Triangular part of inferior frontal gyrus

At a glance

English

Triangular part of inferior frontal gyrus

Latin (TA2)

Pars triangularis gyri frontalis inferioris

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the triangular part of inferior frontal gyrus

The triangular part (pars triangularis) is the central one of three portions of the inferior frontal gyrus, itself the lowest of the three horizontal gyri on the lateral surface of the frontal lobe. It is marked off by the short anterior branches of the lateral sulcus: the ascending ramus behind it and the horizontal (anterior) ramus below it. Because these two branches spread apart from a common stem, the cortex between them makes a triangle whose apex points down towards the stem of the lateral sulcus. Behind the ascending ramus lies the opercular part; in front of and below the horizontal ramus lies the orbital part. Above, the inferior frontal sulcus separates the gyrus from the middle frontal gyrus.

In cytoarchitecture the triangular part corresponds to Brodmann area 45, while the opercular part behind it is area 44 and the orbital part is area 47. Areas 44 and 45 together are Broca's area in the dominant hemisphere, which is the left in most people. The triangular part is connected with the temporal and parietal lobes by long association fibres, including the arcuate fasciculus and the extreme capsule pathway, and with the motor cortex behind it.

Its functions lie in language. Area 45 is thought to deal more with meaning and the selection of words, while area 44 deals more with sound, grammar and the planning of speech movements. Imaging also links the pars triangularis with reading, verbal working memory and translation, including moving from a second language back into the mother tongue. The corresponding area in the non-dominant hemisphere is involved in the melody and emotional tone of speech.

A stroke in the territory of the superior division of the left middle cerebral artery that involves this region and its neighbours causes Broca's (expressive) aphasia. Speech becomes slow, effortful and reduced to key words, though comprehension is relatively preserved, and writing is affected in the same way. Neurosurgeons map this region carefully before removing tumours from the left frontal lobe.

Function and supply

Function

Part of Broca's area in the dominant hemisphere, concerned mainly with choosing words and the meaning of language; also active in reading, verbal working memory and translation.

Blood supply

Branches of the superior division of the middle cerebral artery, especially the prefrontal (candelabra) branches.