Which sequence correctly describes the facial touch pathway from primary neuron to cortex?

• A. Thalamic neuron extends to sensory cortex; primary neurons in DRG synapse with second neuron in brain stem (pons); second neuron decussates
• B. Primary neurons in DRG synapse with second neuron in brain stem (pons); second neuron decussates via medial lemniscus and ascends contralateral to synapse in the thalamus; thalamic neuron extends to sensory cortex
• C. Primary neurons in DRG synapse in Cortex; second neuron decussates
• D. Secondary decussates via medial lemniscus; primary neurons in DRG synapse

Answer: (B)

Facial touch uses a relay system with three neurons: the first-order neuron has its cell body in a sensory ganglion (for the face this is the trigeminal ganglion), it synapses in the brainstem with a second-order neuron in the principal (chief) sensory nucleus in the pons. The second-order neuron then crosses to the opposite side and ascends to the thalamus, where it synapses in the ventral posteromedial nucleus. A third-order neuron in the thalamus projects to the primary somatosensory cortex.

The sequence described in this option matches that flow: the primary neuron in a sensory ganglion synapses in the brainstem, the second-order neuron crosses to the contralateral side and ascends to the thalamus, and the thalamic neuron then extends to the sensory cortex. In precise anatomy, facial sensation is via the trigeminal pathway and the crossing occurs through the trigeminothalamic tract rather than the medial lemniscus, and the relevant ganglion is the trigeminal ganglion rather than a dorsal root ganglion. But the overall order—ganglion to brainstem, cross and ascend to thalamus, then to cortex—is the correct sequence.