Speech anatomy refers to the physical structures in the body involved in producing spoken language, from the brain to the vocal apparatus.
Speech: the verbal expression of thoughts and feelings using articulated sounds, typically organized into words and sentences. It involves the physical production of sounds through the vocal apparatus.
Vocal Apparatus: the system of the body responsible for producing speech, which includes the following parts of the body:
Articulation: the physical process of producing speech sounds, i.e. the process of using the vocal apparatus.
Word: a unit of language that carries meaning and consists of one or more sounds or letters. Words are combined to form sentences and convey ideas.
Phoneme: the smallest unit of sound in a language that can distinguish words from each other. Phonemes are abstract representations of speech sounds and vary from language to language. For example, in English, the sounds /p/ and /b/ are distinct phonemes because they change the meaning of words (e.g., "pit" vs. "bit").
Sentence: a grammatical unit that expresses a complete thought, consisting of one or more words arranged in a coherent structure, typically including a subject and a predicate.
Grammar: the set of rules and principles that govern the structure of sentences in a language. It includes syntax (the arrangement of words and phrases), morphology (the structure of words), punctuation, and the conventions for combining words and phrases to convey meaning effectively.
Frontotemporal dementia (FTD): a group of neurodegenerative disorders characterized by progressive damage to the frontal and temporal lobes of the brain. This damage leads to a range of cognitive, behavioral, and language impairments.
Speech production refers to the process whereby a human translates thoughts in their brain into speech.
There are three main stages of speech production:
Conceptualization of speech is performed by a part of the brain known as Broca's area, which is located in the frontal lobe of the left hemisphere. Other parts of the brain, such as Wernicke's area also contribute to the formation of speech.
Conceptualization of speech is unaffected by ALS, however, it can be severely affected by frontotemporal dementia (FTD).
The formulation of speech involves several interconnected regions in the brain, primarily within the left hemisphere. The key areas include:
The premotor cortex is affected by ALS, therefore ALS does affect formulation of speech.
Articulation is the final stage of speech, and is the realization of the muscle movements that were prepared in the formulation stage previously. The process of speech articulation is as follows:
Exhalation from the Lungs: Speech begins with the exhalation of air from the lungs and into the trachea (windpipe).
Phonation (Voice Production): As the exhaled air passes through the larynx, the vocal cords vibrate, producing sound waves. The pitch and volume of the sound depend on the tension and length of the vocal cords.
Resonance: The sound produced by the vibrating vocal cords travels up through the pharynx (throat) and into the oral and nasal cavities. The shape and size of these cavities, including the mouth, nose, and throat, modify the sound by resonance. The resonance chambers can amplify or dampen different frequencies, contributing to the timbre and quality of the voice.
Articulation: The final shaping of speech sounds occurs as the sound wave is modified by the movement of the articulators: primarily the tongue, lips, teeth, and palate. For example:
Speech Output: The modified sound exits the mouth (and sometimes the nose for nasal sounds) as spoken words. This process involves the precise coordination of all these structures to produce clear and intelligible speech.
ALS affects speech articulation because the destruction of motor neurons negatively affects the ability to control the muscles of the vocal apparatus, coupled with the progressive weakness of those muscles.
Respiratory issues caused by ALS also affect speech: reduced lung volume may make speech quieter; the need to use ventilation to assist with breathing may interfere with speaking.
An individual with ALS may have difficulty with certain specific phonemes before others, or they may have difficulty with speech in general, such as reduced volume or speed.
The English language has 44 to 45 distinct phonemes, depending on dialect. There are 24 consonant sounds, 12 pure vowel sounds, and 8 complex vowel sounds.
Individual phonemes in English (or any other language) vary in how difficult they are to articulate. Some require careful coordination of a large number of parts of the vocal apparatus, such as the tongue, jaw, and larynx. Others are quite straightforward and are easily articulated by babies within their first few weeks of life.
The most challenging phonemes in English are generally considered to be the following: