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(also midbrain), the part of the brainstem posterior to the diencephalon and anterior to the pons and cerebellum. The mesencephalon comprises the tegmentum, the cerebral peduncles (which resemble longitudinal strands), and the lamina quadrigemina, which consists of two pairs of colliculi. The mesencephalic cavity forms the aqueduct of Sylvius—a narrow channel connecting the third and fourth ventricles of the brain.
A section of the mesencephalon shows the tegmental lamina formed by the lamina quadrigemina, the tegmentum, which extends even to the upper parts of the cerebral peduncles, and a base formed by the ventral part of the peduncles. The tegmental lamina and tegmentum consist primarily of gray matter (a mass of nerve cell bodies), which forms the nuclei of the colliculi of the corpora quadrigemina, the central gray matter surrounding the aqueduct of Sylvius, the reticular formation, the nuclei of the third and fourth pairs of cranial nerves, the nucleus rubber, substantia nigra, and some other nuclei.
The tegmentum includes white matter, which is formed by axons (processes of neurons). The white matter includes different sensory tracts (skin, muscle-joint, visceral, acoustic), the medial and lateral lemnisci, the spinothalamic tract, branches of the optic tracts to the superior colliculi, the pathways from the cerebellum to the nucleus rubber, and the dorsal longitudinal fasciculus, which connects the nuclei of all oculomotor nerves with vestibular nuclei in the metencephalon. The white matter also contains a continuation of the medial longitudinal fasciculus of the prosencephalon, which unites the structures of the limbic lobe (gyrus fornicatus), diencephalon, and mesencephalon in the limbic system and in monoaminoergic neurons.
The base of the mesencephalon (the cerebral peduncles proper), which is separated from the tegmentum of the substantia nigra, consists of white matter. Descending fibers pass through the base from the cortex of the cerebral hemispheres to the motor apparatus of the spinal cord (corticospinal tract), the cerebellum (with a gap in the nuclei of the pons), and the motor nuclei of the cranial nerves. The cerebral peduncles diverge near the diencephalon, forming the interpeduncular space with a corresponding nucleus.
Mesencephalic structures participate in the performance of the functions of vision and hearing and in the regulation of movements and posture, muscle tone, states of wakefulness and sleep, and emotional and motivational activity. Signals reaching the nuclei of the superior colliculi from the optic tracts affect the oculomotor nuclei, which adjust the eye’s optic system by changing the diameter of the pupil (pupillary reflex) and focusing the image on the retina. The cells of the superior colliculi also receive signals from the higher parts of the brain, including the cortical regions, and from the reticular formation; these signals regulate the selection of visual information. In lower vertebrates with a poorly developed prosencephalon, visual information is primarily analyzed at the level of the mesencephalic tegmentum.
The nuclei of the inferior colliculi are associated with the processes of locating the source of a sound in space. The superior and inferior colliculi participate in orienting responses, which in their primitive form are known as starting reflexes. Starting reflexes include quivering, orientation of the sense organs to the source of a sound, and pupil dilatation. They may be observed in animals after the surgical separation of the mesencephalon from the higher parts of the brain.
The impulses from the motor area of the cortex are transmitted to the mesencephalic reticular formation through branches of the fibers of the pyramidal tract and cerebellum. These impulses then modulate cerebrospinal motor cells, which coordinate movement and determine muscle tone. These influences travel from the mesencephalon via reticulospinal pathways, altering the excitability of motor cells directly, through interneurons, or through the gamma motor system, which regulates the sensitivity of muscle proprioceptors. Transection of the mesencephalon between the superior and inferior colliculi results in decerebrate rigidity manifested by the pronounced extension of the extremities and neck. Electrostimulation of certain areas of the mesencephalic reticular formation enables a paralyzed animal to walk and run.
The substantia nigra, striate nucleus, thalamus, and premotor area of the cortex of the cerebral hemispheres form part of the cerebral system, which regulates plastic tone. Injury to this system results in the development of parkinsonism. The reticular formation has a substantial number of cells of the ascending activating system, through which wakefulness is maintained. Injury to the mesencephalic tegmentum may increase drowsiness, for example, as it does in cases of lethargic encephalitis.
Stimulation of an animal’s central gray matter causes pronounced affective behavior characterized by such emotions as rage, aggression, and fear. The continuation in the mesencephalon of the prosencephalic medial longitudinal fasciculus contains the main mass of fibers ascending from the cells of the medulla oblongata, pons, and mesencephalon. These cells produce the mediators serotonin and the catecholamines (including noradrenaline and dopamine). The continuation of the prosencephalic medial longitudinal fasciculus ensures the transmission of subconscious activities and emotional (nonspecific) supportive processes. The central gray matter and mesencephalic reticular formation participate in the regulation of blood circulation, respiration, and excretion.
L. P. LATASH