A B S T R A C TA functional analysis of the striated swim-bladder muscles engaged in the sound production of the toadfish has been performed by simultaneous recording of muscle action potentials, mechanical effects, and sound. Experiments with electrical nerve stimulation were made on excised bladder, while decerebrate preparations were used for studies of reflex activation of bladders in situ. The muscle twitch in response to a single maximal nerve volley was found to be very fast. The average contraction time was 5 msec. with a range from 3 to 8 msec., the relaxation being somewhat slower. The analysis of muscle action potentials with surface electrodes showed that the activity of the muscle fibers running transversely to the long axis of the muscle was well synchronized both during artificial and reflex activation. With inserted metal microelectrodes monophasic potentials of 0.4 msec. rise time and 1.2 to 1.5 msec. total duration were recorded. The interval between peak of action potential and onset of contraction was only 0.5 msec. Microphonic recordings of the characteristic sound effect accompanying each contraction showed a high amplitude diphasic deflection during the early part of the contraction. During relaxation a similar but smaller deflection of opposite phase could sometimes be distinguished above the noise level. The output from the microphone was interpreted as a higher order derivative function of the muscle displacement. This interpretation was supported by complementary experiments on muscle sound in mammalian muscle. The dependence of the sound effects on the rate of muscle contraction was demonstrated by changing the temperature of the preparation and, in addition, by a special series of experiments with repeated stimulation at short intervals. Results obtained by varying the pressure within the bladder provided further evidence for the view that the sound initiated in the muscle is reinforced by bladder resonance. Analysis of spontaneous grunts confirmed the finding of a predominant sound frequency of about 100 per second, which was also found in reflexly evoked grunts. During these, muscle action potentials of the same rate as the dominant sound frequency were recorded, the activity being synchronous in the muscles on both sides. Some factors possibly contributing to rapid contraction are discussed.

The contraction times of laryngeal muscles in the dog were found to differ from 14 and 16 msec for the thyroarytenoid and lateral cricoarytenoid muscles to 30 and 35 msec for the posterior cricoarytenoid and the cricothyroid muscles respectively. The same relation between the contraction times of fast and slow muscles was characteristic also of some other species investigated. The view is advanced that the short contraction time of the two sphincter girdle muscles primarily serves the purpose of closing the laryngeal aperture in protective larynx reflexes. The thyroarytenoid and cricothyroid muscles of dog, chosen as representatives of fast and slow muscle, were subjected to a more detailed analysis. Determinations of the active state plateau by the method of superimposition of tension curves from one and two stimuli gave values of 3.0–4.0 msec for the thyroarytenoid and 3.5–4.5 msec for the cricothyroid muscle. After determinations of the tetanus‐twitch ratios for the two muscles, the rate of decay of the active state could be estimated and was shown to be faster in the thyroarytenoid muscle. Tetanic direct stimulation showed that complete fusion may not occur until above 300 per sec in the cricothyroid and above 400 in the thyroarytenoid muscle. Analogous analyses performed by indirect stimulation of the fast and slow muscle gave very similar results with respect to mechanical separation time and fusion frequency. The results from an action potential analysis during short‐interval nerve stimulation are discussed from the point of view of the active state conception.

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Summary. Intracellular recordings have been performed from interneurons in the lumbar segments of the spinal cord of non‐anesthetized spinal cats. A full account is given of the experimental technique used in this and other investigations in the same series. Section I. Continuous inkwriter recordings of the membrane potential level obtained from interneurons in various parts of the cord cross section are presented and compared with records from afferent fibres and motoneurons. Data of the average amplitude values and recording times for the different types of units are given. Various factors influencing the values of the recorded membrane potential are discussed and an attempt has been made to find a relation hetween the degree of injury, cell size and recorded membrane potential. Section II. The most common types of interneuron discharges in response to synchronous afferent volleys are described and typical variations in prepotential configuration are illustrated. The firing level at which an impulse is set up by afferent single shock stiniulation has been analysed in some detail in an experiment in which physiological variations in the membrane potential level were induced by natural sensory stimulation. It was found that the firing level is not constant but is reduced with lowering of the membrane potential value in an approximately linear way. The spike repolarization level was also found to vary with the nienibrane potential level in a similar way which may result in levels of increased or decreased polarization as compared with the pre‐spike level. Records of spontaneous and naturally induced activity are presented showing the relation between slow depolarization processes and spike discharges. In experiments including both natural and artificial activation records have been obtained showing no distinct prepotentials. In such recordings the spikes have usually been of short duration (average 0.8 msec as compared with 1.3 msec for spikes accompanied by slow potentials). Some of these cases may represent axon recordings, but possible explanations of an absence of recordable prepotentials from soma are also discussed. In the majority of interneuron recordings the spike amplitude has been 10–20 per cent lower than the membrane potential value; overshooting has only been observed in about 10 per cent of the recordings. Some factors influencing the spike amplitude have been taken up for discussion. Special attention has been given to variations in spike configuration in the form of pronounced notches and a splitting up into sub‐spikes; these variations have been interpreted as signs of a fractionated function of the cell membrane.