Midbrain, also called mesencephalon, region of the developing that is composed of the tectum and tegmentum. The midbrain serves important functions in motor movement, particularly movements of the , and in auditory and visual processing. It is located within the and between the two other developmental regions of the , the and the ; compared with those regions, the midbrain is relatively small.
The tectum (from Latin for "roof") makes up the rear portion of the midbrain and is formed by two paired rounded swellings, the superior and inferior colliculi. The receives input from the and the cortex and participates in a variety of visual reflexes, particularly the tracking of objects in the visual field. The receives both crossed and uncrossed auditory fibres and projects upon the medial geniculate body, the auditory relay nucleus of the .
The tegmentum is located in front of the tectum. It consists of fibre tracts and three regions distinguished by their colour—the , the periaqueductal gray, and the . The is a large structure located centrally within the tegmentum that is involved in the coordination of sensorimotor information. Crossed fibres of the superior cerebellar peduncle (the major output system of the cerebellum) surround and partially terminate in the red nucleus. Most crossed ascending fibres of that bundle project to thalamic nuclei, which have access to the primary motor cortex. A smaller number of fibres synapse on large cells in caudal regions of the red nucleus; those give rise to the crossed fibres of the rubrospinal tract, which runs to the and is influenced by the motor cortex.
The is a large pigmented cluster of neurons that consists of two parts, the and the . Cells of the pars compacta contain the dark pigment ; these cells synthesize and project to either the or the , both of which are structures of the and are involved in mediating movement and motor coordination. These two structures, in addition to the globus pallidus, form the . By inhibiting the action of neurons in the caudate nucleus and the putamen, the dopaminergic cells of the pars compacta influence the neuronal output of the GABA (). The neurons in turn project to the cells of the pars reticulata, which, by projecting fibres to the thalamus, are part of the output system of the corpus striatum.
The periaqueductal gray region of the tegmentum is made up of gray matter (neural tissue with relatively few axons covered in ) and surrounds the cerebral aqueduct, a short canal that runs between the third and fourth ventricles of the brain. The periaqueductal gray appears to function primarily in suppression, a result of its naturally high concentrations of .
Also within the midbrain are the , tracts made up of neurons that connect the cerebral hemispheres to the . The midbrain also contains a portion of the reticular formation, a neural network that is involved in arousal and alertness. in the midbrain that stimulate the controlling eye movement, shape, and diameter form the nuclear complex of the oculomotor nerve and the trochlear nucleus.
Hindbrain, also called rhombencephalon, region of the developing vertebrate that is composed of the , the , and the . The hindbrain coordinates functions that are fundamental to survival, including respiratory rhythm, motor activity,, and wakefulness. It is one of the three major developmental divisions of the brain; the other two are the and .
Each of the three different regions of the hindbrain helps coordinate specific functions and activities. The transmits signals between the and the higher parts of the brain; it also controls autonomic functions such as heartbeat and . The is partly made up of tracts that connect the with higher levels, and it also contains groups that transfer information from the to the . Some of those cell groups are part of the reticular formation, a network of neurons extending throughout the that regulates alertness, sleep, and wakefulness. The medulla likewise houses a portion of the reticular formation.
The third area of the hindbrain, the cerebellum, is enriched with and granule cells. Purkinje cells are large neurons that serve a critical role in coordinating motor activity. Granule cells, in contrast, are very small neurons; their function is unclear, though they are thought to have an important role in motor learning.
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