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D R Stokes

Publications and source records attributed to D R Stokes.

11 recordsLinked to original sources
PubMed2001

The efficiency of an asynchronous flight muscle from a beetle.

Mechanical power output and metabolic power input were measured from an asynchronous flight muscle, the basalar muscle of the beetle Cotinus mutabilis. Mechanical power output was determined using the work loop technique and metabolic power input by monitoring CO(2) production or both CO(2) production and O(2) consumption. At 35 degrees C, and with conditions that maximized power output (60 Hz sinusoidal strain, optimal muscle length and strain amplitude, 60 Hz stimulation frequency), the peak mechanical power output during a 10 s burst was approximately 140 W kg(-1), the respiratory coefficient 0.83 and the muscle efficiency 14-16 %. The stimulus intensity used was the minimal required to achieve a maximal isometric tetanus. Increasing or decreasing the stimulus intensity from this level changed mechanical power output but not efficiency, indicating that the efficiency measurements were not contaminated by excitation of muscles adjacent to that from which the mechanical recordings were made. The CO(2) produced during an isometric tetanus was approximately half that during a bout of similar stimulation but with imposed sinusoidal strain and work output, suggesting that up to 50 % of the energy input may go to muscle activation costs. Reducing the stimulus frequency to 30 Hz from its usual value of 60 Hz reduced mechanical power output but had no significant effect on efficiency. Increasing the frequency of the sinusoidal strain from 60 to 90 Hz reduced power output but not CO(2) consumption; hence, there was a decline in efficiency. The respiratory coefficient was the same for 10 s and 30 s bursts of activity, suggesting that there was no major change in the fuel used over this time range. The mass-specific mechanical power output and the efficiency of the beetle muscle were each 2-3 times greater than values measured in previous studies, using similar techniques, from locust flight muscles, which are synchronous muscles. These results support the hypothesis that asynchronous flight muscles have evolved in several major insect taxa because they can provide greater power output and are more efficient than are synchronous muscles for operation at the high frequencies of insect flight.

Animals↗
PubMed2000

Power output by an asynchronous flight muscle from a beetle.

The basalar muscle of the beetle Cotinus mutabilis is a large, fibrillar flight muscle composed of approximately 90 fibers. The paired basalars together make up approximately one-third of the mass of the power muscles of flight. Changes in twitch force with changing stimulus intensity indicated that a basalar muscle is innervated by at least five excitatory axons and at least one inhibitory axon. The muscle is an asynchronous muscle; during normal oscillatory operation there is not a 1:1 relationship between muscle action potentials and contractions. During tethered flight, the wing-stroke frequency was approximately 80 Hz, and the action potential frequency in individual motor units was approximately 20 Hz. As in other asynchronous muscles that have been examined, the basalar is characterized by high passive tension, low tetanic force and long twitch duration. Mechanical power output from the basalar muscle during imposed, sinusoidal strain was measured by the work-loop technique. Work output varied with strain amplitude, strain frequency, the muscle length upon which the strain was superimposed, muscle temperature and stimulation frequency. When other variables were at optimal values, the optimal strain for work per cycle was approximately 5%, the optimal frequency for work per cycle approximately 50 Hz and the optimal frequency for mechanical power output 60-80 Hz. Optimal strain decreased with increasing cycle frequency and increased with muscle temperature. The curve relating work output and strain was narrow. At frequencies approximating those of flight, the width of the work versus strain curve, measured at half-maximal work, was 5% of the resting muscle length. The optimal muscle length for work output was shorter than that at which twitch and tetanic tension were maximal. Optimal muscle length decreased with increasing strain. The curve relating work output and muscle length, like that for work versus strain, was narrow, with a half-width of approximately 3 % at the normal flight frequency. Increasing the frequency with which the muscle was stimulated increased power output up to a plateau, reached at approximately 100 Hz stimulation frequency (at 35 degrees C). The low lift generated by animals during tethered flight is consistent with the low frequency of muscle action potentials in motor units of the wing muscles. The optimal oscillatory frequency for work per cycle increased with muscle temperature over the temperature range tested (25-40 degrees C). When cycle frequency was held constant, the work per cycle rose to an optimum with increasing temperature and then declined. We propose that there is a temperature optimum for work output because increasing temperature increases the shortening velocity of the muscle, which increases the rate of positive work output during shortening, but also decreases the durations of the stretch activation and shortening deactivation that underlie positive work output, the effect of temperature on shortening velocity being dominant at lower temperatures and the effect of temperature on the time course of activation and deactivation being dominant at higher temperatures. The average wing-stroke frequency during free flight was 94 Hz, and the thoracic temperature was 35 degrees C. The mechanical power output at the measured values of wing-stroke frequency and thoracic temperature during flight, and at optimal muscle length and strain, averaged 127 W kg(-1)muscle, with a maximum value of 200 W kg(-1). The power output from this asynchronous flight muscle was approximately twice that measured with similar techniques from synchronous flight muscle of insects, supporting the hypothesis that asynchronous operation has been favored by evolution in flight systems of different insect groups because it allows greater power output at the high contraction frequencies of flight.

Animals↗
PubMed2000

Asynchronous muscle: a primer.

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.

Animals↗
PubMed1997

Functional role of Ca2+ currents in graded and spike-mediated synaptic transmission between leech heart interneurons.

We used intracellular recording and single electrode voltage-clamp techniques to explore Ca2+ currents and their relation to graded and spike-mediated synaptic transmissions in leech heart interneurons. Low-threshold Ca2+ currents (activation begins below -50 mV) consist of a rapidly inactivating component (I(CaF)) and a slowly inactivating component (I(CaS)). The apparent inactivation kinetics of I(CaF) appears to be influenced by Ca2+; both the substitution of Ca2+ (5 mM) with Ba2+ (5 mM) in the saline and the intracellular injection of the rapid Ca2+ chelator, bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA), from the recording microelectrode, significantly increase its apparent inactivation time constant. The use of saline with a high concentration of Ba2+ (37.5 mM) permitted exploration of divalent ion currents over a broader activation range, by acting as an effective charge carrier and significantly blocking outward currents. Ramp and pulse voltage-clamp protocols both reveal a rapidly activating and inactivating Ba2+ current (I(BaF)) and a less rapidly activating and slowly inactivating Ba2+ current with a broad activation range (I(BaS)). Low concentrations of Cd2+ (100-150 microM) selectively block I(BaS), without significantly diminishing I(BaF). The current that remains in Cd2+ lacks the characteristic delayed activation peak of I(BaS) and inactivates with two distinct time constants. I(BaF) appears to correspond to a combination of I(CaF) and I(CaS), i.e., to low-threshold Ca2+ currents, that can be described as T-like. I(BaS) appears to correspond to a Ca2+ current with a broad activation range, which can be described as L-like. Cd2+ (100 microM) selectively blocks spike-mediated synaptic transmission between heart interneurons without significantly interfering with low-threshold Ca2+ currents and plateau formation in or graded synaptic transmission between heart interneurons. Blockade of spike-mediated synaptic transmission between reciprocally inhibitory heart interneurons with Cd2+ (150 microM), in otherwise normal saline, prevents the expression of normal oscillations (during which activity in the two neurons consists of alternating bursts), so that the neurons fire tonically. We conclude that graded and spike-mediated synaptic transmission may be relatively independent processes in heart interneurons that are controlled predominantly by different Ca2+ currents. Moreover, spike-mediated synaptic inhibition appears to be required for normal oscillation in these neurons.

Action Potentials↗
PubMed1992

Structural organization of two fast, rhythmically active crustacean muscles.

The organization of the flagellum abductor muscle and of a scaphognathite levator muscle of the green crab, Carcinus maenas, has been compared quantitatively using light and electron microscopy. These muscles are rhythmically active at relatively high frequencies and for long durations. Fibers of both muscles are interconnected to form fascicles of 50 or more fibers within which there is cytoplasmic continuity. A single muscle is made up of 8-12 fascicles. Individual fibers consist of a peripheral rind of densely packed mitochondria, a thick region of glycogen granules, and myofibrils arranged into scattered central islands. Less than half the volume-density of these muscles is contractile material, the balance being largely mitochondria and glycogen. The fibers within a muscle are structurally similar. They have short sarcomeres (about 2 microns), thin to thick filament ratios of about 3:1, and junctions between the sarcoplasmic reticulum and the transverse tubules at the M line. Sarcoplasmic reticulum occupies about 10% of the myofibrillar volume-density. A well developed sarcoplasmic reticulum appears to underlie the capacities of these two muscles for high frequency contraction; extensive mitochondria and glycogen stores should confer fatigue resistance under both aerobic and anaerobic conditions.

Animals↗
PubMed1979

Ultrastructural heterogeneity of the mesocoxal muscles of Periplaneta americana.

Ultrastructural studies have been performed upon the posterior coxal depressor muscle (136) and a coxal branch of the main depressor group (135d') from the mesocoxa of the cockroach, Periplaneta americana. The quantitative stereometric analyses performed have shown the latter muscle to consist of a dorsal band of fibers having 25.% mitochondria and 13.6% sarcoplasmic reticulum (SR) and T-tubules (TTS), and a ventral group of fibers with only 4.4% mitochondria and 26.6% SR/TTS. The volume fractions characteristic of the ventral fibers of muscle 135d' are also typical of muscle 136.

Animals↗
PubMed1979

Enzyme histochemistry of the mesocoxal muscles of Periplaneta americana.

Histochemical techniques have been employed to characterize enzymatic activity in the mesocoxal muscles of the cockroach, Periplaneta americana. Through our studies of the enzymes myosin-ATPase, NADH reductase, succinic dehydrogenase (SDH), and lactic dehydrogenase (LDH), we were able to classify fibers within these muscles according to criteria established for muscle fibers of vertebrates. Many of the mesocoxal muscles possess two different and distinct populations of fibers, whereas the remaining muscles are homogeneous with respect to their constituent fibers. The data presented here indicate biochemical heterogeneity for muscles of differing structural and functional features and possible neurotrophic influences upon oxidative enzymes and myosin-ATPase isozymes.

Adenosine Triphosphatases↗
PubMed1975

The neural control of contraction in a fast insect muscle.

The wing muscles used in singing by the katydid, Neoconocephalus robustus, are extraordinarily fast. At 35 degrees C, the animal's thoracic temperature during singing, an isometric twitch lasts only five to eight msec (onset to 50% relaxation) and the fusion frequency of these muscles is greater than 400 Hz. Stimulating the motornerve to a singing muscle initiates a short (2.5 msec at 35 degrees C), sometimes overshooting depolarization of the muscle fibers. Despite their spike-like appearance, the electrical responses are largely synaptic potentials. The muscle membrane appears to be capable of only weak, electrically-excitable, depolarizing electrogenesis. The short synaptic potentials result in part from rapidly-developing delayed rectification, in part from a low resting membrane resistance (Rm = 162 omega cm2) and a concomitantly short membrane time constant (about 1.5 msec).

Animals↗
PubMed1975

Structural and functional heterogeneity in an insect muscle.

Singing muscles of the katydid, Neoconocephalus robustus (Insecta, Tettigoniidae) are neurogenic, yet perform at contraction-relaxation frequencies as high as 212 Hz (Josephson and Halverson, '71). The mechanical and electrical responses of different bands of one of these muscles (the dorsal longitudinal muscle, DLM) has been examined with respect to ultrastructural features of each part which may be related to muscle performance. The DLM is composed of three bands and is innervated by four motoneurones. The cell bodies of three of these motoneurones occur ipsilaterally in the prothroracic ganglion; the cell body of the other motoneurone is contralateral in the mesothoracic ganglion. Three of the motoneurones (as yet unidentified fast axons) initiate extraordinarily fast twitches (rise time equal 7.3 msec, half duration equals 14.3 msec, 25 C), the fourth (an unidentified slower axon) evokes twitches which are considerably slower (rise time equals 18.9 msec, half duration equals 5.10 msec). Whereas the ventral and medial bands of the muscle are innervated only by fast axons (some fibers of the medial band are doubly innervated), the dorsal band is innervated by both a fast axon and the slower axon. A few fibers of the dorsal band are doubly innervated. The structure of fibers from the ventral and medial bands is very similar, with short sarcomeres (4.0 and 4.3 mum, respectively) and thin strap-like myofibrils delineated by well-developed sarcoplasmic reticulum (SR). Twenty-four percent of the volume of ventral band fibers is SR and the diffusion distance from SR to the center of the adjacent myofibril averages 0.083 mum. Twenty percent of the medial band fiber volume is SR, with a diffusion distance of 0.118 mum. Ventral and medial band fibers contain about 40% mitochondria, and 33% myofibrils. The dorsal band fibers have longer sarcomeres (9.5 mum), and only 10% of the fiber volume is SR. The muscle fibrils of the dorsal band are larger and consequently the diffusion distance is greater (0.227 mum) than in the ventral and medial bands. Mitochondria comprise 23% of the volume of dorsal band fibers. Most dorsal band mitochondria are aggregated into distinct clumps. Although some dorsal band fibers are innervated by a fast axon and some by the slower axon, the dorsal band fibers are structurally homogeneous, suggesting that neurotrophic effects are not important in maintaining the structure of dorsal band fibers. The mechanical-electrical performance and ultrastructure of the ventral and medial bands suggest their roll as fast, metabolically active but weak muscles, used in singing; the dorsal band as a slower but stronger muscle, perhaps involved in postural movements of the wing during singing.

Animals↗