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The brain stem is the lower part of the brain, adjoining and structurally continuous with the spinal cord. Most sources consider the pons, medulla oblongata, and midbrain all to be part of the brainstem.
Differentiation of the brain stem from the cerebrum is complex, with regard to both anatomy and taxonomy. Some taxonomies describe the brain stem as the medulla and mesencephalon, whereas others include diencephalic regions.
Ventral view/medulla and pons
The most medial part of the medulla is the anterior median fissure. Moving laterally on each side are the pyramids. The pyramids contain the fibers of the corticospinal tract, or the upper motor neuronal axons as they head inferiorly to synapse on lower motor neuronal cell bodies within the ventral horn of the spinal cord.
Dorsal view/medulla and pons
The most medial part of the medulla is the posterior median fissure. Moving laterally on each side is the fasciculus gracilis, and lateral to that is the fasciculus cuneatus. Superior to each of these, and directly inferior to the obex, are the gracile tubercles and cuteanus tubercles, respectively. Underlying these are their respective nuclei. The obex marks the end of the 4th ventricle and the beginning of the central canal. The posterior intermediate sulci separates the fasciculi gracilis from the fasciculi cuneatus. Lateral to the fasciculi cuneatus is the lateral funiculus.
Spinal Cord to Medulla Transitional Landmark: From a ventral view, there can be seen a decussation of fibers between the two pyramids. This decussation marks the transition from medulla to spinal cord. Superior to the decussation is the medulla and inferior to it is the spinal cord.
The midbrain is divided into three parts. The first is the tectum, which is "roof" in Latin. The tectum includes the superior and inferior colliculi and is the dorsal covering of the cerebral aqueduct. The inferior colliculus, involved in the special sense of hearing sends its inferior brachium to the medial geniculate body of the diencephalon. Superior to the inferior colliculus, the superior colliculus marks the rostral midbrain. It is involved in the special sense of vision and sends its superior brachium to the lateral geniculate body of the diencephalon. The second part is the tegmentum and is ventral to the cerebral aqueduct. Several nuclei, tracts and the reticular formation is contained here. Last, the ventral side is comprised of paired cerebral peduncles. These transmit axons of upper motor neurons.
Midbrain internal structures
Periaqueductal Gray: The area around the cerebral aqueduct, which contains various neurons involved in the pain desensitization pathway. Neurons synapse here and, when stimulated, cause activation of neurons in the raphe nucleus magnus, which then project down into the dorsal horn of the spinal cord and prevent pain sensation transmission.
The adult human brainstem emerges from two of the three primary vesicles formed of the neural tube. The mesencephalon is the second of the three primary vesicles, and does not further differentiate into a secondary vesicle. This will become the midbrain. The third primary vesicle, the rhombencephalon, will further differentiate into two secondary vesicles, the metencephalon and the myelencephalon. The metencephalon will become the cerebellum and the pons. The myelencephalon will become the medulla.
There are three main functions of the brainstem. The first is its role in conduit functions. That is, all information related from the body to the cerebrum and cerebellum and vice versa, must traverse the brain stem. The ascending pathways coming from the body to the brain are the sensory pathways, and include the spinothalamic tract for pain and temperature sensation and the dorsal column, fasciculus gracilis, and cuneatus for touch, proprioception, and pressure sensation (both of the body). (The facial sensations have similar pathways, and will travel in the spinothalamic tract and the medial lemniscus also). Descending tracts are upper motor neurons destined to synapse on lower motor neurons in the ventral horn and intermediate horn of the spinal cord. In addition, there are upper motor neurons that originate in the brainstem's vestibular, red, tactile, and reticular nuclei, which also descend and synapse in the spinal cord. Second, the cranial nerves 3-12 emerge from the brain stem. Third, the brain stem has integrative functions (it is involved in cardiovascular system control, respiratory control, pain sensitivity control, alertness, and consciousness). Thus, brain stem damage is a very serious and often life-threatening problem.
Physical signs of brainstem disease
Diseases of the brainstem can result to abnormalities in the function of cranial nerves, which may lead to visual disturbances, pupil abnormalities, changes in sensation, muscle weakness, hearing problems, vertigo, swallowing and speech difficulty, voice change, and co-ordination problems. Localizing neurological lesions in the brainstem may be very precise, although it relies on a clear understanding on the functions of brainstem anatomical structures and how to test them.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Brain_stem". A list of authors is available in Wikipedia.|