Midbrain, mesencephalon, develops in the process of phylogenesis under the predominant influence of the visual receptor, therefore its most important formations are related to the innervation of the eye. Here, centers of hearing were formed, which, together with the centers of vision, subsequently expanded in the form of four mounds of the roof of the midbrain.
With the appearance in higher animals and humans of the cortical end of the auditory and visual analyzers in the forebrain cortex, the auditory and visual centers of the midbrain themselves fell into a subordinate position and became intermediate, subcortical. With the development of the forebrain in higher mammals and humans, pathways began to pass through the midbrain, connecting the cerebral cortex with the spinal cord (brain stems).
As a result, in the middle brain of a person has:
1) subcortical centers of vision and nerve nuclei that innervate the muscles of the eye,
2) subcortical auditory centers,
3) all the ascending and descending pathways connecting the cerebral cortex with the spinal cord and passing through the midbrain,
4) beams of white matter connecting the midbrain with other parts of the central nervous system.
Accordingly midbrain, which is the smallest and most simply arranged part of the brain in humans, has two main parts: the roof, where the subcortical centers of hearing and vision are located, and the legs of the brain, where the pathways predominantly pass.
Dorsal part, roof of the midbrain, tectum mesencephali.
It is hidden under the posterior end of the corpus callosum and is subdivided by two criss-cross grooves - longitudinal and transverse - into four mounds in pairs.
Top two mounds, colliculi superiores, are subcortical centers of vision, both lower, colliculi inferiores, - subcortical hearing centers. The pineal gland lies in the flat groove between the upper tubercles. Every hillsto the so-called knob, brachium colliculiheading laterally, anteriorly and upward, to the diencephalon. Upper knob, brachium colliculi superioris, goes under pillow, pulvinar, thalamus to lateral geniculate body, corpus geniculatum laterale.
Lower knoll handle, brachium colliculi inferioris, passing along the upper edge of trigonum lemnisci to sulcus lateralis mesencephali, disappears under medial geniculate body, corpus geniculatum mediale... The named geniculate bodies already belong to the diencephalon.
Ventral part, legs of the brain, pedunculi cerebri, contains all the pathways to the forebrain.
Brain legs look like two thick semi-cylindrical white strands that diverge from the edge of the bridge at an angle and plunge into the thickness of the cerebral hemispheres.
Midbrain cavity which is the remainder of the primary cavity of the middle cerebral bladder, has view of a narrow channel and called aqueduct of the brain, aqueductus cerebri... It is a narrow, ependymal lined canal 1.5-2.0 cm long, connecting the IV ventricle with the III. Dorsal water supply limited to the roof of the midbrain, ventrally - the lining of the legs of the brain.
On a transverse section of the midbrain, there are three main parts:
1) roof plate, lamina tecti,
2) a tire, tegmentum, representing the upper section of the pedunculi cerebri,
3) the ventral pedunculi cerebri, or the base of the brain stem, basis pedunculi cerebralis.
According to the development of the midbrain under the influence of the visual receptor, it contains various nuclei related to the innervation of the eye.
In lower vertebrates upper colliculus serves as the main endpoint of the optic nerve and is the main visual center. In mammals and in humans, with the transfer of visual centers to the forebrain, the remaining connection of the optic nerve with the upper mound is important only for reflexes. In the nucleus of the lower mound, as well as in the medial geniculate body, the fibers end auditory loop (lemniscus lateralis)... The roof of the midbrain has a two-way communication with the spinal cord - tractus spinotectalis and tractus tectobulbaris et tectospinalis... The latter, after crossing in the tire, go to the muscle nuclei in the medulla oblongata and spinal cord. This is the so-called visual-sound reflex pathway, which was mentioned when describing the spinal cord. Thus, the lamina of the midbrain roof can be considered as a reflex center for various kinds of movements, arising mainly under the influence of visual and auditory stimuli.
Brain plumbing surrounded by a central gray matter, which in its function is related to the vegetative system. In it, under the ventral wall of the aqueduct, nuclei are laid in the lining of the brain stem two motor cranial nerves - n. oculomotorius (III pair) at the level of the upper colliculus and n. trochlearis (IV pair) at the level of the lower colliculus. The nucleus of the oculomotor nerve consists of several sections, respectively, of the innervation of several muscles of the eyeball.
Medially and posterior to it, another small, also paired, vegetative accessory nucleus, nucleus accessories, and unpaired median nucleus... Accessory nucleus and unpaired median nucleus innervate involuntary muscles of the eye, m. ciliaris and m. sphincter pupillae... This part of the oculomotor nerve belongs to the parasympathetic system. Above (rostral) the nucleus of the oculomotor nerve in the tectum of the cerebral peduncle is the nucleus of the medial longitudinal bundle.
Lateral to the aqueduct of the brain is the nucleus of the midbrain tract of the trigeminal nerve, nucleus mesencephalicus n. trigemini.
The legs of the brain are dividing, as already noted, to the ventral part, or base of the brain peduncle, basis pedunculi cerebralis, and tire, tegmentum... The border between them is substantia nigra, owes its color to the contained in its constituent nerve cells black pigment - melanin.
Lining of the midbrain, tegmentum mesencephali, - the part of the midbrain located between its roof and the substantia nigra of the legs of the brain.
Departs from her tractus tegmentalis centralis - central tire path - the projection descending nerve pathway located in the central part of the midbrain lining. It contains fibers coming from the thalamus, pallidus, red nucleus and reticular formation of the midbrain to the reticular formation and the olive of the medulla oblongata, belongs to the extrapyramidal system.
Substantia nigra extends along the entire length of the brain stem from the bridge to the diencephalon, in its function it belongs to the extrapyramidal system.
Located ventrally from substantia nigra the base of the brain stem contains longitudinal nerve fibers descending from the cerebral cortex to all the underlying parts of the central nervous system (tractus corticopontmus, corticonuclearis, corticospinalis, etc.).
Tegmentumlocated dorsally from substantia nigra, contains mainly ascending fibers, including the medial and lateral loops. As part of these loops, all sensory pathways, with the exception of the visual and olfactory pathways, ascend to the large brain.
Among gray matter nuclei the most significant is red nucleus, nucleus ruber... This elongated sausage-like formation extends in the operculum of the brain stem from the hypothalamus of the diencephalon to the lower colliculus, where an important descending tract, tractus rubrospinalconnecting red core with the anterior horns of the spinal cord. This bundle after leaving red kernel intersects with a similar bundle of the opposite side in the ventral part of the median suture - the ventral intersection of the tire.
Nucleus ruber is a very important focal point of the extrapyramidal system, connected with the rest of its parts. Fibers from the cerebellum as part of the upper legs of the latter pass to it after they cross under the roof of the midbrain, ventrally from aqueductus cerebrias well as from pallidum - the lowest and most ancient of the subcortical nodes of the brain, which are part of the extrapyramidal system. Thanks to these connections, the cerebellum and the extrapyramidal system, through the red nucleus and the tractus rubrospinal extending from it, influence the entire skeletal muscles in the sense of regulating unconscious automatic movements.
The lining of the midbrain also continues reticular formation, formatio reticularis, and fasciculus longitudindlis medialis... The latter originates in various places. One of its parts begins from the vestibular nuclei, passes on both sides along the sides of the midline, directly under the gray matter of the bottom of the aqueduct and IV ventricle, and consists of ascending and descending fibers going to the nuclei of the III, IV, VI and XI cranial nerves ...
Medial longitudinal fasciculus is an important associative path that connects the various nuclei of the nerves of the eye muscles with each other, which determines the combined movements of the eyes when they are deflected in one direction or another. Its function is also associated with eye and head movements that occur when the balance apparatus is irritated.