Pain, temperature, and gross touch sense information of the face, and a few other parts of the head, enters the brainstem in axons of the trigeminal nerve. These axons descend in the brainstem prior to synapsing in an ipsilateral nucleus called the spinal trigeminal nucleus. This is a long nucleus that extends from about midway up the brainstem through the inferior brainstem and into the upper levels of the spinal cord. Axons from this nucleus decussate and ascend in the trigeminothalamic tract to synapse in the thalamus. Abnormalities of these functions will be ipsilateral if a lesion occurs before the decussation of this pathway, or contralateral with lesions after it. Axons of the third neuron in the chain, called the somatosensory thalamocortical tract, travel from the thalamus to the primary somatosensory cortex, from which neurons project to association somatosensory cortices.
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Introduction to Neurology 