Anesthetic and calcium action in the voltage-clamped squid giant axon

Changes in spike configuration and in the inward and outward currents of voltage-clamped axons agree in indicating that the increases in permeability to sodium and potassium ions during activity are depressed by procaine and cocaine and augmented by calcium. At low levels of depolarization, the effect of the multivalent ion is similar to that of the local anesthetics, in keeping with their similar effects on the threshold of excitability. The reduction of membrane conductance at rest requires a higher concentration of the drugs than that needed to affect the increase in permeability with activity.

Voltage clamp of motoneuron soma

Concentric microelectrodes were used to voltage-clamp spinal motoneurons. With depolarizing voltage steps, current transients often appear, with some latency and in all-or-none fashion. Voltage-current relations indicate a two- to threefold reduction in resistance between inside and outside when the cell fires. These results suggest that activity does not involve the whole soma-dendritic membrane.

Synaptic potential in the motor giant axon of the crayfish

Some electrical properties of the synapses between central giant axons (presynaptic) and the motor giant axon (postsynaptic) of the crayfish abdominal nerve cord have been investigated. Postsynaptic potential change in response to presynaptic volleys contains two components: a spike potential and a synaptic potential of very long time course. Amplitude of the synaptic potential is graded according to the number of active presynaptic axons. Conductance increase in the synaptic membrane endures over most of the period of potential change, and it is this rather than the "electrical time constant" of the membrane that in large measure determines the form of the synaptic potential. Temporal summation of synaptic potential occurs during repetitive presynaptic stimulation, and after such stimulation the rate of decay of synaptic potential is greatly slowed.

Effects of calcium lack on action potential of motor axons of the lobster limb

The effects of external calcium deprivation on certain characteristics of the action potential of the lobster motor axon have been studied. Upon exposure to calcium-free solution the spike amplitude is rapidly decreased within a few minutes and is followed by a slow linear decline. The rates of spike rise and fall are proportionally reduced more than the spike but follow similar time courses during calcium lack. Associated with these phenomena are the loss in the normal slow spike repolarization process, the development of a large and lengthy undershoot, and the appearance of a high degree of refractoriness. The mean increase in the refractory period is 525 per cent upon 10 minutes' exposure to calcium-free solution. These effects are completely reversible upon returning the axons to normal solution. These results are compared to similar effects of calcium deprivation on frog myelinated axons and squid and lobster giant axons recently observed by other workers.

Extracellular potentials from single spinal motoneurons

Extracellular action potentials found close to the surface of motoneurons are related to the intracellular spikes. Evidence is cited to support the assumption that the extracellular spikes have the same time course as the membrane current at the site of recording. Simultaneously recorded intracellular and extracellular spikes are compared. Intracellular spikes are transformed, by means of a circuit which is equivalent to the extracellular recording situation, into transients that are like those appearing extracellularly. Evidence is given that the recordings are from the cell bodies of motoneurons. The results show that the membrane at the extracellular recording site does not produce a spike since the time course of the extracellular potentials is determined by the passive properties of the membrane.

Theoretical stability properties of a space-clamped axon

A mathematical method for determining the stability properties of a uniform nerve membrane is developed. Two basically similar tests of stability are considered: examination of the real characteristic roots of the linearized equations and application of a modified Nyquist criterion to the linearized alternating current admittance. The method is applied to the Hodgkin-Huxley equations for the squid axon membrane at 6.3 degrees C to decide theoretically whether stable membrane behavior might be expected in a space clamp experiment. The equations are solved for step depolarizations similar to those used in voltage clamp experiments. Each solution can be represented by a trajectory in the phase space of the variables V, m, h, and n. The stability of motion of a phase point on a given trajectory, and hence the adequacy of the control of the membrane potential, is shown to be a function of the effective conductance in series with the membrane. (For a patch of membrane away from the point controlled by feedback, the effective conductance is the combined conductance of the axial current electrode, axoplasm, and an external layer of sea water, all in series.) In particular, there is a (uniquely determined) critical conductance, defined as the minimum effective series conductance consistent with stability, associated with each point on the trajectory. During a step depolarization the critical conductance goes through a maximum. The values of such maxima as a function of voltage are closely similar to the negative slopes of the peak inward current versus voltage curve. This empirical correlation may be helpfup in the prediction of stability in experimental situations.

Characterization of the resting axolemma in the giant axon of the squid

Previous electron microscope studies have shown that the Schwann cell layer is traversed by long and tortuous slit-like channels ∼60Å wide, which provide the major route of access to the axolemma surface. In the present work the restriction offered by the resting axolemma to the passage of six small non-electrolyte molecules has been determined. The radii of the probing molecules were estimated from constructed molecular models. The ability of the axolemma to discriminate between the solvent (water) and each probing molecule was expressed in terms of the reflection coefficient σ. σ was then used to calculate an effective pore size for the resting axolemma. The value of 4.25 Å found for the pore radius is in excellent agreement with the 1.5 to 8.5 Å limiting values previously calculated from our measurements of water fluxes. The presence of pores with 4.25 Å radius in the resting axolemma is compatible with restricted diffusion of Na. The present paper leads to the conclusion that the axolemma is the only continuous barrier across which the ionic gradient responsible for the normal functioning of the nerve can be maintained. The combined findings of electron microscopy, water permeability, and molecular restricted filtration indicate that in all probability the axolemma is the "excitable membrane" of the physiologists.

Effect of the method of leading on the recording of the nerve fiber spectrum

A study has been made of the modifications of the shape of a nerve action potential dependent upon the placement of the two electrodes, always necessary for a lead. In a classic diphasic lead separation of the electrodes brings out, in addition to a separation of the phases, the appearance of a positive deflection traceable to the passage of an impulse between the electrodes. This phenomenon, called the lead separation effect (1.s.e.), must be considered as an expression of a feature of normal nerve fiber biophysics. It regularly appears and it can be analyzed with respect to the position of the sink maximum. Also it cannot be eliminated by a block at the second electrode. The advantage of approximating the leads was shown by the absence of a 1.s.e. following spikes recorded by electronic integration of tangents, which with validity can be derived from threshold fibers. Since tangent leads are not adaptable to recording a spectrum, a block at the second electrode is required. The making of such blocks and the configuration of records obtained with them are described. Conditions for an optimal lead, but not an ideal lead, were delimited. In an optimal lead only two major elevations appear in the spectrum of a skin nerve: those known as alpha and delta. A reference to maps of fiber size analyses shows that the fibers in the delta elevation have velocities of conduction slower than they would have if following in linear sequence the fiber diameters belonging to the alpha elevation.

Non-linear current-potential relations in an axon membrane

The membrane current density, I_m, in the squid giant axon has been calculated from the measured external current applied to the axon, I_o, by the equation See PDF for Equation where V_m is the membrane potential under the current electrode and r₁ and r₂ are the external and internal longitudinal resistances. The original derivation of this equation included in one step an assumption of a linear relation between I_m and V_m. It is shown that the same equation can be obtained without this restricting assumption.

Substrates supporting activity in immature nerve fibers

The ‘A’ fibers within mixed nerve trunks from immature chicks (16–21 days incubation) become inexcitable after 2 hours in nutrient-free Ringer's solution. Conduction in the ‘C’ fibers persists up to 8 hours under the same conditions. In contrast, nerves from adult chickens or rats are capable of transmitting impulses up to 14 hours in the same media. Addition of 5 mm/l. glucose or pyruvate rapidly restores conduction; however, there is no restoration when a variety of carbohydrate intermediates are present for 5 hours. Anaerobic survival of conduction with paired nerves is longer in 5 mm/l. glucose than in 10 mm/l. pyruvate and is inversely proportional to the frequency of stimulation. Our findings suggest that: a) a glycolytic energy source in immature axons is available for activity during anaerobiasis; b) the axon membrane is relatively impermeable to certain carbohydrate intermediates; c) in developing nerve the fraction of the total metabolism that is essential for the maintenance of the excitatory state is relatively small.

Nerve fiber activity in heavy water

Replacement of normal Ringer's surrounding the node of Ranvier of Bufo marinus with Ringer's prepared with D2O instead of H2O resulted in an increase in the duration of the response. In the squid giant axon immersed in D2O a rise in threshold membrane potential and a prolongation of the spike potential were observed.

Site of origin and propagation in spike in the giant neuron of Aplysia

The form and time sequence of spikes generated by orthodromic, antidromic, and direct stimulation and during spontaneous activity have been studied with intracellular electrodes simultaneously introduced in the soma and in different parts of the axon of the giant nerve cell of Aplysia. Evidence was obtained that under normal conditions of excitability, the spike originates at some distance from the soma in an axonal region with a higher excitability surpassing that of the surrounding membranes. Between the trigger zone and the soma is situated a region of transitional excitability where the conduction of the spike towards the soma may be blocked at a functionally determined and variable locus. The cell body is electrically excitable, but has the highest threshold of all parts of the neuron. The inactivation or even the removal of the cell body does not suppress synaptic transmission.

Analysis of repetitive transient response of lobster motor axons

An analysis was made to determine the relation between spike timing and the intensity of a constant current evoking a repetitive discharge in the single lobster motor axon. Accurate measurements of repetition intervals during the transient phase showed that an intensity increase of about 10–3 rheobase units produces a significantly different change in spike interval timing at the 0.005 probability level. Applications of excitation theory to the latency-intensity data have produced an equation which predicts the latency to the nth spike in a repetitive sequence as a function of stimulus intensity. The equation implies that the excitation process producing the nth spike is similar to the process producing the first spike in the repetitive sequence. Influences of supernormality and refractoriness were incorporated into the analysis. Also repeated stimulation at a fixed intensity indicated an inherent variability in the timing of the repetitive response which was shown to be a function of the magnitude of the latency. To explain this result a fixed uncertainty in the level of the initiating excitatory state was postulated.

Interactions of calcium with sodium and potassium in membrane potentials of the lobster giant axon

Experiments were performed on the lobster giant axon to determine the relation between intracellular spike amplitude and external calcium ion concentration. Action potential decline in low external calcium is greatly accelerated by simultaneous removal of external sodium ion. Correlation of the time course of spike decline in low calcium-low sodium solution with the time courses of spike decline in low calcium alone and in low sodium alone indicates that the effect of simultaneous removal of both ions is significantly greater than the sum of the individual effects. For a given time of treatment, spike amplitude was a function of external calcium concentration. While spike height is proportional to the log of the external calcium concentration over the range 2.5 to 50 millimolar, the proportionality constant is dependent upon the sodium concentration. Under the conditions of low external sodium (50 per cent reduction) the slope of the linear relationship between the spike height and the log of the external calcium concentration is about 5 times greater than in normal external sodium. Decreasing external calcium concentration and simultaneously increasing external potassium concentration produce a greater spike reduction than the arithmetic sum of spike reductions in low calcium alone and in high potassium alone. It is suggested that calcium interacts strongly with sodium and potassium in the spike-generating mechanism. A theoretical basis for these results is discussed.

An approach to the quantitative analysis of electrophysiological data from single neurons

The application of a digital computer to the processing of data from single neurons is described. Examples from experimental data are presented to demonstrate the usefulness of certain types of computations. These methods are placed in a descriptive mathematical framework. Other easily attainable computations are suggested.

Spontaneous activity in crustacean neurons

Single units which discharged with regular spontaneous rhythms without intentional stimulation were observed in the ventral nerve cord by intracellular recording close to the sixth abdominal ganglion. These units were divided into two groups: group A units in which interspike intervals varied less than 10 msec.; group B units in which interspike intervals varied within a range of 10 to 30 msec. Group A units maintained "constant" interspike intervals and could not be discharged by sensory inputs, while the majority of group B units could be discharged by appropriate sensory nerve stimulation. Both group A and B units discharged to direct stimulation when the stimulating and recording electrodes were placed in the same ganglionic intersegment, and directly evoked single spikes reset the spontaneous rhythm. In group B units, presynaptic volleys reset the spontaneous rhythm of some units; but in others, synaptically evoked spikes were interpolated within the spontaneous rhythm without resetting. The phenomenon of enhancement could also be demonstrated in spontaneously active units as a result of repetitive stimulation. It is concluded that endogenous pacemaker activity is responsible for much of the regular spontaneous firing observed in crayfish central neurons, and that interaction of evoked responses with such pacemaker sites can produce a variety of effects dependent upon the anatomical relationships between pacemaker and synaptic regions.

Electrophysiology of supramedullary neurons in Spheroides maculatus. I. Orthodromic and antidromic responses

This series of three papers presents data on a system of neurons, the large supramedullary cells (SMC) of the puffer, Spheroides maculatus, in terms of the physiological properties of the individual cells, of their afferent and efferent connections, and of their interconnections. Some of these findings are verified by available anatomical data, but others suggest structures that must be sought for in the light of the demonstration that these cells are not sensory neurons. Analysis on so broad a scale was made possible by the accessibility of the cells in a compact cluster on the dorsal surface of the spinal cord. Simultaneous recordings were made intracellularly and extracellularly from individual cells or from several, frequently with registration of the afferent or efferent activity as well. The passive and active electrical properties of the SMC are essentially similar to those of other neurons, but various response characteristics have been observed which are related to different excitabilities of different parts of the neuron, and to specific anatomical features. The SMC produce spikes to direct stimuli by intracellular depolarization, or by indirect synaptic excitation from many afferent paths, including tactile stimulation of the skin. Responses that were evoked by intracellular stimulation of a single cell cause an efferent discharge bilaterally in many dorsal roots, but not in the ventral. Sometimes several distinct spikes occurred in the same root, and behaved independently. Thus, a number of axons are efferent from each neuron. They are large unmyelinated fibers which give rise to the elevation of slowest conduction in the compound action potential of the dorsal root. A similar component is absent in the ventral root action potential. Antidromic stimulation of the axons causes small potentials in the cell body, indicating that the antidromic spikes are blocked distantly to the soma, probably in the axon branches. The failure of antidromic invasion is correlated with differences in excitability of the axons and the neurite from which they arise. As recorded in the cell body, the postsynaptic potentials associated with stimulation of afferent fibers in the dorsal roots or cranial nerves are too small to discharge the soma spike. The indirect spike has two components, the first of which is due to the synaptically initiated activity of the neurite and which invades the cell body. The second component is then produced when the soma is fired. The neurite impulse arises at some distance from the cell body and propagates centrifugally as well as centripetally. An indirect stimulus frequently produces repetitive spikes which are observed to occur synchronously in all the cells examined at one time. Each discharge gives rise to a large efferent volley in each of the dorsal roots and cranial nerves examined. The synchronized responses of all the SMC to indirect stimulation occur with slightly different latencies. They are due to a combination of excitation by synaptic bombardment from the afferent pathways and by excitatory interconnections among the SMC. Direct stimulation of a cell may also excite all the others. This spread of activity is facilitated by repetitive direct excitation of the cell as well as by indirect stimulation.