The metencephalon or hindbrain is part of the rhombencephalon and is composed of the cerebellum and bridge (pons). Numerous centers and nuclei contribute to motor function, coordination, and learning processes. Pathologic relevance to the metencephalon is possessed primarily by malformations and lesions that can lead to deficits in functional areas.
What is the metencephalon?
The metencephalon is a part of the brain that belongs to the rhombic brain (rhombencephalon). Because the metencephalon is located in the back of the head, it is also known as the hindbrain. In the embryo, the neural tube represents the precursor of the entire human nervous system. From it, the so-called cerebral vesicles develop within the first 25 days. During embryonic development, the metencephalon forms a coherent structure as the 4th cerebral vesicle, which only later divides into the cerebellum and pons and subsequently forms the finer structures.
Anatomy and structure
The metencephalon consists of two subunits: Cerebellum and Bridge (Pons). The cerebellum has two hemispheres. In cross-section, three layers of the cerebellar cortex can be distinguished, which not only differ histologically, but also each contain specific types of nerves. Beneath the cortex, in the medulla, is the white matter of the cerebellum, which is characterized by numerous nerve fibers. Here are located various nuclei, which represent nodes in information processing. They include the nucleus emboliformis (also known as the nucleus interpositus anterior) and the nucleus globosus (or nucleus interpositus posterior), which are located close to each other, the nucleus dentatus, and the nucleus fastigii. The other part of the metencephalon is the pons or bridge. This structure contains numerous nerve tracts and forms the main link between the medulla oblongata, spinal cord, and peripheral nervous system on the one hand, and the rest of the brain on the other. Also located in the pons are various nuclei: the nuclei motorii, the bridge nuclei (nuclei pontis), the vestibular nuclei (nuclei vestibulares), and the nucleus sensibilis pontinus. Part of the fourth ventricle is also part of the metencephalon; this is a fluid-filled cavity in the brain.
Function and tasks
The tasks of the metencephalon vary by region; overall, motor functions and coordination processes are the main focus. The pons is mainly responsible for the transmission of nerve signals and, in its function as a bridge, represents a bottleneck of the central nervous system. Various cranial nerves originate in the pons. Physiology summarizes the motor nuclei as nuclei motorii. They play a crucial role in the coordination of the supporting muscles and are active, for example, during walking. In the bridge nuclei (nuclei pontis), nerve fibers converge that are involved in learning new movement sequences as well as in correcting movements. Also located in the pons are the vestibular nuclei (nuclei vestibulares); they interconnect information from the vestibular organ in the inner ear with other signals and contribute to processes that require coordination. In addition to supporting motor movements, eye movements also rely on the vestibular nuclei. Sensory fibers of the trigeminal nerve converge in the nucleus sensibilis pontinus. The processing of these stimuli serves protective and defensive mechanisms, for example, when onion vapors irritate the eyes. The cerebellum is characterized by a high diversity of tasks, which has not yet been fully explored. Its four nuclei, numerous synapses and high overall nerve density – the cerebellum contains half of all neurons in the brain – contribute to learning and cooperate with higher cognitive areas. In addition, the cerebellum controls numerous motor processes. In doing so, it also controls very fine muscles that humans need to speak. Coordination, support motor skills, postural motor skills and movement planning are other tasks of the cerebellum. Specific tasks of the nuclei in the cerebellum include control of target motor activity in nucleus dentatus, the largest of the nuclei in the cerebellum. Nucleus emboliformis and nucleus globosus also contribute to target motor function; in addition, they tune support motor function. The nuclei fastigii participate in postural motor function – both in the case of static postures and in dynamic adaptation of movement sequences.Special fibers contribute to appropriate adjustments for eye movements.
Diseases
Diseases of the metencephalon manifest depending on the area affected. Permanent limitations usually result from congenital malformations or acquired lesions due to circulatory disorders, traumatic brain injury, stroke, increased intracranial pressure, tumors, and other underlying diseases. Neurodegenerative diseases such as multiple sclerosis can also affect the metencephalon. In this demyelinating disease, nerve fibers lose their insulating layer due to inflammation; as a result, information processing is impaired. The cerebellum, which is part of the metencephalon, may also be affected. Lesions due to multiple sclerosis typically lead to ataxia: affected persons are no longer able to coordinate or correctly execute movements, although the muscles are completely intact. Gait disturbances are a particularly common form of ataxia. Millard-Gubler syndrome represents an example of symptoms resulting from a Pons lesion, whereby the damage is due to a circulatory disorder. Characteristic signs of this clinical picture are facial paralysis and paralysis of the eye muscle responsible for outward turning movements (abducens paresis); both symptoms manifest on the side of the body damaged by the lesion. In Millard-Gubler syndrome, the other side of the body is incompletely paralyzed (hemiparesis) and shows spastic symptoms. Foville syndrome also results from damage to the pons, often due to a tumor or circulatory disturbance. Symptoms are similar to those seen in Millard-Gubler syndrome, but hemiparesis is not accompanied by spasticity but by loss of sensation (hemianesthesia).
