Cranial versus Somatic Nerves

CRANIAL VERSUS SOMATIC NERVES

Somatic sensory nerves contain axons from different sensory receptors. These are called

afferent fibers. Within the same nerve are efferent “motor” or outgoing nerves. The nerve

splits at the spinal cord into dorsal (sensory) roots and ventral (motor and autonomic) roots.

Spinal nerves, you should know, involve contralateral innervation. This means that the right

side of the body is connected to the left brain, while the left side of the body is connected to

the right brain. This is not true of cranial nerves. These are mostly ipsilateral, meaning the

nerve fibers do not cross over.

In the spinal cord, there are tracts that are specifically for the transfer of sensory information to

the brain. These are the dorsal column, the medial lemniscus, and the spinothalamic tract.

The fasciculus gracilis is the part of the dorsal column that contains axons from the legs and

lower body, while the fasciculus cuneatus contains axons from the upper body and arms. Each

of these axons end in the medulla of the brain, sending another neuron back up toward the rest

of the brain. Shortly after this, the crossing over or “decussation” happens. The axons then go

up to the thalamus. The third neuron starts in the thalamus and ends in the postcentral gyrus of

the cerebral cortex. This means that three nerves are necessary for sensory information

processing. This controls light touch and proprioception.

The spinothalamic tract is responsible for pain and temperature sensations. Everything

happens the same as with the dorsal tracts except that they decussate in the spinal cord at the

level in which they first enter the spinal cord. The trigeminal pathway (a cranial nerve pathway)

also has three nerves involved in controlling the sensation but there are different places for the

nerves to go: the second nerve starts in the spinal trigeminal nucleus (in the medulla), the chief

sensory nucleus (in the pons) or in the mesencephalic nucleus in the midbrain. There is

decussation after that, with ascension into the thalamus and later, the cerebrum.

As mentioned, not all cranial nerves have ipsilateral communication with the brain. Vision, and

the optic nerve, is one of these. Axons from the medial side of the retina will decussate, while

the axons of the lateral side of the retina do not decussate. In some cases, the peripheral vision