Human Anatomy Part 2 - K. A. Diubenko A. K. Kolomiitsev Yu. B. Chaikovskyi 2008

Special Part
Nervous system, systema nervosum - Special Part
Brain - Embryogenesis of the brain - Midbrain

Class="center">The Midbrain, mesencephalon, is derived from the mesencephalic vesicle and belongs to the Brainstem, truncus cerebri (Fig. 143 A, B; 143 B). It is the shortest part of the brainstem, measuring about 2.5 cm in length. The anterior part of the midbrain is located between the Pons and the Diencephalon and consists of two main parts: the tectum, tectum mesencephali, which contains the subcortical auditory and visual centers, and the cerebral peduncles, pedunculi cerebri, through which the tracts pass.

The cerebral peduncles are located on the ventral surface of the midbrain, while the dorsal surface is formed by the corpora quadrigemina. The BOUNDARIES OF THE midbrain are arbitrary: the posteroinferior boundary runs below the exit point of the cerebral peduncles from the pons, the posterosuperior boundary lies behind the mammillary bodies, and the dorsal boundary is at the level of the posterior commissure beneath the Pineal Gland.

The cavity of the midbrain is represented by the cerebral aqueduct, aqueductus cerebri (aqueductus mesencephali). This is a remnant of the primary cavity of the midbrain

Internal Structure of the midbrain

In cross-section, the midbrain is conventionally divided into three parts (Fig. 145 A):

1. The Base of the cerebral peduncles, basis pedunculi cerebri;

2. the tectum, tectum;

3. the tegmentum, tegmentum.

The base of the cerebral peduncles, basis pedunculi cerebri, consists of White matter. The fibers gather to form the central part of the peduncles, which contains the corticospinal and corticonuclear fibers; medial to them are fibers, and laterally are the parietotemporooccipitopontine fibers.

Fig. 145 A. Internal STRUCTURE OF THE midbrain, Horizontal section of the midbrain at the level of the superior colliculi

Fig. 145 B. Section of the midbrain at the level of the superior colliculi (diagram)

The boundary between the base of the cerebral peduncles and the tegmentum is the substantia nigra. The substantia nigra occupies the space along the cerebral peduncles from the pons to the diencephalon; it is band-shaped, wide in the middle and tapered at the ends. The substantia nigra is divided into two parts: the compact part, pars compacta, formed by Nerve Cells rich in the pigment melanin, and the reticular part, pars reticularis, formed by a network of myelinated fibers.

The substantia nigra has extensive connections with other Brain structures: the Cerebral Cortex, striatum, subthalamic Nucleus, thalamus, and red nucleus.

The substantia nigra, together with the red nucleus and the reticular Formation of the midbrain, plays an important role in motor Functions and the maintenance of Muscle tone. Together with the globus pallidus system, it provides statokinetic function. Along with the Regulation of Muscle tone, the substantia nigra is involved in the coordination of swallowing and mastication. Neurons of the substantia nigra release the neurotransmitter dopamine. In patients suffering from Parkinson's disease (paralysis agitans), degeneration of substantia nigra neurons occurs, leading to a dopamine deficiency. L-DOPA is a therapeutic drug prescribed to these patients. It promotes dopamine synthesis and alleviates the course of the disease.

The tegmentum of the midbrain, tegmentum mesencephali, is located between the tectum and the substantia nigra. In the midbrain tegmentum, dorsal to the substantia nigra, sensory pathways to the cerebral hemispheres pass, including the lateral and medial lemnisci, with the exception of the olfactory and visual pathways.

In the angles of the midbrain tegmentum formed by the lateral and medial lemnisci, there are the superior cerebellar peduncles, pedunculi cerebellaris, which originate in the cerebellar nuclei and decussate in the midbrain to form Wernekink's decussation.

Rostrally (above) to the decussation, There are two round structures—the red nuclei, nucleus ruber. These are the largest nuclei of the midbrain tegmentum, where a significant portion of the fibers of the superior cerebellar peduncles, globus pallidus, thalamus, and Branches of the lateral and medial lemnisci, as well as the nuclei of the midbrain tectum, terminate. The rubrospinal tract, tractus rubrospinalis, originating from the red nuclei, reaches the motor neurons of the anterior horns of the Spinal Cord after decussating (Forel's decussation)*—the ventral tegmental decussation, decussatio tegmenti centralis (Fig. 146 B).

The red nuclei are closely connected with the Cerebellum, vestibular nuclei, Basal Ganglia, and the motor cortex. Working in conjunction with the reticular formation, the red nuclei ensure normal body posture in space, regulate posture, inhibit tonic antigravity Reflexes of the spinal cord, and regulate muscle tone.

*Auguste Henri Forel (1848-1931) was a Swiss psychiatrist.

Thus, the tectum of the midbrain is a reflex center for various movements initiated by visual and auditory stimuli.

Alongside the posterior longitudinal fasciculus lies the tectospinal tract, tractus tectospinalis. Its fibers originate from the Cells of the superior colliculi and partially from the inferior colliculi. Emerging from the colliculi, the fibers arch around the central Gray matter dorsal to Forel's decussation and form Meynert's fountain-like decussation*, decussatio tractum tegmenti (Fig. 145 B). The tectobulbar fibers of this tract terminate in the brainstem, in the nuclei of the nerves innervating the Muscles of the Eyeball, while the tectospinal fibers terminate in the cells of the anterior horns of the spinal cord.