The tegmentum is a part of the brainstem that includes the midbrain, bridge (pons), and medulla oblongata. It contains numerous nuclear areas (nuclei) and nerve tracts, some of which perform motor functions and others sensory or sensitive functions. Nonspecific lesions to the tegmentum may occur, for example, in the setting of stroke, neurodegenerative disease, or as a result of mechanical injury.
What is the tegmentum?
The word “tegmentum” comes from Latin and literally means “hood.” The tegmentum owes its name to the fact that it forms a continuous layer that appears to be attached. The anatomical structure is composed of three parts that span the midbrain, bridge and medulla oblongata. All three areas are part of the brainstem, also known as the truncus cerebri or truncus encephali. In the embryo, this brain area develops from the second and third cerebral vesicles, but the cerebellum is excluded from the brainstem. The tegmentum is contiguous with the internal cerebrospinal fluid space formed by the four cerebral ventricles together with the spinal canal (canalis centralis).
Anatomy and structure
When speaking of the tegmentum in general, the tegmentum mesencephali or midbrain cap is often meant; however, in a strict sense, the tegmentum mesencephali is only one of three parts of the tegmentum. The other two parts are the tegmentum myelencephali (afterbrain cap) and the tegmentum pontis (bridge cap). Here, the tegmentum myelencephali belongs to the medulla oblongata (medulla oblongata), while the tegmentum pontis is part of the bridge (pons). Adjacent to the tegmentum mesencephali in the midbrain are the cerebral crura cerebri and the aquaeductus mesencephali, on the other sides of which the tectum mesencephali faces the tegmentum mesencephali. In the bridge and the medulla oblongata, the tegmentum lies posterior to each of the structures. All three parts of the tegmentum house different nuclear areas and neural pathways. Because of the insulating layer surrounding the nerve fibers, neurology also refers to them as white matter, whereas the nuclear areas consist of clusters of nerve cell bodies known as gray matter.
Function and tasks
The fifth cranial nerve (triplet nerve, trigeminal nerve) originates in the tegmentum. Its motor fibers originate from the nucleus motorius nervi trigemini, whereas its sensory fibers originate from the nucleus spinalis nervi trigemini in the medulla oblongata, the nucleus principalis nervi trigemini in the pons, or the nucleus mesencephalicus nervi trigemini in the midbrain. Like the trigeminal nerve, the medial loop pathway (lemniscus medialis) passes throughout the brainstem. The core areas of the medial loop pathway are in the medulla oblongata, but its course takes it through the bridge and midbrain to the thalamus. The caeruleal nucleus or locus caeruleus is located in the tegmentum pontis. It belongs to the formatio reticularis, which is a network of nuclei and other neurons. Its function includes attention and orientation control, for which, however, it is not the only one responsible. The sixth cranial nerve (abducens nerve) controls certain eye movements, while the seventh cranial nerve (facial nerve) carries sensory, sensitive and parasympathetic nerve fibers in addition to motor ones. The motor fibers of the facial nerve participate in the control of facial expressions, hearing, speech, jaw opening, and swallowing. The nuclei of the eighth cranial nerve (auditory nerve, vestibulocochlear nerve) are also located in the tegmentum pontis. The substantia nigra, which belongs to the extrapyramidal motor system and is thus involved in movement control, is located in the tegmentum mesencephali, among others. The nucleus ruber is also part of this system. In addition, the tegmentum mesencephali is home to the nucleus nervi oculomotorii and the nucleus accessorius nervi oculomotorii; both are responsible for eye movements. The nucleus nervi trochlearis is the motor nucleus of the fourth cranial nerve (nervus trochlearis), which participates in the control of eye movements. In addition, numerous other fibers pass through the tegmentum mesencephali.
Diseases
The nuclei of the abducens nerve in the tegmentum pontis can suffer damage as part of a skull base fracture, resulting in double vision and impaired eye movements.The trochlear nerve is also a possible cause of disorders affecting eye movements: Typically, when there is damage to the trochlear nerve, the eye turns medially upward. Lesions to the facial nerve may be due to tumors, hemorrhage, Lyme disease, temporal bone fracture, or stroke, and often result in facial paralysis. The trigeminal nerve may also be responsible for paralysis; damage to it also allows sensory disturbances in the face. Functional limitations of the nerves and their core areas can occur in a variety of ways. One possible cause that can affect any area of the tegmentum is stroke. In this case, the occlusion of a blood vessel supplying the brain leads to undersupply and corresponding failures in the affected brain areas. This damage can be permanent if the undersupply results in the death of nerve cells, which is the case with prolonged circulatory disorders. Rapid action is therefore required in the event of a stroke to limit the damage. Another example of a disease affecting the tegmentum is Parkinson’s disease. It is based on the loss of nerve cells in the substantia nigra and manifests itself in the form of muscle rigidity (rigor), muscle tremor (tremor), slowing of movements (bradykinesis) and postural instability. Furthermore, other neurodegenerative diseases such as Alzheimer’s dementia or multiple sclerosis may affect the gray and white matter of the tegmentum.
