Chapter 28: Brainstem Anatomy

5 centimeters long and 3 to 4 centimeters wide, situated in the posterior cranial fossa. Anatomically, the brainstem is divided rostrocaudally into the midbrain (mesencephalon), the pons, and the medulla oblongata (myelencephalon), with each segment generally possessing a ventral basis and a dorsal tegmentum, plus the midbrain's unique dorsal tectum. The brainstem is densely packed with essential components, including numerous intrinsic neuronal bodies, the reticular formation—a complex network critical for regulating consciousness, respiratory activities, and cardiovascular control—and the nuclei for ten cranial nerves (III through XII) that govern sensory, motor, and autonomic functions of the head and neck. Major ascending sensory pathways, such as the medial lemniscus, the anterolateral system (spinal lemniscus), and the trigeminothalamic tracts, traverse the brainstem en route to the thalamus, while prominent descending projections like the corticospinal and corticonuclear tracts pass through or terminate within it. Detailed sectional anatomy reveals key structures: the medulla contains the pyramids, the inferior olivary nuclear complex, the decussations of both the motor (pyramidal) and sensory (dorsal column-medial lemniscus) systems, and vital centers. The pons is notable for the abducens nucleus, the facial nerve fibers forming the facial colliculus, and the pontine nuclei, which relay information to the cerebellum via the middle cerebellar peduncle. The rostral midbrain houses the red nucleus, the substantia nigra, and the superior and inferior colliculi in the tectum, responsible for visual and auditory processing, respectively. Clinically, the concentration of these critical pathways means small lesions can produce devastating outcomes, such as irreversible cardiac arrest or the 'locked-in' syndrome, emphasizing the necessity of accurate anatomical knowledge for diagnosis. Furthermore, the sources introduce modern proposals for brainstem organization, suggesting nomenclature aligned with developmental gene expression patterns that partition the region into neuromeres (mesomeres and rhombomeres), moving away from traditional gross anatomical divisions.