Table of Contents
Embryonic Development of the Rhombencephalon
During embryonic development, the brain, spinal cord, and central nervous system grow from the neural tube, which develops from the dorsal surface ectoderm. Three primary cerebral ganglia develop from the cranial part of the neural tube.
One of these cerebral ganglia becomes the rhombencephalon. The other two cerebral ganglia form the prosencephalon (forebrain) and the mesencephalon (midbrain). The cerebellum, medulla oblongata, and pons grow from the rhombencephalon.
Structure of the Metencephalon
Topography of the cerebellum
Topographically, the cerebellum is located dorsal to the medulla oblongata and the pons. The mesencephalon and medulla oblongata are connected to this via the cerebellar veil (superior and inferior medullary velum).
In addition, the cerebellum forms the roof of the fourth ventricle. The tentorium cerebelli, which forms a duplication of the dura mater, is located apical to the cerebellum.
Because the dura mater consists of taut conjunctive tissue, an increase in pressure from cerebral edema may result in impaction of the cerebellum in the tentorial incisure. This constriction is also referred to as upper impaction.
Lower impaction is the rostral movement of the brain with the impaction of the cerebellum in the foramen magnum. During lower impaction, the pressure of the cerebellar tonsils on the respiratory center may be fatal.
Macroscopic structure of the cerebellum
The cerebellum arises from the rhombic lips or alar plates of the metencephalon. On gross examination, it appears as an ovoid structure that resides in the posterior cranial fossa inferior to the tentorium cerebelli. In an anatomical specimen, the cerebellum can be divided macroscopically into the vermis (cerebellar worm) and the two hemispheres. The outer surface of the cerebellum exhibits numerous grooves (fissures) and twists (folia), which lead to a significant increase in the cerebellum’s surface area.
The cerebellum can also be divided into three sections. These are the cerebellar vermis (worm); the nodulus, located in the lower part of the vermis; and the flocculus, located to the side of the nodulus. The nodulus and flocculus are jointly referred to as the lobus flocculonodularis.
The vermis connects the two cerebellar hemispheres, and each hemisphere can be divided into three lobes. These are the lobus flocculonodularis, the lobus anterior, and the lobus posterior.
The fissura prima is located between the two lobes. Two additional fissures in the cerebellum are the fissura horizontalis and the fissura posterolateralis, which separates the lobus posterior from the lobus flocculonodularis.
Depending on its function, the cerebellum can also be divided into three entities. These are the vestibulocerebellum, the spinocerebellum, and the pontocerebellum. Their names stem from the main portion of the afferents that enter each respective entity.
The vestibulocerebellum is found in the flocculonodular lobe and draws most of its afferents from the vestibular system. The spinocerebellum receives its afferents primarily from the spinal cord and consists of the cerebellar vermis and the surrounding areas. The afferents from the pontine nuclei are drawn to the pontocerebellum, which consists of the two cerebellar hemispheres.
The efferents and afferents of the cerebellum pass through the three cerebellar peduncles—the pedunculus cerebellaris superior, the pedunculus cerebellaris medius, and the pedunculus cerebellaris inferior.
Structure of the cerebellar cortex
The neurons of the cerebellum are located in the cerebellar cortex. Histologically, the cerebellar cortex is divided into three layers. From the inside out, there is the granular layer (stratum granulosum), the Purkinje layer (stratum purkinjense), and the molecular layer (stratum moleculare).
As the name implies, the granular layer consists mostly of granule cells. These cells transmit their information through the stimulating neurotransmitter glutamate. The mossy fibers that contain all the afferents of the cerebellum end at the granule cells—except for afferents from the olivary bodies. The fibers extending from the olivary bodies are called ‘climbing fibers.’
The Purkinje layer consists of the Purkinje cells, which are the largest in the cerebellar cortex. Their cell bodies (somata) are located in the cerebellar hemispheres, and their cell processes (axons) advance to the cerebellar nuclei (see below) and the molecular layer.
The stratum moleculare is the outermost layer of the cerebellar cortex.
The cerebellar nuclei consist of four halved areas. These are the nucleus dentatus, the nucleus emboliformis, the nucleus globosus, and the nucleus fastigii.
The nucleus dentatus (toothed) is located in the center of the cerebellum. Medial to the nucleus dentatus lies the nucleus emboliformis (plug shaped), and medial to this is the nucleus globosus (sphere shaped). The nucleus fastigii (gable) is the most medially located area.
Afferents of the cerebellum
The cerebellum receives many afferents that pass through the three cerebellar peduncles to the cerebellar cortex. On the way, some collaterals branch out into the cerebellar nuclei.
The tractus vestibulocerebellaris, the tractus olivocerebellaris, the tractus reticulocerebellaris, and the tractus spinocerebellaris posterior are among the afferents that pass through the inferior cerebellar peduncle (pedunculus cerebellaris inferior).
The tractus vestibulocerebellaris receives its information through the nuclei vestibulares. For the most part, this information ends in the area of the lobus flocculonodularis.
The fibers of the olivary bodies cross to the other side at the level of the brain stem and then pass through the tractus olivocerebellaris on their way to the cerebellum. Thus, they reach the Purkinje cells of the cerebellar cortex in the form of climbing fibers.
Proprioceptive information from the spinal cord is forwarded to the cerebellum via the tractus reticulocerebellaris.
The tractus spinocerebellaris posterior stems from the posterior horn of the spinal cord, where the nucleus dorsalis is located. The information passed through the tractus originates in the ipsilateral half of the body and reaches the granular layer in the area of the spinocerebellum as mossy fibers.
The fibrae pontocerebellaris of the nuclei pontis pass through the pedunculus cerebellaris medius and are a continuation of the corticopontine pathways. The fibers cross over to the other side before passing through the middle cerebellar peduncle and move to the cortex of the cerebellar hemispheres, i.e. the area of the pontocerebellum. The cerebellum receives information on planned movements of the cerebrum via the fibrae pontocerebellaris.
The tractus spinocerebellaris anterior is an afferent of the pedunculus cerebellaris superior. It feeds information from the ipsilateral half of the body to the spinocerebellum.
Efferents of the cerebellum
The efferent pathways of the cerebellum originate from the cerebellar nuclei to the thalamus, the vestibular nuclei, the nucleus ruber, and the formatio reticularis.
Usually, fibers from a specific functional unit of the cerebellum end at their respective cerebellar nuclei, whereby the fibers originate in the cerebellar cortex and inhibit the cerebellar nuclei. The afferent pathways to the cerebellar cortex, in turn, have an excitatory effect on the cerebellar nuclei.
The fibers from the vestibulocerebellum primarily run to the nucleus fastigii, those from the spinocerebellum run to the nucleus globosus and the nucleus emboliformis, and the axons from the pontocerebellum run to the nucleus dentatus. The nucleus globosus and nucleus emboliformis are jointly referred to as the nucleus interpositus.
The efferent pathways move through the cerebellar peduncles to their respective destinations in a manner similar to that of the afferents of the cerebellum. The fibers of the vestibulocerebellar tract, which originate in the core of the nucleus fastigii, pass through the pedunculus cerebellaris inferior.
The tractus cerebellothalamicus and the tractus cerebellorubralis pass through the pedunculus cerebellaris superior.
Because of its origins in the nucleus dentatus, the tractus cerebellothalamicus is also referred to as the ‘dentatothalamic tract,’ and its fibers primarily extend to the nucleus ventralis lateralis of the thalamus. On the way to this position, they pass to the contralateral side, immediately after entering the tegmentum. From the thalamus, the information continues to the motor cortex.
The tractus cerebellorubralis contains fibers from the nucleus interpositus and the nucleus dentatus. These fibers also cross over to the opposite side before ending at the nucleus ruber at the level of the mesencephalon. The pathways cross again from the nucleus ruber to the spinal system so that the information of the ipsilateral half of the body is transmitted.
Function of the cerebellum
The cerebellum plays a crucial role in the coordination and storing of movement processes, primarily those concerning fine motor processes and balance. To facilitate this role, the cerebellum receives numerous afferents from the motor areas of the brain and subsequently returns the information through its efferents.
Each of the three functional entities plays a significant role based on their specific regulation. For instance, the vestibulocerebellum is primarily responsible for maintaining balance and relaying eye movements, the spinocerebellum transmits information on the respective position of the body, and the pontocerebellum is important for coordinating movement, mainly with regard to quick, successive, partially antergic actions.
Damage to the cerebellum may result in symptoms such as dysarthria, nystagmus, intention tremor, ataxia, dysdiadochokinesia, dysmetria, or rebound phenomenon.
Structure of the pons
The brain stem is made up of the pons, the mesencephalon (midbrain), and the medulla oblongata (elongated medulla).
The word pons means ‘bridge,’ alluding to the fact that the pons provides a connection between the medulla oblongata, the cerebellum, and the mesencephalon. Together, the pons and medulla oblongata border the rhomboid fossa (see below), the base of the fourth ventricle. The cerebellum is connected to the pons via the pedunculus cerebellaris medius.
The exit points of cranial nerves VII (nervus facialis) and VIII (nervus vestibulocochlearis) are found at the point where the pons, medulla oblongata, and cerebellum meet. This point is also called the ‘cerebellopontine angle.’ Cranial nerve VI (nervus abducens) and cranial nerve V (nervus trigeminus) also exit in the area of the pons.
The nuclei pontis and several cranial nerve nuclei are located inside the pons. The cranial nerve nuclei located here are the abducens nucleus, the facial nucleus, the vestibulocochlear nuclei, the nucleus principalis nervi trigemini, and the nucleus motorius nervi trigemini.
The nucleus salivatorius, which transmits parasympathetic influences, is found in the area of the facial nucleus.
Structure of the Myelencephalon
The medulla oblongata forms the lower part of the brain stem and has a caudal connection to the spinal cord. By definition, it extends from the pons to the end of C1, the first cervical nerve. Along with the pons, the cranial segment forms the base of the fourth ventricle (fossa rhomboidea).
Contents of the rhomboid fossa (fossa rhomboidea)
The colliculus facialis, formed by the inner abducens nerve, is found in the middle of the rhomboid fossa. Lateral to this, the protrusion of the vestibular nuclei forms the colliculus facialis and the area vestibularis, and the protrusion of the nucleus cochlearis posterior forms the tuberculum acusticum.
Furthermore, the parasympathetic nucleus areas of cranial nerve X (nervus vagus) and cranial nerve XII (nervus hypoglossus) protrude caudally into the rhomboid fossa as the trigonum nervi vagi and trigonum nervi hypoglossi, respectively.
In addition, the area postrema (vomiting center)—which is one of the circumventricular organs and part of the formatio reticularis (see below)—is located in the caudal area of the rhomboid fossa.
Topography of the medulla oblongata
The medulla oblongata is cranially connected with the cerebellum via the velum medullare inferius (inferior medullary velum) and the pedunculus cerebellaris inferior.
The pyramids, which contain the tractus corticospinalis (pyramidal tracts), border the medulla oblongata on the ventral side, i.e. toward the frontal lobe. A majority (approximately 80%) of the fibers of the pyramidal tracts cross over to the contralateral site at the level of the medulla oblongata, causing this location to be called the decussatio pyramidum (pyramidal decussation).
The remaining fibers cross over further along, and because of this, motor phenomena and failures on the left side of the body are due to damage to the right hemisphere.
The olivary bodies (olivae), which are connected to the cerebellum via the tractus olivocerebellaris, are located next to the pyramids. Information between the motor centers and the cerebellum is passed from here through the nucleus olivaris inferior to the olivary bodies.
Cranial nerve XII (nervus hypoglossus) exits between the two olivary bodies in the sulcus preolivaris. Other cranial nerves that exit in the area of the olivary bodies are IX (nervus glossopharyngeus), X (nervus vagus), and XI (nervus accessorius).
The tuberculum gracile and the tuberculum cuneatum (which contain the nucleus gracilis and nucleus cuneatus, respectively) are located dorsal to the medulla oblongata. The tuberculum gracile lies medial to the tuberculum cuneatum and is the synapse of the epicritic afferents of the trunk and lower half of the body.
Crossover of the afferents from the arm and throat area occurs in the tuberculum cuneatum. The lemniscus medialis travels from the tuberculi to the contralateral thalamus as a cohesive efferent. The thalamus acts as the third synapse and the third neuron to subsequently forward the information to the somatosensory cortex.
Circulatory and respiratory center
The circulatory and respiratory center is another functional center located next to the area postrema in the vicinity of the medulla oblongata.
In addition, autonomic centers for regulating heartbeat, metabolism, the width of blood vessels, and individual reflexes (e.g., coughing) are located here. These centers are functionally ascribed to the formatio reticularis (see below).
Structure of the Formatio Reticularis
The formatio reticularis serves to regulate motor and autonomic functions. To this end, it extends across the entire brain stem and contains multiple regulatory centers and nuclei. It also plays a crucial role in regulation of the circadian rhythm.
Nuclei of the formatio reticularis
The nuclei of the formatio reticularis are—in contrast to the nuclei of the cranial nerves—diffuse in their differentiation. The nuclei can be categorized into medial and lateral groups by their localization and into other groups by the transmitter they contain.
The nuclei of the medial group contain rather large neurons, whereas those of the lateral group consist mainly of small neurons. The larger neurons form ascending and descending pathways with their axons, while the axons of the small neurons usually do not extend through the brain stem.
Serotonin, noradrenaline, adrenaline, acetylcholine, and dopamine are some of the transmitters of the nuclei.
One example of a serotoninergic nucleus is the raphe nuclei, which extends into the hypothalamus, the neocortex, and the limbic system. These nuclei are located in the medulla oblongata and the pons, as well as the mesencephalon, wherein they are each located on either side of the midline. They play a crucial role in the modulation of emotions and moods. A release of serotonin also results in deep sleep.
The locus coeruleus contains the transmitter noradrenaline and modulates the control of motor processes in the cerebellar area. The release of noradrenaline occurs mainly during the waking state. It sends stimulation throughout the entire cortex area of the brain, resulting in increased alertness.
The transmitter dopamine is located mainly in the area tegmentalis ventralis (ventral tegmental area), which is found in the mesencephalon. The fibers chiefly project into the limbic system, which includes the amygdala and the hippocampus, and into the mesolimbic system (hypothalamus). Dopamine, which is transmitted through the nucleus accumbens, has a euphoric effect, among others.
Nuclei in the vicinity of the formatio reticularis that contain acetylcholine include the pedunculopontine nucleus and the dorsal tegmental nucleus. The cholinergic neurons are activated in the waking state and during rapid-eye-movement (REM) sleep, although they are inactive during deep sleep.
The fibers of the formatio reticularis, which are responsible for the waking state, are jointly referred to as the ascending reticular activating system (ARAS). Damage to or disruption of these fibers can lead to a comatose state.
However, systems other than the formatio reticularis are also responsible for the waking state. The other systems that have a waking effect are the locus coeruleus (transmitted by the transmitter noradrenaline), the cholinergic fibers from the basal forebrain (e.g., the nucleus basalis of Meynert), the histaminergic fibers from the thalamus (nucleus centromedianus), and the glutamatergic fibers from the thalamus (intralaminar nuclei).
Substances that inhibit the systems mentioned above can cause sedation or a reduced state of alertness. For instance, benzodiazepine or barbiturates decrease the activity of the formatio reticularis. Alcohol also has a suppressive effect on the activity of the ARAS.
Respiratory center of the formatio reticularis
Two groups of neurons regulate respiration. One group is made up of the inspiratory neurons and the other the expiratory neurons.
The rhythmic alteration between inspiration and expiration is regulated by a cellular complex located in the vicinity of the ventral medulla oblongata (pneumotaxic center). This is called the pre-Bötzinger complex. Failure of this cellular complex results in breathing disorders.
Circulatory center of the formatio reticularis
The neurons for circulatory regulation are divided into blood pressure–increasing and blood pressure–decreasing neurons. Multiple neurons with the same function are found in specific regions. The neurons that raise blood pressure tend to be located laterally.
The circulatory center is regulated by fibers from the hypothalamus, which pass through the fasciculus medialis telencephali (medial forebrain bundle).
Structure of the area postrema (vomiting center)
The area postrema is one of the circumventricular organs. It is also located in the area of the formatio reticularis.
A characteristic of the circumventricular organs is the fact that the blood–brain barrier is usually interrupted here—substances from the blood can pass directly into the cerebral parenchyma (and vice versa).
Certain substances can trigger a vomiting reaction, and the stimulus is transmitted by a serotoninergic receptor of the subtype 5-HT3. An increased concentration of serotonin in the blood, e.g., as a result of chemotherapy, can thus stimulate these receptors, thereby inducing a vomiting response. The vomiting stimulus can, in turn, be effectively suppressed by 5-HT3 antagonists.
The dopaminergic D2 receptor and the histamine H1 receptor are two other receptors located in the area postrema, and stimulation of these results in illness and vomiting. Therefore, antagonists can be applied as antiemetics against D2 and H1 receptors.
Gaze centers of the formatio reticularis
The gaze centers are divided into the pontine and the vertical. The pontine gaze center, which regulates horizontal movements of the eye, is made up of the paramedian pontine reticular formation (PPRF), whereas the vertical gaze center comprises the rostral interstitial nucleus of medial longitudinal fasciculus (riFLM).
The medial longitudinal fasciculus connects the three eye muscle nuclei of cranial nerves III, IV, and VI, and both sides run ventrally to the aqueductus mesencephali up to the cervical spinal cord.
Damage to the medial longitudinal fasciculus results in internuclear ophthalmoplegia (INO). The most common cause of damage to the bundle is multiple sclerosis.
A clinical symptom of internuclear ophthalmoplegia is that when looking to the side, the contralateral eye remains in the center and the ipsilateral eye exhibits monocular nystagmus. The convergence reaction, however, is regular.