The brain stem is the portion of the encephalon which occupies the middle and caudal fosse of the floor of the cranial cavity. It constitutes the axis of the encephalon and is continuous with the spinal cord via the foramen magnum. It is formed by nuclei and nerve fibers, the latter of which form tracts[1], lemnisci[2] and fasciculi[3]. All of the cranial nerves, with the exception of the first (olfactory nerves) exit and/or enter the brain stem.


The brain stem serves as a route for ascending and descending pathways that connect the spinal cord with the cerebrum and cerebellum. It also regulates reflex functions (micturition, respiration and cardiovascular activity), receives sensory information from cranial structures, and controls the head muscles.


In the central portion of the brain stem, from the caudal portion of the diencephalon to the medulla oblongata and in the spinal cord between the dorsal and lateral horns, is a diffuse network of neurons formed by gray matter and white matter called reticular[4] formation. There is an extensive part of the reticular formation in the brain stem known as the ascending reticular activating system (ARAS). It receives extensive projections from conscious pathways and stimulates the entire cerebral cortex. Its function is to prepare the cerebral cortex to receive sensory inputs from any modality.


The primary divisions of the brain stem, from rostral to caudal, are: diencephalon, mesencephalon, pons and medulla oblongata.


[1] From the Latin tractus, “line”.

[2] From the Greek lemniskos, “ribbon”.

[3] From the Latin fasciculus, “fascicule, bundle”.

[4] From the Latin reticulum, “net”.