Overview
This lecture covers the anatomy and function of the spinal cord and spinal nerves, including key structures, protective meninges, nerve organization, and clinically relevant features.
Spinal Cord Anatomy & Function
- The spinal cord is part of the central nervous system (CNS), along with the brain.
- It processes and integrates information from the body and can mediate reflexes independently of the brain.
- Rapid reflexes, like withdrawing from heat, occur at the spinal cord level without brain input.
- The spinal cord measures about 45 cm and extends from the foramen magnum to L1-L2 vertebrae.
- Cervical and lumbar enlargements occur due to increased neuron numbers for limb control.
- The conus medullaris marks the end of the spinal cord; nerves below form the cauda equina (“horse’s tail”).
- The filum terminale, an extension of pia mater, anchors the spinal cord to the sacrum.
Cross-Sectional Anatomy
- Gray matter (inner H-shape) contains neuron cell bodies; white matter (outer) contains axon tracts.
- Dorsal (posterior) root: sensory (afferent) input; ventral (anterior) root: motor (efferent) output.
- The dorsal root ganglion (DRG) houses sensory neuron cell bodies.
- Dorsal=Afferent, Ventral=Efferent (mnemonic: “SAME DAVE”).
Spinal Meninges & Protection
- Three meninges: pia mater (innermost), arachnoid mater (middle, web-like), dura mater (outermost, tough).
- Denticulate ligaments anchor the spinal cord within vertebral column.
- Spinal tap/epidural is performed below L2, usually between L3-L5, to avoid spinal cord injury.
- Epidural goes into epidural space; spinal tap into subarachnoid space for CSF collection.
Spinal Nerves & Dermatomes
- Thirty-one pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
- Spinal nerves carry both sensory and motor information between CNS and body.
- Peripheral nerve anatomy: axon (covered by endoneurium), fascicle (perineurium), whole nerve (epineurium).
- Dermatomes: body surface regions innervated by specific spinal nerves; used to diagnose nerve damage.
Nerve Plexuses
- Four major plexuses: cervical, brachial, lumbar, and sacral (lumbar and sacral may combine as lumbosacral).
- Brachial plexus (C5-T1) gives rise to key upper limb nerves: musculocutaneous, median (linked to carpal tunnel syndrome), ulnar ("funny bone"), axillary, and radial.
- Lumbosacral plexus (T12-L4): includes genitofemoral, femoral, and lateral femoral cutaneous nerves.
Key Terms & Definitions
- Reflex Arc — A neural pathway that mediates a reflex action via the spinal cord.
- Conus Medullaris — Tapered end of the spinal cord near L1-L2.
- Cauda Equina — Bundle of lumbar and sacral nerves below spinal cord end.
- Filum Terminale — Pia mater extension anchoring spinal cord to sacrum.
- Dorsal Root Ganglion (DRG) — Cluster of sensory neuron cell bodies in the dorsal root.
- Meninges — Three protective membranes covering CNS: pia, arachnoid, dura mater.
- Dermatome — Skin area supplied by a single spinal nerve.
- Plexus — Network of interlacing nerves from spinal nerves.
Action Items / Next Steps
- Review and fill out learning objectives for chapter 14.
- Memorize the three meningeal layers and spinal nerve organization.
- Be able to identify major nerves of the brachial and lumbosacral plexuses.