Overview
This lecture explains the structure and function of a typical spinal nerve, its components, and the organization of the spinal cord, emphasizing the thoracic nerves as representative examples.
Spinal Nerve Organization
- The spinal cord gives rise to 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
- Lumbar and cervical enlargements occur where nerves serve the limbs.
- The thoracic spinal nerves are considered "typical" because they have all common components found in spinal nerves.
Spinal Cord Structure
- The spinal cord has enlargements (cervical and lumbar) and narrows at the conus medullaris.
- The filum terminale is a connective tissue anchoring the spinal cord to the sacrum.
- The cauda equina ("horse's tail") is a bundle of sacral and coccygeal nerve roots.
Typical Spinal Nerve Anatomy
- Each spinal segment has a ventral root (motor fibers) and a dorsal root (sensory fibers with a dorsal root ganglion).
- The gray matter of the spinal cord is centrally located (butterfly shape), with white matter on the outside.
- A mixed spinal nerve forms where dorsal and ventral roots join.
- The spinal nerve splits into a dorsal ramus (serves back muscles and skin) and a ventral ramus (serves sides and front).
Special Features of Thoracic Nerves
- Only thoracic nerves (T1–L2) have a lateral horn, which is part of the autonomic nervous system.
- The sympathetic chain runs parallel to the spinal cord; nerves enter via the white ramus communicans and exit via the gray ramus communicans.
Sensory and Motor Pathways
- Sensory neurons enter the spinal cord via the dorsal root and synapse in the dorsal horn.
- Motor impulses originate in the ventral horn, exit via the ventral root, and stimulate muscles.
- The spinal nerve is "mixed," containing both sensory and motor fibers.
Clinical Relevance
- Damage to the ventral root results in loss of motor function but preserved sensation.
- Damage to the dorsal root results in loss of sensation but preserved motor function.
- Understanding symptoms helps localize spinal injuries.
Key Terms & Definitions
- Spinal nerve — a mixed nerve carrying sensory and motor fibers from the spinal cord.
- Dorsal root — carries sensory fibers to the spinal cord.
- Ventral root — carries motor fibers from the spinal cord.
- Dorsal root ganglion — cluster of sensory neuron cell bodies on the dorsal root.
- Ramus (rami) — branches off the spinal nerve (dorsal and ventral), serving different body regions.
- Lateral horn — part of gray matter in T1–L2 involved in autonomic (sympathetic) output.
- Sympathetic chain — chain of ganglia for autonomic nervous system communication.
- Conus medullaris — tapered end of the spinal cord.
- Filum terminale — connective tissue anchoring the spinal cord.
- Cauda equina — bundle of lumbar, sacral, and coccygeal nerve roots.
Action Items / Next Steps
- Review and memorize the terms and structures of a typical spinal nerve for the lecture exam and final.
- Watch the provided video resources on spinal nerve anatomy under helpful resources.