View source for Brainstem

{{short description|Posterior part of the brain, adjoining and structurally continuous}}
{{more citations needed|date=January 2013}}
{{Infobox brain
| Name            = Brainstem
| Latin           = truncus encephali
| Image           = 1311 Brain Stem.jpg
| Caption         = The three distinct parts of the brainstem are colored in this sagittal section of a human brain.
| IsPartOf        = [[Brain]]
| Components      = [[Medulla oblongata|Medulla]], [[pons]], [[midbrain]]
| Artery          =
| Vein            =
}}
[[File:3D Medical Animation Mid-Brain Different Parts.jpg|alt=3D Medical Animation Still Shot Showing Different Parts of Mid-Brain|thumb|261x261px|A 3D medical animation still shot showing different parts of the midbrain.]]
The '''brainstem''' (or '''brain stem''') is the posterior stalk-like part of the [[brain]] that connects the [[cerebrum]] with the [[spinal cord]].<ref name="Singh">{{cite book |last1=Singh |first1=Vishram |title=Textbook of Anatomy Head, Neck, and Brain; Volume III |date=2014 |isbn=9788131237274 |page=363 |publisher=Elsevier India |edition=2nd}}</ref> In the [[human brain]] the brainstem is composed of the [[midbrain]], the [[pons]], and the [[medulla oblongata]].<ref name="Singh"/> The midbrain is continuous with the [[thalamus]] of the [[diencephalon]] through the [[tentorial notch]], and sometimes the diencephalon is included in the brainstem.<ref name="Haines">{{cite book |last1=Haines |first1=D |last2=Mihailoff |first2=G |title=Fundamental Neuroscience for Basic and Clinical Applications |date=2018 |isbn=9780323396325 |page=152|publisher=Elsevier |edition=5th}}</ref>

The brainstem is very small, making up around only 2.6 percent of the brain's total weight.<ref name="Haines1">{{cite book |last1=Haines |first1=D |last2=Mihailoff |first2=G |title=Fundamental Neuroscience for Basic and Clinical Applications |date=2018 |isbn=9780323396325 |page=195|publisher=Elsevier |edition=5th}}</ref> It has the critical roles of regulating [[heart]] and [[respiratory system|respiratory]] function, helping to control [[heart rate]] and [[breathing rate]].<ref name="Encyclopedia Britannica">{{Cite web|title=Brainstem {{!}} Definition, Structure, & Function|url=https://www.britannica.com/science/brainstem|website=Encyclopedia Britannica|language=en|access-date=2020-05-13}}</ref> It also provides the main motor and sensory nerve supply to the [[face]] and [[neck]] via the [[cranial nerves]]. Ten pairs of cranial nerves come from the brainstem.<ref>{{Cite web|title=Cranial Nerve Nuclei and Brain Stem Circulation|url=https://nba.uth.tmc.edu/neuroanatomy/L10/Lab10p01_index.html|website=Neuroanatomy Online|access-date=2020-05-13}}</ref> Other roles include the regulation of the [[central nervous system]] and the body's [[sleep cycle]].<ref name="Encyclopedia Britannica" /> It is also of prime importance in the conveyance of motor and sensory pathways from the rest of the brain to the body, and from the body back to the brain.<ref name="Encyclopedia Britannica" /> These pathways include the [[corticospinal tract]] (motor function), the [[dorsal column-medial lemniscus pathway]] ([[Fine touch#Fine touch and crude touch|fine touch]], [[Cutaneous receptor#Modalities|vibration sensation]], and [[proprioception]]), and the [[spinothalamic tract]] ([[pain]], temperature, itch, and [[Somatosensory system#Fine touch and crude touch|crude touch]]).<ref>Kolb, B. & Whishaw, I. Q. (2009). ''Fundamentals of human neuropsychology'': 6th ed., New York: Worth Publishers.{{ISBN?}}</ref>

==Structure==
The parts of the brainstem are the midbrain, the pons, and the medulla oblongata; the diencephalon is sometimes considered part of the brainstem.<ref name="Haines" />

The brainstem extends from just above the [[tentorial notch]] superiorly to the [[first cervical vertebra]] below the [[foramen magnum]] inferiorly.<ref name="Sinnatamby-2011">{{Cite book |last=Sinnatamby |first=Chummy S. |title=Last's Anatomy |publisher= Elsevier Australia|year=2011 |isbn=978-0-7295-3752-0 |edition=12th |pages=474}}</ref>

===Midbrain===
{{Main|Midbrain}}
[[File:Brainstem and thalamus ja ja 135.png|thumb|upright=1.4|Diagram showing the position of the colliculi. Superior shown in green and inferior in orange.]]
[[File:Midbrain-axial-showing-tectum-and-tegmentum.jpg|thumb|View of the midbrain showing covering tectum and tegmental floor]]
The [[midbrain]] is further subdivided into three parts: [[tectum]], [[tegmentum]], and the [[ventral tegmental area]].  The [[tectum]] forms the ceiling. The tectum comprises the paired structure of the [[superior colliculus|superior]] and [[inferior colliculus|inferior colliculi]] and is the dorsal covering of the [[cerebral aqueduct]]. The [[inferior 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 the [[medial geniculate nucleus]] of the [[diencephalon]]. The [[superior 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 the [[lateral geniculate body]] of the diencephalon.

The [[tegmentum]] which forms the floor of the midbrain, is ventral to the [[cerebral aqueduct]].  Several [[Nucleus (neuroanatomy)|nuclei]], [[neural tract|tracts]], and the [[reticular formation]] are contained here.

The [[ventral tegmental area]] (VTA) is composed of paired [[cerebral peduncle]]s. These transmit axons of [[upper motor neuron]]s.

====Midbrain nuclei====
The midbrain consists of:
*[[Periaqueductal gray]]:  The [[gray matter]] around the [[cerebral aqueduct]] contains neurons involved in the [[pain]] desensitization pathway.  Neurons synapse here. When stimulated by a signal, the synaptic connections activate neurons in the [[nucleus raphe magnus]]. The pathway then projects down into the [[posterior grey column]] of the spinal cord, inhibiting pain sensation transmission.
*[[Oculomotor nerve]] nucleus:  This is the third [[cranial nerve nucleus]].
*[[Trochlear nerve]] nucleus:  This is the fourth cranial nerve.
*[[Red nucleus]]: This is a motor nucleus that sends a descending tract to the [[lower motor neuron]]s.
*[[Substantia nigra pars compacta]]: This is a concentration of neurons in the ventral portion of the midbrain that uses [[dopamine]] as its [[neurotransmitter]] and is involved in both motor function and emotion. Its dysfunction is implicated in [[Parkinson's disease]].
*[[Reticular formation]]: This is a large area in the [[midbrain]] 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 the [[locus coeruleus]], which is involved in intensive [[alertness]] modulation and in [[Autonomic nervous system|autonomic]] reflexes.
*[[Central tegmental tract]]: Directly anterior to the floor of the [[fourth ventricle]], this is a pathway by which many tracts project up to the cortex and down to the spinal cord.
*[[Ventral tegmental area]]: A [[dopaminergic]] [[nucleus (neuroanatomy)|nucleus]], known as [[Dopaminergic cell groups#Cell group A10|group A10 cells]]<ref name="Hegarty">{{cite journal |last1=Hegarty |first1=SV |last2=Sullivan |first2=AM |last3=O'Keeffe |first3=GW |title=Midbrain dopaminergic neurons: a review of the molecular circuitry that regulates their development. |journal=Developmental Biology |date=15 July 2013 |volume=379 |issue=2 |pages=123–38 |doi=10.1016/j.ydbio.2013.04.014 |pmid=23603197|doi-access=free }}</ref> is located close to the midline on the floor of the midbrain.
*[[Rostromedial tegmental nucleus]]: A [[GABAergic]] nucleus located adjacent to the ventral tegmental area.

<gallery>

Midbrain - superior colliculus.svg|Cross-section of the midbrain at the level of the superior colliculus
Midbrain - inferior colliculus.svg|Cross-section of the midbrain at the level of the inferior colliculus
</gallery>

===Pons===
{{Main|Pons}}
{{expand section|date=April 2014}}
The pons lies between the midbrain and the [[medulla oblongata]]. It is separated from the midbrain by the [[superior pontine sulcus]], and from the medulla by the [[inferior pontine sulcus]].<ref name="Carpenter">{{cite book |last1=Carpenter |first1=Malcolm |title=Core text of neuroanatomy |date=1985 |publisher=Williams & Wilkins |isbn=0683014552 |pages=42–43 |edition=3rd}}</ref> It contains tracts that carry signals from the [[cerebrum]] to the medulla and to the [[cerebellum]] and also tracts that carry sensory signals to the [[thalamus]]. The pons is connected to the cerebellum by the [[cerebellar peduncles]]. The pons houses the respiratory [[pneumotaxic center]] and [[apneustic center]] that make up the pontine respiratory group in the [[respiratory center]]. The pons co-ordinates activities of the [[Cerebellum|cerebellar hemispheres]].<ref>{{Cite book |title=Discovering the Brain |last= Ackerman| first= Sandra |chapter=Major Structures and Functions of the Brain |year=1992 |publisher= National Academies Press|location= Washington (DC)|doi= 10.17226/1785 |pmid= 25121239|isbn= 978-0-309-04529-2|url=https://www.ncbi.nlm.nih.gov/books/NBK234157/}}</ref>
The [[pons]] and [[medulla oblongata]] are parts of the [[hindbrain]] that form much of the brainstem.

<gallery>

Pons - Middle.svg|Cross-section of the middle pons (at the level of cranial nerve V)
Pons - Inferior.svg|Cross-section of the inferior pons (at the level of the facial genu)
</gallery>

===Medulla oblongata===
{{Main|Medulla oblongata}}
{{expand section|date=April 2014}}
The [[medulla oblongata]], often just referred to as the medulla, is the lower half of the brainstem continuous with the spinal cord. Its upper part is continuous with the pons.<ref name="Alberts">{{cite book|last1=Alberts|first1=Daniel|title=Dorland's Illustrated Medical Dictionary.|date=2012|publisher=Saunders/Elsevier|isbn=978-1-4160-6257-8|edition=32nd|location=Philadelphia, PA}}</ref>{{rp|1121}} The medulla contains the [[cardiovascular centre|cardiac]], dorsal and ventral [[respiratory group]]s, and [[vasomotor center|vasomotor]] centres, dealing with [[heart rate]], [[breathing]] and [[blood pressure]]. Another important medullary structure is the [[area postrema]] whose functions include the control of [[vomiting]].

<gallery>

Medulla - Rostral level cross section.svg|Cross-section of the rostral (superior) medulla
Medulla - Middle level cross section.svg|Cross-section of the middle medulla
Medulla - Inferior level cross section.svg|Cross-section of the inferior medulla
</gallery>

===Pontomedullary junction===
The pons meets the medulla at the pontomedullary junction.<ref name="Mercier">{{cite journal |last1=Mercier |first1=PH |last2=Brassier |first2=G |last3=Fournier |first3=HD |last4=Picquet |first4=J |last5=Papon |first5=X |last6=Lasjaunias |first6=P |title=Vascular microanatomy of the pontomedullary junction, posterior inferior cerebellar arteries, and the lateral spinal arteries. |journal=Interventional Neuroradiology |date=30 March 2008 |volume=14 |issue=1 |pages=49–58 |doi=10.1177/159101990801400107 |pmid=20557786|pmc=3313705 }}</ref> This region is supplied by the joining of the [[Basilar artery|basilar]], [[vertebral artery|vertebral arteries]]. The [[posterior 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.<ref name="Mercier"/>

===Appearance===
;From the front
[[File:Slide2RAFA.JPG|thumb|The appearance of a [[cadaver]]ic brainstem from the front, with major parts labelled]]

In the medial part of the [[Medulla oblongata|medulla]] is the [[anterior median fissure of the medulla oblongata|anterior median fissure]].  Moving laterally on each side are the [[medullary pyramids (brainstem)|medullary pyramids]].  The pyramids contain the fibers of the [[corticospinal]] 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 the [[anterior grey column]] of the [[spinal cord]].

The [[anterolateral sulcus of medulla|anterolateral 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 the [[Olivary body|olives]].  The olives are swellings in the medulla containing underlying inferior nucleary nuclei<ref>{{Cite journal |last1=Ding |first1=Song-Lin |last2=Royall |first2=Joshua J. |last3=Sunkin |first3=Susan M. |last4=Ng |first4=Lydia |last5=Facer |first5=Benjamin A. C. |last6=Lesnar |first6=Phil |last7=Guillozet-Bongaarts |first7=Angie |last8=McMurray |first8=Bergen |last9=Szafer |first9=Aaron |last10=Dolbeare |first10=Tim A. |last11=Stevens |first11=Allison |last12=Tirrell |first12=Lee |last13=Benner |first13=Thomas |last14=Caldejon |first14=Shiella |last15=Dalley |first15=Rachel A. |date=2017-02-01 |title=Comprehensive cellular-resolution atlas of the adult human brain |journal=The Journal of Comparative Neurology |volume=525 |issue=2 |pages=407 |doi=10.1002/cne.24130 |issn=1096-9861 |pmc=6886049 |pmid=27917481}}</ref> (containing various nuclei and afferent fibers).  Lateral (and dorsal) to the olives are the rootlets for [[Cranial nerves|CN]] IX ([[glossopharyngeal]]), CN X ([[vagus]]) and CN XI ([[accessory nerve]]).  The pyramids end at the [[pons|pontine]] [[medulla oblongata|medulla]] junction, noted most obviously by the large [[basal 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 (the [[trigeminal nerve]]) emerges.  Cranial nerve III (the [[oculomotor nerve]]) emerges ventrally from the midbrain, while the CN IV (the [[trochlear nerve]]) emerges out from the dorsal aspect of the midbrain.

Between the two pyramids can be seen a [[decussation]] 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
[[File:Slide4RAFA.JPG|thumb|The appearance of a [[cadaver]]ic brainstem from behind, with major parts labelled]]
The most medial part of the [[medulla oblongata|medulla]] is the [[posterior median sulcus of medulla oblongata|posterior median sulcus]].  Moving laterally on each side is the [[gracile fasciculus]], and lateral to that is the [[cuneate fasciculus]].  Superior to each of these, and directly inferior to the [[obex]], are the gracile and cuneate tubercles, respectively.  Underlying these are their respective nuclei.  The obex marks the end of the [[fourth ventricle]] and the beginning of the [[central canal]].  The posterior intermediate sulcus separates the gracile fasciculus from the cuneate fasciculus.  Lateral to the cuneate fasciculus is the [[lateral funiculus]].

Superior to the obex is the floor of the [[fourth 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 the [[vagal trigone]] and superior to that it the [[hypoglossal 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 the [[striae medullares]].  Continuing in a [[Anatomical terms of location#Directional terms|rostral]] direction, the large bumps are called the facial colliculi.  Each [[facial 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, or [[sulcus (anatomy)|sulcus]] that runs rostrally, and is known as the [[sulcus limitans]].  This separates the medial motor neurons from the lateral sensory neurons.  Lateral to the sulcus limitans is the area of the [[vestibular 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 the [[inferior 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.

===Blood supply===
[[File:Blausen 0114 BrainstemAnatomy.png|thumb|The brainstem receives blood via the [[vertebral artery|vertebral arteries]], shown here.]]
The main supply of blood to the brainstem is provided by the [[basilar artery|basilar arteries]] and the [[vertebral artery|vertebral arteries]].<ref name="Purves" />{{rp|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.<ref>{{Cite web |date=2015-07-21 |title=Brainstem Arteries |url=https://www.umassmed.edu/strokestop/modules/module-3-the-blood-supply-of-the-brain/vertebral-basilar/brainstem-arteries/ |access-date=2024-12-06 |website=UMass Chan Medical School |language=en}}</ref>

===Development===
The human brainstem emerges from two of the three [[brain vesicle|primary brain vesicles]] formed of the [[neural tube]].  The [[mesencephalon]] is the second of the three primary vesicles, and does not further differentiate into a [[brain vesicle|secondary brain vesicle]].  This will become the midbrain.  The third primary vesicle, the [[rhombencephalon]] (hindbrain) will further differentiate into two secondary vesicles, the [[metencephalon]] and the [[myelencephalon]].  The metencephalon will become the [[cerebellum]] and the pons.  The more caudal myelencephalon will become the [[medulla oblongata|medulla]].

==Function==
The brainstem plays important functions in [[breathing]], [[heart rate]], [[arousal]] / [[consciousness]], [[sleep]] / [[wakefulness|wake]] functions and [[attention]] / concentration.<ref>{{citation | title=Simplified Brain Behavior Relationships | publisher=Brain Injury Association of Virginia | location=Alexandria, Virginia, US |year= 2005}}</ref>

There are three main functions of the brainstem:
# 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 the [[spinothalamic tract]] for pain and temperature sensation and the [[dorsal column-medial lemniscus pathway]] (DCML) including the [[gracile fasciculus]] and the [[cuneate 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 the [[Anterior horn of spinal cord|ventral horn]] and [[posterior horn of spinal cord|posterior 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.<ref>{{cite web |url=http://vanat.cvm.umn.edu/NeuroLectPDFs/CranialN-Lect.pdf |title=Lecture 6: Cranial Nerves |access-date=2012-11-10 |url-status=live |archive-url=https://web.archive.org/web/20130418031405/http://vanat.cvm.umn.edu/NeuroLectPDFs/CranialN-Lect.pdf |archive-date=2013-04-18 }}</ref> 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.

===Cranial nerves===
[[File:Brain stem sagittal section.svg|thumb|A cross-section of the brainstem showing the multiple [[cranial nerve nucleus|nuclei]] of the ten pairs of [[cranial nerves]] that emerge from it]]
{{expand section|date=October 2016}}{{Main|Cranial nerves}}

Ten of the twelve pairs of cranial nerves either target or are sourced from the brainstem nuclei.<ref name="Purves">{{cite book|last1=Purves|first1=Dale|title=Neuroscience|date=2011|publisher=Sinauer|location=Sunderland, Mass.|isbn=978-0-87893-695-3|edition=5th.}}</ref>{{rp|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.<ref name="Vilensky">{{cite book|title=The Clinical Anatomy of the Cranial Nerves: The Nerves of 'On Olympus Towering Top'|last1=Vilensky|first1=Joel|last2=Robertson|first2=Wendy|last3=Suarez-Quian|first3=Carlos|publisher=Wiley-Blackwell|year=2015|isbn=978-1-118-49201-7|location=Ames, Iowa}}</ref>

==Clinical significance==
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.

[[Brainstem stroke syndrome]] can cause a range of impairments including [[locked-in syndrome]].

[[Duret haemorrhages]] are areas of bleeding in the midbrain and upper pons due to a downward traumatic displacement of the brainstem.<ref name="Alberts" />{{rp|842}}

Cysts known as [[syrinx (medicine)|syrinxes]] can affect the brainstem, in a condition, called [[syringobulbia]]. These fluid-filled cavities can be congenital, acquired or the result of a tumor.

Criteria for claiming [[brainstem 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 that [[imaging technology]] is unnecessary. The absence of the [[cough reflex|cough]] and [[gag reflex]]es, of the [[corneal reflex]] and the [[vestibulo-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.<ref>Black's Medical Dictionary 39th edition 1999</ref>

==Additional images==
<gallery>

File:Human brain frontal (coronal) section description.JPG|The midbrain, pons, and medulla oblongata are labelled on this coronal section of the human brain.
File:Human brainstem.gif|3D visualization of the brainstem in an average human brain
</gallery>

==See also==
{{Anatomy-terms}}

* [[Triune brain]] – reptilian brain

==References==
{{reflist|30em}}

==External links==
{{Commons category|Brainstem}}
*[[v:Comparative Neuroscience|Comparative Neuroscience]] at [[v:|Wikiversity]]
{{Nervous system}}
{{Medulla}}
{{Pons}}
{{Mesencephalon}}

{{Authority control}}

[[Category:Brainstem| ]]
[[Category:Neurophysiology]]