Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Nervous System (general structural and developmental plan of the nervous system)
Central Nervous System
Brain - Midbrain

The Midbrain is relatively small in size, measuring about 20 mm in length. It is situated between the Pons posteriorly (inferiorly) and the Diencephalon anteriorly (superiorly). It is completely covered by the cerebral hemispheres. The anterior part of the midbrain comprises the cerebral peduncles and the midbrain tectum.

The ventral region of the midbrain consists of paired structures known as the cerebral peduncles. These are rounded White matter tracts emerging from the pons and extending anteriorly and superiorly toward the cerebral hemispheres. Inferiorly, between the cerebral peduncles, lies the interpeduncular fossa, at the floor of which the posterior perforated substance is visible. The rootlets of the Oculomotor nerve (cranial nerve III) emerge from the sulci located on the Medial surface of the cerebral peduncles.

The dorsal region of the midbrain is represented by the tectum. The tectum features the tectal plate (corpora quadrigemina), which consists of four elevations called colliculi. The two superior colliculi contain subcortical centers of the visual analyzer, whereas the two inferior (posterior) colliculi serve as subcortical centers of the Auditory Analyzer. The Pineal Gland, which belongs to the diencephalon and Functions as an endocrine gland, lies in the depression between the superior colliculi. Brachia extend laterally from each colliculus toward the diencephalon: the brachium of the superior colliculus runs to the ipsilateral lateral geniculate body, while the brachium of the inferior colliculus extends to the medial geniculate body.

The cavity of the midbrain is the cerebral aqueduct (mesencephalic aqueduct), a narrow channel connecting the third and fourth ventricles. The cerebral aqueduct serves as the boundary between the midbrain tectum dorsally and the midbrain tegmentum ventrally. The aqueduct is approximately 1.5 cm long. It is surrounded by the periaqueductal Gray matter, which houses the motor nuclei of Cranial Nerves III and IV, the paired autonomic Edinger-Westphal Nucleus (nucleus of Yakubovich), the reticular formation, and the mesencephalic Nucleus of the Trigeminal nerve. A cross-section of the midbrain clearly reveals two parts—the tectum and the cerebral peduncles—separated by the aqueduct. Each cerebral peduncle consists of a tegmentum and a basis pedunculi, demarcated by the substantia nigra. The dark color of the substantia nigra results from the presence of melanin pigment within its Nerve Cells. The basis pedunculi is formed by the nerve fibers of descending Pyramidal Tracts passing from the Cerebral Cortex to the Spinal Cord, Medulla Oblongata, and pons. The tegmentum is composed primarily of ascending and descending tracts, along with accumulations of gray matter forming large and small nuclei.

Structural and functional Organization OF THE midbrain. The midbrain tectum is formed by the two superior and two inferior colliculi, as well as the tectal and pretectal regions. The primary nerve fibers delivering neural impulses to the superior colliculi are the Pathways of the visual analyzer. Efferent fibers pass from the superior colliculi within the brachia to the lateral geniculate body and the pulvinar of the thalamus.

The Afferent Pathways of the inferior colliculi are represented by fibers of the lateral lemniscus, which transmit impulses from auditory nuclei located within the tegmentum of the pons. Efferent fibers originating from the inferior colliculi form bundles directed toward the medial geniculate body on each side. Thus, the superior colliculi function as the subcortical visual center, while the inferior colliculi act as the subcortical auditory center. Fibers emerging from the superior and inferior colliculi of the tectum form the tectospinal tract, which conveys neural impulses from these subcortical visual and auditory centers to the motor nuclei of the anterior horns of the spinal cord. The efferent influences of midbrain tectal Neurons mediate a series of vital unconditional Reflexes. The reflex arcs for orienting visual and auditory reflexes (such as turning the HEAD toward a stimulus or reflex adjustment of the auricle to sound) are integrated within the corpora quadrigemina. All of these anatomical reactions are classified as startle or protective reflexes.

The gray matter of the tegmentum comprises motor association and autonomic nuclei of cranial nerves (III, IV, and V), as well as nuclei of the reticular formation. Throughout the midbrain, near the walls of the cerebral aqueduct, lies the paired mesencephalic nucleus of the trigeminal nerve. It consists of sensory neurons homologous to the neurons of the sensory nuclei of Cranial and Spinal Nerves. This nucleus receives impulses from proprioceptors of the Muscles of Mastication. Efferent fibers from the mesencephalic nucleus of the trigeminal nerve project to the motor neurons of cranial nerves V, VII, IX, and X. Through the action of these fibers, the coordination of movements involving the masticatory, facial, and Neck Muscles, as well as the vocal cords, is achieved.

The motor nuclei of the tegmentum include: the Trochlear nerve nucleus (cranial nerve IV), whose axons innervate the superior oblique Muscle of the eye; and the paired oculomotor nerve nucleus, which innervates three rectus muscles, the inferior oblique muscle of the Eyeball, and the levator palpebrae superioris muscle.

Anterior to the oculomotor nucleus lies its accessory (parasympathetic) nucleus, the axons of which project to the ciliary ganglion. Postganglionic fibers from this ganglion innervate the ciliary muscle and the sphincter pupillae. The interstitial nucleus (nucleus of Cajal) contributes to The formation of the medial longitudinal fasciculus, which interconnects cranial nerve nuclei into a unified functional system that regulates conjugate Eye Movements.

Structures of the extrapyramidal system, which govern complex coordinated motor acts, are located within the tegmental region. These include the substantia nigra and the red nuclei. The substantia nigra is situated in the ventral portions of the tegmentum and appears as a broad layer of pigmented cells. Axons of substantia nigra cells project to the subcortical nuclei (Basal Ganglia) of the cerebral hemispheres. Damage to the substantia nigra leads to impaired fine cooperative movements and the onset of involuntary muscle contractions (tremor). The paired, spindle-shaped red nucleus lies between the substantia nigra and the periaqueductal gray matter surrounding the cerebral aqueduct. The red nuclei receive nerve fibers that are axons of neurons from the cerebellar nuclei and basal ganglia of the Telencephalon. The axons of neurons within the magnocellular division of the red nucleus form the descending rubrospinal tract, which terminates on motor neurons of the spinal cord. This pathway is regarded as a vital component of the extrapyramidal system, integrating cerebral influences onto spinal motor neurons. The red nuclei participate in maintaining Skeletal Muscle tone and executing habitual, repetitive movements.

A significant portion of the tegmentum is occupied by the structures (cells and nuclei) of the reticular formation, which maintain extensive connections with Brainstem structures. The midbrain not only serves as a center for integrating numerous vital reflexes but also performs conductive functions. The basis pedunculi consists exclusively of descending tracts that connect the cerebral cortex with motor neurons of the brainstem and spinal cord. Fibers of the corticopontine tract occupy the most medial and lateral positions, whereas the corticospinal and corticonuclear (corticobulbar) tracts are located in the middle portion of the basis pedunculi.

Ascending and descending pathways, the medial longitudinal fasciculus, the tectospinal tract, the rubrospinal tract, and other pathways originate from the structures of the tegmentum.