Neuro Review

Overview

• Lecture examines the peripheral nervous system arising from the spinal cord.
• Emphasizes roots, spinal nerves, meningeal coverings, root ganglia, and segmental organization.
• Focus on anatomy relevant to motor and sensory pathways and functional implications.

Spinal Cord Location And Coverings

• Spinal cord lies within vertebral canal; cord is shorter than vertebral column.
• Cord typically ends at L1–L2 vertebral level (conus medullaris at tip).
• Spinal meninges (outer to inner): dura mater, arachnoid mater, pia mater.
• Epidural space contains fat and a venous plexus supporting the cord.
• Filum terminale (fine thread) anchors conus medullaris to sacrum.

Internal Spinal Cord Organization

• Gray matter is central (nerve cell bodies); white matter is peripheral (myelinated tracts).
• Gray matter has dorsal (posterior) horns and ventral (anterior) horns.
  • Posterior horn: sensory neurons and interneurons.
  • Anterior horn: motor neurons.
• Cervical and lumbar enlargements correspond to limb innervation density.

Spinal Nerve Roots And Ganglia

• Spinal cord gives off multiple anterior (ventral) and posterior (dorsal) rootlets.
• Anterior roots: motor (efferent) fibers from anterior horn neurons.
• Posterior roots: sensory (afferent) fibers entering posterior horn.
• Dorsal (posterior) root ganglion contains sensory neuron cell bodies.
  • No synapse in dorsal root ganglion; axon continues into cord.

Spinal Nerve Structure And Branches

• Anterior and posterior roots join to form a mixed spinal nerve.
• Spinal nerve divides into:
  • Posterior ramus: supplies intrinsic back muscles and overlying skin.
  • Anterior ramus: supplies the anterolateral trunk, limbs, and often forms plexuses.
• Posterior rami remain segmental; anterior rami often form plexuses (e.g., cervical, brachial, lumbosacral).

Functional Pathways And Reflexes

• Motor pathway: motor neuron in anterior horn → anterior root → spinal nerve → anterior/posterior ramus to target muscle.
• Sensory pathway: peripheral receptor → peripheral axon → dorsal/posterior ramus or anterior ramus → spinal nerve → dorsal root ganglion → posterior root → posterior horn.
• In posterior horn: sensory fibers can synapse with secondary sensory neurons, ascend, cross, or activate interneurons.
• Interneurons can synapse with anterior horn motor neurons producing reflexes (e.g., withdrawal reflex).

Dermatomes And Segmental Organization

• Each spinal nerve supplies a strip of skin (dermatome) with sensory innervation.
• Dermatomes follow the segmental layout down the trunk and into limbs.
• Segmental arrangement remains clear on the back (posterior rami distribution).

Key Terms And Definitions

• Posterior (Dorsal) Root: sensory root entering the spinal cord.
• Anterior (Ventral) Root: motor root leaving the spinal cord.
• Dorsal Root Ganglion: cluster of sensory neuron cell bodies on posterior root.
• Spinal Nerve: mixed nerve formed by union of anterior and posterior roots.
• Posterior Ramus: branch supplying intrinsic back muscles and skin.
• Anterior Ramus: branch supplying trunk, limbs; often forms plexuses.
• Conus Medullaris: tapered end of spinal cord at L1–L2 level.
• Filum Terminale: connective tissue anchor from conus medullaris to sacrum.
• Cauda Equina: bundle of elongated nerve roots below conus medullaris.
• Epidural Space: space external to dura mater containing fat and venous plexus.
• Dermatome: skin area innervated by sensory fibers of a single spinal nerve.

Structured Summary Table

Action Items / Next Steps (if applicable)

• Review diagrams showing roots, ganglia, and rami to reinforce spatial relationships.
• Memorize dermatomal maps and major plexus formation from anterior rami.
• Practice tracing simple reflex arcs showing sensory → interneuron → motor pathways.