The gray matter of the cord is butterfly shaped, with the so-called dorsal (posterior) horns forming the upper wings of the butterfly shape. These are linked by a thin gray commissure in which lies the central canal. In the thoracic and upper lumbar segments the gray matter extends on both sides to form lateral horns. The lower wings of the butterfly shape are formed by the ventral (anterior) horns of the gray matter. (The size of the gray matter is greatest at egments that innervate the most skeletal muscle. These are the cervical and lumbosacral, which innervate upper and lower limb muscles, respectively.)
Structurally, the gray matter is composed of neuronal cell nuclei, their processes, neuroglia and blood vessels. The overall arrangement of the gray matter of the cord was systematized by Rexed, who proposed the generally accepted laminar arrangement, commonly referred to as the cytoarchitectonic organization of the spinal cord. The gray matter is divided arbitrarily into nine visually distinct laminae, labeled I through IX, and an area X, which surrounds the central canal. Most laminae are present throughout the cord, but VI, for example, is apparently absent from T4 to L2.
Lamina I is at the apex of the dorsal horn, and contains the posterior marginal nucleus. These cells respond to thermal and other noxious stimuli, and receive axosomatic connections from lamina II. Near the apex, in lamina II, is the substantia gelatinosa, which is found throughout the length of the cord, and which receives touch, temperature and pain afferents, as well as inputs from descending fibers. Both I and II are rich in substance P, considered to be an excitatory neurotransmitter of pain impulses, in opioid receptors and the enkephalin.
Ventral to the substantia gelatinosa, extending through III and IV, is the largest dorsal horn nucleus, the nucleus proprius, which also exists at all cord levels. This receives inputs concerning movement, position, vibration and two-point discrimination from the dorsal white column. The nucleus reticularis is present in the broad lamina V, which is divided into medial and lateral zones, except in thoracic segments. Lamina VI, seen only at cord enlargements, receives group I muscle afferents in its medial zone, and descending spinal terminations in its lateral zone. Lamina VII contains the nucleus dorsalis of Clark (Clark’s column), a group of relatively large multipolar or oval nerve cells that extends from C8 through L3 or L4. Most of the cells respond to stimulation of muscle and tendon spindles. Layer VIII is a zone of heterogeneous cells most prominent from T1 through L2 or L3, associated with autonomic function.
Lamina IX is situated in the anterior or ventral horn of the gray matter, and contains clusters of large, motor nerve cells. The larger cells send out a efferent motoneuron axons, which innervate the extrafusal skeletal muscle fibers, while smaller cells send out g motoneuron axons, which innervate the intrafusal spindle fibers.