The excitable substances of amphibian muscle

Minding the gap: discovering the phenomenon of chemical transmission in the nervous system

The neuron doctrine, according to which nerves consist of discontinuous neurons, presented investigators with the challenge of determining what activities occurred between them or between them and muscles. One group of researchers, dubbed the sparks, viewed the electrical current in one neuron as inducing a current in the next neuron or in muscles. For them there was no gap between the activities of neurons or neurons and muscles that required filling with a new type of activity. A competing group, the soups, came to argue for chemicals, subsequently referred to neurotransmitters, as carrying out the activities between neurons or between neurons and muscles. But even for them the conclusion that chemicals performed this activity was only arrived over time. I examine the prolonged period in which proponents of chemical transmission developed their account and challenged the sparks. My goal is to illuminate the epistemic processes that led to the discovery of a new scientific phenomenon-chemical transmission between neurons.

The threshold conditions for initiation of action potentials by excitable cells

1. The relation between the strength and duration of a just threshold stimulus (strength-duration curve) is analysed for an excitable membrane polarized uniformly and for an excitable cable polarized at one point.2. The effect on the strength-duration curve of non-linearity in the membrane current-voltage curve has been analysed. The strength-duration curve can be derived if the membrane current-voltage relation is independent of time. The effects of changes in the current-voltage curve with time due to the existence of a finite membrane activation time and membrane accommodation are analysed.3. The strength-duration curve for the Hodgkin-Huxley membrane equations (Hodgkin & Huxley, 1952) is compared to that of Hill's (1936) two-time constant model.4. The relation between membrane current-voltage curves and those for a point-polarized cable are derived for the steady-state condition. The cable properties tend to linearize the current-voltage curve and to sharpen the voltage threshold.5. The strength-duration curve for a point-polarized cable whose membrane obeys the Hodgkin-Huxley equations is computed numerically. There is an additional large effect on the cable strength-duration curve arising from the redistribution of charge during passage of a constant current; and the resulting strength-duration curve lies within the range of curves predicted by Hill's model.6. The conditions required for a constant charge threshold (Hodgkin & Rushton, 1946) are shown to be satisfied for short, intense stimuli applied to a cable at one point.7. The results are discussed with reference to the experimental studies available.

ON THE MEASURE OF EXCITABILITY

Recent time-intensity data by Rushton (1932) on the sciatic nerve of the frog are shown to provide additional support to the writer's suggestion (1932, a) that integrals of the equation See PDF for Equation where V is the applied voltage, p is the local excitatory process and K and k are constants adequately represent the just effective direct current stimuli when the threshold value of p is made a linear function of the voltage of the form h ± α V where h and α are constants. The measurement of excitability is discussed and it is shown that the criteria for "true" measurements are not likely to be found by the agreement of the data with canonical time-intensity functions as suggested by Lapicque (1931) but rather in the establishing of standard experimental conditions. These conditions may permit the use of chronaxie as a measure of excitability, but it seems more likely that the constant k of the above equation will have to be adopted. There is sufficient evidence to cast considerable doubt on the validity of any conclusions drawn from the existing measurements of chronaxie although those derived through a particular technique may be valid. The problem requires a thorough experimental investigation in terms of integrals of the above equation.

Excitability of skeletal muscle during development, denervation, and tissue culture

A quantitative understanding of the bulk excitability of skeletal muscle tissues is important for the design of muscle tissue bioreactor systems, implantable muscle stimulators, and other systems where electrical pulses are employed to elicit contractions in muscle tissue both in vitro and in vivo. The purpose of the present study is to systematically compare the excitability of mammalian (rat) skeletal muscle under a range of conditions (including neonatal development, denervation, and chronic in vivo stimulation of denervated muscle) and of self-organized muscle tissue constructs engineered in vitro from both primary cells and cell lines. Excitability is represented by rheobase (R(50), units = V/mm) and chronaxie (C(50), units = microseconds) values, with lower values for each indicating greater excitability. Adult skeletal muscle is the most excitable (R(50) ~ 0.29, C(50) ~ 100); chronically denervated whole muscles (R(50) ~ 2.54, C(50) ~ 690) and muscle engineered in vitro from cell lines (C2C12 + 10T1/2) (R(50) ~ 1.93, C(50) ~ 416) have exceptionally low excitability; muscle engineered in vitro from primary myocytes (R(50) ~ 0.99, C(50) ~ 496) has excitability similar to that of day 14 neonatal rat muscle (R(50) ~ 0.65, C(50) ~ 435); stimulated-denervated muscles retain excellent excitability when chronically electrically stimulated (R(50) ~ 0.40, C(50) ~ 100); and neonatal rat muscle excitability improves during the first 6 weeks of development, steadily approaching that of adult muscle.

Time, from psychology to neurophysiology: a historical view

Two aspects of psychology and physiology of time are dealt with in this paper: the way time perception was increasingly studied during the 19th century by scientists, including many physicists, and the way the temporal properties of the nervous system were discovered and explored by physiologists. The neurophysiological correlation between both aspects still remains to be explained. The relationship between time consciousness and consciousness mechanisms was often guessed by philosophers and looked for by scientists. It remains a major subject of investigation in neuroscience as well as a philosophical puzzle.

Nerves, alcohol and drugs, the Adrian–Kato controversy on nervous conduction: deep insights from a “wrong” experiment?

Edgar Douglas Adrian, a dominating figure of 20th century electrophysiology, published in 1912 a study on the effects of the conduction block induced by application of alcohol vapours to small segments of nerves from which he derived the conclusion that nerve signals regenerate along the nerve fibre during the conduction process. This conclusion was based on results of experiments in which the time required to produce a conduction block was found to decrease as the length of the nerve segment treated was increased. These results could not be confirmed when similar experiments were performed about 10 years later by Gen'ichi Kato, a leading figure of Japanese physiology and founder of one of the great schools of Japanese electrophysiology. Directly or indirectly, the Adrian-Kato controversy was at the inception of two of the most important advancements of 20th century neurophysiology: the elucidation of the mechanism of nervous conduction in squid giant axon by Hodgkin and Huxley and the discovery of the saltatory conduction in myelinated nerve fibres by Tasaki, Takeuchi, Huxley and Stämpfli. This controversy is also interesting for its epistemological aspects, which is important now to re-evaluate.

Electric current as a stimulus, with respect to its duration and strength

Our experiments on the excitatory effect of an electric current have been carried out by examining the relation between the current-duration and its liminal strength. From the determination of the optimal electric stimuli of amphibian muscle and nerve by the condenser method (1) (2), and from the investigations on the relation of the current-duration to the liminal current-strength (3) (4), Keith Lucas confirmed that there should exist three substances, at least, in a normal sartorius muscle of frogs and toads; each substance being distinguished by its own “excitation time,” that is, the substance α represents the muscle material, γ the intramuscular nerve material, and β the intermediary substance; the first showing the longest, the second the intermediate, the third the shortest “excitation time.” His further investigations upon the excitatory process (5) (6), led him to confirm the validity of A. V. Hill’s excitation formula in all respects (7) (8). The essential part of our present paper consists of the discussion upon these works. Part I.—Experiments on a Muscle as a Whole. Apparatus and Method of Experiment . On the whole, we adopted Lucas' method (3) with some modifications in detail.

Strength-duration characteristics of myelinated and non-myelinated bulbospinal axons in the cat spinal cord

Strength-duration characteristics for the stimulation of 131 raphespinal and reticulospinal axons in the spinal cord were determined using two types of stimulating electrode. Conduction velocity of these fibres ranged from 0.86 to 63 m/s. With silver wire (250 micron diameter) stimulating electrodes, chronaxies were: 0.18 +/- 0.06 ms for axons conducting between 16 and 63 m/s, 0.4 +/- 0.22 ms for axons conducting between 5 and 15 m/s and 2.06 +/- 0.79 ms for those with conduction velocity less than 5 m/s. There was an inverse relationship between chronaxie and conduction velocity. Rheobase values ranged from 7.4 to 400 microA but were independent of conduction velocity. Chronaxies obtained with wire electrodes were compared with those from stimulation of the same fibre through saline-filled micropipettes (2-12 micron tip diameter). Rheobase values with the micropipettes ranged from 1.6 to 20 microA, indicating a close proximity of the pipette to the axon. For these axons, chronaxies from metal wire electrodes ranged from 0.12 to 2.4 ms and for micropipettes from 0.04 to 0.65 ms. In almost all cases, chronaxies for micropipette stimulation were lower than those for metal wire electrodes. Furthermore, with micropipettes chronaxies were independent of conduction velocity. The results are shown to be related to differences in time constant of the activated region of axon and charge requirements of threshold activation. The two stimulating conditions, i.e. micro-electrodes compared with wire electrodes, are analogous to the theoretical point stimulated cable and uniformly polarized membrane cases. The results are discussed in relation to the possibility of determination of fibre type from stimulation characteristics. A distinction between chronaxies of myelinated and non-myelinated fibres can be made using wire electrodes of 250 micron diameter, but not with micro-stimulation, as with micropipettes (2-12 micron diameter).

The distribution of acetylcholine sensitivity at the post-synaptic membrane of vertebrate skeletal twitch muscles: iontophoretic mapping in the micron range

1. The distribution of acetylcholine (ACh) sensitivity was mapped in skeletal twitch muscles of the snake, frog and mudpuppy with iontophoretic methods that provide a resolution in the mum range. 2. The preparations were thin sheets of muscle fibres that were viewed with Nomarski optics, giving sharp definition of cellular detail. The muscles in the snake were especially suitable. Their motor nerves terminate in a compact cluster of synaptic boutons that rest in distinct craters on the muscle surface. After treatment with collagenase the motor nerve and its terminal boutons can be removed, exposing the subsynaptic membrane in the craters. 3. The slopes of dose-response curves obtained by iontophoretic application of ACh were expressed in mV/nC and used as an index of ACh sensitivity. The areas of highest sensitivity, tested either with the terminals in place or removed, were those immediately under the presynaptic terminals. The greatest subsynaptic sensitivities were about 5000 mV/nC, and the time course of the potentials caused by ACh released iontophoretically closely matched that of synaptic potentials set up by ACh released by the nerve. 4. The sensitivity of the extrasynaptic surface less than 2 mum away was at least 50 times lower than that of the subsynaptic membrane. The low extrasynaptic sensitivity declined still further at greater distances. 5. Acetylcholinesterase was shown physiologically to be confined to subsynaptic areas. No activity of the enzyme was detected in extrasynaptic areas beyond about 2 mum from the edge of the synapse. 6. The confinement of high densities of receptors and of acetylcholinesterase to the subsynaptic membrane in muscles is also a feature in parasympathetic neurones. It is suggested that similar specialization may be a widespread property of neurones with chemical synapses.