Biomedical Engineering Reference
In-Depth Information
cases, the functions of muscles are adapted to allow not only singing, but
respiration as well [Suthers 2001]. This can be clearly seen when it comes to
terminating a vocalization. This can be achieved either by withdrawing the
membranes from the airflow, or by adducting them, blocking the airflow. In
birds with a tracheal syrinx, the second possibility is not an option, since
adduction would not allow silent respiration. Some cases have been studied
in detail. Parrots and pigeons are the most thoroughly studied of the non-
oscines. In the following subsection, we shall review the structure of the syrinx
of the pigeons. This example can give us an insight into the similarities and
differences between oscines and nonoscines.
3.3.1 The Example of the Pigeons
In Fig. 3.1b, we display a schematic ventral view of the pigeon syrinx. A
detailed study of the biomechanics of the syrinx in anesthetized pigeons was
performed by Larsen and Goller [Goller and Larsen 1997a], who injected gas
into the subsyringeal air sacs while various muscles were electrically stimu-
lated. The results of those studies allowed them to identify the sound sources:
it was found that air-induced phonation was associated with vibrations of the
lateral tympaniform membranes. The control of the sounds in this case has to
be carried out with a smaller number of muscles. Contraction of the trache-
olateralis muscle (TL) withdraws the lateral tympaniform membranes out of
the lumen, opening the air path, while shortening of the sternotrachealis (ST)
brings the cartilages of the syrinx closer together, which leads to a folding of
the LTMs into the lumen (although even maximal ST contraction does not
lead to closure of the syrinx).
In order to start phonation, the pressure in the air sacs is increased. In
particular, the interclavicular sac is inflated, which pushes the LTMs into the
syringeal lumen, creating a sort of valve. The air passing through these folded
membranes sets them into vibration, which modulates, in turn, the airflow
going into the trachea. It was observed [Goller and Larsen 1997b] that strong
stimulation of the TL muscle leads to termination of the phonation, by the
abduction of the membranes from the air pathway. Interestingly enough,
weak stimulation of the same muscle was observed to lead to changes in the
acoustic features of the phonation. In fact, recent work with doves has shown
that TL activity correlates with frequency changes even during fast trills,
showing that syringeal muscles have superfast kinetics [Elemans et al. 2004].
Beyond what is known for the pigeons, the birds lacking the complex
set of intrinsic muscles that oscines have control the fundamental frequen-
cies of their vocalizations by processes which are not completely understood
[Suthers 2001]. It is known that the extrinsic muscles gating the sound affect
the tension of the oscillating tissues, as does the subsyringeal pressure, by
stretching them [Beckers et al. 2003b]. It is likely that different species have
different ways of controlling their vocalizations by combinations of these basic
gestures.
