Final answer:
The somatotopy of the first neuron in the dorsal column is characterized by a spatial organization that reflects the origin of sensory input on the body. Axons from the lower body enter medially in the fasciculus gracilis, and axons from the upper body enter laterally in the fasciculus cuneatus, maintaining this arrangement until synapsing in the medulla.
Step-by-step explanation:
The somatotopy of the first neuron in the dorsal column refers to the orderly mapping of body regions in the central nervous system. This mapping is evident in the way axons from different parts of the body arrange themselves within the dorsal column of the spinal cord. Specifically in the dorsal column system, axons from lower levels of the body, like the legs and lower body, position themselves medially within the fasciculus gracilis, while axons from upper levels, involving the upper body and arms, position themselves laterally in the fasciculus cuneatus.
Upon entering the dorsal column, these axons maintain their spatial organization as they travel upwards to the brain. They ultimately terminate in the medullary nuclei such as the nucleus gracilis and nucleus cuneatus, where they synapse with the second-order neurons. These axons from the two tracts then decussate or cross over to the opposite side of the medulla, ascend in the form of the medial lemniscus, and eventually synapse with the third-order neurons in the thalamus. The third-order neurons then project to the somatosensory cortex located in the postcentral gyrus of the cerebral cortex where the sensory information is processed to become conscious perception.