Brainstem

Original Editor - Wendy Walker

Lead Editors - Lucinda hampton, Stacy Schiurring, Jess Bell, Olajumoke Ogunleye, Kim Jackson, Evan Thomas, Pacifique Dusabeyezu, Wendy Walker, Garima Gedamkar, WikiSysop, Rucha Gadgil, Aya Alhindi, Tarina van der Stockt, Daniele Barilla and Carina Therese Magtibay

Brainstem Overview

Brainstem anatomy

The brainstem is a stalk-like projection of the brain extending caudally from the base of the cerebrum. It is one of the most ancient parts of the brain, and it acts to bridge communication between the cerebrum, cerebellum, and spinal cord:

• the brainstem has three parts: midbrain, pons, and medulla oblongata  
• it maintains functions necessary for life, such as (1) breathing, (2) consciousness, (3) maintaining blood pressure, (4) heart rate, and (5) sleep regulation
• the brainstem contains collections of white and grey matter:
  • the grey matter consists of nerve cell bodies and includes important brainstem nuclei (e.g., the nuclei of ten of the twelve cranial nerves originate from the brainstem
  • the white matter tracts of the brainstem consist of neuron axons that travel between the cerebrum, cerebellum, and spinal cord. Afferent tracts carry information to the brain (e.g., somatosensory pathways) and efferent pathways carry information from the brain (e.g., corticospinal tract)

Brainstem Anatomy and Structure

The brainstem is located in the posterior cranial fossa of the skull. While the nervous system is a continuous neuronal network and functions in a continuous manner, the brainstem has distinct anatomical areas and boundaries:

• superiorly (or ventrally): the midbrain is continuous with the cerebral hemisphere
• inferiorly (or caudally): the medulla is continuous with the spinal cord
• posteriorly: the pons and medulla are separated from the cerebellum by the fourth ventricle

Midbrain (Mesencephalon)

The midbrain is the smallest portion of the brainstem and is continuous with the diencephalon (the area of the brain which contains the epithalamus, thalamus, hypothalamus, ventral thalamus and the third ventricle).

Anterior Midbrain

The anterior midbrain contains the cerebral peduncles. The cerebral peduncles are a pair of fibre bundles that are separated by the interpeduncular fossa. They originate in the cerebral cortex. The corticospinal tract and the corticobulbar tract travel through the cerebral peduncles.

Posterior Midbrain

The tectum (i.e. the roof) of the midbrain is formed by two pairs of rounded protrusions, the superior and inferior colliculi:

• superior colliculi are involved in visual processing and eye movements
• inferior colliculi are involved in auditory processing

The posterior midbrain connects and communicates with the cerebellum posteriorly through the superior cerebellar peduncles.

Clinically Relevant Brainstem Nuclei of the Midbrain

• Red nuclei: located anteromedially in the midbrain at the level of the superior colliculus. The exact function of the red nuclei is still being investigated, but they are likely involved in locomotor functions, in the execution of "simple, stereotyped movements," and specialised movements of the upper limb. The red nuclei is the origin of the descending rubrospinal tract. The rubrospinal tract functions primarily to transmit signals into the red nucleus from the cerebral motor cortex and cerebellum to the spinal cord.
• Substantia nigra: located at the base of the midbrain. The substantia nigra has two parts: the pars compacta and the pars reticulata. It is rich in dopamine neurons and is considered part of the basal ganglia. The substantia nigra plays a critical role in modulating motor movement and reward functions. In Parkinson's, neurodegeneration occurs due to the death of dopamine-producing cells in the substantia nigra. This causes the hallmark movement dysfunction associated with Parkinson's.
• Cranial Nerve IV (CN IV, trochlear nerve): located just inferior to the inferior colliculi. CN IV is unique among its fellow cranial nerves as it is the only one to emerge from the posterior surface of the brainstem.
• Cranial nerve III (CN III, oculomotor nerve): originates at the oculomotor sulcus within the interpeduncular cistern.

Pons (Metencephalon)

Pons and medulla, anterior.

Moving, rostrally, the next portion of the midbrain is the pons. It is located between the midbrain and the medulla. The pons is notable for its rounded shape and "bridges" (pons is Latin for bridge). It connects the two cerebellar hemispheres and forms important connections with the cerebellum via fibre bundles known as the cerebellar peduncles.

Anterior Pons

The basilar artery runs through the basilar groove on the anterior surface of the pons. The anterior pons contains various nerve tracts:

• the pontocerebellar tracts: run through the middle cerebellar peduncles, and assist with precise movement coordination, in particular the dextrous movements of the hands and upper extremities
• the corticospinal tracts: run through the base of the pons

Posterior Pons

The posterior pons connects and communicates with the cerebellum via the middle cerebellar peduncle. The posterior portion of the pons also forms part of the floor of the fourth ventricle.

Clinically Relevant Brainstem Nuclei Located in the Pons

• Cranial nerve V (CN V, trigeminal nerve), exits from the superior anterior lateral pons
• Cranial nerve VI (CN VI, abducens nerve)
• Cranial nerve VII (CN VII, facial nerve)
• Cranial nerve VIII (CN VIII, vestibulocochlear nerve)
  • CN VI, VII, and VIII emerge at the pontomedullary junction (a groove formed where the pons meets the medulla)

Medulla (Myelencephalon)

Pyramidal decussation

The last and more distal, or caudal, section of the brainstem is the medulla (also known as the medulla oblongata). The medulla is continuous with the spinal cord and joins at the level of the foramen magnum.

Please note that "bulb" is an old term for the medulla oblongata. You will see the word "bulbar" used to describe conditions associated with the medulla (e.g. bulbar palsy). It is also used to describe structures connected to the medulla, such as the muscles innervated by the cranial nerves that originate in the medulla. For example, the muscles of the tongue, pharynx and larynx are known as "bulbar muscles".

Anterior Medulla

The anterior median fissure runs down the front of the medulla, separating the paired enlargements known as the pyramids. The pyramids carry motor fibres from the motor cortex of the cerebrum and to the spinal cord. The majority of these motor fibres cross (or decussate) at the level of the medulla to the opposite side (making up the lateral corticospinal tract). The motor fibres which do not cross at this point make up the medial (or anterior) corticospinal tract:

• the olivary bodies (olives) are lateral to the pyramids: these oval protrusions are formed by the underlying olivary nuclei. This group of nuclei is located near the border of the medulla and the pons. There are two olivary nuclei:
  • the inferior olives are more prominent than the superior olives. This group of nuclei coordinates information between the spinal cord and cerebellum to regulate fine motor control and movement coordination. The inferior olives may also play a role in motor learning. This group of nuclei is located in the medulla.
  • the superior olives are involved in the auditory nervous system and assist in the perception of sound. This group of nuclei are located in the pons.
• the inferior cerebellar peduncles connect the medulla on each side to the cerebellum
• the spinothalamic tract ascends through the lateral medulla outside of the pyramids, carrying information about pain and temperature sensations

Posterior Medulla

The posterior portion of the medulla connects to the spinal cord and forms a portion of the floor of the fourth ventricle.

The posterior median sulcus separates two fibre bundles: fasciculus gracilis and fasciculus cuneatus. These bundles contain the ascending posterior column tracts carrying sensory information up from the spinal cord:

• fibres carrying information from the lower extremities and the trunk synapse at the gracilis nucleus and fibres carrying information from the upper extremities and the upper body above T6 (except for the face and ears) synapse at the cuneate nucleus
• fibres leaving these nuclei decussate at the level of the medulla after which they form the medial lemniscus tracts and continue travelling up towards the thalamus. Together, this is known as the posterior column-medial lemniscus pathway which carries information about fine touch, proprioception, and vibration sensations

Clinically Relevant Brainstem Nuclei Located in the Medulla

• Cranial nerve IX (CN IX, glossopharyngeal nerve)
• Cranial nerve X (CN X, vagus nerve)
• Cranial nerve XI (CN XI, accessory or spinal accessory nerve)
• Cranial nerve XII (CN XII, hypoglossal nerve)

Brainstem Nerve Tracts

The reticular formation is a network of brainstem nuclei and neurons with a vast number of connections to different and far-reaching systems. It extends from the spinal cord, through the brainstem and into the diencephalon. It plays a role in vital functions necessary for survival, such as consciousness, heart rate, respiratory rate, autonomic regulation of blood pressure, as well as movement coordination, postural reflexes, taste, and wakefulness and sleep:

• the reticular formation does not have clearly defined boundaries and receives input from various nerve tracts including: (1) spinothalamic tracts, (2) spinoreticular tracts, (3) dorsal column-medial lemniscus pathway, (4) visual pathways, (5) auditory pathways, (6) vestibular pathways, and (7) cerebelloreticular pathways
• the reticular formation has efferent fibres to the (1) thalamic nuclei, (2) cerebellum, (3) red nucleus, (4) corpus striatum, (5) substantia nigra, (6) hypothalamus, and (7) subthalamic nucleus

Motor Tracts

• Corticospinal tracts: a collection of axons that carry movement-related information from the cerebral cortex to the spinal cord. These tracts allow the cerebral cortex to connect to spinal motor neurons and control movement of the torso, and upper and lower limbs
• Corticobulbar tracts: arise from the upper motor neurons of the cranial nerves. The muscles of the face, head, and neck are controlled by the corticobulbar system

Sensory Tracts

• Spinothalamic tract: part of the anterolateral system (which also includes the spinoreticulothalamic tract and the spinotectal tract). This tract carries sensory information from the trunk and limbs to the sensory cortex of the cerebellum. The spinothalamic tract is made of two separate yet indistinct pathways:
  • anterior spinothalamic tract: carries sensory information about course (or crude) touch and pressure
  • lateral spinothalamic tract: carries sensory information on pain and temperature
• Dorsal column (also known as the dorsal column-medial lemniscus pathway): an ascending tract that carries sensory information from the peripheral nerves to the cerebral cortex. This pathway travels from the periphery through the dorsal column of the spinal cord, through the medial lemniscus of the brainstem, to the postcentral gyrus in the cerebral cortex:
  • it relays sensory information about fine touch, 2-point discrimination, conscious proprioception, and vibration sense from the skin and joints of the body (this does not include information from the head)
• Trigeminal lemniscus and spinotrigeminal tract
  • carries information about pain and temperature from the face
  • this information enters the brainstem via CN V
• Lateral lemniscus
  • carries auditory information from the cochlear nuclei via CN VIII

Blood Supply of the Brainstem

The blood supply into the brain comes from the vertebral and internal carotid arteries. The vertebral arteries supply the brainstem directly. The paired vertebral arteries come together on the anterior brainstem to form the basilar artery, which lies in the basilar groove on the pons. The network formed by the vertebral and basilar arteries is called the vertebral-basilar system:

• the midbrain is supplied by the superior cerebellar arteries and posterior cerebral arteries
• the pons is supplied by the basilar artery and its branches. This includes the anterior inferior cerebellar arteries, the pontine branches or paramedian arteries, and superior cerebellar arteries
• the medulla oblongata is supplied by the vertebral arteries and their branches, the anterior spinal artery, the posterior spinal arteries, and the posterior inferior cerebellar arteries

Clinical Significance

Common medical or rehabilitation diagnoses that affect the brainstem can present with significant clinical impairments and limitations. Diagnoses include stroke, malignancy, and demyelinating processes. Below is limited list of diagnoses associated with damage or injury to the brainstem:

• multiple sclerosis (MS): cerebellar and brainstem (pyramidal) symptoms are common in the early stages of MS
  • a study published in 2020 suggests that patients who present with early brainstem symptoms have a better prognosis, and develop disability slower, than those who present with early cerebellar symptoms
• stroke (or cerebral vascular accident, CVA) that affects the brainstem can cause severe symptoms, including:
  • difficulty chewing and swallowing
  • impaired speaking ability
  • weakness or paralysis in the arms, legs, and/or face
  • balance impairments
  • changes or impairments in sensation to include hearing loss and vision problems
  • vertigo
  • locked-in syndrome
  • coma
  • changes in the regulation of vital functions, such as involuntary breathing ability
  • Wallenberg syndrome: a Wallenberg stroke can present with a wide variety of impairments, but typically includes motor, sensory, cognitive, perceptual, speech and language deficits
  • medial pontine syndrome (also known as Foville syndrome): results from an infarction of the medial inferior pons. Individuals with this condition present with contralateral hemiplegia/hemiparesis, contralateral loss of proprioception and vibration, ipsilateral ataxia, ipsilateral facial weakness (CN VII), and lateral gaze paralysis and diplopia (CN VI).

• cerebellar tonsillar herniation results in sudden respiratory and cardiac arrest due to compression of the medulla
• central pontine myelinolysis, from the rapid correction of hyponatraemia, can result in seizures, ataxia, and disturbed consciousness
• diffuse intrinsic pontine glioma (DIPG) is a type of fast-growing brain tumour that can grow in the pons of the brainstem. It is a form of childhood cancer.
• recent research into Long COVID, a post-viral illness affecting COVID-19 survivors, suggests a link between symptoms such as fatigue, dyspnoea, gastrointestinal and cardiac issues, cognitive impairments, and myalgia with viral-induced dysfunction in the brainstem

The following optional video provides a summary of brainstem stroke.

Additional Resources

• Neuroanatomy - Brainstem
• Know your brain