Gray Matter

The gray matter of the spinal cord is composed of neuron cell bodies, their dendrites and the initial part of the axon, the axon terminals of fibers synapsing in this area, and glial cells. Because this area has few myelinated fibers, it appears distinctly light and has a characteristic shape in myelin-stained sections.

The spinal gray matter is divided into a posterior (dorsal) horn, an anterior (ventral) horn, and the region where these meet, commonly called the intermediate zone (or intermediate gray). On the basis of the shape, size, and distribution of neurons located in these areas, the gray matter is divided into laminae (Rexed laminae) I to IX and an area X around the central canal. These laminae are also characterized by the input they receive and the trajectory of axons originating from each lamina.

The posterior horn is composed of laminae I to VI. The most distinct structure in the posterior horn, the substantia gelatinosa (lamina II), is capped by cells of the posteromarginal nucleus (lamina I). Laminae III to VI are located sequentially below the substantia gelatinosa. Laminae III and IV may also be called the nucleus proprius (posterior or dorsal proper sensory nucleus); their cells have elaborate dendrites that extend into lamina II. Laminae V and VI, which form the base of the posterior horn, are usually divided into medial and lateral portions.

The intermediate zone, lamina VII, extends from the area of the central canal to the lateral edge of the spinal gray and varies in shape at different levels of the cord. Particularly characteristic of lamina VII at thoracic levels are the posterior thoracic nucleus (dorsal nucleus of Clarke) and the lateral horn, which contains the intermediolateral nucleus, frequently called the intermediolateral cell column.

The anterior horn is made up of laminae VIII and IX. Lamina VIII contains a population of smaller cells that are interneurons and tract cells. Lamina IX consists of several distinct clusters of large motor neurons whose axons directly innervate skeletal muscle.