Contributions of syringeal muscles to respiration and vocalization in the zebra finch

Acute and chronic electromyographic (EMG) recordings from individual syringeal muscles were used to study syringeal participation in respiration and vocalization. In anesthetized birds, all syringeal muscles recorded were active to some degree during the expiratory phase of respiration, following activity in the abdominal musculature and preceding the emergence of breath from the nostril. In awake birds, the ventralis (V) muscle fired a strong, consistent burst, but the dorsalis (D) was variable both in strength and timing. Denervation of V is sufficient to produce the wheezing respiration originally seen in birds with complete bilateral section of the tracheosyringeal nerve. Complete syringeal denervation also removed almost all the acoustic features that distinguish individual song syllables, but had a minor effect on the temporal structure of song. When activity in V and D was recorded in awake, vocalizing birds, D was active before and during sound production, and V showed a small burst before sound onset and a vigorous burst timed to the termination of sound. During song, V was consistently active at sound offset, but also participated during sound for narrow bandwidth syllables. For some syllables (simple harmonic stacks), neither muscle was active. These data suggest that V contributes to syllable termination during vocalization and may silence the syrinx during normal respiration. D contributes to the acoustic structure of most syllables, and V may contribute to a special subset of syllables. In summary, the syringeal muscles show different activity patterns during respiration and vocalization and can be independently activated during vocalization, depending on the syllable produced.     

The neuromuscular control of birdsong.

Birdsong requires complex learned motor skills involving the coordination of respiratory, vocal organ and craniomandibular muscle groups. Recent studies have added to our understanding of how these vocal subsystems function and interact during song production. The respiratory rhythm determines the temporal pattern of song. Sound is produced during expiration and each syllable is typically followed by a small inspiration, except at the highest syllable repetition rates when a pattern of pulsatile expiration is used. Both expiration and inspiration are active processes. The oscine vocal organ, the syrinx, contains two separate sound sources at the cranial end of each bronchus, each with independent motor control. Dorsal syringeal muscles regulate the timing of phonation by adducting the sound-generating labia into the air stream. Ventral syringeal muscles have an important role in determining the fundamental frequency of the sound. Different species use the two sides of their vocal organ in different ways to achieve the particular acoustic properties of their song. Reversible paralysis of the vocal organ during song learning in young birds reveals that motor practice is particularly important in late plastic song around the time of song crystallization in order for normal adult song to develop. Even in adult crystallized song, expiratory muscles use sensory feedback to make compensatory adjustments to perturbations of respiratory pressure. The stereotyped beak movements that accompany song appear to have a role in suppressing harmonics, particularly at low frequencies.     

Sexual Dimorphism of the Zebra Finch Syrinx Indicates Adaptation for High Fundamental Frequencies in Males

Background

In many songbirds the larger vocal repertoire of males is associated with sexual dimorphism of the vocal control centers and muscles of the vocal organ, the syrinx. However, it is largely unknown how these differences are translated into different acoustic behavior.

Methodology/Principal Findings

Here we show that the sound generating structures of the syrinx, the labia and the associated cartilaginous framework, also display sexual dimorphism. One of the bronchial half rings that position and tense the labia is larger in males, and the size and shape of the labia differ between males and females. The functional consequences of these differences were explored by denervating syringeal muscles. After denervation, both sexes produced equally low fundamental frequencies, but the driving pressure generally increased and was higher in males. Denervation strongly affected the relationship between driving pressure and fundamental frequency.

Conclusions/Significance

The syringeal modifications in the male syrinx, in concert with dimorphisms in neural control and muscle mass, are most likely the foundation for the potential to generate an enhanced frequency range. Sexually dimorphic vocal behavior therefore arises from finely tuned modifications at every level of the motor cascade. This sexual dimorphism in frequency control illustrates a significant evolutionary step towards increased vocal complexity in birds.     

Peripheral control and lateralization of birdsong

Recent studies on several species of oscine songbirds show that they achieve their varied vocal performances through coordinated activity of respiratory, syringeal, and other vocal tract muscles in ways that take maximum advantage of the acoustic flexibility made possible by the presence of two independently controlled sound sources in their bipartite syrinx (vocal organ). During song, special motor programs to respiratory muscles alter the pattern of ventilation to maintain the supply of respiratory air and oxygen to permit songs of long duration, high syllable repetition rates, or maximum spectral complexity. Each side of the syrinx receives its own motor program that, together with that sent to respiratory muscles, determines the acoustic properties of the ipsilaterally produced sound. The acoustic expression of these bilaterally distinct, phonetic motor patterns depends on the action of dorsal syringeal adductor muscles that, by opening or closing the ipsilateral side of the syrinx to airflow, determine the amount each side contributes to song. The syringeally generated sound is further modified by muscles that control the shape of the vocal tract. Different species have adopted different motor strategies that use the left and right sides of the syrinx in patterns of unilateral, bilateral, alternating, or sequential phonation to achieve the differing temporal and spectral characteristics of their songs. As a result, the degree of song lateralization probably varies between species to form a continuum from unilateral dominance to bilateral equality. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 632–652, 1997     

Electromyographic studies of the syrinx in parrots (Aves,Psittacidae)

Summary The vocal organ (syrinx) of a bird may contain either extrinsic muscles alone or both extrinsic and intrinsic muscles. The former arise and insert on the trachea and affect the syrinx only indirectly; the latter also arise on the trachea but insert directly on syringeal elements. It is widely supposed that syringeal muscles can affect modulations of the sounds the birds make, and further, that the intrinsic muscles are closely associated with such a function. However, the exact roles of the two groups of muscles have not been directly observed.The psittacid syrinx, which has one (for practical purposes) pair of extrinsic and two pairs of intrinsic muscles, is about as simple as one can find in birds capable of uttering a wide variety of sounds. We have taken electromyograms from the syringeal muscles of five species of parrots. In all of these, the extrinsic sternotrachealis showed the simple activation pattern activity previously described from several non-passerine species that possess only extrinsic muscles. The intrinsic muscles, however, showed a variety of activity patterns. The relatively simple call of Cyanoliseus patagonus again showed the simple activation pattern. In Myiopsitta monachus, the muscles showed a string of pulses that matched to pulses of sound in a strongly amplitude modulated call. Agapornis roseicollis used at least two distinct patterns, each associated with a different call.The results are consistent with an hypothesis that, because of their indirect attachment of the syrinx, extrinsic muscles are poorly suited to the production of precise, rapid changes in syringeal action, but rather will function in an on-off switch capacity. Intrinsic muscles are so situated that, given proper neurological stimulus, they can effect a variety of alterations in the sound pattern. Hence, intrinsic muscles are necessary for the evolution of large vocabularies and variable vocal behavior.     

Syringeal specialization of frequency control during song production in the Bengalese finch (Lonchura striata domestica)

Background

Singing in songbirds is a complex, learned behavior which shares many parallels with human speech. The avian vocal organ (syrinx) has two potential sound sources, and each sound generator is under unilateral, ipsilateral neural control. Different songbird species vary in their use of bilateral or unilateral phonation (lateralized sound production) and rapid switching between left and right sound generation (interhemispheric switching of motor control). Bengalese finches (Lonchura striata domestica) have received considerable attention, because they rapidly modify their song in response to manipulations of auditory feedback. However, how the left and right sides of the syrinx contribute to acoustic control of song has not been studied.

Methodology

Three manipulations of lateralized syringeal control of sound production were conducted. First, unilateral syringeal muscular control was eliminated by resection of the left or right tracheosyringeal portion of the hypoglossal nerve, which provides neuromuscular innervation of the syrinx. Spectral and temporal features of song were compared before and after lateralized nerve injury. In a second experiment, either the left or right sound source was devoiced to confirm the role of each sound generator in the control of acoustic phonology. Third, air pressure was recorded before and after unilateral denervation to enable quantification of acoustic change within individual syllables following lateralized nerve resection.

Significance

These experiments demonstrate that the left sound source produces louder, higher frequency, lower entropy sounds, and the right sound generator produces lower amplitude, lower frequency, higher entropy sounds. The bilateral division of labor is complex and the frequency specialization is the opposite pattern observed in most songbirds. Further, there is evidence for rapid interhemispheric switching during song production. Lateralized control of song production in Bengalese finches may enhance acoustic complexity of song and facilitate the rapid modification of sound production following manipulations of auditory feedback.     

Effect of tracheosyringeal denervation on call in greenfinch (Carduelis sinica)

After sections of left or right tracheosyringeal nerve (NXIIts), greenfinches may repeat everyday calls, with no effect on temporal properties. It is suggested that either side of syrinx may produce sound alone and ipsilateral innerration of NXIIts for the syringeal muscles. After section of left NXIIts, the bird produces the vocal pattern of partial tone increase, and effects on the sound intensity and sentence length average 1.4 and 2.8 times those after section of right NXIIts, suggesting that the innervation of NXIIts has left side dominance. After bilateral section of NXIIts, the call rhythm in company with expiratory motions is 98–146 times/min, on an average, and lose all sentence types and syllable structure of normal call. But the call spectra produced by tympaniform membrane vibrations without innervation still reserve frequency components similar to the tonic frequency and harmonics of normal calls.     

Sexual dimorphism and bilateral asymmetry of syrinx and vocal tract in the European starling (Sturnus vulgaris)

Sexually dimorphic vocal behavior in zebra finches (Taeniopygia guttata) is associated with a 100% larger syrinx in males and other morphological adaptations of the sound source. The songbird syrinx consists of two independent sound sources, whose specialization for different spectral ranges may be reflected in morphological properties, but the morphology of labia and syringeal skeleton have not been investigated for lateralized specializations. Similarly, little is known whether the morphology of the songbird vocal tract reflects differences in vocal behavior. Here, we tested the hypothesis that different vocal behavior and specialization is reflected in the morphology. We investigated syringeal and upper vocal tract morphology of male and female European starlings (Sturnus vulgaris). Female starlings exhibit smaller vocal repertoires and sing at lower rates than males. In males, the left syrinx produces mostly low frequencies, while the right one is used for higher notes. Macroscopic and histological techniques were used to record nineteen measurements from the syrinx and the vocal tract which were tested for sexual differences in syrinx and vocal tract and for lateral asymmetry within the syrinx. Sexually dimorphic vocal behavior is reflected in the morphology of the starling syrinx. Males have a larger syrinx with the size difference attributable to increased muscle mass and three enlarged elements of the syringeal skeleton. The upper vocal tract, however, does not differ between males and females. Distinct lateralization was found in two elements of the syringeal skeleton of females, and the labia in the left syrinx are larger than those on the right in both sexes. The sexual dimorphism of the syringeal size is smaller in starlings (35%) than in zebra finches (100%), which is consistent with the different vocal behavior of females in both species. The morphological differences between the two sound sources are discussed in relation to their vocal performance. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

The acoustic effect of vocal tract adjustments in zebra finches

Vocal production in songbirds requires the control of the respiratory system, the syrinx as sound source and the vocal tract as acoustic filter. Vocal tract movements consist of beak, tongue and hyoid movements, which change the volume of the oropharyngeal–esophageal cavity (OEC), glottal movements and tracheal length changes. The respective contributions of each movement to filter properties are not completely understood, but the effects of this filtering are thought to be very important for acoustic communication in birds. One of the most striking movements of the upper vocal tract during vocal behavior in songbirds involves the OEC. This study measured the acoustic effect of OEC adjustments in zebra finches by comparing resonance acoustics between an utterance with OEC expansion (calls) and a similar utterance without OEC expansion (respiratory sounds induced by a bilateral syringeal denervation). X-ray cineradiography confirmed the presence of an OEC motor pattern during song and call production, and a custom-built Hall-effect collar system confirmed that OEC expansion movements were not present during respiratory sounds. The spectral emphasis during zebra finch call production ranging between 2.5 and 5 kHz was not present during respiratory sounds, indicating strongly that it can be attributed to the OEC expansion.     

Bilaterally symmetrical respiratory activity during lateralized birdsong

We investigated whether activity of expiratory muscles reflects lateralized activity of the vocal organ during production of birdsong. Respiration and syringeal motor activity were assessed in brown thrashers by monitoring bilateral airflow and subsyringeal air sac pressure, together with the electromyographic activity of expiratory abdominal muscles and vocal output. Activity of expiratory muscles was always present on both sides, regardless of whether song was produced bilaterally or on only one side of the syrinx. The average amplitude of expiratory EMG of one side does not change significantly, even if that side is silent during phonation. The temporal pattern of the electromyogram (EMG) was similar on both sides. Bilateral bursts of EMG activity on both sides accompanied changes in the rate of syringeal airflow, even when these flow fluctuations were generated only by one side of the syrinx. Motor commands to the respiratory muscles therefore appear to be bilaterally distributed, in contrast to the lateralized motor control of the syrinx.     

Unilateral nervous control of the syrinx in java sparrows (Padda oryzivora)

Summary The syrinx of song birds contains two sound sources, the internal tympaniform membranes located one in each bronchus, that are controlled by the syringeal musculature. These muscles are innervated by the cervicalis descendens superior (CDS) branch of the hypoglossal nerve. Unilateral sections of the left CDS nerve in Java sparrows markedly disrupted tonal quality of the songs, although temporal parameters were unaltered. Bilateral CDS nerve sections caused greater disruption in frequency characteristics and temporal parameters were altered. Most birds died due to asphyxia soon after the operation. Right CDS nerve sections had much less effect, except on long whistles where extra low frequency sounds appeared, probably from the denervated right bronchus. Intact song pattern was restored within four months without re-innervation of the right syringeal musculature. This unilateral dominance in the control of the syrinx agrees with previous work and adds to the support for the two voice theory of sound production in birds (Greenewalt, 1968; Stein, 1968).Abbreviation CDS Nervus cervicalis descendens superior     

Acoustic structure of vocalization and stapedius muscle activity during vocal development in chickens (Gallus gallus)

The link between stapedius muscle activity and acoustic structure of vocalization was analysed in cocks of age 20–30 to 90–100 days old. The results show that stapedius muscle activation depends on the acoustic structure of vocalization and changes during vocal development. This dependence was observed in spontaneous calls and in vocalizations elicited by stimulating the mesencephalic calling area. In 30-day-old cocks stapedius muscle EMG response is never associated with vocalizations with an acoustic energy content which is always distributed at frequencies higher than 2000 Hz. The coupling between vocalization and stapedius muscle activity begins later, when birds produce vocalizations with acoustic energy shifted towards lower frequencies. Overall, stapedius muscle activity is related to a bird's production of high amplitude low frequencies. These results support the hypothesis that the primary role of the stapedius muscle during normal vocal development is to dampen the amplitude of low frequency energy that reaches the cochlea during vocalization.     

An Intermediate in the evolution of superfast sonic muscles

Background

Intermediate forms in the evolution of new adaptations such as transitions from water to land and the evolution of flight are often poorly understood. Similarly, the evolution of superfast sonic muscles in fishes, often considered the fastest muscles in vertebrates, has been a mystery because slow bladder movement does not generate sound. Slow muscles that stretch the swimbladder and then produce sound during recoil have recently been discovered in ophidiiform fishes. Here we describe the disturbance call (produced when fish are held) and sonic mechanism in an unrelated perciform pearl perch (Glaucosomatidae) that represents an intermediate condition in the evolution of super-fast sonic muscles.

Results

The pearl perch disturbance call is a two-part sound produced by a fast sonic muscle that rapidly stretches the bladder and an antagonistic tendon-smooth muscle combination (part 1) causing the tendon and bladder to snap back (part 2) generating a higher-frequency and greater-amplitude pulse. The smooth muscle is confirmed by electron microscopy and protein analysis. To our knowledge smooth muscle attachment to a tendon is unknown in animals.

Conclusion

The pearl perch, an advanced perciform teleost unrelated to ophidiiform fishes, uses a slow type mechanism to produce the major portion of the sound pulse during recoil, but the swimbladder is stretched by a fast muscle. Similarities between the two unrelated lineages, suggest independent and convergent evolution of sonic muscles and indicate intermediate forms in the evolution of superfast muscles.     

Mutually exclusive muscle designs: the power output of the locomotory and sonic muscles of the oyster toadfish (Opsanus tau)

Animals perform a vast array of motor activities. Although it has generally been accepted that muscles are well suited to the function that they must perform, specialization for performing one function may compromise their ability for carrying out another. We examined this principle in the toadfish muscular system: slow-twitch red and fast-twitch white myotomal muscles are used for powering swimming at relatively low frequencies, while the superfast swimbladder muscle powers mating calls by contracting at 100 Hz. We measured muscle power output over a wide range of frequencies. The red and white locomotory muscles could not generate power over ca. 2.2 and 12 Hz, respectively and, hence, could not power sound production. In contrast, the swimbladder muscle has many specializations that permit it to generate power at frequencies in excess of 100 Hz. However, these specializations drastically reduce its power output at low frequencies: the swimbladder muscle generated only one-twentieth of the power of the red muscle and one-seventh of the power of the white muscle at the frequencies used during swimming. To generate the same total power needed for swimming would require unfeasibly large amounts of swimbladder muscle that could not fit into the fish. Hence, the designs of the swimbladder and locomotory muscles are mutually exclusive.     

A comparison of the origin and temporal arrangement of pulsed sounds in the songs of the Grasshopper and Sedge warblers, Locustella naevia and Acrocephalus schoenobaenus

The songs of the two British warblers Locustella naevia and Acrocephalus schoenobaenus were examined by means of oscillographic recordings. Both songs are composed of pulsed elements arranged in rhythmical successions. The song of L. naevia is simple and stereotyped consisting of a succession of paired pulses with a repetition frequency of 26 Hz. The song of A. schoenobaenus contains many different phrases each consisting of a succession of identical chirps. Chirps consist of successions of pulses, the number and repetition rate of which vary from phrase to phrase.
The songs are discussed in relation to the physiology of sound production and to the way in which information is encoded into sound signals. It is proposed that pulses are produced by the syringeal muscles, whilst the rhythm, tempo and duration of chirps and phrases are controlled by the respiratory muscles. A distinction is drawn between the action of the extrinsic syringeal muscles, which may produce low frequency pulses, and the intrinsic syringeal muscles, which may produce high frequency pulses.
The codal format in both songs is shown to be highly redundant, being based on the repetition of identical units of information. This device reduces the possibility of distortion of meaning by interference from environmental noise but limits the information carrying capacity of the code. The method of pulse coding is shown to be particularly well suited to the avian auditory system which is adapted to receiving and processing rapid transient signals. This ability is in part attributable to the fine discrimination of time and amplitude changes in the cochlea.     

The anatomy of the syrinx in passerine birds

The macroscopical structure of the organ of voice in songbirds has long been known, but detailed information on the microscopical anatomy of the syrinx has generally been lacking. Observations based largely on macroscopical evidence have led to a number of erroneous interpretations of function of various syringeal components, and lacking microscopical information, the vocal mechanism of birds cannot be adequately understood.
A wide variety of passeriform bird syrinxes have been studied by means of serial sections. Although there is much less variation in syringeal anatomy amongst songbirds than there is in the other orders of birds, and although all songbird syrinxes conform to the same basic pattern, there is nevertheless marked variation in various syringeal components between different passerine groups. Variations in syringeal structure within families Corvidae ( Corvus corone, C. frugilegus ), Sturnidae ( Sturnus vulgaris, Gracula religiosa ), Turdidae ( Turdus merula, Erithacus rubecula ), Hirundinidae ( Delichon urbica ), Ploceidae ( Passer domesticus ) and Paridae ( Parus major, Aegithalos caudatus ) are described and discussed. The significance of these findings in relation to bird sound production is discussed.     

Sensitive period for sensorimotor integration during vocal motor learning

Sensory experience during sensitive periods in development may direct the organization of neural substrates, thereby permanently influencing subsequent adult behavior. We report a sensitive period during the imitative motor learning phase of sensorimotor integration in birdsong development. By temporarily and reversibly blocking efference to the vocal muscles, we disrupted vocal motor practice during selected stages of song development. Motor disruption during prolonged periods early in development, which allows recovery of vocal control prior to the onset of adult song, has no effect on adult song production. However, song disruption late in development, during the emergence of adult song, results in permanent motor defects in adult song production. These results reveal a decreased ability to compensate for interference with motor function when disturbances occur during the terminal stage of vocal motor development. Temporary disruption of syringeal motor control in adults does not produce permanent changes in song production. Permanent vocal aberrations in juveniles are evident exclusively in learned song elements rather than nonlearned calls, suggesting that the sensitive period is associated with motor learning.     

The songbird syrinx morphome: a three-dimensional, high-resolution, interactive morphological map of the zebra finch vocal organ

Background

Like human infants, songbirds learn their species-specific vocalizations through imitation learning. The birdsong system has emerged as a widely used experimental animal model for understanding the underlying neural mechanisms responsible for vocal production learning. However, how neural impulses are translated into the precise motor behavior of the complex vocal organ (syrinx) to create song is poorly understood. First and foremost, we lack a detailed understanding of syringeal morphology.

Results

To fill this gap we combined non-invasive (high-field magnetic resonance imaging and micro-computed tomography) and invasive techniques (histology and micro-dissection) to construct the annotated high-resolution three-dimensional dataset, or morphome, of the zebra finch (Taeniopygia guttata) syrinx. We identified and annotated syringeal cartilage, bone and musculature in situ in unprecedented detail. We provide interactive three-dimensional models that greatly improve the communication of complex morphological data and our understanding of syringeal function in general.

Conclusions

Our results show that the syringeal skeleton is optimized for low weight driven by physiological constraints on song production. The present refinement of muscle organization and identity elucidates how apposed muscles actuate different syringeal elements. Our dataset allows for more precise predictions about muscle co-activation and synergies and has important implications for muscle activity and stimulation experiments. We also demonstrate how the syrinx can be stabilized during song to reduce mechanical noise and, as such, enhance repetitive execution of stereotypic motor patterns. In addition, we identify a cartilaginous structure suited to play a crucial role in the uncoupling of sound frequency and amplitude control, which permits a novel explanation of the evolutionary success of songbirds.     

Effect of tracheosyringeal denervation on call in greenfinch ( Carduelis sinica)

After sections of left or right tracheosyringeal nerve (NXⅡts), greenfinches may repeat everyday calls, with no effect on temporal properties. It is suggested that either side of syrinx may produce sound alone and ipsilateral innervation of NXⅡts for the syringeal muscles. After section of left NXIIts, the bird produces the vocal pattern of partial tone increase, and effects on the sound intensity and sentence length average 1.4 and 2.8 times those after section of right NXIIts, suggesting that the innervation of NXIIts has left side dominance. After bilateral section of NXIIts, the call rhythm in company with expiratory motions is 98-146 times/min,on an average, and lose all sentence types and syllable structure of normal call. But the call spectra produced by tympaniform membrane vibrations without innervation still reserve frequency components similar to the tonic frequency and harmonics of normal calls.     

Movement and sound generation by the toadfish swimbladder

Although sound-producing (sonic) muscles attached to fish swimbladders are the fastest known vertebrate muscles, the functional requirement for such extreme speed has never been addressed. We measured movement of the swimbladder caused by sonic muscle stimulation in the oyster toadfish Opsanus tau and related it to major features of the sound waveform. The movement pattern is complex and produces sound inefficiently because the sides and bottom of the bladder move in opposite in and out directions, and both movement and sound decay rapidly. Sound amplitude is related to speed of swimbladder movement, and slow movements do not produce perceptible sound. Peak sound amplitude overlaps fundamental frequencies of the male's mating call because of muscle mechanics and not the natural frequency of the bladder. These findings suggest that rapid muscle speed evolved to generate sound from an inefficient highly damped system.