Anatomy of the brainstem

Our central nervous system is made up of the cerebrum, the cerebellum, the brainstem and the spinal cord. The brainstem is a trunk-like part that sits in the posterior cranial fossa and connects the spinal cord inferiorly with the forebrain superiorly.

The brainstem can be divided into three parts. From caudal to rostral these parts are: the medulla oblongata, the pons and the midbrain.

The brainstem is made up of white and gray matter. The white matter contains many ascending and descending fibers that act like a highway, allowing information to travel to and from the spinal cord and the higher parts of the central nervous system.

Scattered amongst the white matter tracts, there are islands of gray matter that consist of neuronal cell bodies, many that are the nuclei associated with cranial nerves.

Some of these collections of nuclei serve as centers for life sustaining reflexes, like those involved with breathing and our heartbeat, others coordinate states of alertness or arousal, while others mediate motor activities and relay sensory information.

First, let’s look at the medulla oblongata, specifically its ventral aspect. Right in the middle, there is the anterior median fissure.

On either side of it, there are two bumps called the pyramids, which contain axons of the corticospinal, or pyramidal, tract.

Before entering the spinal cord, these fibers cross over to the opposite side, forming the decussation of the pyramids.

Lateral to each pyramid, there are two oval bumps called the olives. They contain the inferior olivary nuclei, which have rich connections to the cerebellum and are involved in motor coordination and learning.

Now, on the dorsal aspect of the medulla oblongata, it’s visible that the rostral medulla contains the inferior part of the fourth ventricle, a space filled with cerebrospinal fluid, or CSF.

Across the floor of the fourth ventricle, spreading transversely, there are the striae medullaris, which are raised stripes that contain arcuatocerebellar fibers.