Physiology of Speech
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Direct answer
Speech is powered by exhaled air held at a subglottic pressure of roughly 5-10 cm of water by adducted vocal folds, which chop the stream into puffs at the fundamental frequency — about 100-120 Hz in men, 200-220 in women and up to 300 in children, set by fold length, mass and tension, with the cricothyroid muscle as chief pitch-raiser. The larynx supplies the buzz; the pharyngeal, oral and nasal cavities filter it into formants that characterise vowels and voiced consonants, while tongue, lips, velum and jaw shape the rest (articulation). Cortically, Wernicke's area in the superior temporal gyrus (area 22) decodes language, Broca's area in the inferior frontal gyrus (areas 44, 45) programmes production, and the arcuate fasciculus links them. Dysarthria — mechanical failure of the speech apparatus — is not aphasia, a distinction every examiner insists upon.
What you must remember
- Three-stage engine: respiration provides power (subglottic pressure 5-10 cm H2O), the larynx provides the source (phonation), and the vocal tract provides the filter (articulation and resonance).
- Pitch mechanics: vocal fold tension, length and mass set frequency; the cricothyroid lengthens and tenses the folds to raise pitch; loudness tracks subglottic pressure and adduction force.
- Frequency numbers: men about 100-120 Hz, women 200-220 Hz, children up to 300 Hz — quotable in one line for viva.
- Language network: Broca's (44, 45) — non-fluent, effortful speech with preserved comprehension and frustrated patient; Wernicke's (22) — fluent, paraphasic output with poor comprehension and little insight; arcuate fasciculus — conduction aphasia with near-normal comprehension and abolished repetition.
- Neural supply of articulation: trigeminal for jaw, facial for lips, vagus for velopharyngeal closure and larynx, hypoglossal for tongue — each palsy flavours dysarthria differently; recurrent laryngeal palsy gives a breathy, hoarse voice.
- Nasal resonance: velopharyngeal port closure separates oral from nasal airflow; cleft palate and vagal palsy give hypernasality with nasal escape.
- Breath and speech: speech prolongs expiration and shortens inspiration at grammatical boundaries; counting aloud in one breath declines measurably in neuromuscular weakness — a bedside spirometry of sorts.
Localising three aphasias at the bedside
A patient says only "tan-tan" following a left frontal stroke: effortful, non-fluent output, but he follows commands and taps furiously when misunderstood — Broca's aphasia from inferior frontal damage. A second patient chats fluently, yet the sentences are empty jargon ("spooner... wentish the forbing"), he cannot name a pen and answers questions off-target without noticing — Wernicke's aphasia from superior temporal damage. A third repeats nothing you say despite understanding it and speaking fluently — conduction aphasia from arcuate fasciculus disconnection; ask her to repeat "today is a sunny day" and the sentence collapses. The examination logic is a three-way table: fluency, comprehension, repetition. Fluency lost with comprehension kept is Broca; fluency kept with comprehension lost is Wernicke; repetition alone lost is conduction — and pure dysarthria, where language is intact but the machine is broken (as in bulbar palsy), sits outside the table entirely.
Where students slip
Dysarthria is offered as a type of aphasia; correcting it earns immediate credit — aphasia is a language disorder regardless of output route, dysarthria is a motor execution disorder of the same machinery that chews and swallows. Second, the Broca-Wernicke axis is memorised without the connecting fibre, yet repetition is the bedside test that exposes the arcuate fasciculus, making conduction aphasia the examiner's favourite third option. Third, pitch is attributed to "vocal cord vibration rate" vaguely; naming the cricothyroid as the tensor that raises pitch, and noting that recurrent laryngeal palsy spars... rather that a unilateral recurrent laryngeal lesion leaves a breathy voice while bilateral adductor paralysis threatens the airway, shows the anatomy has actually been learnt. Indian viva convention: demonstrate hoarseness workup by asking the patient to say "ee" and count to twenty — breathy breaks reveal fatigable adduction, and in myasthenia they worsen as counting proceeds.
Frequently asked questions
What are the three physiological components of speech?
Respiratory power (subglottic pressure of 5-10 cm H2O), laryngeal phonation generating the fundamental frequency, and articulation-resonance shaping that sound in the vocal tract.
What is the typical fundamental frequency of the human voice?
About 100-120 Hz in men, 200-220 Hz in women and up to 300 Hz in children, determined by vocal fold length, mass and tension.
How do Broca's and Wernicke's aphasias differ clinically?
Broca's is non-fluent with preserved comprehension and frustrated awareness; Wernicke's is fluent and paraphasic with impaired comprehension and poor insight.
What is conduction aphasia and which structure is damaged?
Disconnection of the arcuate fasciculus between Broca's and Wernicke's areas: fluent speech and adequate comprehension, but repetition is profoundly impaired.
How does dysarthria differ from aphasia?
Dysarthria is a motor-execution disorder of articulatory muscles (cranial nerves V, VII, X, XII) with intact language; aphasia is a disorder of language processing itself.