The inferior colliculi (Latin, lower hills) together with the superior colliculi form the eminences of the corpora quadrigemina, and also part of the tectal region of the midbrain. The inferior colliculus lies caudal to its counterpart - the superior colliculus - above the trochlear nerve, and at the base of the projection of the medial geniculate nucleus (MGN) and the lateral geniculate nucleus (LGN).
The inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several more peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. The inferior colliculus has three subnuclei.
Relationship to auditory system
The inferior colliculi of the midbrain are located just below the visual processing centers known as the superior colliculi. The inferior colliculus is the first place where vertically orienting data from the fusiform cells in the dorsal cochlear nucleus can finally synapse with horizontally orienting data. This homecoming of the aural dimensions puts these dual mesencephalic bumps in the position to fully integrate all the sound location data.
The inferior colliculus function as a master computer both in regard to its hardware (complex connections) and its software (internal organization). IC are large auditory nuclei on the right and left sides of the midbrain. It is divided into three parts, the Central Nucleus of IC (CNIC), dorsal cortex and lateral cortex; however, CNIC is the principal way station for ascending auditory information in the IC.
1. Input and Output Connection of IC
The input connections to the inferior colliculus are composed of many brainstem nuclei. All nuclei except the contralateral ventral nucleus of the lateral lemniscus (LL) send projections to the central nucleus (CNIC) bilaterally. It has been shown that great majority of auditory fibers ascending in the lateral lemniscus terminate in the CNIC. In addition, the IC receives descending inputs from the auditory cortex, medial geniculate body (MGB), and superior colliculus (SC).
The inferior colliculus receives input from both the ipsilateral and contralateral cochlear nucleus and respectively the corresponding ears. Of course, there is some lateralization, the dorsal projections (containing vertical data) only project to the contralateral inferior colliculus. This inferior colliculus contralateral to the ear it is receiving the most information from, then projects to its ipsilateral medial geniculate nucleus.
The medial geniculate body (MGB) is the output connection from interior colliculus and the last subcortical way station. The MGB is composed of ventral, dorsal, and medial divisions, which are relatively similar in humans and other mammals. The ventral division receives auditory signals from the central nucleus of the IC (1).
CNIC: Central Nucleus of IC
AVCN: Anterior Ventral Cochlear Nucleus
PVCN: Posterior Ventral Cochlear Nucleus
SCN: Superior Colliculus Nucleus
LSO: Lateral Superior Olive
MSO: Medial Superior Olive
DNLL: Dorsal Nucleus of the Lateral Lemniscus
MGB: Medial Geniculate Body
SC: Superior Colliculus
TB: Trapezoid Body.......................................................................................