Shape, Size, and Distribution of Ca2+ Release Units and Couplons in Skeletal and Cardiac Muscles

▪ Abstract Bird song provides an unusually impressive illustration of vertebrate behavioral diversification. Research on bird song evolution traditionally focuses on factors that enhance song diversity, such as cultural transmission and sexual selection. Recent advances in the study of proximate mechanisms of vocal behavior, however, provide opportunities for studying mechanistic constraints on song evolution. The main goal of this review is to examine, from both conceptual and empirical perspectives, how proximate mechanisms might temper patterns of song evolution. We provide an overview of the two “substrates” of song evolution, memes and vocal mechanisms. We argue that properties of vocal mechanisms (control, production, and ontogeny) constrain vocal potential and may thus limit pathways of meme evolution. We then consider how vocal mechanisms may constrain song evolution under five scenarios of drift and selection and examine four specific song traits for which mechanistic constraints appear to counte...

http://www.bio.umass.edu/biology/podos/Pubs/2004Podos+AREES.pdf

Bird Song: The Interface of Evolution and Mechanism

The asynchronous muscles of insects are characterized by asynchrony between muscle electrical and mechanical activity, a fibrillar organization with poorly developed sarcoplasmic reticulum, a slow time course of isometric contraction, low isometric force, high passive stiffness and delayed stretch activation and shortening deactivation. These properties are illustrated by comparing an asynchronous muscle, the basalar flight muscle of the beetle Cotinus mutabilis, with synchronous wing muscles from the locust, Schistocerca americana. Because of delayed stretch activation and shortening deactivation, a tetanically stimulated beetle muscle can do work when subjected to repetitive lengthening and shortening. The synchronous locust muscle, subjected to similar stimulation and length change, absorbs rather than produces work.

Asynchronous muscle: a primer.

Experiments were carried out to compare the amplitude and time course of Ca2+ release from the sarcoplasmic reticulum (SR) in intact slow-twitch and fast-twitch mouse fibres. Individual fibres within small bundles were injected with furaptra, a low-affinity, rapidly responding Ca2+ indicator. In response to a single action potential at 16 °C, the peak amplitude and half-duration of the change in myoplasmic free [Ca2+] (Δ[Ca2+]) differed significantly between fibre types (slow-twitch: peak amplitude, 9.4 ± 1.0 μM (mean ± S.E.M.); half-duration, 7.7 ± 0.6 ms; fast-twitch: peak amplitude 18.5 ± 0.5 μM; half-duration, 4.9 ± 0.3 ms). SR Ca2+ release was estimated from Δ[Ca2+] with a computational model that calculated Ca2+ binding to the major myoplasmic Ca2+ buffers (troponin, ATP and parvalbumin); buffer concentrations and reaction rate constants were adjusted to reflect fibre-type differences. In response to an action potential, the total concentration of released Ca2+ (Δ[CaT]) and the peak rate of Ca2+ release ((d/dt)Δ[CaT]) differed about 3-fold between the fibre types (slow-twitch: Δ[CaT], 127 ± 7 μM; (d/dt)Δ[CaT], 70 ± 6 μM ms−1; fast-twitch: Δ[CaT], 346 ± 6 μM; (d/dt)Δ[CaT], 212 ± 4 μM ms−1). In contrast, the half-duration of (d/dt)Δ[CaT] was very similar in the two fibre types (slow-twitch, 1.8 ± 0.1 ms; fast-twitch, 1.6 ± 0.0 ms). When fibres were stimulated with a 5-shock train at 67 Hz, the peaks of (d/dt)Δ[CaT] in response to the second and subsequent shocks were much smaller than that due to the first shock; the later peaks, expressed as a fraction of the amplitude of the first peak, were similar in the two fibre types (slow-twitch, 0.2–0.3; fast-twitch, 0.1–0.3). The results support the conclusion that individual SR Ca2+ release units function similarly in slow-twitch and fast-twitch mammalian fibres.

Sarcoplasmic reticulum calcium release compared in slow‐twitch and fast‐twitch fibres of mouse muscle

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. 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. 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.

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

Vertebrate sound producing muscles often operate at frequencies exceeding 100 Hz, making them the fastest vertebrate muscles. Like other vertebrate muscle, these sonic muscles are ''synchronous,'' necessitating that calcium be released and resequestered by the sarcoplasmic reticulum during each contraction cycle. Thus to operate at such high frequencies, vertebrate sonic muscles require extreme adap- tations. We have found that to generate the ''boatwhistle'' mating call ('200 Hz), the swimbladder muscle fibers of toadfish have evolved (i) a large and very fast calcium transient, (ii) a fast crossbridge detachment rate, and (iii) probably a fast kinetic off-rate of Ca 21 from troponin. The fibers of the shaker muscle of rattlesnakes have independently evolved similar traits, permitting tail rattling at '90 Hz. Skeletal muscle fibers perform a wide range of activities, and different fiber types are accordingly designed to operate at different speeds and frequencies (1). A number of modifica- tions appear to underlie this diversity. For example, in loco- motory muscle, compared with slow twitch fibers, fast twitch fibers have a faster myosin with a higher maximum velocity of shortening (Vmax) (2, 3), a greater content of sarcoplasmic reticulum (SR), and its associated Ca 21 pumps (4, 5), a different isoform of the SR Ca 21 pump (SERCA1 in fast versus SERCA2 in slow) (6, 7) and a greater concentration of parvalbumin (a soluble protein that binds both calcium and magnesium) (5, 8). There is also evidence that fast fibers have a briefer myoplasmic free Ca 21 concentration ((Ca 21 )) tran- sient (9, 10) and less sensitive force-pCa relationship (11, 12). To understand the physiological modifications that underlie very rapid contractions, we have studied two of the fastest vertebrate muscles known. Both of these ''sonic'' muscles are used to produce sounds at the frequency at which the muscle contracts. The ''boatwhistle'' mating call of the male toadfish (Opsanus tau) is generated by '200 Hz contractions (25°C) of the muscles encircling the fish's gas-filled swimbladder (13- 15). The familiar ''rattle'' of the venomous western diamond- back rattlesnake (Crotalus atrox) is generated by '90 Hz contractions (35°C) of the shaker muscles at the base of the tail (16-19). The operational frequencies of these sonic muscles are 1-2 orders of magnitude higher than those of the loco- motory muscles in the same animals (0.5-5 Hz) (20). For fibers to operate at such high frequencies, they must activate and relax rapidly. Based on morphological and bio- chemical evidence from swimbladder of a very large SR Ca 21 pump density, a high density of SR Ca 21 release sites, and a large parvalbumin concentration, it has been proposed that an important modification of these sonic fibers is unusually rapid Ca 21 cycling (5, 21). By measuring myoplasmic free (Ca 21 ), we found that these fibers do indeed have unusual Ca 21 tran- sients—in fact the largest and fastest ever recorded. How- ever, our results showed that a fast Ca 21 transient alone is not sufficient for high frequency operation. By measuring Vmax, an index of crossbridge detachment rate, and the force-pCa relationship in skinned fibers, a possible index of troponin kinetics, we found that rapid activation and relax- ation likely also require a modification of the crossbridge kinetic rate, and probably a modification of the kinetics of Ca 21 -troponin binding. In reaching these conclusions, we first compared the above measurements in three fiber types from toadfish, ranging from slow twitch swimming fibers to the superfast twitch swimbladder fibers. We then compared the properties of rattlesnake shaker fibers with those of swimbladder.

A. Syme

Papers

The whistle and the rattle: The design of sound producing muscles (rattlesnakeytoadfishycalcium transientsymuscle mechanicsyswimbladder)