Nervous System & Special Senses

The dorsal column–medial lemniscus pathway. First-order neurons ascend ipsilaterally in the dorsal columns and synapse in the nucleus gracilis/cuneatus of the caudal medulla; second-order fibers decussate there as the internal arcuate fibers and ascend as the medial lemniscus.

It carries pain and temperature (and crude touch). Second-order neurons decussate within 1–2 levels of entry at the anterior white commissure of the spinal cord, then ascend contralaterally.

~90% of fibers decussate at the pyramidal decussation in the caudal medulla. Lesions above the decussation cause contralateral weakness; lesions below (in the cord) cause ipsilateral weakness.

Below the lesion: ipsilateral loss of motor (corticospinal) and dorsal column (touch/vibration/proprioception), with contralateral loss of pain/temperature (spinothalamic, beginning 1–2 levels below). At the level: ipsilateral LMN signs and a band of ipsilateral sensory loss.

UMN: spasticity, hyperreflexia, increased tone, positive Babinski, minimal atrophy, no fasciculations. LMN: flaccid paralysis, hyporeflexia, decreased tone, atrophy, fasciculations, no Babinski.

Midbrain: CN III, IV (IV is the only CN to exit dorsally). Pons (pontomedullary junction): CN V (pons proper), VI, VII, VIII. Medulla: CN IX, X, XII; CN XI arises from the upper cervical cord. CN I and II are not true brainstem nerves.