Biomedical Engineering Reference
In-Depth Information
3.4 Respiration
The production of song requires precise respiratory patterns, in order to both
accomplish adequate gas exchange and provide the airflow needed for phona-
tion. In order to achieve these goals, birds have a respiratory system which
is unique in structure and e
ciency. The avian lungs are small and relatively
rigid (with a nearly fixed volume), and do not move freely [Hildebrand 1995].
Their ventilation is carried out by a set of
air sacs
, thin-walled structures
which connect to the trachea and lungs in a complex way. The typical struc-
ture consists of a pair of abdominal sacs (where the primary bronchi termi-
nate), a pair of posterior thoracic sacs, a pair of anterior thoracic sacs, paired
cervical sacs and an unpaired interclavicular air sac.
The motion of these sacs is driven by inspiratory and expiratory mus-
cles. By moving the sternum ventrally, the inspiratory muscles expand the
air sacs, decreasing the pressure. The motion is reversed by the expiratory
muscles, which therefore compress the air sacs, increasing the pressure. It is
during expiration that most vocalizations are achieved. The pressure during
singing can be over an order of magnitude larger than during normal, silent
respiration, reaching values of up to 30 cm H
2
O in some syllables sung by a
canary [Hartley and Suthers 1989].
Despite the wide variety of anatomical features found in the avian vocal
organs, it is interesting that some motor patterns are found across different
avian groups. For example, in the case of both oscines and nonoscines, the
production of repeated syllables at a moderate rate involves a brief inspira-
tion, or minibreath [Calder 1970], which allows the replacement of the air
used for the vocalization. Notice that the neural instructions controlling the
activity of the inspiratory and expiratory muscles will be interacting with the
mechanics of the body during the process. On one hand, stretching receptors
will modulate the activity of the neurons controlling inspiration. On the other
one, both inspiratory and expiratory muscles will be operating in conjunction
with the forces of thoracic elastic recoil: at small air sac volumes, the recoil
will help the inspiratory muscles, while at volumes of the air sac larger than
the equilibrium value, the elastic recoil will help expiration [Suthers 2004].
A second respiratory pattern frequently used across avian groups is the pul-
satile one, used at very high repetition rate. In this pattern, there are no
minibreaths, and the expiration is maintained through some level of activ-
ity in the expiratory muscles. The critical rate at which a bird will turn to
a pulsatile respiratory regime depends on its body size, probably owing to
the natural frequency of the mechanical parts of the body involved in the
respiration. This hypothesis was tested by tutoring mockingbirds with ca-
naries singing using minibreaths [Zollinger and Suthers 2004]. The result of
the experiment was that mockingbirds could copy the high-repetition-rate
syllables, but using a gesture that involved pulsatile expiration. This is a
beautiful example of the subtle interaction between the nervous system and
the body, which we shall explore further in this topic.
