Page 83 - Edited - Webster HEAD AND NECK - part 2-Merge PDF

HN 275

VOCALISATION

Vocalization is a complex process which involves sensory feedback from the larynx, pharynx and mouth
and lips (vagus, glossopharyngeal and trigeminal nerves) and motor outflow through the vagus nerves (from the
paired medullary nucleus ambiguus) to the larynx, pharynx and palate; to the tongue (hypoglossal nerves); to
the jaw-moving muscles (trigeminal nerves) and muscles of the cheeks and lips (facial nerves); and voluntary
over-ride of respiratory centres to control the flow of air: this last is achieved through the appropriate spinal
cord motor neurons supplying the respiratory muscles (SI 83). (A conscious patient with completely paralysed
respiratory muscles who is kept alive on a respirator or "iron lung" cannot speak or make a noise, except during
the expiratory phase as determined by the machine.) The basic hindbrain mechanisms are operated on by ..
higher centres" first to produce sounds of different pitch and quality (motor cortex); and secondly to manipulate
these into oral language (speech or song) i.e. the language areas of the cortex (Neuro notes Vol. I, pp.74–76).
Thus:

Phonation. Sounds of varying pitch are produced by blowing air past the vocal folds of the larynx: th· s is
phonation. Its failure/absence is dysphonia: absent or low volume/hoarse voice, as in laryngitis or
laryngeal paralysis. It is also as well to remember that a patient with a tracheotostomy (below the vocal folds,
through the trachea or anterior crico-thyroid ligament) must block the tube (using one or more finger tips) before
he/she can phonate.

The larynx, like the bassoon, oboe and cor anglais is a double reed instrument. The "reeds" (vocal folds)
of the larynx can be varied in stiffness, tension, and even shape. The gap between them - the glottis - is also
variable, changing reflexly with the respiratory cycle (SI 79) and voluntarily in response to demands for shouting
(widening of glottis) or whispering (narrowing). All necessary movements are brought about by using the
skeletal laryngeal muscles (nucleus ambiguus, cranial nerve X on each side). The pitch of notes for woodwind
instruments is changed by varying the length of a coupled vibrating column of air. However, in a similar way to
brass instruments (for which the player uses his/her "embouchure" - the lips - as controlled reeds), the pitch of
laryngeal notes is changed by varying the stiffness, shape and tension of the vocal folds: for the latter, the
laryngeal cartilage is rocked backwards (low pitch) and forwards (high pitch) on the cricoid cartilage. (Try a
simple experiment: locate your larynx - "Adam's apple" - and hold its upper "beak" - i.e. the laryngeal
prominence gently between finger and thumb. Open your mouth and make the highest and lowest notes you can.
Which way does the larynx move in each case? What is happening to the vocal folds?)

The quality of these sounds is changed by induced vibrations in resonant cavities - i.e. harmonics are
emphasised and/or added. These are chambers of fixed size (the nasal cavity, and the four sets of paranasal air
sinuses which open into it - hence the change in voice quality when one has a cold and/or sinusitis) and chambers
of variable size (the pharynx and oral cavity - by changing the size of the oro-pharyngeal isthmus, moving the
soft palate, the tongue and the muscular pharyngeal walls) Note that the contribution of the fixed resonators can
1
be varied to some extent by raising and lowering the soft palate and so controlling the pathway from the source
of the sound into the nasal passages. (It is perfectly easy to demonstrate the contribution uf the soft palate to this
process: open your mouth, look at its interior in a mirror, and say 'AH'. The soft palate automatically rises - there
is nothing you can do about it - enlarging the oral cavity and changing the shape of the entrance to the naso-
pharynx and nasal cavity.) Simple phonation depends essentially on the medullary nucleus ambiguus in each side
of the medulla oblongata. Loudness is controllable (e.g. greater volume and singing of prolonged notes) by using
the abdomino-thoracic "pump" to produce a more powerful air flow, and widening the glottis. People "go red in
the face" under such circumstances because raised intrathoracic pressure (the glottis, while not closed, is at least
a restriction) reduces venous return (the jugular veins become engorged, and the external jugular vein can be seen
in the neck).

1 Note: The average fundamentals of the adult human speaking voice are ~145 Hz in males and ~230 Hz in females, although
partials (harmonics) extend as high as ~8 to ~10 kHz, and are most important in producing understandable speech. The
resonant frequency of the pharynx is ~500Hz, and emphasis of this frequency gives a voice a rounded and attractive
quality. The maximum range of fundamental notes in the human voice is from ~66 Hz for an (exceptional) bass singer at
the lowest end of his register, to ~1056 Hz for the very highest soprano note - a range of ~4 octaves, C - c"'. Compare
with the range of fundamentals on a modern concert grand piano - about seven octaves, from ~30 Hz to ~5.5 kHz,
although partials extend to ~15 kHz.

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