Peripheral mechanisms for vocal production in birds - differences and similarities to human speech and singing

Song production in songbirds is a model system for studying learned vocal behavior. As in humans, bird phonation involves three main motor systems (respiration, vocal organ and vocal tract). The avian respiratory mechanism uses pressure regulation in air sacs to ventilate a rigid lung. In songbirds sound is generated with two independently controlled sound sources, which reside in a uniquely avian vocal organ, the syrinx. However, the physical sound generation mechanism in the syrinx shows strong analogies to that in the human larynx, such that both can be characterized as myoelastic-aerodynamic sound sources. Similarities include active adduction and abduction, oscillating tissue masses which modulate flow rate through the organ and a layered structure of the oscillating tissue masses giving rise to complex viscoelastic properties. Differences in the functional morphology of the sound producing system between birds and humans require specific motor control patterns. The songbird vocal apparatus is adapted for high speed, suggesting that temporal patterns and fast modulation of sound features are important in acoustic communication. Rapid respiratory patterns determine the coarse temporal structure of song and maintain gas exchange even during very long songs. The respiratory system also contributes to the fine control of airflow. Muscular control of the vocal organ regulates airflow and acoustic features. The upper vocal tract of birds filters the sounds generated in the syrinx, and filter properties are actively adjusted. Nonlinear source-filter interactions may also play a role. The unique morphology and biomechanical system for sound production in birds presents an interesting model for exploring parallels in control mechanisms that give rise to highly convergent physical patterns of sound generation. More comparative work should provide a rich source for our understanding of the evolution of complex sound producing systems.

New perspectives on the physics of birdsong

In this work, we revisit the path that has been travelled during the last few years towards the modelling of the avian vocal organ, the syrinx, using numerical and theoretical techniques from bifurcation theory as analysing tools and present experimental support for the models. This fruitful perspective allowed the retracing of many acoustic features of syllables to intrinsic properties of the syrinx, thereby relocating the bird phonatory organ from the role of a mere vocal instrument of the nervous system to a central source of complex acoustical behaviour.

Nontraumatic cervicothoracic syrinx as a cause of progressive neurologic dysfunction

BACKGROUND/OBJECTIVE

Syringomyelia is characterized by a fluid-filled cavity within the spinal cord. While its pathogenesis is currently debated, the relationship of syringomyelia with other conditions, such as Chiari I malformation and cord/column trauma, is well accepted. Despite these common associations, a nidus for syrinx formation has not been identified in a subset of patients. We report 2 patients with idiopathic cervicothoracic syringomyelia who presented with progressive neurologic dysfunction. Diagnostic and treatment algorithms used in the care of these patients are presented.

METHODS

Retrospective review, including preoperative and postoperative studies, intraoperative findings, and the patients' surgical outcomes.

RESULTS

Patients underwent laminectomy, lysis of adhesions, untethering of spinal cord, fenestration of syrinx, and duraplasty after preoperative studies demonstrated evidence of focal cerebrospinal fluid flow block at the level of the syrinx. One patient's neurologic condition improved after surgery, whereas the other's remained unchanged without further deterioration; both showed radiographic decrease in the syrinx on immediate postoperative magnetic resonance imaging.

CONCLUSIONS

These 2 cases illustrate patients who develop a cervicothoracic syrinx in the absence of any trauma, infection, previous manipulation of the neuraxis, or malformations known to be associated with a syringomyelia. Whereas there is no consensus on the optimal management of these patients, the patients reported here experienced arrest in deterioration or improvement of their neurologic examination, making the identification of this condition important as a potentially reversible cause of neurologic deficits. Long-term follow-up is required to determine the efficacy, durability, and lifestyle impact of the procedure.

Experience affects recruitment of new neurons but not adult neuron number

It is not known whether the addition of new neurons to the high vocal center (HVC) of juvenile zebra finches permits vocal learning or is the consequence of it. To tease apart these two, we performed surgery on 26-d-old juveniles. The operations were removal of both cochleae and unilateral or bilateral denervation of the syrinx. Ability to imitate a tutor song was little affected by unilateral syringeal denervation but was severely hindered by bilateral denervation or deafening. Recruitment of new HVC neurons was studied by injecting BrdU, a cell birth marker, on post-hatching days 61-65 and killing the animals 30 d later. Deafening or bilateral denervation did not alter the number of BrdU-labeled neurons in HVC, but unilateral denervation nearly doubled this number in the intact side. This doubling was transient, was blocked by deafening, and was not seen in birds that received BrdU injections earlier or later in vocal ontogeny. The adult number of HVC neurons was not affected by any of our surgical procedures. Apparently experience does not affect the total number of neurons in adult HVC, but some kinds of experience can, during narrowly defined times, influence the recruitment of new HVC neurons.