Diagram showing the position of the colliculi. Superior shown in green and inferior in orange.View of the midbrain showing covering tectum and tegmental floor
Themidbrain is further subdivided into three parts:tectum,tegmentum, and theventral tegmental area. Thetectum forms the ceiling. The tectum comprises the paired structure of thesuperior andinferior colliculi and is the dorsal covering of thecerebral aqueduct. Theinferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei, as well as inputs from the auditory cortex. Its inferior brachium (arm-like process) reaches to themedial geniculate nucleus of thediencephalon. Thesuperior colliculus is positioned above the inferior colliculus, and marks the rostral midbrain. It is involved in the special sense of vision and sends its superior brachium to thelateral geniculate body of the diencephalon.
Reticular formation: This is a large area in themidbrain that is involved in various important functions of the midbrain. In particular, it contains lower motor neurons, is involved in the pain desensitization pathway, is involved in the arousal and consciousness systems, and contains thelocus coeruleus, which is involved in intensivealertness modulation and inautonomic reflexes.
Central tegmental tract: Directly anterior to the floor of thefourth ventricle, this is a pathway by which many tracts project up to the cortex and down to the spinal cord.
The pons meets the medulla at the pontomedullary junction.[12] This region is supplied by the joining of thebasilar,vertebral arteries. Theposterior inferior cerebellar artery also joins from which a large number of perforating arteries arise. Lateral spinal arteries also emerge to supply the posterior surface of the medulla oblongata.[12]
The appearance of acadaveric brainstem from the front, with major parts labelled
In the medial part of themedulla is theanterior median fissure. Moving laterally on each side are themedullary pyramids. The pyramids contain the fibers of thecorticospinal tract (also called the pyramidal tract), or the upper motor neuronal axons as they head inferiorly to synapse on lower motor neuronal cell bodies within theanterior grey column of thespinal cord.
Theanterolateral sulcus is lateral to the pyramids. Emerging from the anterolateral sulci are the CN XII (hypoglossal nerve) rootlets. Lateral to these rootlets and the anterolateral sulci are theolives. The olives are swellings in the medulla containing underlying inferior nucleary nuclei[13] (containing various nuclei and afferent fibers). Lateral (and dorsal) to the olives are the rootlets forCN IX (glossopharyngeal), CN X (vagus) and CN XI (accessory nerve). The pyramids end at thepontinemedulla junction, noted most obviously by the largebasal pons. From this junction, CN VI (abducens nerve), CN VII (facial nerve) and CN VIII (vestibulocochlear nerve) emerge. At the level of the midpons, CN V (thetrigeminal nerve) emerges. Cranial nerve III (theoculomotor nerve) emerges ventrally from the midbrain, while the CN IV (thetrochlear nerve) emerges out from the dorsal aspect of the midbrain.
Between the two pyramids can be seen adecussation of fibers which marks the transition from the medulla to the spinal cord. The medulla is above the decussation and the spinal cord below.
From behind
The appearance of acadaveric brainstem from behind, with major parts labelled
The most medial part of themedulla is theposterior median sulcus. Moving laterally on each side is thegracile fasciculus, and lateral to that is thecuneate fasciculus. Superior to each of these, and directly inferior to theobex, are the gracile and cuneate tubercles, respectively. Underlying these are their respective nuclei. The obex marks the end of thefourth ventricle and the beginning of thecentral canal. The posterior intermediate sulcus separates the gracile fasciculus from the cuneate fasciculus. Lateral to the cuneate fasciculus is thelateral funiculus.
Superior to the obex is the floor of thefourth ventricle. In the floor of the fourth ventricle, various nuclei can be visualized by the small bumps that they make in the overlying tissue. In the midline and directly superior to the obex is thevagal trigone and superior to that it thehypoglossal trigone. Underlying each of these are motor nuclei for the respective cranial nerves. Superior to these trigones are fibers running laterally in both directions. These fibers are known collectively as thestriae medullares. Continuing in arostral direction, the large bumps are called the facial colliculi. Eachfacial colliculus, contrary to their names, do not contain the facial nerve nuclei. Instead, they have facial nerve axons traversing superficial to underlying abducens (CN VI) nuclei. Lateral to all these bumps previously discussed is an indented line, orsulcus that runs rostrally, and is known as thesulcus limitans. This separates the medial motor neurons from the lateral sensory neurons. Lateral to the sulcus limitans is the area of thevestibular system, which is involved in special sensation. Moving rostrally, the inferior, middle, and superior cerebellar peduncles are found connecting the midbrain to the cerebellum. Directly rostral to the superior cerebellar peduncle, there is the superior medullary velum and then the two trochlear nerves. This marks the end of the pons as theinferior colliculus is directly rostral and marks the caudal midbrain. Middle cerebellar peduncle is located inferior and lateral to the superior cerebellar peduncle, connecting pons to the cerebellum. Likewise, inferior cerebellar peduncle is found connecting the medulla oblongata to the cerebellum.
The main supply of blood to the brainstem is provided by thebasilar arteries and thevertebral arteries.[14]: 740 It is important to note that there is a bit of variability in how these arteries connect and supply blood to the brain, such as where the arteries fuse or are reinforced. The variability that exists allows for syndromes to be introduced if certain vessels are excluded from where they should normally be. Syndromes can be in fragments or combinations depending on how the vessels are arranged and if the brain is getting adequate blood supply.[15]
The human brainstem emerges from two of the threeprimary brain vesicles formed of theneural tube. Themesencephalon is the second of the three primary vesicles, and does not further differentiate into asecondary brain vesicle. This will become the midbrain. The third primary vesicle, therhombencephalon (hindbrain) will further differentiate into two secondary vesicles, themetencephalon and themyelencephalon. The metencephalon will become thecerebellum and the pons. The more caudal myelencephalon will become themedulla.
The brainstem plays a role in conduction. That is, all information relayed from the body to the cerebrum and cerebellum and vice versa must traverse the brainstem. The ascending pathways coming from the body to the brain are the sensory pathways and include thespinothalamic tract for pain and temperature sensation and thedorsal column-medial lemniscus pathway (DCML) including thegracile fasciculus and thecuneate fasciculus for touch,proprioception, and pressure sensation. The facial sensations have similar pathways and will travel in the spinothalamic tract and the DCML. Descending tracts are the axons of upper motor neurons destined to synapse on lower motor neurons in theventral horn andposterior horn. In addition, there are upper motor neurons that originate in the brainstem's vestibular, red, tectal, and reticular nuclei, which also descend and synapse in the spinal cord.
The cranial nerves III-XII emerge from the brainstem.[17] These cranial nerves supply the face, head, and viscera. (The first two pairs of cranial nerves arise from the cerebrum).
The brainstem has integrative functions being involved in cardiovascular system control, respiratory control, pain sensitivity control, alertness, awareness, and consciousness. Thus, brainstem damage is a very serious and often life-threatening problem.
Ten of the twelve pairs of cranial nerves either target or are sourced from the brainstem nuclei.[14]: 725 The nuclei of the oculomotor nerve (III) and trochlear nerve (IV) are located in the midbrain. The nuclei of the trigeminal nerve (V), abducens nerve (VI), facial nerve (VII) and vestibulocochlear nerve (VIII) are located in the pons. The nuclei of the glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI) and hypoglossal nerve (XII) are located in the medulla. The fibers of these cranial nerves exit the brainstem from these nuclei.[18]
Diseases of the brainstem can result in abnormalities in the function of cranial nerves that 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.
Duret haemorrhages are areas of bleeding in the midbrain and upper pons due to a downward traumatic displacement of the brainstem.[11]: 842
Cysts known assyrinxes can affect the brainstem, in a condition, calledsyringobulbia. These fluid-filled cavities can be congenital, acquired or the result of a tumor.
Criteria for claimingbrainstem death in the UK have developed in order to make the decision of when to stop ventilation of somebody who could not otherwise sustain life. These determining factors are that the patient is irreversibly unconscious and incapable of breathing unaided. All other possible causes must be ruled out that might otherwise indicate a temporary condition. The state of irreversible brain damage has to be unequivocal. There are brainstem reflexes that are checked for by two senior doctors so thatimaging technology is unnecessary. The absence of thecough andgag reflexes, of thecorneal reflex and thevestibulo-ocular reflex need to be established; the pupils of the eyes must be fixed and dilated; there must be an absence of motor response to stimulation and an absence of breathing marked by concentrations of carbon dioxide in the arterial blood. All of these tests must be repeated after a certain time before death can be declared.[19]
^Vilensky, Joel; Robertson, Wendy; Suarez-Quian, Carlos (2015).The Clinical Anatomy of the Cranial Nerves: The Nerves of 'On Olympus Towering Top'. Ames, Iowa: Wiley-Blackwell.ISBN978-1-118-49201-7.
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