Spinal Cord Organization Overview

Overview

Lecture covers gross anatomy of the spinal cord and spinal nerves, their segments, landmarks, pathways, and basic functions within the CNS and PNS.

Nervous System Organization

• Central nervous system includes brain and spinal cord; peripheral nervous system includes spinal nerves.
• PNS has sensory and motor components; both somatic and autonomic contribute to spinal nerves.
• Spinal cord and spinal nerves are separate systems but tightly integrated.

Spinal Cord Extent and Segments

• Begins at level just below foramen magnum near C1 vertebra.
• Ends around L1–L2 vertebral levels as a tapered point.
• Segments named like vertebrae: cervical, thoracic, lumbar, sacral, coccygeal.
• Terminal tip called conus medullaris; below it, bundled roots form cauda equina (L2 to Co1).

Spinal Nerves Count and Exit Patterns

• Cervical nerves: 8 pairs; C1–C7 exit above corresponding vertebra; C8 exits above T1.
• Thoracic and below: nerves exit below corresponding vertebra.
• Total pairs: 31 (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal).
• Sacrum is one fused bone from five embryologic segments; five sacral nerve pairs persist.

Spinal Enlargements

• Cervical enlargement: C5–T1; increased ventral gray matter for upper limb muscles.
• Lumbar enlargement: L2–S3; increased ventral gray matter for lower limb muscles.

Cross-Section: White vs Gray Matter

• White matter: myelinated axons; carries ascending sensory and descending motor information.
• Gray matter: neuronal cell bodies and dendrites; forms posterior, lateral, anterior horns.
• Trend: white matter decreases from cervical to coccygeal; gray matter increases inferiorly.

Neuron Structure and Myelination

• Cell body and dendrites: unmyelinated; contribute to gray matter.
• Axon: often myelinated; myelin appears white, forms white matter.
• CNS myelin by oligodendrocytes; PNS myelin by Schwann cells.

Spinal Cord Landmarks and Columns

• Posterior median sulcus marks dorsal midline.
• Anterior median fissure marks ventral midline.
• White columns (funiculi): dorsal (posterior), lateral, ventral; contain tracts.
• Lateral gray horn present only T1–L2; holds sympathetic preganglionic motor neurons.

Functional Organization of Horns and Columns

• Posterior gray horn: sensory neurons and processing.
• Anterior (ventral) gray horn: somatic motor neurons to skeletal muscle.
• Lateral gray horn (T1–L2): sympathetic preganglionic motor neurons.
• White columns: ascending sensory tracts and descending motor tracts.

CNS vs PNS Groupings

• CNS: group of cell bodies called nucleus; bundle of axons called tract.
• PNS: group of cell bodies called ganglion; mixed nerves contain sensory and motor fibers.

Spinal Nerve Anatomy

• Dorsal root: sensory fibers entering posterior horn; contains dorsal root ganglion.
• Ventral root: motor fibers exiting anterior horn.
• Roots merge to form a mixed spinal nerve.
• Spinal nerve divides into dorsal rami (back/neck) and ventral rami (anterior-lateral trunk and limbs).
• Ventral rami connect via white and gray rami communicantes to sympathetic ganglia.

Sensory-Motor Pathway (Basic Flow)

• Skin receptors detect injury; sensory signals travel via dorsal or ventral rami.
• Signals pass through spinal nerve, dorsal root, dorsal root ganglion.
• Central process enters posterior horn, synapses; interneurons activate motor neurons.
• Motor neurons in anterior horn send axons through ventral root to spinal nerve.
• Motor output distributed via dorsal rami (back/neck muscles) or ventral rami (trunk/limb muscles).

Spinal Segments and Nerves Summary

Key Terms & Definitions

• Central nervous system: brain and spinal cord.
• Peripheral nervous system: nerves outside CNS; includes spinal nerves.
• Conus medullaris: tapered end of spinal cord at L1–L2.
• Cauda equina: bundle of spinal roots from L2 to Co1, resembling a horse tail.
• Funiculus: large white matter column (dorsal, lateral, ventral).
• Tract: bundle of CNS axons within white matter.
• Nucleus: group of CNS neuron cell bodies within gray matter.
• Ganglion: group of PNS neuron cell bodies (e.g., dorsal root ganglion).
• Rami communicantes: connections between ventral rami and sympathetic ganglia.

Action Items / Next Steps

• Memorize 31 spinal nerve pairs and exit rules (especially C8 exception).
• Associate horn functions: posterior sensory; anterior somatic motor; lateral sympathetic (T1–L2).
• Understand enlargements and limb innervation correlations.
• Review pathways of dorsal and ventral roots, rami, and mixed spinal nerves.