Sonic Muscle Fibers Research Articles

Sound variation by hypertrophy and atrophy sonic muscle in the male southern black drum (Pogonias courbina).

The male and female southern black drum possess highly specialized, bilateral, striated sonic muscles used in sound production during courtship by males and in the production of disturbance calls by both males and females. Androgen-driven hypertrophy of the sonic muscles in males during the late spring spawning period results in increased growth of sonic muscle mass followed by post-spawning atrophy of sonic muscles. We examined changes in sonic muscle morphology and in the sound characteristics of males and females underlying seasonal changes in sonic muscle mass and muscle contraction as measured by sound production. In males, the sound pressure level increased while sound pulse duration decreased with increasing sonic muscle mass, indicating that sonic muscle fibers contract with greater force and shorter duration during the spawning season. Interpulse interval and the total number of pulses varied seasonally with muscle mass, which suggests that the effects of steroids on male southern black drum sound characteristics are more pronounced peripherally than in the central nervous system. In females, no increase in sonic muscle mass was found, and therefore, a change was not observed in the acoustic variables analyzed. Seasonal sonic muscle hypertrophy in males likely functions as a secondary sexual characteristic that maximizes vocalization amplitude or loudness during the spawning period.

Functional Adaptation of Vocalization Revealed by Morphological and Histochemical Characteristics of Sonic Muscles in Blackmouth Croaker (Atrobucca nibe).

Simple SummarySound production is common in numerous fish species. Some species can emit calls through the contraction of specialized muscles called sonic or drumming muscles. The sonic muscles of fish are among the fastest muscles in vertebrates. Although numerous studies have investigated the mechanism underlying sound production in fish, only the distinct features of the sonic muscles of a few species have been investigated. We demonstrated that the sonic muscles have functionally adapted for fast twitching and fatigue resistance, which support vocalization in the blackmouth croaker (Atrobucca nibe).Sound production in the blackmouth croaker (Atrobucca nibe) was characterized using acoustic, morphological, and histochemical methods. Their calls consisted of a train of two to seven pulses; the frequency ranged from 180 to 3000 Hz, with a dominant frequency of 326 ± 40 Hz. The duration of each call ranged from 80 to 360 ms. Male A. nibe possess a pair of bilaterally symmetric sonic muscles attached to the body wall adjacent to the swim bladder. The average diameter of the sonic muscle fibers was significantly shorter than that of the abdominal muscle fibers. Semithin sections of the sonic muscle fibers revealed a core-like structure (central core) and the radial arrangement of the sarcoplasmic reticulum and myofibrils. Numerous mitochondria were distributed within the central core and around the periphery of the fibers. Most of the fibers were identified as Type IIa on the basis of their myosin adenosine triphosphatase activities, but a few were identified as Type IIc fibers. All sonic muscle fibers exhibited strong oxidative enzyme activity and oxidative and anaerobic capabilities. The features suggest that the sonic muscles of A. nibe are morphologically and physiologically adapted for fast twitching and fatigue resistance, which support fish vocalization.

Fast drum strokes: Novel and convergent features of sonic muscle ultrastructure, innervation, and motor neuron organization in the pyramid butterflyfish (hemitaurichthys polylepis)

Sound production that is mediated by intrinsic or extrinsic swim bladder musculature has evolved multiple times in teleost fishes. Sonic muscles must contract rapidly and synchronously to compress the gas-filled bladder with sufficient velocity to produce sound. Muscle modifications that may promote rapid contraction include small fiber diameter, elaborate sarcoplasmic reticulum (SR), triads at the A-I boundary, and cores of sarcoplasm. The diversity of innervation patterns indicate that sonic muscles have independently evolved from different trunk muscle precursors. The analysis of sonic motor pathways in distantly related fishes is required to determine the relationships between sonic muscle evolution and function in acoustic signaling. We examined the ultrastructure of sonic and adjacent hypaxial muscle fibers and the distribution of sonic motor neurons in the coral reef Pyramid Butterflyfish (Chaetodontidae: Hemitaurichthys polylepis) that produces sound by contraction of extrinsic sonic muscles near the anterior swim bladder. Relative to adjacent hypaxial fibers, sonic muscle fibers were sparsely arranged among the endomysium, smaller in cross-section, had longer sarcomeres, a more elaborate SR, wider t-tubules, and more radially arranged myofibrils. Both sonic and non-sonic muscle fibers possessed triads at the Z-line, lacked sarcoplasmic cores, and had mitochondria among the myofibrils and concentrated within the peripheral sarcoplasm. Sonic muscles of this derived eutelost possess features convergent with other distant vocal taxa (other euteleosts and non-euteleosts): small fiber diameter, a well-developed SR, and radial myofibrils. In contrast with some sonic fishes, however, Pyramid Butterflyfish sonic muscles lack sarcoplasmic cores and A-I triads. Retrograde nerve label experiments show that sonic muscle is innervated by central and ventrolateral motor neurons associated with spinal nerves 1-3. This restricted distribution of sonic motor neurons in the spinal cord differs from many euteleosts and likely reflects the embryological origin of sonic muscles from hypaxial trunk precursors rather than occipital somites.

Sound production mechanism in carapid fish: First example with a slow muscle

Fish sonic swimbladder muscles are the fastest muscles in vertebrates and have fibers with numerous biochemical and structural adaptations for speed. Carapid fishes produce sounds with a complex swimbladder mechanism including skeletal components and extrinsic sonic muscle fibers with an exceptional helical myofibrillar structure. To study this system we stimulated the sonic muscles, described their insertion and action, and generated sounds by slowly pulling the sonic muscles. We find the sonic muscles contract slowly, pulling the anterior bladder and thereby stretching a thin fenestra. Sound is generated when the tension trips a release system that causes the fenestra to snap back to its resting position. The sound frequency does not correspond to the calculated resonant frequency of the bladder, and we hypothesize that it is determined by the snapping fenestra interacting with an overlying bony swimbladder plate. To our knowledge this tension release mechanism is unique in animal sound generation.

Sound production mechanism in carapid fish: first example with a slow sonic muscle

Fish sonic swimbladder muscles are the fastest muscles in vertebrates and have fibers with numerous biochemical and structural adaptations for speed. Carapid fishes produce sounds with a complex swimbladder mechanism, including skeletal components and extrinsic sonic muscle fibers with an exceptional helical myofibrillar structure. To study this system we stimulated the sonic muscles, described their insertion and action and generated sounds by slowly pulling the sonic muscles. We find the sonic muscles contract slowly, pulling the anterior bladder and thereby stretching a thin fenestra. Sound is generated when the tension trips a release system that causes the fenestra to snap back to its resting position. The sound frequency does not correspond to the calculated resonant frequency of the bladder, and we hypothesize that it is determined by the snapping fenestra interacting with an overlying bony swimbladder plate. To our knowledge this tension release mechanism is unique in animal sound generation.

Myosin flares and actin leptomeres as myofibril assembly/disassembly intermediates in sonic muscle fibers

The sonic muscle of type 1 male midshipman fish produces loud and enduring mating calls. Each sonic muscle fiber contains a tubular contractile apparatus with radially arranged myofibrillar plates encased in a desmin-rich cytoskeleton that is anchored to broad Z bands (approximately 1.2 micro m wide). Immunomicroscopy has revealed patches of myosin-rich "flares" emanating from the contractile tubes into the peripheral sarcoplasm along the length of the fibers. These flares contain swirls of thick filaments devoid of associated thin filaments. In other regions of the sarcoplasm at the inner surface of the sarcolemma and near Z bands, abundant ladder-like leptomeres occur with rungs every 160 nm. Leptomeres consist of dense arrays of filaments (approximately 4 nm) with a structure that resembles myofibrillar Z band structure. We propose that flares and leptomeres are distinct filamentous arrays representing site-specific processing of myofibrillar components during the assembly and disassembly of the sarcomere. Recent reports that myosin assembles into filamentous aggregates before incorporating into the A band in the skeletal muscles of vertebrates and Caenorhabditis elegans suggest that sonic fibers utilize a similar pathway. Thus, sonic muscle fibers, with their tubular design and abundant sarcoplasmic space, may provide an attractive muscle model to identify myofibrillar intermediates by structural and molecular techniques.

Concentric intermediate filament lattice links to specialized Z-band junctional complexes in sonic muscle fibers of the type I male midshipman fish

Type I male midshipman fish produce high-frequency hums for prolonged durations using sonic muscle fibers, each of which contains a hollow tube of radially oriented thin and flat myofibrils that display extraordinarily wide (∼1.2 μm) Z bands. We have revealed an elaborate cytoskeletal network of desmin filaments associated with the contractile cylinder that form interconnected concentric ring structures in the core and periphery at the level of the Z bands. Stretch and release of single fibers revealed reversible length changes in the elastic desmin lattice. This lattice is linked to Z bands via novel intracellular desmosome-like junctional complexes that collectively form a ring, termed the “Z corset,” around the periphery and within the core of the cylinder. The junctional complex consists of regularly spaced parallel ∼900-nm-long cytoskeletal rods, or “Z bars,” interconnected with slender (3–4 nm) plectin-positive filaments. Z bars are linked to the Z band by plectin filaments and on the opposite side to a dense mesh of desmin filaments. Adjacent Z bands are linked by slender filaments that appear to suspend sarcotubules. We propose that the highly reinforced elastic desmin cytoskeleton and the unique Z band junctions are structural adaptations that enable the muscles’ high-frequency and high-endurance activity.

Weakfish sonic muscle: influence of size, temperature and season.

The influence of temperature, size and season on the sounds produced by the sonic muscles of the weakfish Cynoscion regalis are categorized and used to formulate a hypothesis about the mechanism of sound generation by the sonic muscle and swimbladder. Sounds produced by male weakfish occur at the time and location of spawning and have been observed in courtship in captivity. Each call includes a series of 6-10 sound pulses, and each pulse expresses a damped, 2-3 cycle acoustic waveform generated by single simultaneous twitches of the bilateral sonic muscles. The sonic muscles triple in mass during the spawning season, and this hypertrophy is initiated by rising testosterone levels that trigger increases in myofibrillar and sarcoplasmic cross-sectional area of sonic muscle fibers. In response to increasing temperature, sound pressure level (SPL), dominant frequency and repetition rate increase, and pulse duration decreases. Likewise, SPL and pulse duration increase and dominant frequency decreases with fish size. Changes in acoustic parameters with fish size suggest the possibility that drumming sounds act as an 'honest' signal of male fitness during courtship. These parameters also correlate with seasonally increasing sonic muscle mass. We hypothesize that sonic muscle twitch duration rather than the resonant frequency of the swimbladder determines dominant frequency. The brief (3.5 ms), rapidly decaying acoustic pulses reflect a low-Q, broadly tuned resonator, suggesting that dominant frequency is determined by the forced response of the swimbladder to sonic muscle contractions. The changing dominant frequency with temperature in fish of the same size further suggests that frequency is not determined by the natural frequency of the bladder because temperature is unlikely to affect resonance. Finally, dominant frequency correlates with pulse duration (reflecting muscle twitch duration), and the inverse of the period of the second cycle of acoustic energy approximates the recorded frequency. This paper demonstrates for the first time that the dominant frequency of a fish sound produced by a single muscle twitch is apparently determined by the velocity of the muscle twitch rather than the natural frequency of the swimbladder.

Continuous adult development of multiple innervation in toadfish sonic muscle.

The sonic muscle of the oyster toadfish Opsanus tau produces unfused contractions at over 200 Hz for mating call production, requiring extreme muscle fiber synchronization. This multiply innervated muscle is sexually dimorphic and grows for life by fiber proliferation and hypertrophy. Previous descriptions of its multiple innervation did not consider fish size or sex. We examined neuromuscular junction (NMJ) development in adult fish of both sexes between 123 and 343 mm in total length (24.7790 g in mass). The NMJ was a tubelike trough that varied in length from 8 to 178 microm. Troughs were usually straight, although some consisted of consecutive ovals and some were branched. Median length of NMJs increased linearly with fish length (r2=.40; p=.002) from 58 to 75 microm. Modal lengths were mostly between 50 and 60 microm and did not increase ontogenetically, indicating that the median increase was caused by a greater number of large junctions in older fish. Median interval between NMJs (measured from the beginning of one junction to the next) ranged from 92 to 116 microm and did not vary with fish size (r2=.06; p=.285). Considering muscle fiber elongation, the data indicate an increase from 60 to 140 NMJs per fiber during fish growth. There were no sexual differences in NMJ length or spacing. In view of the slow conduction velocity of sonic muscle fibers, the addition of new NMJs and the relatively constant distance between them supports rapid and synchronized contraction necessary for sound production in both sexes.

Embryonic, juvenile, and adult development of the toadfish sonic muscle.

Sonic muscle fibers intrinsic to the swim bladder of the oyster toadfish Opsanus tau proliferate throughout adult life and have an unusual radial morphology: alternating ribbons of sarcoplasmic reticulum (SR) and myofibrils surround a central core of sarcoplasm. Large fibers in adults form multiple cores, fragment, and appear to divide into smaller, more energy efficient units. We examined embryonic to adult development of sonic muscle using electron and light microscopy and focused on the incidence of satellite cells (SC). Muscle fibers form late in the larval period from myoblasts, which do not appear to fuse into myotubes, but enlarge and differentiate myofibrils in a single patch. The SR differentiates from the outside inward, separating the myofibrils into bundles of varying thickness, which often exceed the thickness seen in adults. SCs in juveniles and adults have a sparse cytoplasm and a heterochromatic nucleus. The % SC nuclei (SC nuclei/total nuclei) decreases from a high of 88% in larvae to a low of 1% in adults although the adult average is 10%. No embryonic type fibers in the process of differentiating myofibrils were seen in adults. Small immature fibers, which had not yet formed the central core, have a complete radially organized contractile cylinder. Immature muscle fibers formed embryonically in the larval period have a different morphology from immature fibers in adults, suggesting that splitting rather than SCs is a major source of new fibers in adults.

The effects of seasonal hypertrophy and atrophy on fiber morphology, metabolic substrate concentration and sound characteristics of the weakfish sonic muscle.

Male weakfish Cynoscion regalis possess highly specialized, bilateral, striated sonic muscles used in sound production associated with courtship. Androgen-driven hypertrophy of the muscles during the late spring spawning period results in a tripling of sonic muscle mass followed by post-spawning atrophy. This study examined the morphological and biochemical changes underlying seasonal changes in sonic muscle mass and the functional effects of these on contraction as measured by sound production. Sonic muscle fiber cross-sectional area (CSA) increased significantly during the period of hypertrophy and then decreased by nearly 60%. Both the CSA of the contractile cylinder and that of the peripheral sarcoplasm decreased significantly by late summer, with the peripheral ring of sarcoplasm virtually disappearing. Muscle protein content followed a similar trend, suggesting a major loss of structural elements during atrophy. Muscle glycogen and lipid content decreased precipitously in early June during the period of maximal sound production. Sound pressure level increased and sound pulse duration decreased with increasing sonic muscle mass, indicating that sonic muscle fibers contract with greater force and shorter duration during the spawning season. Neither the pulse repetition rate nor the number of pulses varied seasonally or with muscle mass, suggesting that the effects of steroids on the acoustic variables are more pronounced peripherally than in the central nervous system. Seasonal sonic muscle hypertrophy, therefore, functions as a secondary sexual characteristic that maximizes vocalization amplitude during the spawning period.

Effects of Exogenous Testosterone on Sonic Muscle Mass in the Weakfish, Cynoscion regalis

Three testosterone (T) implant experiments were carried out to investigate the hormonal control of seasonal hypertrophy in the sound producing, or sonic, muscles of the weakfish Cynoscion regalis. The first experiment examined the role if T in the seasonal increase in sonic muscle mass noted in adult male weakfish during the spring and summer spawning period. Specimens held over-winter were implanted with T or vehicle prior to the natural spawning season. The sonic muscle mass of the T implanted group increased two and a half-fold over a period of 3 weeks, while that of the sham-implanted group did not change. At the end of the experiment, the cross-sectional area of the sonic muscle fibers in the T-implanted group averaged 500 μm2, while that of the sham-implanted group averaged 300 μm2. In a second experiment, the elevated sonic muscle mass of spawning male weakfish was sustained with pharmacological levels of plasma T for a period of 4 months following the spawning season. The sonic muscle mass of the sham-implanted group decreased to typical postseason values. In a third experiment, male and female age-0 weakfish were held over-winter and then implanted with T in advance of their first natural spawning season. The sonic muscles of the T treated male specimens attained twice the mass of those of the sham-implanted male specimens in 3 weeks time. No sonic muscle development was noted in any of the female specimens.

Functional morphology of toadfish sonic muscle fibers: relationship to possible fiber division

Sexually dimorphic sonic muscles, which vibrate the swimbladder for sound production in the oyster toadfish (Opsanus tau), are among the fastest vertebrate muscles. Previous work has shown that sonic muscle fibers are smaller in males, have an unusual morphology, and increase in number and size for life. We now report evidence consistent with the hypothesis that mature, presumably postmitotic, sonic fibers divide, and suggest that division, which returns fibers to small energy-efficient units, is necessary because mitochondria are excluded from the fiber's contractile cylinder. Large fibers, potential candidates for division, develop fragments of contractile cylinder separated by channels of an expanded sarcoplasmic reticulum; these channels can assume the appearance of the sarcoplasm (glycogen granules and mitochondria) beneath the sarcolemma. Measurements indicate that contractile cylinder diameter does not increase with fish size and that diameters are approximately 21% larger in females (p < 0.0001). Fiber fragmentation, possible division, and the presence of smaller fibers with smaller diameter contractile cylinders in males are seen as adaptations for repeated rapid contraction and fatigue resistance during production of the male's courtship boatwhistle call.

Acoustic signaling in teleost fishes: Model systems for vertebrate vocalization

Teleost fishes can vocalize by contracting striated muscles attached to the lateral walls of their swimbladder. The rhythmic, oscillatory‐like, firing properties of a brain‐stem sonic motor circuit determine the fundamental frequency of the acoustic signals. In the midshipman Porichthys notatus only large, nest‐building, egg‐guarding males generate long duration (up to 1 h) “hums” that attract females to the nests. Females, and smaller, “sneak‐spawning” males, do not hum. Behavorial sex differences in sound production are correlated with: (a) the soma‐dendritic dimensions and discharge frequency of individual sonic motoneurons and their presynaptic pacemaker neurons, (b) the functional architecture of sonic muscle fibers and myofibrils, and (c) the density of neuromodulator inputs to the sonic motor nucleus. The sonic motor circuit can serve as a “simple” model system for a detailed analysis of the synaptic and electro‐responsive properties of single neurons controlling the generation of sex‐ and species‐typical acoustic communication signals. The collaboration of Harriet Baker, Robert Baker, Richard Brantley, and Margaret Marchaterre is acknowledged. [Work supported by NSF, NIH, and Hatch Grants.]

Embryonic, larval and adult development of the sonic neuromuscular system in the oyster toadfish.

Unlike the case in birds and mammals where addition of neurons and muscle fibers is almost exclusively an embryonic process, fish form new sensory receptors, central neurons and muscle fibers postnatally. We have investigated the development of the sonic neuromuscular system, consisting of a spinal sonic motor nucleus (SMN) and a pair of sonic muscles intrinsic to the swimbladder, in the oyster toadfish Opsanus tau. The SMN differentiates late in embryonic development, and neurons increase in number and size through the larval period and for 7-8 years of adult life. Neuron number increases from a median of 35 to 322 cells, from the time of differentiation through the larval period, and to almost 3,000 cells in large fish. Immature neurons appear to originate from precursor cells in the base of the ventricular zone of the central canal and are added throughout the SMN. No evidence of cell death was seen. The number and diameter of sonic muscle fibers increase for life (about 16- and 3-fold, respectively); fiber number ranges from almost 31,000 in a 1-year-old juvenile to almost 500,000 in large animals. Fiber splitting provides a possible mode of fiber addition. Multiyear addition of neurons and muscle fibers indicates that embryonic processes continue beyond the termination of larval life.

Concentration and Location of Metabolic Substrates in Fast Toadfish Sonic Muscle

We measured glycogen, lipid and water concentrations in toadfish (Opsanus tau) fast sonic muscle and white body muscle, and glycogen and lipid were localized histochemically in the sonic muscle. Sonic muscle had 5.05 jg of glycogen per mg wet weight, 15.64% fat and 81% water. These values were 2.2, 6.2 and 0.18 times higher than the corresponding concentrations in white body muscle. The presence of abundant glycogen and fat provides potential fuel for anaerobic and aerobic metabolism and agrees with the designation of sonic muscle as fast oxidative glycolytic. No sexual or ontogenetic differences were present in glycogen, fat or water concentrations in sonic or body muscle, and these molecules cannot be used to explain sexual differences in sound production. Glycogen is localized in sarcoplasm under the sarcolemma, in sarcoplasmic reticulum and throughout the sarcoplasmic core. Lipids have a similar distribution, except that their distribution in the core is restricted to the periphery. These findings are in accord with ultrastructure of sonic muscle fibers.