑divisions – Sympathetic (fight‑or‑flight, NE/EPI), Parasympathetic (rest‑and‑digest, ACh), Enteric (local gut control). Peripheral Neuropathy – Damage to peripheral nerves; “glove‑and‑stocking” sensory loss; may involve autonomic fibers. 📌 Must Remember PNS Protection – No skull/vertebral column; no blood‑brain barrier → more toxin‑sensitive. Cranial Nerve Exception – Olfactory & optic nerves are CNS, not PNS. Phrenic Nerve Roots – C3‑C5 (C3 + C4 + C5 = “keep the diaphragm alive”). Sympathetic Neurotransmitters – Norepinephrine & epinephrine ↑ heart rate, ↑ skeletal‑muscle blood flow, ↓ digestion. Parasympathetic Neurotransmitter – Acetylcholine (ACh) → ↓ heart rate, ↑ salivation & digestion. Mononeuropathy – Single nerve/root damage (injury, tumor, compression). Compression Neuropathies – Carpal tunnel (median nerve) & tarsal tunnel (posterior tibial nerve). Peripheral Neuropathy Pattern – Distal → proximal “glove‑and‑stocking”. 🔄 Key Processes Signal Flow in Somatic System Motor cortex → upper motor neuron → spinal cord → lower motor neuron (somatic spinal nerve) → skeletal muscle (voluntary contraction). Sympathetic Activation (Fight‑or‑Flight) Stress → hypothalamus → spinal cord (T1‑L2) → pre‑ganglionic neuron → sympathetic ganglion → post‑ganglionic neuron → target organ (release NE/EPI). Parasympathetic Activation (Rest‑and‑Digest) Visceral stimulus → brainstem or sacral spinal cord → long pre‑ganglionic fiber → ganglion near/within organ → short post‑ganglionic fiber → ACh release on target. Enteric Reflex Local stretch/chemical stimulus in gut → intrinsic enteric neurons → coordinated muscle contraction/relaxation without CNS input (modulated by sympathetic/parasympathetic). 🔍 Key Comparisons Somatic vs. Autonomic Control: Voluntary vs. involuntary. Effectors: Skeletal muscle vs. smooth muscle, cardiac muscle, glands. Neurotransmitters: ACh at both NMJ & parasympathetic; NE/EPI for sympathetic. Sympathetic vs. Parasympathetic State: Fight‑or‑flight vs. rest‑and‑digest. Neurotransmitter: NE/EPI vs. ACh. Heart: ↑ rate vs. ↓ rate. Digestive activity: ↓ vs. ↑. Cervical vs. Thoracic Spinal Nerve Exit Cervical: Above vertebral body (C1‑C7). Thoracic‑coccygeal: Below vertebral body. ⚠️ Common Misunderstandings “All cranial nerves are peripheral.” – Olfactory (I) & optic (II) are CNS. “Sympathetic always excites, parasympathetic always relaxes.” – Some sympathetic actions (e.g., vasoconstriction) can be inhibitory; parasympathetic can also cause glandular secretion (active). “Enteric system is separate from autonomic.” – It is a subdivision of the autonomic nervous system, though it can function independently. “Peripheral neuropathy always painful.” – May be painless sensory loss; pain is common but not universal. 🧠 Mental Models / Intuition “C‑C‑C” for diaphragm – C3, C4, C5 = keep the diaphragm alive. “Fight‑or‑flight = NE/EPI, Rest‑and‑digest = ACh” – neurotransmitter cue for state identification. Plexus “Tree” Image – Roots → trunks → divisions → cords → branches = predictable order (e.g., Brachial plexus). 🚩 Exceptions & Edge Cases Vagus nerve (CN X) – Though a cranial nerve, it carries visceral sensory fibers to thorax/abdomen, blurring somatic/autonomic lines. Accessory nerve (CN XI) – Motor to sternocleidomastoid & trapezius (outside head) – unique peripheral target for a cranial nerve. Autonomic Independence – Sympathetic & parasympathetic can act on different organs simultaneously (e.g., ↑ heart rate, ↓ gut activity). 📍 When to Use Which Identify nerve involvement → Look at distribution: Median nerve → carpal tunnel symptoms (thenar weakness, sensory loss thumb‑index‑middle). Sciatic nerve → L4‑S3 distribution, posterior thigh & leg pain. Choose autonomic state → Clinical context: Stress/trauma → sympathetic‑dominant signs (tachycardia, dilated pupils). Post‑prandial → parasympathetic‑dominant signs (↑ salivation, ↑ gut motility). Select plexus level for injury → Upper‑limb weakness → consider brachial plexus (C5‑T1). Lower‑limb weakness → consider lumbosacral plexus (L1‑Co1). 👀 Patterns to Recognize “Glove‑and‑stocking” → peripheral neuropathy, especially diabetic. “Cervical root exit above vertebra” → C1‑C7 pattern (useful for spinal injury localization). Sympathetic “NE/EPI → ↑ HR, ↓ GI” vs. Parasympathetic “ACh → ↓ HR, ↑ GI”. Compression neuropathy → symptoms confined to nerve’s anatomical tunnel (e.g., wrist for carpal tunnel). 🗂️ Exam Traps Mistaking optic/olfactory nerves as peripheral – remember they are CNS. Assuming all autonomic fibers use the same neurotransmitter – sympathetic uses NE/EPI; parasympathetic uses ACh. Confusing “sympathetic” with “always excitatory” – sympathetic can cause vasoconstriction (inhibition of blood flow to skin). Attributing “rest‑and‑digest” to all parasympathetic actions – some actions are active (e.g., salivation). Over‑generalizing “mononeuropathy = compression – mononeuropathy also includes trauma, tumor, or inflammatory causes. --- Study tip: Review the C‑C‑C mnemonic, the plexus tree diagram, and the neurotransmitter‑state pairing. Spot the pattern, eliminate the distractors, and you’ll ace the PNS section!