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The pars triangularis refers to the triangular shaped cortical region of the inferior frontal gyrus in the frontal lobe of the brain. It sits in between the more rostral pars orbitalis and caudal pars opercularis which altogether make up the inferior frontal gyrus. When coupled with the pars opercularis, the pars triangularis is most commonly associated with Broca’s area and is well known in its involvement in speech production 1-2.
The pars triangularis is separated rostrally from the pars orbitalis by the anterior horizontal ramus and caudally from the pars opercularis by the ascending ramus of the lateral sulcus (a.k.a. Sylvian fissure). Dorsally the pars triangularis is delimited by the inferior frontal sulcus distinguishing itself from the middle frontal gyrus. The pars triangularis is usually split into an anterior and posterior portion by the triangular sulcus (a.k.a. incisura capitis) which superficially merges with the inferior frontal sulcus 4-6.
It is supplied by the frontal branches of the middle cerebral artery.
Less commonly seen is a distinct sulcus that extends towards the pars triangularis to form its rostral end, called the pretriangular sulcus (a.k.a. sulcus radiatus of Eberstellar) which is not to be confused as a continuation of the inferior frontal sulcus 4,5.
Radiographically the pars triangularis can be identified as the middle triangular shaped portion of the inferior frontal gyrus which grossly takes on a characteristic “M” configuration. This is also referred to as the M sign when identifying the inferior frontal gyrus on neuroimaging 3.
The pars triangularis is often referred to by its functional and cytoarchitectural title of Brodmann’s area 45. In the dominant hemisphere, it is one of two regions that make up Broca’s area together with Brodmann’s area 44 (a.k.a. pars opercularis). Broca’s area is well known to be involved in expressive aspects of spoken and written language. More specifically, the pars triangularis is involved with semantic processing of language. In the non-dominant hemisphere, the same cortical region is involved in non-verbal communication such as gesticulation, facial expression and modulation of timing and intonation of speech 1,5,7.
- 1. A L Foundas, C M Leonard, R L Gilmore, E B Fennell, K M Heilman. Pars triangularis asymmetry and language dominance. Proceedings of the National Academy of Sciences. 93 (2): 719. Pubmed
- 2. Tomaiuolo F, MacDonald JD, Caramanos Z, Posner G, Chiavaras M, Evans AC, Petrides M. Morphology, morphometry and probability mapping of the pars opercularis of the inferior frontal gyrus: an in vivo MRI analysis. The European journal of neuroscience. 11 (9): 3033-46. Pubmed
- 3. M. Wagner, A. Jurcoane, E. Hattingen. The U Sign: Tenth Landmark to the Central Region on Brain Surface Reformatted MR Imaging. American Journal of Neuroradiology. 34 (2): 323. doi:10.3174/ajnr.A3205 - Pubmed
- 4. Michael Petrides. Neuroanatomy of Language Regions of the Human Brain.(2014) ISBN: 9780124059313
- 5. Juergen K. Mai, George Paxinos. The Human Nervous System.(2012) ISBN: 9780080921303
- 6. Stephan Ulmer, Olav Jansen. fMRI. ISBN: 9783642343421
- 7. Paul Johns. Clinical Neuroscience. ISBN: 9780443103216