Cortical Speech Centers
The physiological basis of speech is ensured by the coordinated work of two main centers located in the dominant (usually left) hemisphere.
Sensory Center (Wernicke's Area) Localized in the posterior part of the superior temporal gyrus. Anatomically, this zone is associated with the planum temporale, the size of which directly correlates with hemisphere dominance. The main function of Wernicke's area is the formation of the semantic meaning of an utterance. This is where the core meaning of what a person intends to say or write originates.
Motor Center (Broca's Area) Located in the posterior part of the inferior frontal gyrus of the same dominant hemisphere. Its task is the programming of vocalization. Having received information about the meaning, this center constructs a detailed motor scheme for the future speech.
Stages of Oral Speech Formation
The realization of oral speech is a relay race of neural impulses consisting of several key steps:
- Generation of meaning: Wernicke's area creates the semantic structure of the future text.
- Information transit: Excitation carrying information about the goal of the utterance is transmitted via the arcuate fibers (arcuate fasciculus) of the cerebrum from Wernicke's area to Broca's area.
- Programming: Broca's area forms the motor program for vocalization.
- Detailing in the motor cortex: From the frontal lobe, impulses are directed to the facial areas of the motor cortex (precentral gyrus) in both the left and right hemispheres. At this stage, precise commands are created for the motor centers of the cranial nerves (CN).
- Executive stage: Signals reach the cranial nerve nuclei in the brainstem. Innervation of the speech musculature occurs: muscles of the tongue, lips, mouth, and larynx. Their coordinated contraction ensures proper articulation.
Written Speech and Clinical Correlations
The physiology of written speech relies on the same initial stages (from efferent synthesis to Broca's area). However, at the motor execution stage, the pathways diverge. Instead of the facial areas of the cortex, impulses from Broca's area are directed to the motor representations of the hand (right or left) in the cerebral cortex, initiating the writing process.
Pathology of Speech Centers Disruptions in the described zones lead to specific forms of aphasia:
- When Wernicke's area is damaged, the patient can speak and their speech is phonetically correct, but completely meaningless (creation of semantic structure is impaired).
- Damage to Broca's area causes motor aphasia: for a certain period, the person either completely loses the ability to speak or suffers from severe phonetic disorders because the motor program for vocalization itself is destroyed.