Inferior colliculus

At a glance

English

Inferior colliculus

Latin (TA2)

Colliculus inferior

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the inferior colliculus

The inferior colliculi (Latin for lower hills) are two rounded bumps on the dorsal surface, or tectum, of the midbrain. Together with the superior colliculi above them they make up the four mounds of the corpora quadrigemina, or tectal plate. Each inferior colliculus lies below and slightly behind the superior colliculus of its side, just above the point where the trochlear nerve leaves the back of the brainstem, and in front of the superior medullary velum and the upper end of the fourth ventricle. From its lateral side a ridge, the inferior brachium, runs forwards to the medial geniculate body of the thalamus.

Inside, each colliculus has three parts. The central nucleus is the core, a stack of layered sheets of cells each tuned to a narrow band of sound frequencies. Around it lies a thin dorsal cortex, and to the side an external (lateral) cortex. Nearly all ascending fibres of the lateral lemniscus end here, so the colliculus collects input from the cochlear nuclei, the superior olivary complex and the nuclei of the lateral lemniscus of both sides. It also receives descending fibres from the auditory cortex and the medial geniculate body, and from somatosensory nuclei.

The colliculus brings together the separate cues that the lower brainstem extracts from sound. Information about differences in timing and loudness between the two ears, and about spectral cues from the outer ear, converges here, giving an integrated map of where a sound comes from. Its cells also respond to the rate at which sound is modulated, which may help in judging pitch and rhythm. Its output goes mainly through the brachium to the ventral medial geniculate nucleus, then on to the auditory cortex. Links with the superior colliculus and the reticular formation let it drive reflexes such as turning the head and eyes towards a sound and the startle response.

It is one of the most metabolically active parts of the brain. Damage to both inferior colliculi is rare but can cause difficulty understanding speech and locating sounds, and lesions here or along the lateral lemniscus alter the brainstem auditory evoked response used to test hearing pathways. Tumours of the tectal plate press on it and on the nearby cerebral aqueduct.

Function and supply

Function

Integrates ascending auditory signals from both ears, building sound location and frequency maps, relays them to the medial geniculate body and drives auditory orienting and startle reflexes.

Blood supply

Branches of the superior cerebellar artery, with quadrigeminal (collicular) branches from the posterior cerebral artery.