Overview
This lecture covers the structure and function of the spinal cord, its role in transmitting sensory and motor information, and the basics of spinal reflexes.
Spinal Cord Structure
- The spinal cord links the brain to spinal nerves, which control the trunk and limbs.
- It is protected by three layers of meninges and the vertebral column.
- The spinal cord extends from the brainstem to the upper lumbar vertebrae.
- In lower regions, nerve roots form the cauda equina as they descend before exiting the canal.
- Cross-sections show central gray matter (neuron cell bodies, dendrites) and surrounding white matter (axon bundles).
Organization and Function of Nervous Tissue
- Gray matter is the site for neuron synapses and information transfer.
- White matter contains spinal tracts—bundles of axons that relay information along the cord.
- Spinal tracts are grouped by function and occur on both sides of the cord.
- Ascending tracts send sensory information up to the brain.
- Descending tracts send motor instructions down from the brain.
- Some tracts cross to the opposite side (contralateral transmission); others stay on the same side (ipsilateral transmission).
Spinal Nerves and Pathways
- There are 31 pairs of mixed spinal nerves, containing both sensory and motor fibers.
- Sensory fibers enter via the dorsal root; motor fibers exit via the ventral root.
- Sensory pathways usually involve three neurons: first-order (from receptor), second-order (in the cord or brainstem), and third-order (to sensory cortex).
- Motor pathways typically involve two neurons: upper motor neuron (from motor cortex/brainstem) and lower motor neuron (to effector).
Somatic Reflexes
- The spinal cord mediates automatic, involuntary skeletal muscle responses known as somatic reflexes.
- Reflex arcs include: somatic receptor, sensory neuron, interneuron, motor neuron, and effector muscle.
- Some reflexes are complex, requiring coordination between both sides of the cord and input from the brain.
Key Terms & Definitions
- Cauda equina — Bundle of nerve roots at the lower end of the spinal cord.
- Gray matter — Central region of the spinal cord containing neuron cell bodies and dendrites.
- White matter — Outer region of the spinal cord made of axon bundles (spinal tracts).
- Spinal tract — Bundles of axons conducting information in one direction along the cord.
- Contralateral — Information crosses to the opposite side of the cord or brain.
- Ipsilateral — Information remains on the same side.
- Dorsal root — Entry point for sensory fibers into the spinal cord.
- Ventral root — Exit point for motor fibers from the spinal cord.
- Somatic reflex — Fast, involuntary muscle response mediated by the spinal cord.
Action Items / Next Steps
- Review diagrams of spinal cord cross-sections to identify gray and white matter.
- Memorize the sequence of neurons in sensory and motor pathways.
- Read about major ascending and descending spinal tracts and their functions.