Medial geniculate body

At a glance

English

Medial geniculate body

Latin (TA2)

Corpus geniculatum mediale

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the medial geniculate body

The medial geniculate body is the thalamic station of the auditory pathway. It forms a small, oval bump on the lower and back surface of the thalamus, beneath the pulvinar and medial to the lateral geniculate body, and it lies beside the upper midbrain. The brachium of the inferior colliculus runs up to it from the midbrain tectum, bringing almost all of its ascending input. Within the swelling lies the medial geniculate nucleus, the last subcortical relay before the cortex.

The nucleus has three main divisions that differ in their cells and their connections. The ventral division is the core of the classic hearing pathway. Its principal relay cells are arranged in parallel layers, each layer tuned to a narrow range of frequencies, so that sound is mapped by pitch, low tones laterally and high tones medially. Their dendrites form clusters, or synaptic nests, where they are excited by fibres from the central nucleus of the inferior colliculus and held in check by inhibitory (GABA) interneurons. Most of its cells respond to both ears. The dorsal division has broadly tuned cells, some of which respond only to complex sounds and some also to touch, and it lacks a clear frequency map. The medial (magnocellular) division has large cells with wide dendrites, responds to intensity and duration of sound, and receives touch, vestibular and other inputs as well as hearing.

Output from the ventral division travels through the auditory radiation, which passes below the lentiform nucleus in the sublenticular part of the internal capsule, to the primary auditory cortex in the transverse temporal gyri. The dorsal and medial divisions project more widely to auditory association cortex and, in the medial division's case, to the amygdala, giving a fast route by which sounds can trigger fear and emotional learning. The cortex sends strong feedback back down to the nucleus, and the nucleus is thought to help direct and sustain attention to sounds.

Because each medial geniculate body receives from both ears, a lesion of one side does not cause deafness in either ear; it may instead affect locating sounds and understanding speech in noise. Abnormal activity in the auditory thalamus and its loop with the cortex is thought to contribute to tinnitus.

Function and supply

Function

Thalamic relay for hearing: passes frequency, intensity and two-ear information from the inferior colliculus to the auditory cortex, and links sound with emotion and attention.

Blood supply

Thalamogeniculate branches of the posterior cerebral artery, with a contribution from the posterior choroidal arteries.