The brainstem is the lower portion of the brain. It is elongated like a stem and lies vertically below the cerebrum (telencephalon) and in front of the cerebellum, above the foramen magnum of base of skull. From top to bottom, it is composed of the midbrain, the pons varolii, and the medulla oblongata. Two of these three anatomical structures contain a series of grey nuclei, which are the nerve centers of ten of the twelve cranial nerves (from CN III to XII), which supply the face, eye, tongue, larynx, scalp, and cardiac plexus. Blood supply: branches from the vertebral and basilar artery.
Anatomical Description
Aside from the grey nuclei, the brainstem is also formed by efferent and afferent myelinated nerve fibers that arise from the cerebral cortex and the spinal cord respectively. Giving shape to its ventral surface, there are two bands of motor (efferent) fibers, which originate from pyramidal Betz cell axons in the cortex of each cerebral hemisphere. These myelinated axons cluster together to form the two pyramidal tracts, which run down through the cerebral peduncles on each side of the anterior surface of brainstem. When they reach the medulla oblongata, the tract on the right side switches to the left, while the one on the left goes obliquely to the right side. This part of the brainstem is called the decussation of the pyramids.
Laterally and posteriorly, the brainstem is constituted by the spinocerebellar tracts and the lemniscus pathway, which is formed in turn by the cuneate and gracile fasciculus. These are bundles of myelinated sensory nerve fibers. The midbrain, which is the top structure of the brainstem, consists of the tegmentum, on the ventral portion, the tectum, on top, and the cerebral peduncles.
Function
The function of the brainstem is to govern cardiac and respiratory activity. It adjusts and modulates heart rate (number of ventricular contractions/systoles) per minute. It also supplies motor and sensory nerve fibers to the face, tongue, larynx, and the cardiac plexus through the cranial nerves.
Below, lateral view (midsagittal section) of human brain, showing the brainstem, with its three different parts.
