Nerve pathways The dorsal column nuclei each have an associated
nerve tract in the
spinal cord, the
gracile fasciculus and the
cuneate fasciculus, together forming the
dorsal columns. Both dorsal column nuclei contain synapses from
afferent nerve fibers that have travelled in the spinal cord. They then send on second-order
neurons of the
dorsal column–medial lemniscal pathway. Neurons of the dorsal column nuclei eventually reach the
midbrain and the
thalamus. They send
axons that form the
internal arcuate fibers. These cross over at the
sensory decussation to form the
medial lemniscus. These are present at the bottom of the
medulla oblongata.
Gracile nucleus The
gracile nucleus is medial to the cuneate nucleus. Its neurons receive afferent input from
dorsal root ganglia sensory neurons of the lower
torso and the
lower limbs. The gracile nucleus and
gracile fasciculus carry
epicritic,
kinesthetic, and conscious
proprioceptive information from the lower part of the body (below the level of T6 in the spinal cord). Because of the large population of neurons in the gracile nucleus they give rise to a raised area called the
gracile tubercle on the posterior side of the closed medulla at the floor of the
fourth ventricle.
Cuneate nucleus The
cuneate nucleus is lateral to the gracile nucleus. It carries the same type of information, but from the upper body and the
upper limbs (except the
face, which is carried by the
principal sensory nucleus of trigeminal nerve). The cuneate nucleus is wedge-shaped and located in the closed part of the medulla. It lies lateral to the gracile nucleus and medial to the spinal trigeminal nucleus in the medulla. The large number of neurons found there give rise to the
cuneate tubercle seen on viewing the posterior aspect of the medulla on the side of the
brainstem. == Function ==