In the periphery, the primary neuron is the sensory receptor
that detects sensory stimuli like touch or temperature. The cell
body of the primary neuron is housed in the dorsal root ganglion of
a spinal nerve or, if sensation is in the head or neck, the ganglia
of the trigeminal or cranial nerves.
The secondary neuron acts as a relay and is located in either
the spinal cord or the brainstem. This neuron’s ascending axons
will cross, or decussate, to the opposite side of the spinal cord
or brainstem and travel up the spinal cord to the brain, where most
will terminate in either the thalamus or the cerebellum.
Tertiary neurons have cell bodies in the thalamus and project
to the postcentral gyrus of the parietal lobe, forming a sensory
homunculus in the case of touch. Regarding posture, the tertiary
neuron is located in the cerebellum.