Speech is essential for communication and depends on the ability to produce complex sequences of movements of the vocal cords, tongue, lips, soft palate and other structures, coordinated by Broca's area of the frontal lobe.
The complex sounds are generated by Broca's speech area, within the frontal lobe; it is adjacent to lateral part of the motor cortex (M1) which projects to the cranial nerves innervating many of the structures involved in vocalisation.
In the majority of people, the Speech area is in the left hemsphere. The complex sequences of movements coordinated by Broca's area utilise auditory feedback from Wernicke's area at the temporo-parietal junction, adjacent to the primary auditory area.
The diagram shows the functions of different parts of the respiratory tract in speech or singing.
Image source: mayo foundation
The diagram shows the position of the vocal cords during inspiration, and during speech or singing.
Speech: Sound Generation and Resonance
Speech is controlled by the frontal cortex and involves regulating the flow of air from the lungs through the vocal cords in the larynx, which vibrate and make sounds.
Movements of the diaphragm and other muscles cause air to move into the lungs. During this phase the vocal cords are separated so as to allow air to flow into the lungs easily.
Rhythmical Respiratory movements are generated by the 'respiratory centre' in the medulla. During speech the respiratory rhythm is interrupted in order to regulate the flow of air through the larynx.
Different frequencies of sound are generated according to the length and tension on the vocal cords, controlled by skeletal muscles of the larynx, using axons in the glossopharyngeal and vagal nerves. The high pitch of the female voice is due to the vocal cords being shorter than those of males.
When breathe is taken into the lungs, the vocal cords are separated (abducted), and during speaking or singing, air passes from the lungs through the vocal cords which are opposed to each other (adducted) and sound is generated.
The oral and nasal cavities act as resonators, as a result of changes in their size by movements of structures such as the tongue and soft palate.
Sounds produced in the larynx are modulated by the tongue and lips, which alter the size of the oral and nasal cavities and airflow through the mouth and nose. These structures are innervated by the hypoglossal and facial nerves respectively, and are particularly important in forming consonants and vowels.
Speech involves complex movements of the larynx, tongue, palate and lips. Individual movements within this complex region are controlled from the most lateral parts of the motor cortex (M1), but the coordinated changes involved in speech or singing are generated by Broca's area, which projects to the oro-pharyngeal area of M1
What parts of the brain are responsible for communication using speech?
Image source: kalaidoscopeindia.blogspot
The diagram shows the links between parts of the cortex concerned with the recognition of speech sounds (Wernicke's area) and with generation of speech (Broca's area)
Broca's area and Wernicke's area
Broca's area is present predominantly on the left side of the frontal lobe in the majority of people, and stroke patients with a right sided paralysis commonly have problems with speech; whereas speech is not often affected in patients with a left hemiparesis.
Lesions of Broca's area are associated with a condition (motor dysphasia), in which the patient is able to understand commands, but cannot form spoken words, or has slurred speech and words that are imperfectly formed.
Auditory feedback is essential in forming words, and Broca's area is connected to auditory areas in the superior temporal lobe.
Wernicke's area is one region of the cortex associated the the understanding of the spoken word. It is located in the superior temporal gyrus behind the primary auditory area, and adjacent to the parietal lobe.
Damage to Wernicke's area results in an inability to understand commands, written or spoken, known as a sensory dysphasia (or aphasia).
It receives inputs from the adjacent primary auditory area, and sends its output to Broca's area; this is achieved through the arcuate fasciculus, a C-shaped subcortical band of association fibres running from the temporal lobe to the frontal lobe.
Language skills depend on areas in the junction of the parietal and temporal lobes that develop late-on during human development.
Broca's and Wernicke's areas are not precisely defined, and it is likely that they contain functionally significant subdivisions, but the names of these regions are still in common use.