Sensory Pathway: Brainstem & Spinal Cord to Thalamus - Neuron Order

Sensory impulses that reach the medulla of the brainstem (where all major ascending sensory pathways terminate), after their entry through the spinal cord, are conveyed through the brainstem to the thalamus by the following chain of neurons:

1. First-order neurons enter the spinal cord through dorsal root ganglia. These are pseudo-unipolar neurons, with the cell body lying outside the cord, in the ganglion. Centrally, their axons form the dorsal root, enter the spinal cord through the medial dorsal septum of the pia mater, and synapse in the spinal cord gray matter with:

- cells in the dorsal horn of the spinal cord to give rise to spinal reflexes, as part of the 'final common pathway', or with

- second-order neurons in the dorsal horn of the spinal cord, which cross over in the white commissure to ascend ipsilaterally in the contralateral spinothalamic tract, and synapse in the thalamus.

2. Second-order neurons: these travel ipsilaterally in the medulla as the medial lemniscus and synapse in the thalamus.

3. Third-order neurons: then project from the thalamus as part of the thalamocortical tract to the appropriate cortical projection area.