Overview
This lecture introduces the structure and function of the spinal cord within the peripheral nervous system, focusing on anatomy, pathways, and reflexes.
Spinal Cord Structure & Regions
- The spinal cord is about 18 inches long and 0.5 inches wide.
- Major regions: cervical, thoracic, lumbar, and sacral; named same as vertebrae.
- Enlargements (C4-T1, T9-T12, L2-S5) correspond to areas with increased neural processing.
- The conus medullaris is found around the first or second lumbar vertebra.
- The terminal filum, an extension of the pia mater, anchors the cord to the coccyx.
- The cauda equina ("horse's tail") consists of spinal nerves extending below the spinal cord.
Meninges and Surrounding Structures
- Spinal cord is covered by three meninges: dura mater (outer), arachnoid (middle), pia mater (inner).
- The epidural and subdural spaces contain adipose tissue and blood vessels.
- Denticulate ligaments (extensions of pia mater) suspend and stabilize the cord within cerebrospinal fluid (CSF).
Internal Anatomy and Roots
- Cross-section: central gray matter (cell bodies/dendrites), outer white matter (myelinated axons).
- Horns: dorsal (sensory), ventral (motor), lateral (present in thoracic/lumbar for autonomic function).
- Dorsal root ganglion contains sensory neuron cell bodies near the cord.
- Ventral roots carry motor (efferent) information out to muscles and glands.
- Dorsal roots carry sensory (afferent) information into the spinal cord.
- Commissures (gray/white) are crossing points at the center of the cord.
Spinal Nerves and Pathways
- Each spinal segment has a pair of spinal nerves (mixed sensory and motor).
- Integration of signals occurs within the cord or can relay to the brain via ascending/descending tracts.
Reflex Arcs and Types
- Reflexes can occur entirely within the spinal cord, bypassing the brain for faster responses.
- Monosynaptic reflex: sensory neuron directly synapses with a motor neuron (e.g., stretch reflex).
- Polysynaptic reflex: involves one or more interneurons between sensory and motor neurons.
- Key reflexes: muscle stretch reflex (protects muscle from overstretch), flexor withdrawal reflex, crossed extensor reflex, Golgi tendon reflex (protects tendons from excessive force).
Motor Neuron Organization
- Alpha (lower) motor neurons control voluntary skeletal muscle movement; cell bodies are in ventral horns.
- A motor unit: a single motor neuron and all muscle fibers it innervates.
- Flexors decrease joint angles; extensors increase them.
- Gamma motor neurons (intrafusal fibers) regulate muscle spindle sensitivity and muscle tone.
- Golgi tendon organs monitor tension and prevent tendon injury via inhibition of excessive contraction.
Key Terms & Definitions
- Spinal Nerve — Mixed nerve carrying sensory and motor fibers between spinal cord and periphery.
- Meninges — Protective membranes (dura, arachnoid, pia mater) covering the CNS.
- Dorsal Root Ganglion — Cluster of sensory neuron cell bodies just outside the spinal cord.
- Reflex Arc — Neural pathway controlling a reflex action.
- Motor Unit — A motor neuron and all the muscle fibers it controls.
- Muscle Spindle — Sensory receptor within muscle detecting stretch.
- Golgi Tendon Organ — Receptor in tendons that senses muscle tension.
Action Items / Next Steps
- Review diagrams of spinal cord anatomy and reflex arcs.
- Prepare questions on spinal cord pathways and reflex mechanisms for next class.
- Read the next textbook section on peripheral nerve pathways.