Chapter 21 – Peripheral Nervous System (BIO 105)

Learning Objectives

• Review anatomy & physiology of spinal cord, peripheral nerves, and reflexes.
• Identify & describe four major nerve plexuses, with emphasis on cervical & brachial plexuses.
• Define dermatomes & myotomes; locate key surface landmarks.
• List the twelve cranial nerves, their functional categories (Sensory/Motor/Both) & primary actions.
• Explain components & clinical significance of somatic reflex arcs.

Spinal Nerves – Overview

• The human spinal cord gives rise to 31 pairs of spinal nerves ("nn").
  • All exit through intervertebral foramina and are mixed nerves (carry both afferent-sensory & efferent-motor fibers).
  • Example: Musculocutaneous n. – sensory to skin over biceps brachii, motor to biceps brachii muscle.
• Numbering reflects vertebral level of emergence:
  • 8 cervical (C1–C8) – note C1 exits between occiput & atlas; C8 exits between C7 & T1.
  • 12 thoracic (T1–T12).
  • 5 lumbar (L1–L5).
  • 5 sacral (S1–S5).
  • 1 coccygeal (Co1).
• Cauda equina ("horse’s tail")
  • Bundle of lumbar, sacral & coccygeal nerve roots extending below spinal cord termination (conus medullaris ≈ L2).
  • Allows lumbar cistern CSF sampling (lumbar puncture) without cord trauma.

Structure of a Spinal Nerve

• Roots (within vertebral canal)
  • Ventral (anterior) root – motor (somatic & autonomic).
  • Dorsal (posterior) root – sensory; contains dorsal root (spinal) ganglion of sensory neuron cell bodies.
• Rami (immediately after exiting IVF – start of PNS)
  • Dorsal ramus – mixed somatic fibers to skin & deep muscles of posterior head, neck, trunk.
  • Ventral ramus – larger, mixed somatic & autonomic fibers.
  • Somatic branches form plexuses & supply limbs, anterior/lateral trunk.
  • Autonomic fibers peel off to sympathetic chain ganglia (no analogous parasympathetic chain).

Nerve Plexuses – General Principles

• Four principal plexuses: Cervical, Brachial, Lumbar, Sacral (+ Coccygeal extension).
• Why a plexus?
  • Fibers of adjacent ventral rami interweave → each peripheral nerve contains axons from >1 spinal level.
  • Injury to a single spinal root seldom abolishes function of an entire limb region; redundancy enhances preservation.

Cervical Plexus (C1–C4 + branch of C5)

• Lies deep to sternocleidomastoid.
• Key motor/sensory branches (many lesser occipital, great auricular, etc. not tested).
• Phrenic nerve (C3–C5)
  • Sole motor supply to diaphragm – mnemonic: “C3, 4, 5 keep the diaphragm alive.”
  • High cervical spinal cord lesion E phrenic paralysis → ventilatory dependence (clinical tie-in Box 21-2).

Brachial Plexus (C5–T1)

• Supplies entire upper limb.
• Classic diagram: Roots → Trunks → Divisions → Cords → Branches ("Randy Travis Drinks Cold Beer").
• Roots – small motor branches
  • Dorsal scapular (rhomboids, levator scapulae).
  • Long thoracic (serratus anterior) – lesion ✱→ winged scapula.
• Trunks
  • Suprascapular (supraspinatus, infraspinatus).
• Cords – named in relation to axillary a.
  • Lateral cord → Lateral pectoral (pectoralis major).
  • Posterior cord → Upper & lower subscapular (subscapularis, teres major); Thoracodorsal (latissimus dorsi).
  • Medial cord → Medial pectoral (pec major & minor).
• Terminal branches (5)
  1. Axillary n. (C5–C6)
  • Motor: deltoid, teres minor.
  • Sensory: “regimental badge” lateral shoulder.
  1. Musculocutaneous n. (C5–C7)
  • Motor: coracobrachialis, biceps brachii, brachialis.
  • Continues as lateral cutaneous n. of forearm.
  1. Radial n. (C5–T1)
  • Motor: triceps, anconeus, brachioradialis, ALL extensor muscles of forearm/hand.
  • Sensory: posterior arm, forearm, dorsolateral hand.
  • Injury → wrist-drop.
  1. Median n. (C5–T1)
  • Motor: most forearm flexors (except FCU), thenar muscles, lateral lumbricals.
  • Sensory: palmar digits 1–3 & half 4.
  • Carpal tunnel syndrome = entrapment.
  1. Ulnar n. (C8–T1)
  • Motor: FCU, ulnar half FDP, intrinsic hand muscles.
  • Sensory: digits 4–5 anterior & medial forearm.
  • "Funny bone" = ulnar n. at medial epicondyle.

Lumbar Plexus (L1–L4) – awareness only

• Supplies anterolateral abdominal wall, genitalia, anterior/medial thigh.
• Major branches & key actions/sensation:
  • Iliohypogastric (L1) – lower abdominal wall, buttock skin.
  • Ilioinguinal (L1) – medial thigh, external genitalia sensation.
  • Genitofemoral (L1–L2) – anterior thigh & genitalia.
  • Lateral femoral cutaneous (L2–L3) – sensory lateral thigh; compression = meralgia paresthetica.
  • Femoral (L2–L4) – motor: quadriceps, iliacus, sartorius; sensory: anterior thigh, medial leg.
  • Obturator (L2–L4) – motor: adductor group; sensory: medial thigh.

Sacral & Coccygeal Plexus (L4–S4)

• Pelvis, posterior thigh, leg & foot.
• Superior gluteal (L4–S1) – gluteus medius/minimus, TFL.
• Inferior gluteal (L5–S2) – gluteus maximus.
• Sciatic n. (L4–S3) – largest nerve in body; splits ≈ popliteal fossa:
  1. Common fibular/peroneal (L4–S2):
  • Motor: anterior & lateral leg (dorsiflexors, evertors, toe extensors).
  • Sensory: lateral leg, dorsum foot.
  • Lesion → foot-drop.
  1. Tibial (L4–S3):
  • Motor: posterior leg (plantar-flexors) & intrinsic foot flexors.
  • Sensory: calf, plantar foot.
• Posterior femoral cutaneous (S1–S3) – sensation buttock & posterior thigh.

Dermatomes – Cutaneous Sensory Map

• Each spinal nerve (except C1) supplies a band-like skin region.
• Key test points (quick neuro exam):
  • C5: lateral shoulder (deltoid patch).
  • C6: dorsum hand – anatomical snuff box.
  • C7: tip of middle finger.
  • C8: medial hand – digits 4–5.
  • T1: medial forearm.
  • T5: nipple line (useful landmark).
  • T10: umbilicus.
  • L1: proximal anterior thigh.
  • L2: mid-anterior thigh.
  • L3: distal anterior thigh.
  • L4: medial malleolus.
  • L5: dorsum foot (big toe web space).
  • S1: lateral plantar foot/posterolateral leg.
  • S2: posteromedial leg.
• Peripheral neuropathy (eg, diabetes) → stocking-glove sensory loss pattern.

Cranial Nerves (CN) – Summary Table

Mnemonic (names): On Old Olympus’ Towering Tops A Friendly Viking Grows Vines And Hops.
Mnemonic (function): Some Say Marry Money But My Brother Says Big Business Matters More (S=sensory, M=motor, B=both).

Somatic (Motor) Nervous System Principles

• Pathway: single lower motor neuron (LMN) cell body in CNS → axon exits via ventral root → peripheral nerve → neuromuscular junction; neurotransmitter \text{ACh}.
• Reflex = predictable, rapid response to specific stimulus through a reflex arc.
  • Components: receptor → afferent neuron → integration center (spinal cord or brain) → efferent neuron → effector.
  • Classified by integration site (spinal vs cranial), laterality (ipsilateral vs contralateral), receptor depth (deep vs superficial), etc.

Clinically Important Somatic Reflexes

• Knee-jerk (patellar)
  • Stretch (myotatic) reflex; monosynaptic; L2–L4 (femoral n.).
  • Ipsilateral, spinal, segmental, deep, extensor reflex.
• Ankle-jerk (Achilles)
  • Monosynaptic; S1–S2 (tibial n.).
• Plantar reflex
  • Normal adult response: toe flexion.
  • Babinski sign (toe extension) normal ≤ 18 months; thereafter indicates corticospinal tract lesion.
• Corneal reflex
  • Afferent: CN V₁ (ophthalmic); Efferent: CN VII → blink.
• Abdominal reflex
  • Light stroke T9–T12 dermatomes → umbilicus deviation toward stimulus.
  • Absence: possible upper motor neuron lesion, obesity, pregnancy, or post-op.

Integrative & Clinical Connections

• Plexus overlap & redundancy critical in trauma – e.g., partial brachial plexus injury may spare distal muscles.
• High cervical lesions (above C3) compromise phrenic n. → respiratory arrest.
• Dermatomal testing assists in localizing spinal cord or root lesions rapidly in ER/ICU settings.
• Cranial nerve examination is a cornerstone of neuro screening; each CN deficit localizes lesion site (brainstem stroke, skull base tumor, etc.).
• Reflex grading (0–4+) provides objective LMN vs UMN information in neurologic exams.

Study Tips & Mnemonics Recap

• "Randy Travis Drinks Cold Beer" = brachial plexus hierarchy.
• "C3, 4, 5 keep the diaphragm alive." = phrenic nerve levels.
• Dermatomes: "T4 nipple, T10 belly button, L4 knee, S1 lateral foot" – anchor points.
• Cranial Nerve names / functions mnemonics (above) – practice aloud.
• Draw plexus & dermatomal maps repeatedly → visual memory.