Principal Fiber Tracts of Spinal Cord

The schematic cross section in the lower part of the illustration shows the location of the principal fiber tracts within the spinal white matter. As indicated by the colors, the tracts can be divided into ascending (blue) and descending (red) pathways linking the spinal cord with the brain, and propriospinal (purple) pathways made up of fibers interconnecting different levels within the spinal cord itself.

The ascending pathways include the fasciculus gracilis and fasciculus cuneatus (part of the medial lemniscus system), which convey fine discriminative sensation from the lower and upper parts of the body, respectively. Less discriminative, higher-threshold sensations are carried by the anterior and lateral spinothalamic tracts; the latter is particularly important in conveying the sensations of pain and temperature. Other ascending pathways, which are more closely involved in reflex activity and motor control, include the posterior and anterior spinocerebellar tracts and the spino-olivary, spinotectal, and spinoreticular tracts.

The descending pathways are divided into two groups. The first group includes the corticospinal tracts and the rubrospinal tract. It terminates preferentially in the posterolateral regions of the spinal cord, which contain the neurons controlling the distal muscles of the limbs. Damage to these pathways results in loss of fine-fractionated control of the extremities. The second group includes the anterior and lateral reticulospinal tracts, the tectospinal tract, the lateral and medial vestibulospinal tracts, and the interstitiospinal tract (from the interstitial nucleus of Cajal and pretectal area) that runs in the medial longitudinal fasciculus and terminates preferentially in the anteromedial regions of the spinal cord. These regions contain the neurons controlling axial and proximal limb muscles and regulate posture and righting. In addition to their motor action, both sets of descending pathways also include fibers that modulate sensory transmission by spinal pathways.

Propriospinal Pathways. Some of the propriospinal pathways consist of afferent fibers, which enter the spinal cord via the posterior roots and then ascend or descend in the oval bundle, comma tract, posterolateral fasciculus (of Lissauer), fasciculus gracilis, or fasciculus cuneatus to terminate on spinal neurons at other levels of the spinal cord. Other propriospinal fibers originate from interneurons in the spinal gray matter itself. Collectively, propriospinal fibers are important in mediating spinal reflexes and coordinating activity at different levels of the spinal cord.

Only gold members can continue reading. Log In or Register to continue