Brainstem is the tubular stalk-like part of the brain made up of midbrain, pons and medulla oblongata from above downward. It is so called because it is like stem of a tree. Main mass of the brain, cerebrum with cerebellum rests on the brainstem and through it, is connected to spinal cord below. Long axis of brainstem is oblique, directed downward and backward.

• Extent: Above, upper end of brainstem (midbrain) is continuous with diencephalon of forebrain.
• Below: Lower end of brainstem (medulla oblongata) passes out of cranial cavity through foramen magnum to become continuous with spinal cord at the level of upper border of first cervical vertebra.

Relations of Brainstem

• With cranial cavity: Brainstem lies in posterior cranial fossa of skull and rests on the slope of clivuswhich is formed by posterosuperior surface of basilar parts of sphenoid and occipital bones.
• With tentorium cerebelli: Tentorium cerebelli is a crescentic horizontal shelf of dura mater of brain lying between posterior part of cerebrum (occipital lobe) and cerebellum. It posseses peripheral convex border. In front of concave anterior border (tentorial notch), brainstem passes downwards. Midbrain is the supratentorial part and, pons with medulla oblongata is the infratentorial part of brainstem lying above and below the tentorium cerebelli respectively.
• With cerebrum and cerebellum: Cerebrum with thalamus (diencephalon) is above and, cerebellum is behind the brainstem. Ventral compact partof midbrain, composed of bundle of descending fibers connects the brainstem (midbrain) above with cerebrum. It is called cerebral peduncle having right and left identical halves. Cerebellum is connected to midbrain, pons and medulla oblongata of brainstem by three pairs of compact bundle of white matter. These are called superior, middle and inferior cerebellar peduncles respectively.
• With fourth ventricle of brain: Fourth ventricle is the cavity of hindbrain. It is related anteriorly to pons and medulla oblongata and posteriorly to cerebellum.

Cavity related to brainstem is of different shapes and natures at different level as follows:

• Midbrain– A narrow linear slit known as aqueduct of Sylvius.
• Pons and upper part of medulla oblongata: A wide tent shaped space forming cavity of hindbrain called fourth ventricle of brain.
• Lower part of medulla oblongata: A narrow central canal of medulla continuous below with central canal of spinal cord.

Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented) (2014)