Peripheral Nervous System – Structure, Function & Repair

Relationship Between CNS & PNS

• Central nervous system (CNS): brain + spinal cord ➔ “central processing center.”

  • Integrates, interprets, makes decisions.

• Peripheral nervous system (PNS): all neural tissue outside CNS.

  • Highway that brings sensory input in and sends motor commands out.

  • Without it, the brain would be “blind & mute.”

Direction of Information Flow

• Sensory (afferent) neurons

  • Mnemonic: "Arriving"—carry information INTO CNS.

  • Tell us what & where things are happening.

• Motor (efferent) neurons

  • Mnemonic: "LEaving"—carry information OUT from CNS to muscles/glands.

  • Execute responses; loss → “Why can’t I move?”

Nerves: Bundles of Axons

• Sensory nerves: only afferent axons → one-way traffic toward CNS.

• Motor nerves: only efferent axons → one-way traffic away from CNS.

• Mixed nerves: carry both; majority of peripheral nerves.

Sub-Classification of Peripheral Nerves

• Cranial nerves (CN): enter/exit brain.

  • Sensory CN: skin, muscles, special senses above the neck + GI tract.

  • Motor CN: muscles & glands of head/neck.

• Spinal nerves (SN): enter/exit spinal cord.

  • Sensory SN: skin & muscles below the neck.

  • Motor SN: trunk & limb musculature, glands.

Sensory Receptors & Coding

Dual Meaning of “Receptor”

1. Molecular protein on dendrites (bind neurotransmitter) – Ch. 12.

2. Entire dendritic ending of a sensory neuron – this chapter.

Transduction Principle

• Dendrites translate physical/chemical energy → electrical language (action potentials).

• No external neurotransmitter at skin; dendrite itself is specialized for stimulus.

Receptor Modalities

• Mechanoreceptors – touch, pressure, vibration, stretch, itch.

• Thermoreceptors – temperature.

• Photoreceptors – light (retina).

• Chemoreceptors – chemicals (odorants, tastants; blood CO₂/O₂).

• Nociceptors – pain from extreme stimuli.

• Proprioceptors – body position, muscle stretch, joint motion.

Receptor Location

• Exteroceptors – external environment (skin, eyes, ears, etc.).

• Interoceptors – internal milieu (viscera, vessels); monitor fullness, BP, pH.

Adaptation Profiles

• Phasic receptors: Rapidly adapt → “turn off.”

  • Example: you stop noticing heavy perfume or chair pressure.

• Tonic receptors: Adapt slowly or not at all.

  • Nociceptors stay active so damage is not ignored.

  • Neurotransmitter chemistry:

  • Pain transmission: glutamate (excitatory).

  • Pain suppression: endorphins; agonist drug = morphine.

Pain Fiber Types

• Sharp pain: fast, thick, myelinated axons; quick withdrawal.

• Dull/throbbing pain: slow, thin, unmyelinated axons; lingering reminder.

Referred Pain Phenomenon

• Visceral pain mislocalized to skin (dermatome overlap).

  • Heart ➔ shoulder/jaw; gallbladder ➔ right shoulder; kidneys ➔ lower back, etc.

  • Theory: visceral & somatic afferents enter the same spinal segments ⇒ CNS assumes somatic origin.

Spinal Nerves: Numbers, Roots, & Branches

• Total: 31 pairs (i.e., 62 individual nerves).

  • Cervical C1 – C8 (8)

  • Thoracic T1 – T12 (12)

  • Lumbar L1 – L5 (5)

  • Sacral S1 – S5 (5)

  • Coccygeal Co1 (1)

• Each spinal nerve = mixed.

• Close to cord, divides into two roots:

  • Dorsal (posterior) root: sensory afferents enter.

  • Ventral (anterior) root: motor efferents exit.

• Roots further subdivide into rootlets before attaching to cord.

• Spinal nerves serve entire body except head/neck (handled by CN).

Plexuses: Overlapping Safety Net

• Plexus = interwoven network of ventral rami; offers redundancy.

• Damaging one spinal nerve rarely abolishes entire limb function.

Cervical Plexus (C1–C5)

• Skin & muscles of back of head, neck, shoulders.

• Named branches: occipital, auricular, supraclavicular, cervical.

• Phrenic nerve (C3-C5): diaphragm & breathing (“C3,4,5 keep the diaphragm alive”).

Brachial Plexus (C5–C8, ±T1/2)

• Entire upper limb.

• Branches: axillary, radial, median, ulnar, subscapular, pectoral, digital.

  • Arrangement: Roots → Trunks → Divisions → Cords → Branches (mnemonic “Randy Travis Drinks Cold Beer”).

Lumbar Plexus (L1–L4)

• Anterior & medial thigh/leg.

• Branches: femoral, obturator, iliohypogastric, ilioinguinal, genitofemoral.

Sacral Plexus (L4–S4)

• Posterior leg, buttock, pelvis, foot.

• Branches: sciatic (→ tibial & common fibular), superior/inferior gluteal, pudendal, sural, plantar.

Dermatomes & Clinical Neurology Without Tech

• Dermatome: skin zone supplied by a single spinal nerve.

• Pin-prick test: sensory loss in a dermatome pinpoint spinal level damage.

  • Overlap means a single nerve lesion rarely causes total numbness.

• Motor correlation:

  • Nerves whose dermatomes cross a joint usually innervate muscles that move that joint.

  • Can’t flex hip? Check lumbar (L2–L4).

  • Can’t extend elbow? Check C6–C8 (radial nerve).

• Real-world relevance: Post-Katrina doctors diagnosed cord injuries using only pins & knowledge of dermatomes.

Peripheral Nerve Regeneration (PNS > CNS)

• Requirement: cell body intact; only axon distal segment dies.

• Step 1 – Injury: distal axon & myelin degenerate; proximal stump remains.

• Step 2 – Cleanup: macrophages phagocytose debris.

• Step 3 – Schwann cell response: proliferate, create regeneration tube guiding sprouts.

• Step 4 – Axonal regrowth (~1 mm/day): one sprout enters tube → elongates → new myelin sheath forms.

• Significance: explains why peripheral nerve injuries can recover while spinal cord injuries (oligodendrocytes, no tubes) rarely do.

Study Strategy Tips (Meta-Advice)

• ORGANIZE information:

  • Chart with columns: Plexus | Function | Spinal Levels | Major Branches.

  • Flash cards for nerve names ↔ sensory regions/motor targets.

• Use mnemonics:

  • “Some Say Marry Money, But My Brother Says Big Brains Matter More” (cranial nerve functions; keeps PNS vs CNS connections fresh).

  • Plexus order: "Catching Bad Lads Sucks" (Cervical, Brachial, Lumbar, Sacral).

• Connect to physiology:

  • Receptor adaptation explains why constant stimuli fade.

  • Pain neurotransmitters link to pharmacology (morphine as endorphin agonist).

• Ethical considerations:

  • Pain’s tonic nature is protective; careless suppression (opioid overuse) has societal consequences.

  • Knowledge of referred pain prevents misdiagnosis (e.g., MI presenting as jaw pain).

These notes condense every concept, detail, and example from the original slides while integrating physiology, clinical application, and study techniques.