Biomedical Engineering Reference
In-Depth Information
that without these nuclei, the birds cannot vocalize. For this reason, these
nuclei are said to be part of the
motor pathway
. Another set of nuclei con-
stitute the
anterior forebrain pathway
. These nuclei can be lesioned without
an immediate degradation of the song. However, lesions of these nuclei affect
the bird's ability to learn or maintain the vocalizations. Let us describe some
features of these pathways.
8.1 The Motor Pathway
It was through lesions and observation of behavior that the nuclei involved in
the generation of motor activities responsible for the production of song were
identified [Nottebohm et al. 1976]. The “motor pathway” is constituted by
two nuclei, called HVC and RA (robustus nucleus of the archistriatum). HVC
sends instructions to RA, which in turn sends instructions to several nuclei:
the tracheosyringeal motor nucleus nXIIts, which innervates the syringeal
muscles, and a set of respiratory nuclei (among which we find the expira-
tory premotor nucleus retroambigualis RAm), which innervates the muscles
involved in respiration (see Fig. 8.1). HVC and RA are necessary for song
production. Lesions in them lead to the suppression of song. The specific func-
tion of each nucleus is more di
cult to unveil. However, there is evidence of
a hierarchical organization in the brain. Microstimulation of HVC is followed
by interruption of singing and restarting of song, while microstimulation of
RA disrupts only the syllable being sung [Yu and Margoliash 1996].
Observed from the peripheral system, the motor pathway has to generate
cyclic instructions to drive the syrinx and the respiratory system. Moreover,
in order to generate the diversity of syllables within a song, the motor path-
way has to be capable of generating a diversity of phase differences between
these oscillations, as discussed in Chap. 5. How does it do this? First of all,
RAm and nXIIts, the brain stem structures most directly involved in phona-
tion since they are the nuclei involved in locking expiratory activity and
controlling the syrinx, are driven by projecting neurons in different regions
of RA [Spiro et al. 1999]. Since RAm also projects onto nXIIts, the phase
difference between the respiratory and syringeal gestures will depend on
both
the nature of this connection and the pattern of activity within RA.
The RA nucleus is innervated by excitatory projection neurons from the
HVC nucleus. Just as RA is closely tied to the peripheral motor system, HVC
is supposed to have connections with higher-level nervous-system functions
[Yu and Margoliash 1996, Suthers and Margoliash 2002]. A song is organized
in terms of minimal continuous utterances that we call syllables, combined
with quiet periods, to build motifs. It has been proposed that this hierarchy in
the song has a neural correlation, on the basis of the observation that electri-
cal stimulation of HVC causes a resetting of the motif being sung, while stimu-
lation of RA causes a distortion of the vocalization [Yu and Margoliash 1996].
