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You are watching: The midbrain pons and medulla oblongata are housed in the
StatPearls
Introduction
The brainstem is the structure that connects the cerebrum of the brain to the spinal cord and cerebellum. It is composed of four sections in descending order: the diencephalon, midbrain, pons, and medulla oblongata. It is responsible for many vital functions of life, such as breathing, consciousness, blood pressure, heart rate, and sleep. The brainstem contains many critical collections of white and grey matter. The grey matter within the brainstem consists of nerve cell bodies and form many important brainstem nuclei. The white matter tracts of the brainstem include axons of nerves traversing their course to different structures; the axons originate from cell bodies located elsewhere within the central nervous system (CNS). Some of the white matter tract cell bodies are located within the brainstem as well. These tracts travel both to the brain (afferent) and from the brain (efferent) such as the somatosensory pathways and the corticospinal tracts, respectively. Ten of the twelve cranial nerves arise from their cranial nerve nuclei in the brainstem. Clinicians can localize lesions of the brainstem with a thorough knowledge of brainstem anatomy and functions. The following activity addresses the nuclei, tracts, and functions of the brainstem.
Structure and Function
The brainstem contains many different nuclei and tracts. This section overviews the major structures located in each area of the brainstem, followed by a summary of the course of the major pathways through the brainstem.
Diencephalon
The diencephalon is the most superior portion of the brainstem and connects to the midbrain inferiorly. This structure encompasses the third ventricle, which connects the interventricular foramen of the lateral ventricles to the cerebral aqueduct. The diencephalon is composed of four major portions: the epithalamus, subthalamus, hypothalamus, and thalamus.<1> Some sources state the subthalamus is one of the four components of the diencephalon while others refer to the pretectum in place of the subthalamus; however, both terms refer to the inferior portion of the diencephalon that is continuous with the midbrain.<1><2> The epithalamus is located in the posterior diencephalon and is composed of the pineal gland, habenular commissure, and posterior commissure.<2> The subthalamus is the upward continuation of the tegmentum of the midbrain and forms the greater part of the inferior portion of the third ventricle. The substantia nigra and red nucleus extend upward from the midbrain into the inferior subthalamus. The acetylcholine-producing neurons of the nucleus of Meynert are located within the subthalamus. The loss of neurons of the nucleus of Meynert occurs in several pathologies that result in dementia.<3> The subthalamus also contains the subthalamic nucleus at the base of the cerebral peduncle. The subthalamic nucleus participates in the regulation of movement with the basal ganglia. The hypothalamus is located in the anterior inferior portion of the diencephalon and includes the infundibulum, mammillary bodies, hypophysis, and many hypothalamic nuclei. The posterior thalamus protrudes and overlaps the superior colliculus of the midbrain, the medial portion of this is the pulvinar. The pulvinar extends laterally to the lateral geniculate bodies which play a role in vision. The medial geniculate bodies are located inferior to the pulvinar and play a role in the auditory pathway.
The thalamus is the most substantial portion of the diencephalon, composing its remainder as two large oval masses flanking the third ventricle. The thalamus serves as a relay point for all sensory input tracts. The thalamus divides into several components composed of different thalamic nuclei and tracts. The anterior thalamus contains the anterior thalamic nuclei that receive communication from the mammillary nuclei via the mammillothalamic tract and play a role in the limbic system. The medial thalamus contains the dorsomedial nucleus and has connections with hypothalamic nuclei and the prefrontal cortex. The medial thalamus integrates an immense amount of sensory data. The lateral thalamus subdivides into dorsal and ventral components. The dorsal portion includes the lateral dorsal nucleus, pulvinar, and the lateral posterior nucleus. The ventral portion contains the ventral anterior and lateral nuclei; these nuclei affect activities of the motor cortex because they have connections to the reticular formation, substantia nigra, and corpus striatum. The ventral portion also contains the ventral posterior medial nuclei which have connections with the ascending trigeminal tracts, as well as the ventral posterior lateral nuclei which serve as the primary sensory relay station for input from the body. The major nuclei have been addressed in this section, but some studies divide the thalamus into 14 major nuclei, and those can be subdivided further into many subnuclei.<4>
Midbrain
The midbrain serves as the connection between the pons and the diencephalon. It also connects posteriorly to the cerebellum via the superior cerebellar peduncles. The anterior part of the midbrain contains the crus cerebri with the interpeduncular fossa located between them. The crus cerebri carry motor cortical spinal fibers, corticonuclear fibers, and pontine fiber tracts. The midbrain contains the cerebral aqueduct centrally which connects the third ventricle superiorly with the fourth ventricle inferiorly. The periaqueductal grey surrounds the cerebral aqueduct. The midbrain is separated relative to the cerebral aqueduct with the posterior portion being the tectum (floor) and anterior to the aqueduct serving as the tegmentum (roof). The posterior surface of the midbrain contains the corpora quadrigemina which are composed of bilateral superior colliculi and bilateral inferior colliculi. The superior colliculi are involved in visual reflexes such as saccadic eye movements. Each superior colliculus sends fibers to the corresponding lateral geniculate body and optic tract through the superior brachium. The inferior colliculi are involved in auditory processing and connect to their corresponding medial geniculate nuclei through the inferior brachium. Just inferior to the inferior colliculi at the posterior midline of the brainstem cranial nerve IV, the trochlear nerve, emerges. The trochlear nerve is unique among cranial nerves as it is the only one to emerge from the posterior surface of the brainstem. The other cranial nerve that arises from the midbrain is cranial nerve III, the oculomotor nerve. The oculomotor nerve arises from the midbrain in the oculomotor sulcus on the medial surface of the crus cerebri, within the interpeduncular cistern.
The midbrain also includes many other important nuclei including but not limited to the substantia nigra at the base of the midbrain, the red nucleus anterior medially at the level of the superior colliculus, and the dorsal raphe nucleus. The largest of the raphe nuclei are in the anterior midline of the periaqueductal grey, and the location of the ventral tegmental area is near the midline medial to the red nucleus. The substantia nigra contains dopaminergic neurons that help to regulate movement associated with the basal ganglia. The ventral tegmental area also contains dopaminergic neurons and plays a role in reward pathways.<5> The raphe nuclei contain serotonergic neurons and project widely throughout the brain. The periaqueductal grey is thought to play a role in pain suppression.<6> The red nucleus is involved with movement and contains many connections with the cerebellum.<7> The medial longitudinal fasciculus lies anterior to the periaqueductal grey and plays a role in coordinating eye movements.
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Pons
The pons connects the medulla oblongata inferiorly to the midbrain superiorly. The anterior portion of the pons is convex and can be easily seen as a visible distention when viewing the brainstem anteriorly. The surface of the anterior distention contains the basilar groove, which is where the basilar artery rests. The posterior pons is connected to the cerebellum by the middle cerebellar peduncles which are the largest of the cerebellar peduncles. The posterior portion of the pons forms the superior portion of the floor of the fourth ventricle. A groove is formed inferiorly where the pons meets the medulla from which cranial nerves VI, VII, and VIII emerge medially to laterally. Cranial nerve V, the largest cranial nerve, exits from the superior anterior lateral pons. Important nuclei of the pons include the cranial nerve nuclei covered in the nerves section, the locus coeruleus, and pontine nuclei. The neurons of the locus coeruleus produce norepinephrine and have projections that spread widely throughout the CNS. The locus coeruleus is located in the posterior lateral pons at the lateral border of the periaqueductal grey and is involved in the reticular activating system. The locus coeruleus also suffers compromise in Alzheimer disease.<8><8> The pontine nuclei are a collection of pontine motor nuclei in the anterior pons that have many connections with the cerebellum via the middle cerebellar peduncle and assist with coordinating movement and help to modulate breathing.<9>
Medulla Oblongata
The most inferior portion of the midbrain is the medulla oblongata, which connects the pons to the spinal cord. It meets the spinal cord at the level of the foramen magnum. The anterior portion of the medulla oblongata contains the pyramids. The pyramids carry motor fibers from the precentral gyrus, or motor cortex, to the grey matter of the spinal cord where they synapse and continue to the muscles of the body through the peripheral nervous system. The pyramids contain a decussation caudally in which the majority of the motor fibers contained cross to the contralateral side of the body. The fibers that decussate become the lateral corticospinal tract in the spinal cord, the fibers that do not decussate become the medial corticospinal tract in the anterior portion of the spinal cord. The pyramids lie on either side of the anterior median fissure, a midline groove that continues caudally along the anterior portion of the spinal cord. Lateral to the pyramids are the olivary bodies that are part of the olivocerebellar system. The hypoglossal nerve, cranial nerve XII, emerges from the anterior surface of the medulla from the sulcus between the olivary bodies and the pyramids. Posterior to the olivary bodies is the postolivary groove. Cranial nerves IX (glossopharyngeal), X (vagus), and XI (accessory) emerge from the postolivary groove in superior to inferior order. Posterior to the postolivary grooves are the inferior cerebellar peduncles that connect the medulla to the cerebellum. The posterior portion of the medulla oblongata inferiorly connects to the spinal cord. The posterior median sulcus is located in the midline on the posterior aspect of the inferior medulla and continues caudally along the posterior spinal cord. The posterior median sulcus in the posterior midline of the spinal cord is flanked by visible localized prominences called the gracile tubercles. On each side, the gracile tubercle contains the gracile nucleus. Similar bilateral localized prominences called the cuneate tubercles (containing the cuneate nucleus on each side) are just lateral to each of the gracile tubercles on the posterior aspect of the spinal cord. The gracile and cuneate tubercles carry second-order neurons of the dorsal column-medial lemniscus system. The gracile nucleus carries fibers for the lower extremities and trunk, and the cuneate nucleus carries fibers for the upper body above T6 except for the face and ears. The superior area of the posterior medulla oblongata forms a portion of the floor of the fourth ventricle.<1>
Major Brainstem Tracts
The Reticular Formation
The reticular formation is found in the anterior portion of the brainstem and is composed of multiple tracts that have a large number of connections. The reticular formation extends from the spinal cord through the brainstem to the diencephalon. It receives input from various tracts including, spinothalamic tracts, spinoreticular tracts, the dorsal column-medial lemniscus pathway, visual pathways, auditory pathways, vestibular pathways, and cerebelloreticular pathways. The reticular formation sends efferent fibers to the thalamic nuclei, cerebellum, red nucleus, corpus striatum, substantia nigra, hypothalamus, and subthalamic nucleus.<1> The vast connections of the reticular formation allow it to modulate many different functions; some of these include movement coordination, autonomic regulation of blood pressure, heart rate, and respiratory rate, postural reflexes, neuro-vegetative reflexes, and taste.<1> It also plays a role in wakefulness and sleep.<10>
The Motor Tracts
Corticospinal Tracts
The majority of the upper motor neurons of the motor tracts originate in the precentral gyrus. The corticospinal fibers descend through the posterior limb of the internal capsule to the crus cerebri and then down the anterior pons to the pyramids of the medulla. At the pyramids, the majority of the corticospinal fibers decussate and descend the spinal cord as the lateral corticospinal tract and eventually continue to supply motor innervation to the limbs and digits. The majority of corticospinal fibers that do not cross over at the medullary pyramids become the medial corticospinal tracts, located anteriorly in the spinal cord, and provide innervation to the muscles of the trunk.<11>
Corticobulbar Tracts
The corticobulbar tracts descend through the genu of the internal capsule and down through a similar course as the corticospinal fibers; however, the corticobulbar fibers exit this course and synapse at the appropriate cranial nerve nuclei at their respective levels. The majority of corticospinal fibers decussate while only some of the corticobulbar fibers decussate as described in the nerves section of this article. The corticobulbar tracts also contain connections with many of the sensory nuclei of the brainstem.<1>
The Sensory Tracts
Spinothalamic Tract or Anterolateral System
The spinothalamic tract is responsible for conveying pain and temperature information from the body to the brain. Peripheral neurons carry sensory information to the posterior column of the spinal cord. After synapsing in the spinal cord, the axons ascend two to three levels before decussating. After decussating, the fibers ascend as the lateral and anterior spinothalamic tracts in the anterior and lateral portions of the spinal cord. When the tracts ascend through the medulla, they merge to form the spinothalamic tract and course along the lateral portion of the medulla. The tract continues up the lateral portion of the anterior pons and midbrain to the ventral posterior lateral thalamus where the axons synapse and continue up through the posterior limb of the internal capsule to enter the post-central gyrus of the cortex.<12>
Dorsal Column-Medial Lemniscus
The dorsal column-medial lemniscus tract is responsible for carrying afferent proprioception, fine touch, two-point discrimination, and vibration to the cortex from the body. Peripheral neurons carry sensory information to the posterior column of the spinal cord and ascend in the posterior portions of the spinal cord as the gracile fasciculus and cuneate fasciculus. The neurons in these fasciculi will synapse of the gracile nucleus and cuneate nucleus at the level of the inferior medulla respectively. The second order neurons will decussate at the level of the medulla and become the medial lemniscus. The medial lemniscus maintains a medial position within the brainstem as it ascends to the ventral posterior lateral thalamus. After synapsing in the thalamus, the fibers continue through the posterior limb of the internal capsule to the post-central gyrus of the cortex.<13>
Trigeminal Lemniscus and Spinotrigeminal Tract
Pain and temperature sensory input from the face enters the brainstem via cranial nerve V. The fibers that carry this information enter the brainstem and descend parallel to the spinal trigeminal nucleus before synapsing in it. Their descent forms the spinotrigeminal tract. After these fibers synapse, they decussate to the contralateral side and ascend as a part of the trigeminal lemniscus.<1>
The trigeminal lemniscus carries sensory axons from the second-order neurons of the principal sensory nucleus of the trigeminal nerve, which contain discriminative touch and oral cavity proprioception. These neurons do not descend before synapsing after entering the brainstem. Most of these fibers decussate to the contralateral side on their course to the ventral posterior medial thalamus and then proceed to the post-central gyrus of the cortex. The fibers of the trigeminal lemniscus ascend the pons and midbrain posterior to the medial lemniscus.
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Lateral Lemniscus
The lateral lemniscus carries auditory information from the cochlear nuclei at the level of the inferior pons superiorly to the superior olivary complex, nuclei of the lateral lemniscus, inferior colliculi, and eventually to the medial geniculate body which sends the auditory information to the temporal lobes of the cerebral cortex. Some of the fibers of the lateral lemniscus decussate while others do not. The lateral lemniscus travels up the posterior lateral portion of the pons and is important for sensory input to the brain.<14>
