Biomedical Engineering Reference
In-Depth Information
in human vocal folds [Goller and Larsen 1997a]. The valve is formed by con-
striction of the bronchial lumen when the labia are drawn together by action
of adductor muscles. The lateral labium is a thick tissue in the anterior lateral
part of the bronchial wall, and it is attached to the third bronchial ring. The
medial part of the valve is a membrane attached to half-rings: one side of
the membrane is the bronchial lumen, and the other the interbronchial space
that opens to the interclavicular air sac. This membrane can be divided into
a thick part known as the medial labium and a very thin part called the
medial tympaniform membrane (MTM) [Fee 2002]. It is interesting to notice
that the MTMs were for a long time assumed to be the sources of sound in
birdsong. However, a series of experiments refuted this belief. By painting the
MTMs in pigeons with adhesive tissue [Goller and Larsen 1997a] or directly
removing them in cardinals and zebra finches, Goller and Larsen established
that the membranes are not actually required for vocalization, although the
intensity and spectral content of the sound did suffer changes after these
alterations.
The identification of the sources of sound is a very important first step
in understanding the mechanisms leading to the production of sound. It is
then important to understand how this vocal organ is driven in order to
generate sounds which can be roughly characterized by their timing, as well
as by their spectral properties. This is achieved by the control of a set of
muscles that alter the configuration of the vocal organ. The study of the
muscular activity during a vocalization is an even harder task than deter-
mining the sources of sound. It requires direct electrophysiological measure-
ments of the muscles controlling the syrinx. Such a research program has
been performed for several species (for the brown thrasher and the cardi-
nal, see [Goller and Suthers 1996b, Suthers et al. 1999]). The measurements
consist in inserting delicate wires that sense the tissue voltage. The voltage
measured in this way is a measure of the degree of activity of the muscle. By
recording these values for several muscles simultaneously with the airflow and
the sound, it was possible to determine the role played by different muscles
in the control of the syrinx.
3.2.2 The Role of the Muscles
Figure 3.4 shows another sketch of the syrinx, this time exhibiting the princi-
pal muscles attached to it. From the studies described above, it was concluded
that some muscles are associated with the active opening or closing of the
interlabial space. In other words, the bird can push one labium against the
other, actively closing the passage of air. It can also open the passage sepa-
rating the labia and keeping them apart as far as possible. In both of these
situations, the labia will not vibrate. Between these extremal situations, with
these muscles relaxed, conditions can be established such that the airflow in-
duces a vibration.
Search WWH ::




Custom Search