The spinal cord begins where the medulla ends, running from the foramen magnum at the base of the skull to about the level of the first lumbar vertebra (L1) (Fig. 5–1). The spinal cord is divided into cervical, thoracic, lumbar, and sacral regions. The cervical and thoracic regions of the spinal cord correspond to the cervical and thoracic regions of the spinal column. However, the spinal cord is shorter than the spinal column, and so the lumbar region of the spinal cord actually corresponds to the lower thoracic spine, and the sacral region of the cord is housed in a short region called the conus medullaris at about the level of the L1-L2 vertebrae. Throughout the spine, dorsal roots enter and ventral roots exit through the neural foramina of the vertebrae that correspond to their spinal cord level of origin/exit. At cervical and thoracic levels, the corresponding foramina are essentially adjacent to the spinal cord levels with which they are associated. Since the spinal cord ends at L1, below L1, the lumbosacral nerve roots (cauda equina) must descend to reach their corresponding exiting foramina (discussed further in Chapters 15 and 17).

The lateral corticospinal tracts, dorsal column pathways, and anterolateral (spinothalamic) tracts are the three most clinically relevant pathways for clinical localization within the spinal cord. Sympathetic and parasympathetic pathways also traverse the spinal cord, and there are a number of other tracts (e.g., tectospinal, rubrospinal, vestibulospinal) that play roles in posture and motor control but are not generally assessed in clinical neurology. The spinocerebellar pathways that bring proprioceptive information to the cerebellum will be discussed in the context of the cerebellum in Chapter 8.

The lateral corticospinal tracts are lateral and posterior in the spinal cord, the dorsal columns are posterior and medial, and the anterolateral (spinothalamic) tracts are anterior and lateral (as their name suggests) (Fig. 5–2).

Lamination refers to the arrangement of fibers within a pathway (Fig. 5–3). For the corticospinal, anterolateral, and dorsal column pathways, this refers to where the arm, leg, and torso fibers run (there are no fibers for the head in the spinal cord since these are carried to/from the brainstem by the cranial nerves).

As the corticospinal tracts descend from the brainstem, arm fibers synapse on lower motor neurons in the cervical cord, but leg fibers must wait until the lumbar cord to do so. Therefore, arm fibers must be most medial to have first access to the alpha motor neurons, and leg fibers are more lateral, gaining exposure to the anterior horn cells only after the arm fibers have done so and departed as the tracts descend. The corticospinal tracts are thus laminated with the arm fibers medial and the leg fibers lateral.

The dorsal column fibers from the feet and legs enter the spinal cord at the lumbar and sacral levels, and are pushed medially by the addition of trunk and arm fibers as the tracts ascend. The dorsal columns are thus laminated with the legs medial and the arms lateral.

The spinothalamic tracts’ feet and leg fibers cross to the contralateral side and are pushed laterally as subsequent crossing fibers for the trunk and arms push them aside and layer more medially. The spinothalamic tracts are thus laminated with the arms medial and the legs lateral like the corticospinal tracts.

In summary, in the corticospinal and anterolateral tracts, the arm fibers are medial and the leg fibers are lateral, and the pattern is reversed for the dorsal columns. As a mnemonic, imagine two people laying feet to feet on top of the spinal cord (dorsal columns: lower extremities medial, upper extremities lateral) and four people (two on each side) falling perilously into the central canal (anterolateral and corticospinal tracts: upper extremities medial, lower extremities lateral). This lamination pattern is particularly important for the understanding of central cord syndrome (e.g., as can be caused by a syrinx) (See “Central Cord Syndrome” later).

There are four important clinical patterns to recognize that signify localization to particular parts of the spinal cord, each with a particular differential diagnosis (Fig. 5-4A and Table 1-1).