Spinal cord gray matter

Macrostructure and horn shape

Gray matter is positioned internally relative to the white matter. Its structure is represented by paired projections called horns, between which lies the intermediate zone. The central canal, lined by ependymocytes, runs through the very center of the spinal cord.

• Posterior horns (cornu posterius): visually relatively narrow and long, diverging outward toward the periphery. They contain association neurons that receive signals from sensory cells of the spinal ganglia.
• Anterior horns (cornu anterius): wider and shorter, oriented anteriorly and slightly inward. The largest cells—motor neurons—are localized here.
• Lateral horns (cornu laterale): small projections from the intermediate zone. They are not present throughout the entire spinal cord, but only at the level of the thoracic and upper lumbar segments ($T_1 – L_3$), where they contain centers of the sympathetic nervous system.

Histological composition and Rexed laminae

The main morphological difference between gray and white matter is the presence of neuron cell bodies. All neurons here are multipolar in structure and form clusters called nuclei.

The glial apparatus is represented mainly by astrocytes (both protoplasmic and fibrous). Their functions are critically important:

1. Formation of the blood-brain barrier (astrocyte processes form glial limiting membranes on the surface of capillaries).
2. Formation of septa (processes of fibrous astrocytes penetrate into the white matter and, together with connective tissue, create its framework).

In addition to the classical division into nuclei, there is a cytoarchitectonic classification. Gray matter is subdivided into 10 vertical Rexed laminae (laminae) based on the density and arrangement of neurons:

• Laminae I–V correspond to the region of the posterior horns.
• Laminae VI–VII occupy the intermediate zone.
• Laminae VIII–IX correspond to the anterior horns.
• Lamina X is the zone surrounding the central canal.

Posterior horn nuclei and projection pathways

Posterior horns participate in closing somatic reflex arcs. Neurons here are divided into groups:

Inhibitory system (Substantia gelatinosa / substantia gelatinosa) Located at the apex of the posterior horns. Small cells form inhibitory synapses on other neurons, providing "gate control" of sensory information. By releasing GABA and opioid neuropeptides (enkephalins/endorphins), they dampen excessive impulse flow, especially pain signals.

Spinocerebellar tract neurons (proprioception)

• Proper nucleus of the posterior horn (nucleus proprius): located in the center of the posterior horn. The axons of these cells form the pathways of conscious sensation (pain, temperature, touch). Having ascended 2–3 segments, they cross over, enter the lateral funiculus, and proceed to the thalamus as the spinothalamic tract.
• Thoracic nucleus (nucleus thoracicus / Clarke's column): lies at the base of the posterior horn. Its axons enter the lateral funiculus of the same side (uncrossed) and are responsible for unconscious coordination, ascending to the cerebellum.

Intermediate zone and lateral horns

The intermediate zone and lateral horns contain essential autonomic and association centers:

• Medial intermediate nucleus (in the intermediate zone): receives signals from visceral receptors. Its neurons send axons to the cells of the lateral horns and to the brain. Feature: in the sacral region, central parasympathetic nervous system neurons are located here.
• Lateral intermediate nucleus (in the lateral horns): this is the central division of the sympathetic nervous system. The axons of these neurons leave the spinal cord via the anterior roots as white communicating branches (rami communicantes albi) and head toward the sympathetic ganglia.

Class and exam questions

What else to study on this topic

• Small diffuse interneurons of local reflex arcs (group A)
• Commissural neurons and formation of the anterior white commissure
• Spinocortical and spinothalamic tracts (pathways of conscious sensation)
• Blood-brain barrier and the role of astrocyte processes

Other topics in "Nervous system: nerves, ganglia, spinal cord"

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