Spinal Cord, Spinal Nerves & Reflexes – Chapter 13 Study Notes

Learning Outcomes

• 13-1 Describe the basic structural and organizational characteristics of the nervous system

• 13-2 Discuss structure & function of spinal cord; enumerate three spinal meninges

• 13-3 Explain roles of white vs. gray matter in sensory & motor pathways

• 13-4 Identify major components of a spinal nerve; define nerve plexus; relate spinal-nerve distribution to region innervated

• 13-5 Explain neuronal pools & five circuit patterns

• 13-6 List steps in a neural reflex & classify reflex types

• 13-7 Differentiate motor responses of reflex classes & show interaction for complex behavior

• 13-8 Describe how higher brain centers supervise & modify reflexes

Structural Organization of the Nervous System

• Central Nervous System (CNS)

  • Brain & spinal cord = processing centers

• Peripheral Nervous System (PNS)

  • Cranial nerves & spinal nerves

  • Carry sensory input to CNS and motor output from CNS

• Reflexes = rapid, automatic responses; spinal reflexes governed by cord alone (e.g., dropping hot pan before pain perceived)

The Spinal Cord – Gross Anatomy

• Housed in vertebral canal + meninges; conduit between brain & body; origin of spinal nerves

• Dimensions: \,45\, \text{cm} long, 14\,\text{mm} wide

• Extends from foramen magnum to level \text{L}1\text{–}\text{L}2 (cord stops lengthening \,\text{age }4; column continues)

• Four regions: cervical, thoracic, lumbar, sacral

• 31 segments, each giving rise to a pair of spinal nerves

• Longitudinal grooves

  • Posterior median sulcus (shallow)

  • Anterior median fissure (deep)

• Central canal \,\rightarrow filled with cerebrospinal fluid (CSF)

Enlargements & Distal Structures

• Cervical enlargement: neurons for shoulders/upper limbs

• Lumbosacral enlargement: pelvis/lower limb

• Conus medullaris: tapered end inferior to lumbar enlargement

• Cauda equina: bundle of nerve roots distal to conus (horse-tail appearance)

• Filum terminale: fibrous strand anchoring cord to coccyx (part of coccygeal ligament)

Roots, Ganglia, Nerves, Rami

• Posterior (dorsal) root – sensory axons

• Anterior (ventral) root – motor axons

• Spinal (dorsal root) ganglion: sensory neuron cell bodies (somata) between vertebral pedicles

• Roots split into rootlets before attaching to cord

• Spinal nerve = union of anterior + posterior roots; mixed (sensory + motor)

• Each spinal nerve branches:

  • White ramus communicans (preganglionic autonomic fibers; myelinated)

  • Gray ramus communicans (postganglionic; unmyelinated)

  • Posterior ramus \,\rightarrow skin & muscles of back

  • Anterior ramus \,\rightarrow lateral & anterior body wall, limbs

• Naming scheme: C1 lies above first cervical vertebra; C8 below C7; rest named by vertebra superior to exit (e.g., T1 below T1 vertebra)

Spinal Meninges

• Continuous with cranial meninges; functions: protect cord & convey blood vessels

• Dura mater (outer)

  • Dense collagen sheet fused to periosteum at occiput; tapers to coccygeal ligament

  • Epidural space (between dura & vertebrae) – adipose & loose CT; analgesic target

  • Subdural space – potential gap deep to dura

• Arachnoid mater (middle)

  • Arachnoid membrane (simple squamous) + trabeculae (collagen/elastic network)

  • Subarachnoid space: CSF reservoir; lumbar puncture withdraws CSF here

• Pia mater (inner)

  • Meshwork bound to neural tissue; carries surface vessels

  • Denticulate ligaments — lateral tooth-like extensions anchoring pia to dura; limit side-to-side motion

• Meningitis: viral/bacterial inflammation of meninges

Internal Anatomy – Gray vs. White Matter

• Gray matter

  • Neuron cell bodies, neuroglia, unmyelinated axons

  • Organized as nuclei inside horns:

  • Posterior horns \,\rightarrow somatic & visceral sensory nuclei

  • Lateral horns (T & L segments) \,\rightarrow visceral motor nuclei

  • Anterior horns \,\rightarrow somatic motor nuclei

  • Gray commissures: axons crossing near central canal

• White matter

  • Myelinated \,\pm unmyelinated axons

  • Columns: posterior, anterior (linked by anterior white commissure), lateral

  • Tracts/fasciculi = axon bundles with same origin/destination/function

  • Ascending tracts \,\rightarrow sensory to brain

  • Descending tracts \,\rightarrow motor from brain

Connective Tissue Layers in Nerves

• Epineurium – outer collagen network

• Perineurium – surrounds fascicles (axon bundles)

• Endoneurium – envelopes each axon

• Peripheral nerves continuous with spinal-nerve layers; all mixed

Dermatomes & Clinical Correlates

• Dermatome = bilateral cutaneous region supplied by single spinal nerve

• Mapping used to localize lesions

• Peripheral neuropathies: localized sensory & motor loss (trauma, compression, disease)

• Shingles (varicella-zoster reactivation) \,\rightarrow rash following dermatome pattern

Nerve Plexuses

• Interwoven networks of anterior rami; redundancy ensures multi-segment innervation

• Cervical plexus (C1–C5)

  • Phrenic nerve (C3–C5) \,\rightarrow diaphragm ("C3,4,5 keep the diaphragm alive")

  • Lesser occipital, great auricular, transverse cervical, supraclavicular nerves

• Brachial plexus (C5–T1)

  • Trunks \,\rightarrow cords (lateral, posterior, medial)

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

  • Median nerve thru carpal tunnel; inflammation \,\rightarrow carpal tunnel syndrome

• Lumbar plexus (T12–L4): iliohypogastric, ilio-inguinal, genitofemoral, femoral, obturator, lateral femoral cutaneous nerves

• Sacral plexus (L4–S4): sciatic (\rightarrow tibial & common fibular), superior/inferior gluteal, pudendal

• Sensory innervation of foot via saphenous, sural, fibular, tibial branches; neurological exams test integrity

Neuronal Pools & Circuit Patterns

• Approximate neuron counts: \text{10}^7 sensory, \text{5} \times \text{10}^5 motor, \text{2} \times \text{10}^{10} interneurons

• Neuronal pool = functional group of interneurons with limited input/output

• Circuit motifs:

  • Divergence – single input \rightarrow multiple outputs (visual info to various brain areas)

  • Convergence – multiple inputs \rightarrow one neuron (breathing controlled by conscious & subconscious centers)

  • Serial processing – sequential pathway (pain to conscious cortex)

  • Parallel processing – same info processed simultaneously in several paths (stepping on bee)

  • Reverberation – positive feedback loops maintain activity (breathing rhythms, posture)

Reflex Arc Basics

• Neural reflex = stereotyped, rapid, automatic motor response to stimulus

• Five components: receptor \rightarrow sensory neuron \rightarrow CNS integration \rightarrow motor neuron \rightarrow effector

• Spinal reflexes processed in cord; cranial reflexes in brainstem

Classification Axes

1. Development

   • Innate (genetic; e.g., withdrawal, suckling)

   • Acquired (learned; e.g., slamming brakes)

2. Motor Response

   • Somatic (skeletal) – superficial & stretch/tendon

   • Visceral (autonomic; smooth, cardiac, glands)

3. Complexity

   • Monosynaptic (one synapse; fastest)

   • Polysynaptic (\geq one interneuron; slower, flexible; intersegmental possible)

4. Processing Site

   • Spinal vs. Cranial

Monosynaptic Stretch Reflexes

• Maintain muscle length & tone; crucial for posture

• Example: patellar reflex

  • Stimulus: tap stretches quadriceps

  • Muscle spindle (intrafusal fibers) \rightarrow sensory neuron \rightarrow direct synapse on \alpha-motor neuron \rightarrow quad contraction

• Gamma motor neurons adjust spindle sensitivity

• Postural reflexes combine monosynaptic & polysynaptic actions for balance

Polysynaptic Reflexes

• Tendon reflex (Golgi tendon organ) prevents over-tension; produces muscle inhibition proportional to tension

• Withdrawal (flexor) reflex: pull limb from painful stimulus; strength linked to stimulus intensity

  • Reciprocal inhibition: antagonistic extensors inhibited to allow flexion

• Crossed-extensor reflex: contralateral extensors activated to support weight when ipsilateral limb flexes (stepping on tack)

• General traits: involve interneuron pools, multiple segments, reciprocal inhibition, reverberating circuits, cooperative reflexes

Higher-Center Modulation of Reflexes

• Descending pathways can facilitate (reinforce) or inhibit spinal circuits

• Voluntary movement may piggy-back on reflex patterns (walking)

• Facilitation: continual excitatory input brings motor neurons closer to threshold

• Inhibition: IPSPs or presynaptic suppression dampen reflex

• Diagnostic plantar reflexes

  • Normal adult: plantar (toes curl)

  • Infant / pathological adult: Babinski (toes fan) indicates absent corticospinal inhibition

Key Numbers & Equations

• Spinal cord length: \approx 45\,\text{cm}

• Segments: 31 (8C, 12T, 5L, 5S, 1Co)

• Neuron counts: sensory \sim 1\times10^7, motor \text{5}\times\text{10}^5, interneurons \text{2}\times\text{10}^{10}

• Conduction speed: myelinated large-fiber stretch reflex \approx 120\,\text{m}\,\text{s}^{-1} (fastest)

Clinical / Practical Connections

• Epidural anesthesia targets epidural space to block spinal nerves during childbirth

• Lumbar puncture (subarachnoid tap) done at \text{L}3\text{–}\text{L}4 to avoid cord

• Carpal tunnel syndrome: median-nerve compression \rightarrow sensory loss + motor deficits in hand

• Varicella-zoster reactivation (shingles) tracks a dermatome

• Patellar reflex testing assesses L2–L4 integrity; Babinski sign screens for corticospinal damage

• Peripheral neuropathies (diabetes, B\text{12} deficiency) present as glove & stocking sensory loss

Ethical / Philosophical Notes

• Reflex testing is cornerstone of neurological exam—non-invasive window into CNS health

• Understanding redundancy (plexuses, convergence) shows evolutionary emphasis on functional resilience

• Autonomic reflexes illustrate subconscious regulation essential for homeostasis (heart rate, digestion)