Cytoarchitecture of Spinal Cord Gray Matter

Posterior Horn. Many neurons in the six laminae of the posterior horn receive direct synaptic input from spinal afferent fibers that enter the spinal cord via the posterior roots and are thus involved in sensation and in the generation of reflex responses to external or proprioceptive signals (see Plate 2-12).

Lamina I of the posterior horn is a thin layer of large cells, which gives origin to the pathway relaying information about painful stimuli to the thalamus. Laminae II and III comprise the substantia gelatinosa, a tightly packed mass of tiny neurons believed to play a role in regulating afferent input to the spinal cord. Lamina IV is a collection of larger neurons (sometimes referred to as the nucleus proprius of the posterior horn) that projects to three sensory structures: the lateral cervical nucleus, the posterior column nuclei, and the thalamus. Thus the connections of laminae I to IV indicate their importance in sensation.

Laminae V and VI contain neurons of medium-to-large size, many of which receive input from afferent fibers carrying proprioceptive information, as well as other sensory information also relayed by neurons in lamina IV. These neurons probably represent an intermediate stage in the transformation of sensory input to motor output. Laminae V and VI are also the sites of origin of ascending projections to higher centers. In spinal segments T1 to L3, lamina VI contains a group of large cells known as Clarke’s column, which projects to the cerebellum via the posterior spinocerebellar tract.

The anterior horn contains the cell bodies of the motor neurons supplying the somatic muscles. These cell bodies are clustered into two distinct groups, referred to by Rexed as lamina IX and IX_M. Lamina IX_M contains the motor neurons supplying the muscles of the trunk and neck, while lamina IX contains motor neurons supplying the limbs. Lamina IX can be further divided into groups of motor neurons supplying flexor and extensor muscles in the proximal and distal parts of the limbs.

The anterior horn also contains laminae VII and VIII. These regions contain interneurons involved in reflex pathways and motor control, as well as neurons that project to motor regions of the brain. The neurons of lamina VIII are particularly related to lamina IX_M and thus participate in movements of the muscles in the trunk and neck. Conversely, neurons of lamina VII are particularly related to lamina IX and therefore participate in movements of the limb muscles. Laminae VII and IX are both highly developed in the spinal enlargements that control the arms and the legs (see Plate 2-8), whereas only laminae VIII and IX_M are found in the high cervical or thoracic segments that control the neck and trunk.

In the thoracic and sacral segments, the intermediolateral cell column, which is not considered part of either the posterior or the anterior horn, contains the neurons of origin of preganglionic autonomic fibers. Lamina X —the small area of gray matter around the central canal—contains neurons that project to the opposite side of the spinal cord, including those in the anterior and posterior commissural nuclei.

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