CNS B – Spinal Cord, Tracts and Trauma

Gross Anatomy of the Spinal Cord

• Continuously extends from the foramen magnum at the base of the skull down to the level of the L2 vertebra.
  • Text frequently abbreviates the overall span as C1! -! L2.
• Inferior to L3 the cord proper ends; only spinal nerves (the cauda equina) descend inside the vertebral canal.
• Sits inside the protective vertebral canal, cushioned further by meninges and cerebrospinal fluid (CSF).

White vs. Gray Matter & the Concept of "Tracts"

• White areas = myelinated axons.
  • Bundles of axons INSIDE the CNS are called tracts (analogous to "nerves" in the PNS).
• Dark central "butterfly" = gray matter composed of unmyelinated cell bodies, dendrites and initial axon segments.
  • Responsible for local integration—receiving, processing, and sending information to other neurons.

Ascending (Sensory) Tracts

• Carry sensory information up the spinal cord toward cortical & subcortical sensory centers.
• Two principal pathways to know for the exam:
  1. Dorsal (Posterior) Column Tracts
  2. Spinothalamic Tracts
• EACH ascending pathway uses three neurons:
  1. Receptor (1°) neuron – in dorsal root ganglion.
  2. (2°) neuron – in spinal cord or medulla.
  3. (3°) neuron – in thalamus projecting to somatosensory cortex.

Dorsal Column Tract (Posterior Column–Medial Lemniscus)

• FUNCTION – conveys:
  • Proprioception (sense of body & limb position)
  • Discriminative/deep touch, pressure, vibration
• CROSSING (decussation): in the medulla oblongata.
• DAMAGE/LESION rules:
  • Unilateral cord injury ➜ loss of proprioception & deep-touch below the lesion on the SAME (ipsilateral) side.
• Clinical pearl: Patient may appear ataxic because they can’t feel limb position even though strength is intact.

Spinothalamic (Anterolateral) Tract

• FUNCTION – conveys:
  • Pain (nociception)
  • Temperature
  • Crude/superficial touch
• CROSSING: immediately (1–2 segments) after entering the spinal cord.
• DAMAGE/LESION rules:
  • Unilateral cord injury ➜ loss of pain & temperature below the lesion on the OPPOSITE (contralateral) side.

Comparison of Ascending Pathways

• "Two different roads to the brain":
  • Spinothalamic fibers cross EARLY (in cord) and ascend contralaterally from the start.
  • Dorsal-column fibers ascend IPSILATERALLY until reaching medulla, then cross.
• Clinically useful for localizing cord hemisections (Brown-Séquard syndrome): ipsilateral proprioception loss + contralateral pain/temperature loss.

Descending (Motor) Tracts

• Deliver motor commands from the brain down the spinal cord to skeletal muscle.
• Focus tract for course/exam: Corticospinal (Pyramidal) Tract.

Corticospinal Tract

• FUNCTION – conscious, precise, “fast & fine” voluntary movements (e.g., writing, buttoning shirt).
• CROSSING: majority decussate at the medullary pyramids.
• DAMAGE/LESION rules:
  • Cord hemisection ➜ loss of voluntary movement below the lesion on the SAME (ipsilateral) side (because crossing already occurred in medulla).

Motor Neuron Hierarchy & Types of Paralysis

• Descending systems employ two neurons:
  1. Upper Motor Neuron (UMN) – soma in cortex; axon descends within corticospinal tract.
  2. Lower Motor Neuron (LMN) – soma in ventral horn; axon exits via ventral root to muscle.
• Injury patterns:
  • LMN damage ➜ flaccid paralysis: muscle limp, no reflexes, atrophy (signal never reaches muscle).
  • UMN damage ➜ spastic paralysis: LMNs intact but lose inhibitory cortical input; reflex arc still active → hyper-reflexia & sustained contractions.

Spinal Cord Trauma Classifications

• Transection = complete severing of cord.
  • Region below cut loses motor (paralysis) and/or sensory (paresthesia) functions.
• Paraplegia – transection between T1 and L1; affects lower limbs only.
• Quadriplegia – transection in cervical region; affects all four limbs.
• Partial transections produce mixed patterns per tract-side rules detailed above.

Meninges & CSF Protection

• Three protective connective tissue membranes:
  1. Dura mater – outermost, tough; two layers around brain (periosteal & meningeal), one layer around cord.
  2. Arachnoid mater – middle, web-like; subarachnoid space deep to it holds CSF.
  3. Pia mater – delicate, directly on neural tissue.
• CSF in subarachnoid space acts as liquid cushion; resists compression and provides nutrient/waste exchange.

Clinical Procedures: Epidural & Lumbar Puncture

• Epidural anesthesia: drug injected above dura (into epidural space) to block pain-carrying sensory nerves.
• Lumbar puncture (spinal tap):
  • Needle inserted below L4 (commonly between L4–L5) to avoid spinal cord.
  • Traverses dura & arachnoid to withdraw CSF for lab analysis (infection, bleeding, pressure).

CSF Production & Circulation

• Choroid plexus – capillary-ependymal complexes in each ventricle secrete CSF.
• Circulatory route:
  1. CSF produced in lateral, third, & fourth ventricles.
  2. Flows through ventricles → exits via median & lateral apertures of fourth ventricle → enters subarachnoid space.
  3. Circulates around brain & spinal cord.
  4. Reabsorbed into dural venous sinuses via arachnoid villi.
• Central canal of spinal cord also filled with CSF; continuous with ventricular system.

Vascular Supply of the CNS

• “Millions” of blood vessels deliver O\textsubscript{2} and glucose; disruption quickly compromises neural tissue (ischemia, infarct).
• CSF + blood supply + meninges + skull/vertebrae = multi-layered protection.

Key Numerical & Anatomical References (Quick Recap)

• Spinal cord span: C1! -! L2.
• Paraplegia lesion window: T1! -! L1.
• Safe lumbar puncture level: below L4.
• Ascending pathways → 3 neurons; Descending (corticospinal) → 2 neurons.
• Dorsal column decussation: medulla; Spinothalamic decussation: spinal cord; Corticospinal decussation: medullary pyramids.