Processing and producing language is a complex process, with several structures within the brain all playing a vital role. The French physician and anatomist Pierre Paul Broca may have discovered the most important part when he identified a common region in the brains of 2 of his speech-impaired patients. This came to be known as Broca area. This region, located in the posterior inferior frontal gyrus of the dominant hemisphere at Brodmann areas 44 and 45, is vital for language.
The primary functions of Broca area are its roles in both language production and comprehension. While the exact role it has in production is still unclear, many believe that it directly impacts the motor movements to allow for speech. Although originally thought to only aid in production, lesions in the area are also related to impairments with understanding language. Different regions of Broca area itself specialize in different aspects of comprehension. The anterior portion helps with semantics, or the meaning of words, while the posterior is associated with phonology or how words sound. Broca area is also necessary for language repetition, gesture production, sentence grammar, and fluidity and is involved in interpreting others’ actions.
As a part of the nervous system, Broca area originates from the ectoderm layer. It begins when a portion of the mesoderm, a transient structure known as the notochord, releases signals to the overlying ectoderm, called the neuroectoderm. This causes the ectoderm to thicken into the neural plate, which will fold to create the neural tube. It is the neural tube that will further thicken to form both the brain and spinal cord. The developing brain first forms 3 vesicles: the forebrain, midbrain, and hindbrain. The forebrain, also known as the prosencephalon, further divides into the telencephalon and the diencephalon. It is from the dorsal telencephalon that the cerebral cortex arises, and that is where Broca area will come to be located.
Due to its location on the lateral cortex of the cerebral hemisphere, Broca area receives its blood supply from the superior division of the middle cerebral artery. With most people being left hemisphere dominant, this means left middle cerebral artery (MCA) most commonly supplies Broca area. In some cases, the callosomarginal artery has served as a collateral artery, providing a double blood supply to the area.
As previously mentioned, the Broca area is usually located in the dominant cerebral hemisphere. Though not a certainty, the dominant hemisphere is commonly found to be opposite one’s handedness. A right-handed individual should have a dominant left hemisphere in most cases. Therefore, a right-handed person should theoretically vary from a left-handed in the location of their Broca area. While the borders of the area may vary, Broca area can reliably be found in the posterior inferior frontal gyrus.
Insults and injury to the brain and Broca area may lead to what is known as Broca aphasia. Also known as expressive aphasia, it is a non-fluent aphasia characterized by partially losing the ability to produce both spoken and written language. Their speech will still contain the important content, but they may omit articles, prepositions, and other words that only have grammatical significance. Thus, they are said to have "telegraphic speech." The aphasia can vary in severity, and in some cases, patients may only be able to speak in single word sentences. In other words, these individuals know what they are trying to say, but they are unable to say it. Patients will also lose the ability of repetition, as an intact Broca area, Wernicke’s area, and arcuate fasciculus are required to repeat words or phrases. These patients do not entirely lose their ability to comprehend, but they have an increased effort of speech. This is because language comprehension is largely handled by Wernicke’s area, which is located in the posterior superior temporal gyrus. Therefore, they are typically aware of their deficits, and Broca aphasia patients are likely to become frustrated and often develop depression. It is also important to differentiate dysphasia from dysarthria. Those with dysarthria have problems speaking because of an inability to move mouth and tongue musculature, while expressive aphasia is more of the issue of word finding.
Broca area has been associated with the speech disorder stuttering. This is also known as stammering, and it is a condition where speech fluidity is interrupted by unintentional prolongations and repetitions of syllables, words, and phrases. Those affected also have instances where they are unable to produce sound, as demonstrated by blocks and silent pauses. When examined with fMRI, hypoactivity is seen in Broca area with hyperactivity in motor areas. Stuttering may not be as severe as Broca aphasia, but it can be very frustrating to those affected by it.
Exceptions and variations following insult to Broca area have been recorded in patients. There have been patients who have suffered damage to the area, and their language expression is intact, while some patients noted to have Broca aphasia have no lesions to that region. This makes it possible that there are other areas of the brain that have a role in language expression. Damage to Broca area can also lead to transient mutism, suggesting it may not be entirely dedicated to processing, but vocalization as well.
Impairments to Broca area are most frequently seen in instances of reduced blood supply to the region, with the greatest instance of this being strokes. Both ischemic and hemorrhagic strokes in the area of the middle cerebral artery can cause damage to the Broca area. Due to its large size and direct route from the internal carotid artery, the middle cerebral artery is the most regularly affected artery in the case of strokes. Therefore, Broca aphasia is seen in many patients who have suffered such cerebrovascular accidents. Other lesions, such as direct trauma, tumors, or infectious masses, can be located in the area, which would have the capability to reproduce Broca aphasia.
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