Overview
This lecture covers the structure, organization, and functions of the gray matter in the spinal cord, focusing on major anatomical regions, Rexed laminae, associated nuclei, and their sensory and motor roles.
Structure & Composition of Gray Matter
- Gray matter consists of unmyelinated neuron parts: cell bodies, dendrites, and some unmyelinated axons.
- Myelin makes neurons appear white; lack of myelin results in gray matter.
- Groups of cell bodies in the CNS are called nuclei; in the PNS, they're called ganglia.
Anatomical Regions of Gray Matter
- Posterior (dorsal) gray horn: mainly sensory function.
- Anterior (ventral) gray horn: mainly somatic motor function.
- Intermediate zone contains the lateral gray horn (present only from T1 to L2) for autonomic motor neurons.
- Gray commissure connects both sides of the spinal cord's gray matter.
Functions of Gray Horns
- Posterior horn processes somatic (from skin, muscles, joints) and visceral (from internal organs) sensory input.
- Intermediate zone (lateral horn T1–L2): autonomic (visceral) motor neurons (sympathetic).
- Anterior horn: somatic motor neurons to skeletal muscle.
Rexed Laminae & Associated Nuclei
- Laminae I–VI: primarily posterior horn; lamina VII: intermediate zone; laminae VIII & IX: anterior horn; lamina X: gray commissure.
- Lamina I: Posterior marginal nucleus (pain & temp, fast pain, A-delta fibers).
- Lamina II: Substantia gelatinosa (pain & temp, slow pain, C fibers, pain modulation, becomes spinal trigeminal nucleus in medulla).
- Laminae III–IV: Nucleus proprius (pain, temp, touch, vibration, proprioception; projects to spinothalamic tract and dorsal columns).
- Lamina V: Processes visceral pain and temperature.
- Lamina VI: Involved in proprioceptive input (ventral spinocerebellar tract) and limb reflexes.
Intermediate Zone (Lamina VII) & Related Nuclei
- Lateral horn (T1–L2): intermediolateral nucleus (preganglionic sympathetic neurons).
- Intermediate zone (S2–S4): intermedial medial nucleus (preganglionic parasympathetic neurons).
- Clarke's column (C8–L2/L3): relay for dorsal spinocerebellar tract (proprioception).
Anterior Horn (Lamina IX) & Nuclear Groups
- Lateral nuclear group: present at cervical (C5–T1) and lumbar (L2–S3) enlargements, supplies distal limb muscles.
- Central nuclear group: contains phrenic (C3–C5, diaphragm), accessory (C1–C5, neck muscles), and Onuf's nucleus (S2–S4, sphincters, sexual function).
- Medial nuclear group: supplies axial (trunk/neck) muscles; posterior portions innervate flexors, anterior extensors.
Lamina VIII & X
- Lamina VIII: relay for subcortical motor tracts (rubrospinal, vestibulospinal, reticulospinal, tectospinal).
- Lamina X: area around the central canal; site for axonal crossover.
Key Terms & Definitions
- Gray Matter — regions of the CNS with unmyelinated neuron parts.
- Nucleus — group of neuronal cell bodies in the CNS.
- Ganglia — group of neuronal cell bodies in the PNS.
- Rexed Laminae — ten histological layers of spinal gray matter.
- Spinothalamic Tract — pathway carrying pain and temperature sensations.
- Clarke’s Column — nucleus for proprioceptive relay to cerebellum.
Action Items / Next Steps
- Review diagrams of spinal cord cross sections.
- Memorize Rexed laminae and their key nuclei/functions.
- Study sensory/motor pathways through the specific horns and nuclei.