Intermediate zone of spinal cord

The intermediate zone of spinal cord refers to the region of gray matter located between the posterior (dorsal) and anterior (ventral) horns of the spinal cord. It consists of two key components: the central intermediate substance (Rexed lamina X) and the lateral intermediate substance (Rexed lamina VII).

The central intermediate substance is the gray matter surrounding the central canal. This region is further divided into the dorsal and ventral commissures, located behind and in front of the central canal, respectively.

Laterally, the central intermediate substance transitions into the lateral intermediate substance, situated between the posterior and anterior horns of the spinal gray matter. This area corresponds anatomically to the base of posterior horn of spinal cord (lamina VII). In the thoracic and upper lumbar region (spinal segments T1 to L2), the lateral intermediate substance extends outward, forming the characteristic lateral horn of the spinal cord.

The lateral intermediate substance (lamina VII) contains several important nuclei, including the posterior thoracic nucleus (also known as the dorsal nucleus of Clarke or Clarke’s column), the visceral afferent nucleus, and the intermediolateral nucleus. These nuclei are primarily concentrated in the thoracic and upper lumbar regions and do not span the entire length of the spinal cord.

• The posterior thoracic nucleus (dorsal nucleus of Clarke, Clarke’s column) is located from spinal segments C8 to L3/L4. It processes subconscious proprioceptive input from muscle spindles and Golgi tendon organs. Axons originating from this nucleus form the dorsal and ventral spinocerebellar tracts, conveying proprioceptive information to the cerebellum.

• The visceral afferent nucleus lies just lateral to Clarke’s column. It is thought to receive afferent sensory input from visceral organs.

• The intermediolateral nucleus is found in the lateral horn of the spinal cord, present in spinal segments T1 to L2. This nucleus consists of preganglionic sympathetic neurons, whose axons project to the ganglia of the sympathetic trunk.

In the sacral spinal segments (S2 to S4), similar cell groups are found within the intermediate zone, giving rise to preganglionic parasympathetic autonomic motor neurons. Unlike the thoracic and upper lumbar regions, these nuclei do not form a distinct lateral horn in the sacral cord.