The midbrain, or mesencephalon, is the first portion of the brainstem, which is located below the diencephalon, at the base of the brain. It lies above the pons and below the thalamus (part of the diencephalon). It is composed of both efferent and afferent nerve fibers, which extends down from the cerebral cortex and runs upwards from the spinal cord, respectively.
Inside the midbrain, there are also a series of nuclei containing nerve cells which are involved in reflex responses to visual and auditory stimuli and some control over awareness. Two of these grey nuclei are the sites of origin of the oculomotor nerve (CN III) and the trochlear nerve (CN IV), both of which innervates the muscles that move the eyeballs.
Anatomical Description
The midbrain consists of four regions: the tectum (roof), the tegmentum, the cerebral peduncles, and the cerebral aqueduct. The tectum is the roof or the superior part of the mesencephalon and it also constitutes its dorsal side; it contains the corpora quadrigemina, which are four rounded elevations on the dorsal aspect that are relay centers for auditory nerve fibers.
The tegmentum is the portion of the midbrain that communicates with the cerebellum by the superior cerebellar peduncles, lying anteriorly to the cerebral aqueduct. Meanwhile the cerebral peduncles are the bundles of myelinated nerve fibers forming the anterior half of the midbrain; these are efferent fibers that run down from the cerebral cortex to the spinal cord. The cerebral aqueduct, on the other hand, is a narrow conduit connecting the third and fourth ventricles; it contains cerebrospinal fluid.
Below, a picture of the brainstem, showing the midbrain.
