Overview
This lecture covers the external and basic internal anatomy of the spinal cord, including its structure, regions, nerve connections, and key anatomical features.
External Structure of the Spinal Cord
- The spinal cord is continuous with the medulla oblongata at its upper end.
- In adults, it extends from the brainstem to between vertebral levels L1 and L2.
- The spinal cord has 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
- Each segment gives rise to a pair of spinal nerves, totaling 31 pairs.
- The spinal cord is mostly cylindrical but has two enlargements: cervical (C4–T1) and lumbar (L1–S3).
Key Anatomical Features
- The cervical and lumbar enlargements correspond to regions where nerve plexuses for upper and lower limbs originate.
- The distal end tapers into the conus medullaris at the L1–L2 level.
- A connective tissue strand, the filum terminale, extends from the conus medullaris to the coccyx.
- The lumbar and sacral nerves hang below the conus medullaris, forming the cauda equina ("horse's tail").
Spinal Nerves and Connections
- Spinal nerves exit the vertebral column through intervertebral foramina.
- Each spinal nerve forms from the convergence of ventral and dorsal rootlets into ventral and dorsal roots.
- The roots merge to form the spinal nerve, which then splits into a thicker ventral ramus (supplies front body) and a thinner dorsal ramus (supplies back body).
- Dorsal root ganglia contain cell bodies of sensory (afferent) neurons; ventral root cell bodies are in the spinal cord's gray matter.
Mnemonics & Functional Organization
- Mnemonic: "Afferent Arrives, Efferent Exits"—afferent brings sensory information in, efferent carries motor instructions out.
- Mnemonic: "SAME DAVE"—Sensory Afferent, Motor Efferent; Dorsal Afferent, Ventral Efferent.
Key Terms & Definitions
- Conus Medullaris — tapered end of the spinal cord at L1–L2.
- Filum Terminale — connective tissue anchoring the conus medullaris to the coccyx.
- Cauda Equina — bundle of spinal nerves below the conus medullaris.
- Dorsal Root Ganglion — a cluster of sensory neuron cell bodies in the dorsal root.
- Ventral Ramus/Dorsal Ramus — branches of spinal nerves supplying front/back of the body, respectively.
Action Items / Next Steps
- Review the next part of the tutorial focusing on internal spinal cord structure and the meninges.