Lateral intermediate substance

The lateral intermediate substance is a component of the intermediate zone of spinal cord, consisting of gray matter situated between the posterior and anterior horns. This region corresponds to the base of the posterior horn, a.k.a. lamina VII. In the thoracic and upper lumbar segments (T1 to L2), the lateral intermediate substance extends outward, forming the lateral horn of the spinal cord.

Within the lateral intermediate substance (lamina VII), several key nuclei play important roles. These include the posterior thoracic nucleus (a.k.a. the Dorsal nucleus of Clarke or Clarke’s column), the visceral afferent nucleus, and the intermediolateral nucleus. Most of these nuclei are confined to the thoracic and upper lumbar spinal segments and do not span the entire length of the spinal cord.

The posterior thoracic nucleus or the Dorsal nucleus of Clarke extends from C8 to L3/L4 spinal segments. It processes subconscious proprioceptive input from muscle spindles and Golgi tendon organs. Neurons in the Clarke’s column give rise to the dorsal and ventral spinocerebellar tracts, which relay this proprioceptive information to the cerebellum.

The visceral afferent nucleus, located just lateral to the Clarke’s column, is thought to be involved in processing sensory input from visceral structures.

An additional group of neuronal cell bodies, called the intermediolateral nucleus is found in the lateral horn - a region formed by the protrusion of the lateral intermediate substance in the T1 to L2 spinal segments. The intermediolateral nucleus gives rise to preganglionic sympathetic neurons, whose axons exit the spinal cord to synapse in the ganglia of the sympathetic trunk. These neurons either synapse immediately or ascend and descend within the trunk to supply autonomic innervation to cervical and pelvic organs.

In the sacral region (S2–S4), a similar group of neurons - the sacral parasympathetic nucleus - is found within the lateral intermediate substance, despite the absence of a prominent lateral horn. These neurons give rise to preganglionic parasympathetic fibers, which provide autonomic innervation to the pelvic organs.