Spinal Cord and Spinal Nerves – Key Vocabulary

Introduction & Chapter Scope

• Purpose of Chapter 13 (Tortora & Derrickson, 16ᵗʰ ed.)

  • Explore how the spinal cord & spinal nerves maintain homeostasis through rapid information flow.

  • Identify every external & internal anatomical landmark of the cord.

  • Explain spinal reflex arcs—why they are life-saving and how they differ from ordinary neural traffic.

  • Continuous theme: integration of structure → function → clinical relevance.

Core Functions of the Spinal Cord

• Two overarching jobs

  • Processes reflexes (fast, involuntary protective responses).

  • Conducts impulses:

  • Sensory (ascending) → brain.

  • Motor (descending) → effectors (muscles & glands).

• Homeostatic link: allows rapid adjustment of heart rate, vessel diameter, posture, pain withdrawal, etc.

Protective Structures

• Triple-layered protection mirrors the brain’s, emphasising damage prevention.

1. Skeletal Framework

• 26 vertebrae form a bony canal; vertebral body anteriorly & laminae/ pedicles posteriorly guard the cord.

2. Meninges (CT membranes)

• Listed superficial → deep:

  1. Dura mater – dense irregular CT; forms the epidural space (fat & BV’s).

  2. Arachnoid mater – elastic & collagen; under it lies the subdural space (interstitial fluid).

  3. Pia mater – thin, vascular; gives off denticulate ligaments (saw-tooth projections anchoring cord laterally).

3. Cerebrospinal Fluid (CSF)

• Sits in subarachnoid space; hydraulic cushion, nutrient & waste conduit.

External Anatomy of the Spinal Cord

• Begins at foramen magnum as medulla extension → ends ~ L2.

• Length: ≈ 42\text{–}45\,\text{cm}; diameter: \approx 2\,\text{cm} cervical, tapering caudally.

• Key landmarks

  • Cervical enlargement (C4–T1) – upper-limb innervation.

  • Lumbar enlargement (T9–T12) – lower-limb innervation.

  • Conus medullaris – cone-shaped terminus at L1\text{–}L2.

  • Cauda equina – dorsal & ventral roots descending like a horse’s tail.

  • Filum terminale – pia extension anchoring conus to coccyx.

Internal Anatomy of the Spinal Cord

• Gray Matter (H-shaped): non-myelinated neuron bodies.

  • Anterior (ventral) horns – somatic motor nuclei.

  • Posterior (dorsal) horns – somatic & visceral sensory nuclei.

  • Lateral horns (T1–L2, S2–S4) – autonomic (symp./parasymp.) nuclei.

  • Gray commissure & central canal – canal carries CSF the cord’s length.

• White Matter (peripheral): myelinated axons organised in funiculi

  • Posterior, lateral, anterior columns.

  • Contain tracts: sensory (ascending) & motor (descending).

Spinal Nerves – Overview

• Part of PNS; connect CNS ↔ receptors/ effectors.

• 31 pairs:

  • Cervical 8, Thoracic 12, Lumbar 5, Sacral 5, Coccygeal 1.

• Each nerve forms by union of:

  • Posterior (dorsal) root – sensory; houses dorsal root ganglion.

  • Anterior (ventral) root – motor fibers.

Connective-Tissue Sheaths (analogous to muscle fascias)

• Epineurium – surrounds whole nerve; dense irregular CT.

• Perineurium – encloses fascicle (bundles of axons).

• Endoneurium – delicate CT around individual axon + myelin.

Primary Branches (Rami) after IVF exit

1. Posterior (dorsal) ramus – deep back muscles + posterior skin.

2. Anterior (ventral) ramus – limbs + anterior/ lateral trunk.

3. Meningeal branch – loops back through IVF → vertebrae, ligaments, meninges.

4. Rami communicantes – autonomic; link to sympathetic trunk ganglia.

Nerve Plexuses & Intercostal Nerves

• Plexus = braided network of anterior rami (except T2–T12).

• Clinical pearl: damage to single spinal segment ≠ total loss because fibers are redistributed.

Cervical Plexus (C1–C4, part C5)

• Motor & sensory to head, neck, diaphragm (via phrenic nerve C3–C5 → “C3,4,5 keep the diaphragm alive”).

Brachial Plexus (C5–T1)

• Supplies shoulders & upper limbs.

• Major nerves: musculocutaneous, axillary, radial, median, ulnar.

• Injuries: Erb-Duchenne palsy (waiter’s tip), wrist-drop, claw-hand, winged scapula.

Lumbar Plexus (L1–L4)

• Femoral (largest), obturator, iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous.

• Innervates anterolateral abdominal wall, external genitalia, anterior thigh.

Sacral Plexus (L4–S4) + Coccygeal

• Sciatic nerve (tibial + common fibular) – body’s longest/ thickest.

• Superior & inferior gluteal, pudendal, posterior femoral cutaneous, etc.

Intercostal (Thoracic) Nerves T2–T12

• No plexus; run along ribs → intercostal muscles, abdominal wall, overlying skin.

Dermatomes

• Skin slices innervated by single spinal nerve; mapping aids in locating cord damage or anaesthetic blocks.

Physiology: Information Flow

• White matter tracts = “superhighways”. Sensory ascend to thalamus/ cortex; motor descend from brain centers.

• Gray matter = in-cord processing → reflexes; integrates EPSPs/IPSPs from multiple sources.

Sensory ↔ Motor Processing Sequence (simplified)

1. Receptors activate → AP in sensory neuron.

2. Cell body in dorsal root ganglion → posterior root.

3. Axon enters posterior horn.

4. Can ascend, synapse with interneuron, or both.

5. Interneuron(s) may cross, ascend, or synapse with motor neurons.

6. Anterior horn motor neurons exit via anterior root.

7. Effectors contract or secrete.

Reflexes & Reflex Arcs

• Reflex: fast, involuntary, stereotyped response.

• Survival tool – limits tissue damage (withdrawal), maintains posture, controls visceral function.

Components (always present)

1. Receptor (e.g., muscle spindle, nociceptor).

2. Sensory neuron (afferent).

3. Integration center (gray matter; mono- or polysynaptic).

4. Motor neuron (efferent).

5. Effector (muscle or gland).

Key Vocabulary

• Ipsilateral vs. contralateral, monosynaptic vs. polysynaptic, reciprocal innervation (agonist contracts while antagonist relaxes).

Stretch Reflex (Patellar, etc.)

• Stimulus: quick stretch → muscle spindle.

• Monosynaptic, ipsilateral; resists length change → maintains muscle tone & posture.

• Clinical use: knee-jerk tests integrity of L2\text{–}L4 segments.

Tendon Reflex (Golgi Tendon)

• Protects tendon from excessive tension.

• Polysynaptic, ipsilateral; Golgi tendon organ → inhibits homonymous muscle, stimulates antagonist.

Flexor (Withdrawal) Reflex

• Nociceptive; polysynaptic, ipsilateral.

• Example: hand withdrawal from hot stove.

Crossed-Extensor Reflex

• Complements flexor reflex to maintain balance.

• Polysynaptic, contralateral; extensor muscles on opposite limb contract.

Comparative Snapshot

Disorders & Clinical Correlations

• Traumatic injuries

  • Monoplegia, hemiplegia, paraplegia, quadriplegia – severity depends on segment & completeness.

  • American Spinal Injury Association (ASIA) grades:

  • A: no motor/sensory below lesion.

  • B–D: progressive preservation (e.g., D = most leg muscles functional).

• Shingles (Herpes zoster)

  • Reactivation of varicella-zoster in dorsal root ganglion → dermatomal rash, burning pain; may cause post-herpetic neuralgia, facial paralysis.

• Spinal cord compression – tumours, herniated discs; ischemia risk.

• Degenerative diseases

  • Multiple sclerosis (MS): immune attack on myelin; sensory & motor deficits, optic neuritis, fatigue.

  • Poliomyelitis: viral destruction of anterior horn motor neurons; vaccine preventable.

Ethical & Practical Implications

• Early vaccination critical: eradicated polio in most regions, shingles vaccine reduces neuralgia risk.

• Injury prevention: seatbelts, sports helmets, ergonomics to limit vertebral compression.

• Rehabilitation & neuroplasticity: PT/OT leverage intact circuits & reflex pathways; electrical-stimulation therapies target residual function.

• CSF sampling (lumbar puncture at L3\text{–}L4) – safe below cord termination; essential for diagnostics (meningitis, MS markers).

Numerical & Miscellaneous Facts

• Total spinal nerves =31\text{ pairs}.

• Ratio gray:white matter increases caudally (because fewer axons remain toward sacrum).

• CSF volume in subarachnoid space ≈ 80\,\text{mL} surrounding cord.

• Conduction velocity in myelinated dorsal column fibers ≈ 50\text{–}100\,\text{m/s} – underlies rapid proprioceptive feedback.

These structured notes encapsulate all slide content, elaborated with mechanisms, clinical ties, and practical considerations—ready to substitute for the original 60-slide deck.