What is the midbrain?
The midbrain (mesencephalon) is a part of the brainstem in the brain. Among other things, it is responsible for the control of coordination. In addition, it plays an important role for hearing and seeing, but also for the sensation of pain.
The midbrain consists of different parts: Towards the back (dorsal) lies the midbrain roof (tectum mesencephali) with the quadruple mound plate (lamina tecti or quadrigemina). In the middle (towards the abdomen = ventral) is the tegmentum mesencephali (hood). In front are two bulges, the cranial crura cerebri, between which is a pit (fossa interpeduncularis) in which the 3rd cranial nerve runs.
Midbrain roof with tetrapod plate.
The tetrapod plate is divided into four mounds by a longitudinal and transverse furrow (two upper: superior colliculi and two lower: inferior colliculi). Between the upper two hills lies the pineal gland (corpus pineale) of the diencephalon.
From each of the four mounds there is a cord that drains into the diencephalon. The upper strand pulls partly into the visual mound, partly into the visual pathway (tractus opticus). A cord from the posterior mound, a primary auditory center, carries fibers from the central auditory pathway. Between the lower two mounds is a strip of white matter, on the side of which the 4th cranial nerve (trochlear nerve) exits.
Midbrain roof
Cerebral peduncles
The cerebral peduncles at the anterior surface of the base of the midbrain are pierced by vessels and another cranial nerve, the oculomotor nerve (3rd cranial nerve), exits here.
The midbrain is traversed by the aquaeductus mesencephali, the thin, canal-like connection between the IIIrd and IVth cerebral ventricles (ventricles). Cerebral ventricle (ventricle).
What is the function of the midbrain?
The midbrain is part of the extrapyramidal system, where the control of movement takes place. For example, the mesencephalon is responsible for controlling almost all eye muscles – for example, opening and closing the eyelids – via the 3rd cranial nerve (oculomotor nerve).
A nucleus of the 5th cranial nerve (trigeminal nerve) is located in the midbrain. It is responsible for the sensitivity of the masticatory muscles, the temporomandibular joint and the external eye muscles.
The cord that pulls from the midbrain quadruple plate into the optic tract carries the pathway for the pupillary reflex.
The nucleus ruber pulls into the spinal cord and influences muscle tone. Signals for movement are mediated in the substantia nigra. Via the midbrain, stimuli coming from the spinal cord and via the diencephalon are transmitted to the cerebrum. In the opposite direction, stimuli from the cerebrum are transmitted to nerve cells in the spinal cord that are responsible for motor function.
Where is the midbrain located?
The midbrain is located between the bridge (pons) and the diencephalon. It surrounds the aquaeductus mesencephali.
What problems can the midbrain cause?
When the midbrain is lesioned (for example, by a tumor), there are disturbances in movement, gait, and concentration. Disturbances in eye movements and pupils may also be indicative of a tumor in the mesencephalon.
Parkinson’s disease is characterized by a deterioration of the cells in the substantia nigra. The neurotransmitter dopamine, which is necessary for stimulus transmission, is missing. The result is dysfunction and disturbances in motor function.
Changes in the substantia nigra of the midbrain are also responsible for attention deficit disorder (ADD) and attention deficit hyperactivity disorder (ADHD).
When the midbrain is damaged, affected individuals are dazed and have a delayed reaction to external stimuli.
