Overview
This lecture covers the anatomy and physiology of a spinal cord cross-section, focusing on signal pathways, reflex arcs, and the organization of white and gray matter.
Spinal Cord Cross-Section Anatomy
- The spinal cord is divided into 31 pairs of spinal nerves, each branching into a dorsal root and a ventral root.
- The dorsal (posterior) side is also called dorsal, and the anterior (front) side is called ventral.
- The dorsal root carries sensory signals into the spinal cord; the ventral root carries motor signals out.
- White matter surrounds the gray matter and consists of myelinated axons for fast signal transmission.
- Gray matter forms the central region and contains unmyelinated neurons for processing signals.
- In the spinal cord, gray matter is central, while white matter is peripheral (the reverse of the brain).
Gray and White Matter Regions
- Gray matter is divided into dorsal horn, lateral horn, and ventral horn.
- The ventral median fissure (anterior) and dorsal median sulcus (posterior) are surface indentations.
- White matter is divided into dorsal, lateral, and ventral funiculi (funiculus = neuron bundle).
Central Canal & Cerebrospinal Fluid
- Central canal runs through the gray matter's center and contains cerebrospinal fluid, connecting to the brain's ventricles and distributing nutrients.
Signal Pathways and Reflex Arcs
- Sensory neurons bring information in via the dorsal root; their cell bodies cluster in the dorsal root ganglion.
- Interneurons in gray matter connect sensory neurons to motor neurons or relay signals upward.
- Motor neurons exit via the ventral root to activate muscles.
- Reflex arcs can be two- or three-neuron loops, enabling fast, automatic responses (e.g., pulling back from a hot surface).
- Interneurons can also send sensory signals up ascending tracts to the brain.
- Ascending tracts carry sensory signals up to the brain (mainly in dorsal and lateral funiculi).
- Descending tracts carry motor commands down from the brain to the body (mainly in lateral and ventral funiculi).
Key Terms & Definitions
- Dorsal/Ventral — posterior (back)/anterior (front) orientation of the spinal cord.
- White matter — nervous tissue with myelinated axons enabling rapid signal transmission.
- Gray matter — nervous tissue with unmyelinated neurons, mainly for processing.
- Horn — protruding regions of gray matter (dorsal, lateral, ventral).
- Funiculus — bundle of axons in white matter (dorsal, lateral, ventral).
- Dorsal root ganglion — cluster of sensory neuron cell bodies.
- Central canal — channel with cerebrospinal fluid for nutrient transport.
- Reflex arc — neural circuit enabling quick, automatic responses.
- Ascending/Descending tract — pathways carrying signals to/from the brain.
Action Items / Next Steps
- Practice labeling a spinal cord cross-section and explaining the functions of its anatomical regions.
- Review the different types of neurons and their pathways in a reflex arc.