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.

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.

Electrical activity generated by electrical stimuli has been recorded through suction electrodes placed on gastrozooid and dactylozooid polyps of the colonial hydroid, Hydractinia echinata Flemming. The polymorphism so characteristic of a Hydractinia colony extends to the physiological machinery controlling the behavior of the different polyp types. Contraction of gastrozooid polyps, where the muscle elements are uniformly distributed within the epidermis, is correlated to an electrical event termed the symmetrical contraction potential (SCP). The SCP is conducted about 4.4 cmlsec. Two kinds of electrical activity are correlated to distinct behavioral events of the specialized asymmetrically coiled dactylozooid polyps: (1) Dactylozooid coiling potentials (DCPs) are conducted at about 4.6 cmlsec and are correlated with contraction of a thin band of muscles on the concave side resulting in asymmetrical contraction and a marked coiling of the polyp. (2) Dactylozooid lashing potentials (DLPs) are conducted at about 9.1 cm/sec and have a threshold 50 per cent or more higher than DCPs. DLPs are correlated with contraction of well-developed muscles on the convex side resulting in a rapid uncoiling or "lashing" of the dactylozooid polyp. Dactylozooid coiling is correlated to an incremental spread of conduction within the colony whereas lashing occurs during colony-wide through-conduc ted excitation. Very small electrical spikes (50-200 WV amplitude) have been recorded from both gastrozooids and dactylozooids. These tiny potentials (TPs) are conducted at 15-21 cmlsec, have a threshold below that of the DCPs and SCPs, and are of unknown behavioral significance. On the basis of the characteristics of the electrical events, the differences in threshold, conduction velocity, and related spread of excitation, it is proposed that these three potentials represent activity within separate conducting systems -an incrementing conducting system (ICS), a through conducting system (TS), and a tiny potential system (TPS). Physiological and microscopical evidence suggest a nerve net origin for the ICS, and epithelial conduction for the TS.

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.