AssessmentsAnatomy of the brainstem
USMLE® Step 1 style questions USMLE
Content Reviewers:Viviana Popa, MD
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 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.
Across the floor of the fourth ventricle, spreading transversely, there are the striae medullaris, which are raised stripes that contain arcuatocerebellar fibers.
Laterally, there are the inferior cerebellar peduncles, which contain fibers that travel between the medulla and the cerebellum.
The fourth ventricle has a diamond, or rhomboid shape, so it’s also called the rhomboid fossa. The ventricle, or fossa, tapers at its caudal aspect to a point called the obex and near this caudal limit is the entrance to the central canal of the spinal cord. In this region, CSF can travel from the fourth ventricle into the central canal.
Looking at the caudal medulla, right on the midline there’s the posterior median sulcus. Next to it, there are two bumps called the gracile tubercles, which contain the gracile nuclei, one on each side.
Lateral to the gracile tubercles, there are two more bumps, on each side, called the cuneate tubercles, which contain the cuneate nuclei.
Now, let’s switch gears and have a look at the embryological development of the medulla oblongata and the spinal cord.
The development is important because it provides a location plan for motor and sensory nuclei and a blueprint for the overall pattern of motor and sensory information flow!
If we look at a transverse section of the part of the neural tube that will develop into the spinal cord, we can see the cavity of the neural tube in the center.
The neural tube has a dorsal, or posterior part, that forms the alar plate, and a ventral, or anterior part, that forms the basal plate.
Inside the neural tube, a groove called the sulcus limitans separates these plates. Now, the alar plate gives rise to sensory nuclei and later develops into the dorsal horns of the spinal cord's gray matter.
The basal plates, on the other hand, give rise to the motor nuclei and later develop into the ventral horns of the spinal cord.
As we continue rostral along the neural tube to the part that will develop into the medulla oblongata, we can see that the cavity of the neural tube extends laterally to form the fourth ventricle.
In doing so, it pushes the alar plates ventrally as well. Even though the basal and alar plates were ventro-dorsal in the spinal cord, they shift to a medio-lateral position in the medulla oblongata.
Medially, the basal plate develops into the motor nuclei of the cranial nerves, while laterally and posteriorly the alar plate will develop into sensory nuclei.
The sulcus limitans that separates them can be seen on the floor of the fourth ventricle, parallel to the posterior median sulcus.
Let’s take a quick break and see if you can identify structures of the dorsal aspect of the medulla oblongata.
Okay, now let’s look at the internal structures of the medulla oblongata, and the easiest way to do that is by making a few transverse sections, starting with the first one at the level of the caudal medulla.
On the ventral aspect, there are the two pyramids, which contain descending fibers of the corticospinal tracts. The majority of these fibers cross over to the opposite side in the pyramidal decussation and form the lateral corticospinal tract as the fibers enter the spinal cord.
Lateral to the pyramids, there’s the anterior spinocerebellar tract, and behind it, the posterior spinocerebellar tract.
These carry proprioceptive input from muscle spindles, golgi tendon organs and joint receptors to the cerebellum. Medial to them, there’s the lateral spinothalamic tract, which carries somatosensory information about pain and temperature, originating from below the face, to the thalamus.
Between these tracts and the decussation of the pyramids, part of the inferior olivary nucleus can be seen. On the dorsal aspect, lateral to the posterior median sulcus, the rostral part of the fasciculus gracilis can be seen on both sides and just lateral to those fibers, the fasciculus cuneatus.
Ventral to them, there are the nucleus gracilis and nucleus cuneatus, which are made up of grey matter. Lateral to the fasciculus cuneatus, the spinal trigeminal tract is present and ventral to it, the spinal trigeminal nucleus.
Finally, between all these nuclei and tracts, there’s the reticular formation, which is a network of nuclei with interspersed afferent and efferent fibers.
It stretches across the entire brainstem, so it acts like an interface between the spinal cord and higher brain centers. The reticular formation influences many functions, such as motor reflexes, eye movements, autonomic functions and even consciousness.
Now let’s look at another transverse section, this time at the level of the decussation of the medial lemnisci. Here, there’s a somatosensory tract decussation: axons originating from the nucleus gracilis and nucleus cuneatus form the internal arcuate fibers that pass around the central gray matter.
Ventral to it, these fibers cross the neural axis to the opposite side. Dorsal to the decussation, in the central gray matter, there are the hypoglossal nuclei. Other structures are similar to those described in the previous section.
Now, even more rostrally, let’s look at the olives and inferior cerebellar peduncles. Here, instead of the central canal dorsally, there is a space created that makes up the fourth ventricle.
The central gray matter is larger and sits under the floor of the fourth ventricle, where it contains nuclei of cranial nerves, next to the posterior median sulcus.
From medial to lateral, these are: the hypoglossal nuclei, dorsal motor nuclei of vagus and the solitary nucleus. Dorsal to the solitary tract, there is the medial vestibular nucleus and laterally the inferior vestibular nucleus.
Ventral to all these nuclei, there’s the reticular formation, and buried deep within it the nucleus ambiguus. Lateral to the reticular formation, there are the spinal trigeminal tracts and nuclei, and most laterally, the inferior cerebellar peduncles.
On the ventral side, next to the anterior median fissure, there are the pyramids. They contain the corticospinal, or pyramidal, fibers that descend to the spinal cord, and the corticobulbar fibers that target motor nuclei of the cranial nerves in the brainstem.
Dorsal and lateral to the pyramids there are the inferior olivary nuclei that look like two big crumpled bags with their openings positioned towards the middle.
These nuclei send fibers across the midline towards the inferior cerebellar peduncles. Also dorsal to the pyramids, and between the inferior olivary nuclei, there are the medial lemnisci, whose axons originated from the nucleus gracilis and nucleus cuneatus in the medulla.
Between the tectospinal tracts and the hypoglossal nuclei there are the medial longitudinal fasciculi, or MLF, which connect the vestibular and cochlear nuclei with the motor nuclei of the cranial nerves III, IV and VI.
Finally, let’s look at the most rostral transverse section, at the level just below the pons. Here, instead of inferior vestibular nuclei there are the lateral vestibular nuclei, positioned between the inferior cerebellar peduncles and the reticular formation.