Physiology of vocal fold vibration
Like the man's lips and tongue in this video, the vocal folds are blown apart by air flow from the lungs, rather than being innervated by a nerve. Nerve innervation would be too slow--the vocal folds vibrate up to 100 times per second. The vocal folds are able to vibrate so rapidly because they are elastic--they come back together after being blown apart. The theory that explains vocal fold movement is the myoelastic aerodynamic theory of phonation, described in your textbook on page 70. The "myoelastic" describes the characteristics of the vocal folds, and the "aeordynamic" describes the movement of air past the vocal folds. Its principles are illustrated in the material that follows on this page of the module.
The diagram below is from your textbook, page 78. Let's look at it to see what happens to the vocal folds as air moves from the lungs to the oral cavity. These 8 frames show 1 cycle of vocal fold vibration. During each cycle, 1 puff of air is emitted.
In A', you can see the vocal folds are being brought together in the midline for phonation. You can see that they are fairly thick. The air is trying to exit the lungs. Subglottal pressure is increasing because the glottis is closed by the vocal folds. Remember Boyle's law from the unit on respiration? When volume gets smaller, pressure increases.
In B', we see that the increasing pressure can force the vocal folds away from the midline at the bottom.
In C', the pressure wave pushes itself upward and a puff of air emerges.
In D', the vocal folds are blown apart.
In F', the vocal folds return quickly to the midline. The arytenoid cartilages are still together, so the vocal folds are stretched. You can see that the vocal folds first return at the bottom. There is an aerodynamic effect here: the space at the bottom is narrower than the space at the top, so because of velocity the bottom closes first. The pressure at the bottom of the vocal folds is more negative than at the top. The two forces that bring the vocal folds togetehr in F' are 1) tissue elasticity, and 2) the Bernoulli effect.
The Bernouilli Effect
When air or liquid flows through a constricted passage, the velocity increases. If a volume of air is held constant, the velocity of air flow through a constriction is increased. You might think of this using the analogy of a creek converging on a constriction, or people converging on a turnstile. Lots of water is coming through a constriction, and lots of people are going through a turnstyle, and velocity increases in the constriction or turnstyle. In nature, no one waits their turn.
The increase in velocity results in a drop in outward pressure. So, Bernouilli's law means that the greater the velocity, the lower the static pressure. The picture in your text, page 78 figure 4.12 illustrates this. In the constriction, pressure is decreasing and becoming negative (a type of suction). Reduction in outward pressure moves the vocal folds towards each other. Blowing them apart creates a narrow space which causes the air pressure within the glottis to decrease. In the picture below, the pressure to the left of the vertical line is less than the pressure to the right of it, as the air passes through the tube.
Airplanes fly due to the Bernouilli effect. The speed of air against an airplane wing causes a pressure drop, which raises the wing. You can see a diagram of this on page 79 in your text. You can illustrate this effect by putting a tissue under your chin and then blowing. The airflow above the tissue is faster than the aiflow below, creating lower pressure on the top of the tissue than the bottom, causing the tissue to rise. Another example your textbook gives is airflow down a hallway. Doors that are open will slam shut, because pressure is lower in the periphery of the hall than the center. This video is a demonstration of the Bernouilli effect.
How does the Bernouilli effect apply to the vocal folds?
The vocal folds are physically closer to each other at the bottom, and the velocity will be greater at that level. Thre is also a phase difference--the patterns of vibration are different between the inferior and superior portions of the vocal folds. This is called the vertical phase difference. The pressure will be less at the bottom due to the lower velocity. This is why the bottom part of the vocal folds comes together more quickly, which you have seen in picture F' in figure 4.11. The vocal folds can start vibrating before they have contact with each other because of the Bernoulli effect (for example, during /h/ in the word "hear"). the vocal folds will come together for the vowel /i/ after /h/.