Midbrain

The midbrain (mesencephalon), unlike other parts of the brain, is less complex. It has a roof and legs. The cavity of the midbrain is the cerebral aqueduct.

The upper (anterior) border of the midbrain on its ventral surface is the optic tracts and mammillary bodies, on the posterior surface - the anterior edge of the pons. On the dorsal surface, the upper (anterior) border of the midbrain corresponds to the posterior edges (surfaces) of the thalami, the posterior (lower) border - the level of the exit of the roots of the trochlear nerve (n. trochlearis, IV pair).

The roof of the midbrain (tectum mesencephalicus, which is a plate of the quadrigeminal body, is located above the cerebral aqueduct. On a brain preparation, the roof of the midbrain can be seen only after removing the cerebral hemispheres. The roof of the midbrain consists of four elevations - hillocks, which have the appearance of hemispheres. The latter are separated from each other by two grooves intersecting at right angles. The longitudinal groove is located in the median plane, the upper (anterior) sections form a bed for the pineal body, and the lower sections serve as a place from which the frenulum of the superior medullary velum begins. The transverse groove separates the superior colliculi (colliculi superiores) from the inferior colliculi (colliculi inferiores). From each of the hillocks in the lateral direction, thickenings in the form of a roller - the knob of the hillock - extend. The knob of the superior colliculus (brachium colliculi cranialis, s. superioris) is located behind the thalamus and is directed toward the lateral geniculate body. The handle of the inferior colliculus (brachium colliculi caudalis, s. inferioris) is directed toward the medial geniculate body.

In humans, the superior colliculi of the midbrain roof (quadruplet) and the lateral geniculate bodies function as subcortical visual centers. The inferior colliculi and medial geniculate bodies are subcortical auditory centers.

The cerebral peduncles (pendunculi cerebri) are clearly visible at the base of the brain as two thick, white, longitudinally striated ridges that emerge from the pons. These fibers are directed forward and laterally (diverge at an acute angle) to the right and left hemispheres of the cerebrum. The depression between the right and left cerebral peduncles is called the interpeduncular fossa (fossa interpeduncularis). The bottom of this fossa serves as a place where blood vessels penetrate the brain tissue. After removing the vascular membrane, a large number of small holes remain in the plate that forms the bottom of the interpeduncular fossa on brain preparations. Hence the name of this gray plate with holes - the posterior perforated substance (substantia perforata interpeduncularis, s. posterior). On the medial surface of each of the cerebral peduncles there is a longitudinal oculomotor groove (sulcus oculomotorius), or the medial groove of the cerebral peduncle. The roots of the oculomotor nerve (III pair) emerge from this groove.

The cerebral peduncles are located anteriorly (ventrally) to the cerebral aqueduct. In a cross-section of the midbrain, the black substance (substantia nigra) is clearly visible in the cerebral peduncle due to its dark color (due to the pigment melanin). It extends in the cerebral peduncle from the pons to the diencephalon. The black substance divides the cerebral peduncle into two sections: the posterior (dorsal) section - the tegmentum of the midbrain (tegmentum mesencephali) and the anterior (ventral) section - the base of the cerebral peduncle (basis pedunculi cerebri). The nuclei of the midbrain are located in the tegmentum of the midbrain and the ascending conduction pathways pass through. The base of the cerebral peduncle consists entirely of white matter, and the descending conduction pathways pass through here.

The aqueduct of the midbrain (aqueductus mesencephali, s. cerebri; Sylvian aqueduct) is a narrow channel about 1.5 cm long. It connects the cavity of the third ventricle with the fourth and contains cerebrospinal fluid. In its origin, the cerebral aqueduct is a derivative of the cavity of the middle cerebral vesicle.

A frontal section of the midbrain shows that the roof of the midbrain (collis) consists of gray matter (gray and white layers of the superior colliculus and the nucleus of the inferior colliculus), which is covered on the outside by a thin layer of white matter.

The central gray matter (substantia grisea centralis) is located around the midbrain aqueduct, in which, in the area of the bottom of the aqueduct, there are nuclei of two pairs of cranial nerves. At the level of the superior colliculus, under the ventral wall of the midbrain aqueduct, near the midline, there is a paired nucleus of the oculomotor nerve (nucleus nervi oculomotorii). It takes part in the innervation of the eye muscles. The parasympathetic nucleus of the autonomic nervous system is localized ventrally to it - the accessory nucleus of the oculomotor nerve (nucleus oculomotorius accessorius; Yakubovich nucleus, Westphal-Edinger nucleus). The fibers extending from the accessory nucleus innervate the smooth muscles of the eyeball (the muscle that constricts the pupil and the ciliary muscle). Anterior and slightly superior to the nucleus of the third pair is one of the nuclei of the reticular formation - the intermediate nucleus (nucleus interstitialis). The processes of the cells of this nucleus participate in the formation of the reticulospinal tract and the posterior longitudinal fasciculus.

At the level of the inferior colliculi in the ventral sections of the central gray matter lies the paired nucleus of the IV pair - the nucleus of the trochlear nerve (nucleus n. trochlearis). The trochlear nerve exits the brain behind the inferior colliculi, on the sides of the frenulum of the superior medullary velum. In the lateral sections of the central gray matter along the entire midbrain is the nucleus of the midbrain tract of the trigeminal nerve (V pair).

In the tegmentum, the largest and most noticeable nucleus in the midbrain cross-section is the red nucleus (nucleus ruber). It is located slightly above (dorsally) the black substance, has an elongated shape and extends from the level of the inferior colliculi to the thalamus. Laterally and above the red nucleus in the tegmentum of the cerebral peduncle, a bundle of fibers that are part of the medial loop is visible in the frontal section. Between the medial loop and the central gray matter is the reticular formation.

The base of the cerebral peduncle is formed by descending conducting pathways. The internal and external sections of the base of the cerebral peduncles form the fibers of the corticopontine tract (see "Conducting pathways..."). The medial 1/5 of the base is occupied by the frontopontine tract, the lateral 1/5part - the temporo-parietal-occipital-pontine tract. The middle part (3/5) of the base of the cerebral peduncle is occupied by the pyramidal tracts.

Corticonuclear fibers pass medially, and corticospinal tracts pass laterally.

The midbrain contains the subcortical centers of hearing and vision, nuclei that provide innervation of the voluntary and involuntary muscles of the eyeball, as well as the midbrain nucleus of the V pair.

The extrapyramidal system includes the black substance, red and intermediate nuclei, etc., which provide muscle tone and control automatic unconscious movements of the body. Ascending (sensory) and descending (motor) conduction pathways pass through the midbrain.

The nerve fibers that make up the medial loop are processes of the second neurons of the proprioceptive sensitivity pathways. The medial loop (lemniscus medialis) is formed by the internal arcuate fibers (fibrae arcuatae internae). The latter are processes of the cells of the nuclei of the cuneate and thin fasciculi and are directed from the medulla oblongata to the nuclei of the thalamus together with the fibers of general sensitivity (pain and temperature), which form the adjacent spinal loop (lemniscus spinalis). In addition, fibers from the sensory nuclei of the trigeminal nerve pass through the tegmentum of the midbrain, called the trigeminal loop (lemniscus trigeminalis); they are also directed to the nuclei of the thalamus.

The processes of the nerve cells of some nuclei form decussations of the tegmentum (decussationes tegmenti) in the midbrain. One of them, the dorsal decussation of the tegmentum, is "fountain-shaped" (Meynert's decussation), and belongs to the fibers of the tegmentospinal tract; the other, the ventral decussation of the tegmentum (Forel's decussation), is formed by the fibers of the Monacan bundle, the rubrospinal tract.

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