Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Spinal cord
Gray matter

Gray matter occupies a central position and, in cross-section, resembles a butterfly or the letter H. It is divided into anterior (ventral) and posterior (dorsal) horns, with lateral horns also present in the thoracic and lumbar regions. White matter is located at the periphery (Atl. Fig. 97).

In the central part of the gray matter lies a narrow cavity—a remnant of the neural tube cavity—called the central canal of the Spinal Cord, which contains CEREBROSPINAL FLUID. Superiorly, the canal communicates with the Fourth ventricle of the Brain, and inferiorly, it opens into the terminal ventricle, an expanded portion of the cavity in the region of the conus medullaris. With age, after 40 years, the diameter of the central canal decreases, and it becomes almost completely obliterated.

The central canal is surrounded by the substantia gelatinosa centralis (central gelatinous substance), consisting mainly of neuroglia and a small number of Neurons. Surrounding the substantia gelatinosa centralis is the central intermediate gray matter, which is bordered anteriorly by the white commissure.

The apex of the posterior horn of the gray matter is surrounded by the substantia spongiosa (spongy zone), beneath which lies the substantia gelatinosa. The latter is formed by small neurons with branching processes that interconnect adjacent segments of the spinal cord and project to the motor neurons of the anterior horns of the same segment. Some axons cross to the contralateral substantia gelatinosa.

Neurons of the spinal cord gray matter form clusters (nuclei) that have a consistent localization (Atl. Fig. 98) and a spindle-like shape, typically spanning several segments. Nerve fibers enter and exit each Nucleus through several spinal roots. Large nuclei formed by motor neurons lie in the anterior horn, and their axons exit via the anterior roots. The nuclei of the posterior horn and intermediate substance are primarily composed of interneurons. In the lateral horns of the thoracic and sacral regions lie the nuclei of the Autonomic Nervous system (lateral intermediate substance). At the Base of the posterior horn is a sizable dorsal nucleus, whose neurons send their axons to the adjacent 2—3 segments of the spinal cord and also form the posterior spinocerebellar tract. Scattered diffusely between the nuclei are individual neurons whose processes project to Cells of the ipsilateral (association bundles) or contralateral (commissural bundles) half of the spinal cord. Axons of neurons projecting to higher Regions of the CNS constitute projection fibers.

The gray matter of the spinal cord is also conventionally divided into layers (laminae) parallel to the dorsal surface of the cord (Atl. Fig. 98). Laminae I—IV represent the primary sensory area. Most afferent fibers from the trunk and limbs terminate here. Their axons form ascending tracts that project to the brain. For instance, lamina I neurons receive nociceptive (pain) information, and their axons enter the spinothalamic tract of the ipsilateral and contralateral sides (via the anterior white commissure) (see Atl.). Neurons of the substantia gelatinosa form laminae II and III. Axons of neurons in laminae IV and VI also travel in the spinothalamic tract but transmit impulses of other modalities (excluding nociceptive). Axons of Nerve Cells located in laminae V—VI form spinocerebellar tracts and project to the reticular formation (see below). Lamina VII receives visceral fibers and those from The Musculoskeletal System, as well as fibers connecting the spinal cord with the Midbrain and Cerebellum. Lamina IX is the primary motor area. Its medial part contains neurons innervating the Muscles of the Trunk, while its lateral part contains those innervating the limbs.

Between the anterior and posterior horns, strands of gray matter penetrate into the white matter, forming a network-like Structure—the reticular formation.