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Common to all vertebrates, the mesencephalon, also known as the midbrain, is the middle division of the brain. Composed of two main regions — the tectum, or “roof” in Latin, and the tegmentum, or “covering” — it plays important roles in the sensory and motor control systems. The midbrain lies between the diencephalon, which includes the thalamus and hypothalamus, and the metencephalon, which includes the pons and medulla. The term "encephalon," meaning brain, comes from the Greek words en, meaning “in,” and kephale, meaning “head.”
The human brain begins as a fluid-filled neural tube that, about four weeks after conception, develops three swellings at its front end. These are the forebrain, midbrain, and hindbrain. Approximately three weeks later, seven weeks after conception, these three swellings become five. The middle of the three swellings becomes the third swelling of the five and is the mesencephalon.
The tectum forms the dorsal, or upper, surface of the midbrain. In mammals, it is divided into two pairs of nuclei: the superior and inferior colliculi. The superior colliculi receive visual information, help coordinate rapid movements of the eyes, and form maps of visual, somatosensory, and auditory space. The inferior colliculi, located under the superior colliculi, receive information about sound.
The size of the tectum often reflects an animal’s ability to see. For example, birds, which require clear, precise sight for flying, have a very large tectum. In addition, some species, instead of seeing with a visual cortex, see only with an “optical tectum,” the equivalent of the superior colliculi in humans. Even humans who are affected by cortical blindness seem to be able to receive visual information from their superior colliculi; cortically blind subjects with intact superior colliculi can correctly identify stimuli at far greater rates than those attributable to chance.
The tectum receives mostly sensory input. The tegmentum, located just below the tectum, is more involved in motor activity. It contains the cerebral aqueduct, the midbrain portion of the reticular formation, cranial nuclei, and several nuclear structures.
The cerebral aqueduct, also known as the ventricular mesocoelia, the iter, or the aqueduct of Sylvius, connects the third and fourth ventricles of the brain. The reticular formation, a loose network of neurons stretching from the midbrain down to the medulla, is involved in various functions, including sleep and arousal, temperature regulation, and motor control. Groups of motoneurons form two cranial nuclei; their axons branch out to create the oculomotor nerve and the trochlear nerve, both of which control eye movement.
Nuclear structures found in the tegmentum include the red nuclei, periaqueductal gray, and substantia nigra. The red nuclei are a pair of pinkish nuclei that play an important role in coordinating motion and are considered to be the main controllers of voluntary movement in most mammals. The periaqueductal gray, which plays a role in the suppression of pain and in systems of defensive behavior, is made up of gray matter located around the cerebral aqueduct. The substantia nigra, composed of two sensorimotor nuclei, gets its name, which means “black substance,” from the dark pigment found in many of its neurons. Extensive degeneration of dopamine-containing neurons in the substantia nigra causes Parkinson’s disease.
Underneath the tegmentum are two tracts that often are considered part of the mesencephalon. These tracts, called the cerebral penduncles, are bundles of nerve fibers passing over the bottom of the brain. They connect the cerebral hemispheres to the spinal cord.