Macrostructure and Horn Shape
Gray matter occupies an internal position relative to white matter. Its structure is represented by paired projections called horns, connected by the intermediate zone. Running through the very center of the spinal cord is the central canal, lined by ependymal cells.
- Posterior horns (cornu posterius): Visually relatively narrow and long, extending posterolaterally. They contain association neurons that receive signals from sensory cells of the dorsal root ganglia.
- Anterior horns (cornu anterius): Broader and shorter, directed anteriorly and slightly medially. The largest cells—motor neurons—are localized here.
- Lateral horns (cornu laterale): Small projections emerging from the intermediate zone. They are not present throughout the entire spinal cord, but only at the thoracic and upper lumbar segments ($T_1 – L_3$), where they contain sympathetic nervous system centers.
Histological Composition and Rexed Laminae
The main morphological difference between gray and white matter is the presence of neuronal cell bodies. All neurons here are multipolar in structure and form clusters called nuclei.
The glial framework is represented mainly by astrocytes (both protoplasmic and fibrous). Their functions are critical:
- Formation of the blood-brain barrier (astroglial end-feet form glial limiting membranes on capillary surfaces).
- Formation of septa (fibrous astrocyte processes penetrate white matter and, together with connective tissue, form its structural framework).
In addition to the classical division into nuclei, there is a cytoarchitectonic classification. Gray matter is divided into 10 vertical Rexed laminae (laminae) based on cell density and morphology:
- Laminae I–V — correspond to the posterior horns.
- Laminae VI–VII — occupy the intermediate zone.
- Laminae VIII–IX — correspond to the anterior horns.
- Lamina X — the area surrounding the central canal.
Posterior Horn Nuclei and Ascending Pathways
Posterior horns participate in closing somatic reflex arcs. Neurons here are divided into groups:
Inhibitory System (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, particularly pain.
Spinocerebellar Pathway Neurons (Proprioception)
- Proper sensory nucleus of posterior horn (nucleus proprius): Located in the center of the posterior horn. Axons of these cells mediate pathways of conscious sensation (pain, temperature, touch). Ascending 2–3 segments, they decussate, enter the lateral funiculus, and travel 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 mediate unconscious coordination, ascending to the cerebellum.
Intermediate Zone and Lateral Horns
The intermediate zone and lateral horns house key autonomic and association centers:
- Medial intermediate nucleus (in the intermediate zone): Receives signals from visceral receptors. Its neurons project axons to lateral horn cells and to the brain. Notable feature: in the sacral region, this area houses the central parasympathetic nervous system neurons.
- Lateral intermediate nucleus (in the lateral horns): Represents the central component of the sympathetic nervous system. Axons of these neurons leave the spinal cord via anterior roots as white communicating branches (rami communicantes albi) and travel to sympathetic ganglia.