Where do the major somatosensory and motor pathways cross the midline?

Where do the major somatosensory and motor pathways cross the midline?

• A. Dorsal columns cross in the spinal cord; the spinothalamic decussates in the midbrain; corticospinal crosses at the optic chiasm.
• B. Dorsal columns decussate in the spinal cord.
• C. Spinothalamic decussates near its entry in the spinal cord; corticospinal decussates at the pyramidal decussation in the medulla.
• D. The dorsal columns cross in the medulla via the internal arcuate fibers; the spinothalamic decussates at or near its entry level in the spinal cord; the corticospinal decussates at the pyramidal decussation in the medulla.

Answer: (C)

Pain and temperature signals in the spinothalamic tract cross almost as soon as they enter the spinal cord, so the contralateral side of the body reflects these sensations. The main motor pathway, the corticospinal tract, crosses at the pyramidal decussation in the lower medulla, so voluntary movements on one side are controlled by the opposite hemisphere. This combination—early crossing for the spinothalamic tract and crossing at the pyramids for the corticospinal tract—best matches the statement about where the major somatosensory and motor pathways cross midline. (Note that the dorsal columns, responsible for fine touch and proprioception, cross later in the medulla, which is a detail not needed to answer this question.)

Pain and temperature signals in the spinothalamic tract cross almost as soon as they enter the spinal cord, so the contralateral side of the body reflects these sensations. The main motor pathway, the corticospinal tract, crosses at the pyramidal decussation in the lower medulla, so voluntary movements on one side are controlled by the opposite hemisphere. This combination—early crossing for the spinothalamic tract and crossing at the pyramids for the corticospinal tract—best matches the statement about where the major somatosensory and motor pathways cross midline. (Note that the dorsal columns, responsible for fine touch and proprioception, cross later in the medulla, which is a detail not needed to answer this question.)