Structural and functional anatomy of the brainstem.

The brainstem connects the forebrain to the spinal cord. It consists of three layers, the ventral basis, the intermediate tegmentum, and the dorsal tectum. Caudo-rostrally, the brainstem is divided into the medulla oblongata or myelencephalon, the metencephalon, and the mesencephalon. The tegmentum is the most conserved part of the brainstem and contains the motor and sensory nuclei of cranial nerves (CN) III-XII. They are arranged in a mediolateral direction, and, unlike the spinal cord, their afferent and efferent fibers are not macroscopically separated. The tegmental reticular formation, the brainstem core, has a lattice structure with manifold longitudinal and transverse fibers and stacked neurons. Its architecture provides maximum convergence and divergence of intrinsic and ascending/descending information from the spinal cord and forebrain. It coordinates brainstem-based vital functions, and its integrity is essential for arousal, consciousness, and goal-directed behavior. Its small aminergic and cholinergic telencephalic projection nuclei have a profound impact on mood, motivation, and cognition. Human telencephalic evolution manifests in volume increases of the cerebral crura, pons, cerebellar hemispheres, olives, pyramidal tract, and medial lemniscus. However, the spatial constraints of the posterior fossa limited the proportional expansion of the brainstem during human telencephalic evolution, rendering this evolutionarily conserved structure increasingly vulnerable to malfunction and disease.