Sensory Pathways in the Dorsal Columns

Abstract

The best-studied sensory pathway of the spinal cord is the dorsal column pathway. The part of this pathway that is actually within the cord consists chiefly of the ascending branches of primary afferent fibers entering through the dorsal roots. The dorsal column is subdivided into two components known as the fasciculus gracilis and the fasciculus cuneatus. The fasciculus gracilis includes the ascending branches of afferents from the midthoracic region and caudally, while the fasciculus cuneatus consists of the branches of afferents from midthoracic to upper cervical levels. The fasciculi gracilis and cuneatus terminate in nuclei of the caudal medulla of the same names, nucleus gracilis and nucleus cuneatus. Collectively, these nuclei are often called the dorsal column nuclei (along with the lateral cuneate nucleus). The nuclei gracilis and cuneatus project (among other places) to the contralateral thalamus via the medial lemniscus. For this reason, the dorsal column pathway is sometimes referred to as the dorsal column—medial lemniscus pathway (Fig. 6.1). Although the dorsal column nuclei are sometimes called “relay” nuclei, this term should not be taken to imply a simple organization or function (G. Gordon, 1973). The dorsal columns also contain axons ascending from tract cells in the spinal cord gray matter (second-order dorsal column pathway), and other second-order neurons project to the dorsal column nuclei via the dorsolateral fasciculus.