Brainstem

BRAINSTEM

The brainstem is the brain structure that connects the cerebrum to the spinal cord and to the cerebellum of the brain. It is divided into four sections: the diencephalon, the midbrain, the pons, and the medulla oblongata (listed from the top down). These structures are essential for many life functions, including sleep, breathing, heart rate, consciousness, and other autonomic functions. There are collections of nuclei and multiple white matter pathways that travel within and outside of the brainstem. A total of ten of the twelve cranial nerves arise directly out of the brainstems nuclei. Let’s look at the different structures that make up the brainstem. Figure 17 shows the structures of the brainstem:

Figure 17.

The diencephalon on the top connect to the midbrain below it and is in close connection with the third ventricle. The thalamus, epithalamus, hypothalamus, and subthalamus are all a part of the diencephalon. The epithalamus is in the back and includes the pineal gland. The subthalamus forms a large part of the third ventricle. The substantia nigra and red nucleus extend up into the subthalamus. The hypothalamus is on the anterior and inferior part of the diencephalon along with the pituitary gland.

The thalamus is the biggest part of the diencephalon; it is two large oval structures on either side of the third ventricle. As you know, there are many functions of this structure. The midbrain is just below this and connects the diencephalon to the pons. It is also attached to the cerebellum posteriorly. There are motor cortical spinal fibers, pontine fiber tracts, and corticonuclear fibers located near the front of this structure, which are different pathways.

The backside of the midbrain has two superior colliculi and two inferior colliculi that are involved with visual reflexes, while the inferior colliculi are involved mainly in auditory processing. Below the inferior colliculi is the area where the fourth cranial nerve exists the brain. It is the only nerve of its type that comes out of the backside of the brainstem rather than the frontside or lateral aspect of the brainstem.

The pons connects the medulla oblongata to the midbrain above it. The backside of the pons is connected to the cerebellum through cerebellar peduncles. Cranial nerves V, VII, and VIII come directly out of the brainstem at the level of the pons. In the pons, there is the locus ceruleus that is involved in alertness by participating in the reticular activating system. The damaged locus ceruleus is commonly seen in Alzheimer’s dementia. Breathing centers are found in the pons.

The medulla or medulla oblongata is the most inferior part of the brainstem. It connects the pons to the spinal cord at the level of the medulla oblongata. There is a predominance of pyramids near the front of the medulla. These carry moor fibers downward from the cortex and is where these motor fibers decussate or cross over so that the right side of the brain affects the left side of the body and vice versa. Not all fibers will decussate but most of them do. Those that do become the lateral corticospinal tract in the spinal cord, while those that don’t form the medial corticospinal tract. The hypoglossal nerve comes from this area and, beneath this, the cranial nerves IX, X, and XI will later emerge.

One major features of the brainstem tracts, which are white matter tracts, is that they pass through on their way above or below the level of the brainstem. One of these is the reticular formation that goes from the spinal cord out to the diencephalon. It receives