Spinal Origin or Termination of Major Descending Tracts and Ascending Pathways

The descending pathways shown in parts A, B, C, and D have a variety of actions on spinal circuitry, including the modulation of somatosensory input and the production of motor output. Actions on the sensory apparatus are typically mediated by descending fibers that terminate upon neurons in the posterior horn (laminae I to VI) of the spinal cord. As shown in A, the specific spinal projections from the somatosensory cortex concerned with sensory control end almost entirely in the posterior horn. Some projections from the brainstem reticular formation, which also has a strong action on the sensory apparatus, also terminate in the posterior horn (C). Conversely, the vestibulospinal tracts (D), which have only a weak action on sensory processes, have relatively few terminals in the posterior horn. The rubrospinal tract (B), which has some inhibitory effect on spinal afferents, terminates in intermediate regions and in the rostral part of the anterior (ventral) horn.

The projections from the brain to the spinal cord have been divided into lateral and medial systems. The lateral descending systems include the lateral (crossed) corticospinal tract fibers originating in the motor cortex (A) and the rubrospinal tract (B) (see Plates 2-8 and 2-9). These tracts terminate predominantly in the lateral parts of laminae V, VI, VII, and IX, which are concerned with the control of the distal musculature of the limbs.

The medial descending systems include the reticulospinal tracts (C) and vestibulospinal tracts (D) (see Plates 2-10 and 2-11). These tracts end most heavily in laminae VIII and IX_M, which are involved in controlling neck and trunk muscles. Endings are also present in the medial parts of laminae VI, VII, and IX, which control the proximal muscles of the limbs. Thus the medial systems act predominantly upon axial and proximal muscles. The functional role of the anterior (direct) corticospinal tract is uncertain, although its endings in lamina VIII suggest that it may be involved in the cortical control of axial muscles.

Connections to Ascending Pathways. Ascending projections from the spinal cord to the brain arise from many parts of the spinal gray matter. In general, however, projections to sensory structures tend to originate in the posterior horn, which is the receiving area for somatosensory input arriving via the posterior roots. The illustration shows the anterior and lateral divisions of the spinothalamic tract, which are continuous with each other. One division originates primarily from neurons in lamina I, which respond chiefly to painful stimuli; the other originates from neurons located mainly in laminae IV to VI, which receive information related to a variety of somatosensory stimuli. Lamina IV also gives rise to projections to other sensory areas, such as the cuneate, gracile, and lateral cervical nuclei. Laminae V to VIII give origin to the spinoreticular tract.

Ascending projections to areas involved in motor control tend to arise from laminae VI to IX, which are related to motor movements. The illustration shows two ascending pathways related to motor activity, both of which terminate in the cerebellum. The posterior spinocerebellar tract originates from Clarke’s column, a group of neurons located in lamina VI, and ascends in the ipsilateral posterolateral funiculus. The anterior spinocerebellar tract originates from spinal border cells at the edge of lamina VII and ascends via the contralateral anterolateral funiculus. As shown, neurons projecting in both tracts receive input from muscle proprioceptors; some also receive indirect cutaneous input. In addition to neurons projecting to the cerebellum, the anterior horn also contains neurons projecting to other structures related to motor control, such as the inferior olive and the reticular formation.

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