Subthreshold Pulses Research Articles

Changes produced in neuronal excitability by subthreshold depolarization as a possible mechanism of interval selective relationships within the central nervous system

A neuronal process was identified inLymnaea stagnalis nerve cells which may be viewed as one of the mechanisms underlying the interval selectivity previously described in research into the functional relationships between mammalian brain cells. This process takes the form of regularly-occurring changes in excitability resulting in a high probability (of 0.6–1) of neuronal spike response to what had previously been subthreshold depolarizing current pulses following similar subthreshold (conditioning) pulses at intervals specific to each individual neuron. It was found that the cycle of change in neuronal excitability following threshold depolarization did not arise from temporal summation of electrotonic local or postsynaptic neuronal potentials; it was an endogenous (cytoplasmic) process insensitive to transmitter (acetylcholine) application but altering irreversibly under the effects of bombesin, one of the modulator peptides.

Phase resetting and annihilation in a mathematical model of sinus node.

A mathematical model of primary sinoatrial pacemaker activity was developed using modifications of Hodgkin-Huxley type equations of voltage- and time-dependent membrane currents. The computer simulation of action potential activity incorporates the results of several existing cardiac models and recent biological data in an attempt to generate a model that more closely approximates the phase-resetting behavior of sinoatrial pacemakers observed biologically in response to subthreshold or electrotonic stimuli. The model was also used to study annihilation, i.e., the cessation of rhythmic activity induced by a critically timed subthreshold stimulus. Perturbation analysis performed by scanning the pacemaker cycle with 50-ms subthreshold current pulses yielded biphasic phase-response relationships that closely resembled the biological data. The pacemaker current, although relatively unimportant in determining the degree of phase 4 depolarization in the simulated sinus nodal pacemaker, was nevertheless prominent in determining phase-resetting behavior. The characteristics of annihilation were studied in normal, "depressed," and hyperpolarized states. In all cases, successful annihilation depended in large part on the dynamic interaction between the slow inward and delayed rectifier outward currents. The annihilation point was found to be an unstable singularity point in which a critical combination of stimulus intensity and timing could cause the pacemaker to approach but never achieve a state of complete quiescence. The model provides the basis for further investigation of complex arrhythmias that may arise as a consequence of multiple-pacemaker interaction within the heart.

Properties of subthreshold response and action potential recorded in layer V neurons from cat sensorimotor cortex in vitro.

Properties of the action potential and subthreshold response were studied in large layer V neurons in in vitro slices of cat sensorimotor cortex using intracellular recording and stimulation, application of agents that block active conductances, and a single-microelectrode voltage clamp (SEVC). A variety of measured parameters, including action-potential duration, afterpotentials, input resistance, rheobase, and membrane time constant, were similar to the same parameters reported for large neurons from this region of cortex in vivo. Action-potential amplitudes and resting potentials were greater in vitro. Most measured parameters were distributed unimodally, suggesting that these parameters are similar in all large layer V neurons irrespective of their axonal termination. The voltage response to subthreshold constant-current pulses exhibited both time and voltage dependence in the great majority of cells. Current pulses in either the hyperpolarizing or subthreshold depolarizing direction cause the membrane potential to attain an early peak and then decay (sag) to a steady level. On termination of the pulse, the membrane response transiently overshoots resting potential. Plots of current-voltage relations demonstrate inward rectification during polarization on either side of resting potential. Subthreshold inward rectification in the depolarizing direction is abolished by tetrodotoxin (TTX). The ionic currents responsible for subthreshold rectification and sag were examined using the SEVC. Steady inward rectification in the depolarizing direction is caused by a persistent, subthreshold sodium current (INaP) (54). Sag observed in response to a depolarizing current pulse is due to activation of a slow outward current, which superimposes on and partially counters the persistent sodium current. Both sag in response to hyperpolarizing current pulses and rectification in the hyperpolarizing direction are caused by a slow inward "sag current" that is activated by hyperpolarizing voltage steps. The sag current is unaltered by TTX, tetraethylammonium, (TEA), Co2+, Ba2+, or 4-aminopyridine. Fast-rising, short-duration action potentials can be elicited by an intracellular current pulse or by orthodromic or antidromic stimulation. Spikes are blocked by TTX. The form of the afterpotential following a directly evoked spike varies among cells with similar resting potentials. Biphasic afterhyperpolarizations (AHPs) with fast and slow components were most frequently seen. About 30% of the cells displayed a depolarizing afterpotential (DAP), which was often followed by an AHP. Other cells displayed a purely monophasic AHP.(ABSTRACT TRUNCATED AT 400 WORDS)

Damage to silicate glass containing impurities from intense radiation

A study has been made of the characteristics of impurities in crystalline quartz and glasses. Threshold values have been determined for glasses in relation to external mechanical loads. The parameters of subthreshold pulses in a damaging series are related to the number of them.

Regional variations in excitability of barnacle neurons

Optical recording techniques using voltage-sensitive dyes were used to examine the initiation and propagation of action potentials within neurons of the supraesophageal ganglion of the giant barnacle, Balanus nubilus. When a neuron was stimulated with current injection into the soma, the site of spike initiation, determined as the location with the earliest time-to-peak, was usually in the axon, 100 to 200 micron from the soma. The soma spike was broader and often later, suggesting that the cell body was less excitable than the axon. The action potential was narrowest at the site of initiation and then widened before propagating uniformly down the axon. In most cases, somatically stimulated action potentials and electrotonic pulses propagated into the dendrites with little delay or change of shape, indicating that the electrotonic length of these processes was great. Several different kinds of experiments indicated that some dendrites of these cells are excitable. These included the observations that (a) spikes could be made to initiate earlier in the dendrites than in the axonal region to which they were connected, and (b) action potentials sometimes decremented less than subthreshold pulses along dendritic processes. However, in other cases a decline in amplitude and a widening of the action potential demonstrated passive propagation into the dendrites, suggesting that not all dendrites are equally excitable.

Calcium transients studied under voltage-clamp control in frog twitch muscle fibres.

1. Intracellular calcium transients were recorded from frog twitch muscle fibres in response to voltage-clamped depolarizing pulses, using arsenazo III as an intracellular calcium monitor. The object was to investigate the time- and voltage-dependent characteristics of the coupling process between membrane depolarization and calcium release from the sarcoplasmic reticulum (s.r.)2. To examine the extent to which the T-tubule membrane potential was controlled during clamp pulses, the dye NK 2367 was used as an optical probe of tubular potential. This indicated that the tubular time constant is about 0.6 msec.3. Strength-duration curves were obtained for depolarizing pulses required to give both threshold mechanical contraction and calcium signal. Curves measured in these two ways were closely similar.4. Changes in holding potential altered the strength-duration curve for calcium release so that at more positive holding potentials a shorter pulse was needed to obtain a response for any given pulse amplitude.5. A latency of a few milliseconds was observed between the onset of depolarization and the initial rise of the calcium signal. This became shorter with stronger depolarizations, but approached a minimum at potentials above about +25 mV.6. Subthreshold depolarizations applied before a test pulse increased the size and decreased the latency of the calcium signal. Conditioning hyperpolarizations had opposite effects.7. The rate of build-up of potentiation or depression of response size seen with subthreshold de- and hyperpolarizing conditioning pulses was examined using conditioning pulses of different durations. For both pulses this process showed a time constant of about 3 msec (at 10 degrees C).8. The rate of decay of potentiation or depression was similarly measured, using a gap of variable duration between conditioning and test pulses. For both de- and hyperpolarizing pulses this showed a time constant of about 5 msec (10 degrees C).9. The relationship between conditioning pulse potential, and the size of calcium signal elicited by a following test pulse was non-linear.10. Subthreshold pulses immediately following a brief test pulse affected the size of the calcium signal in a similar way to preceding conditioning pulses.11. The relationship between potential and size of the calcium signal was examined using pulses of 3 and 20 msec duration. With the long pulse the relation was roughly sigmoid, but with the short pulse continued to rise even at strongly positive potentials.12. The results are discussed in terms of a model in which the exponential build-up of a hypothetical coupler in the excitation-contraction (e.-c.) coupling process is presumed to lead to calcium release when a threshold level is exceeded.

On the absolute magnitude of the human lead vector.

Twelve cases with temporary artificial pacing were used to investigate on the efficacy of a bipolar current source within the heart to generate potential variations in Frank lead ECGs. A constant current, subthreshold square pulse was applied to the catheter electrodes and the artefact was recorded with the Frank system. Assuming that the Frank system was normalized and orthogonal, the lead vector magnitude was calculated as 0.58 ohm/cm on the average. The value of the lead vector was inversely related to the thorax size. The direction of the vector determined from the recorded potentials approximated the direction of the catheter tip on the X-ray film. In addition, similar measurements were made with a homogeneous torso model and the results were compared with those of clinical cases.

Cellular electrophysiologic abnormalities of diseased human ventricular myocardium

Using standard microelectrode techniques, the cellular electrophysiologic features of ventricular myocardium resected from 8 patients with refractory arrhythmias were studied in vitro. Action potentials from damaged myocardium compared with normal myocardium had reduced resting membrane potential, amplitude, and maximal upstroke velocity. Tetrodotoxin, but not verapamil, suppressed 3 action potentials with resting potentials of -60 to -64 mV and Vmax less than 70 V/s. Verapamil, but not tetrodotoxin, suppressed 4 action potentials with resting potentials of -44 to -57 mV and Vmax less than 20 V/s. Unidirectional block, Wenckebach block, and summation occurred in damaged zones. Exit block from and frequency-dependent entrance block into an ectopic focus were noted. Subthreshold responses in the focal area induced by action potentials in the surrounding myocardium and by subthreshold current pulses injected through the recording microelectrode altered the spontaneous discharge rate of the focus, as previously described for modulated parasystole. Pulses early in the spontaneous cycle delayed the next expected discharge, and later pulses accelerated the subsequent discharge. Pulses injected at the singular point completely suppressed automaticity (annihilation). Tetrodotoxin and verapamil suppressed automaticity in some fibers. Single action potentials induced in quiescent fibers triggered and terminated sustained rhythmic activity. These data suggest that depressed fast responses, slow responses, and subthreshold potentials can generate and modulate ectopic activity in damaged human ventricle and that fast- and slow-channel blocking agents and single premature stimuli can terminate such activity.

Pulsed clearing of a synthetic aqueous aerosol by CO2laser radiation

An experimental investigation was made of the action of a microsecond pulse in the 10.6 µ range on the dispersive properties of a fine-droplet synthetic aqueous fog. It was found that when a certain threshold pulse energy was reached, a state of total clearing occurred and remained stable for a time governed by the wind-induced drift of the fog. At subthreshold pulse energies, secondary condensation occurred causing clouding of the medium at the 0.63 µ wavelength. An optothermodynamic analysis was made of the clearing process and the recondensation kinetics was analyzed.

Mechanical activation in slow and twitch skeletal muscle fibres of the frog.

1. Slow and twitch muscle fibres of the frog were studied with a two-micro-electrode point voltage-clamp method. Slow fibres were identified in pyriformis and cruralis muscles by their appearance in the light microscope, electrical characteristics, and rate of sarcomere shortening or of tension development. 2. The relation between the amplitude and duration of threshold depolarizing pulses was determined in sartorius twitch and pyriformis slow fibres. Strength-duration relations for contractile activation are very similar in the two fibre types. 3. The effect of a brief subthreshold pulse on the threshold voltage level decays with a half-time of 1-2 msec at 9 degrees C in both slow and twitch fibres. This fast decay, thought to reflect voltage-dependent deactivation of Ca2+ release following repolarization, is followed by a slower decay of greatly different rates in the two fibre types. the slower components of decay might reflect the rate of background Ca2+ removal by the sarcoplasmic reticulum. 4. Reducing external Ca2+ levels to about about 0.1 microM with 2.5 mM-EGTA has no effect on the shapes of strength-duration curves for both slow and twitch fibres, suggesting that activator Ca2+ in both fibre types originates entirely from intracellular stores. 5. "Tonic' contractions were studied using voltage-clamped short cruralis slow fibres at 20 degrees C. Reducing external Ca2+ to about 0.1 microM had no effect on the steepness of the steady-state tension-voltage relation or on the ability of slow fibres to maintain maximal tension during long (200 sec) depolarizations to membrane potentials of up to +50 mV. 6. Functional similarities in activation kinetics of slow and twitch fibres are discussed in relation to the sensing of tubular membrane potential by the sarcoplasmic reticulum, to Ca2+ release from it, and to possible mechanisms involved in these processes. Processes leading to the rapid turning on and off of Ca2+ release in response to changes in tubular membrane potential are probably similar in slow and twitch fibres. However, the apparent lack of voltage-and time-dependent inactivation of Ca2+ release in slow fibre points to a major difference in the two types of muscle.

Quantitative aspects of ionic conductance mechanisms contributing to firing pattern of motor cells mediating inking behavior in Aplysia californica.

1. The release of ink occurs selectively to long-lasting stimuli (18). There is a good correspondence between the features of the behavior and the firing pattern of the ink motor cells since the initial synaptic input is ineffective in firing the ink gland motor neurons. The features of the firing pattern of these cells have in turn been attributed to a fast K+ current that shunts the initial excitatory input and a late buildup of synaptic input. The purpose of the present paper is to examine these features quantitatively using a kinetic model of the ink gland motor neurons. 2. A modified Hodgkin-Huxley (13) model was developed, which includes seven membrane currents: a ) a fast inward Na+ current, 6) a slow inward Ca2+ current, c) a fast outward K+ current, d) a slow (delayed) outward K+ current, e) a synaptic current, f) a leakage current, and g) a capacitative current. The three ink gland motor neurons are electrically coupled to each other. The model also includes the features of this electronic coupling. 3. The Na+ current underlies the spike initiation, while the delayed K+ current accounts for spike repolarization. The fast transient K+ current also contributes to the repolarization process and slows the rate of rise of the membrane potential during the interspike interval. The slow Ca2+ current makes only a small contribution to the generation of the action potential, but contributes to the rise of the membrane potential during the interspike interval. 4. A depolarizing current pulse to one ink gland motor neuron yields an initial rapid depolarization followed by a slower component, which after a several-second period results in a burst of action potentials. The activation and inactivation sequence of a fast transient K+ current contributes to the cell’s selective response to long-lasting depolarizing current pulses. If the suprathreshold current pulse is preceded by a small subthreshold pulse, spikes are elicited immediately with the application of the suprathreshold pulse, since the subthreshold pulse inactivates the transient outward current. 5. The ink gland motor neurons also show a selective response to long-lasting trains of synaptic input. A train of synaptic input causes an initial large depolarization, but the initial synaptic input is ineffective in firing the cell. As a result, there is a severalsecond silent period or pause before an accelerating burst of spikes is produced. The fast transient K+ current shunts the initial synaptic input, preventing the cell from reaching spike threshold. Over a several-second period this current inactivates and the synaptic current is more effective in firing the cell. 6. While the fast outward current is inactivating, the synaptic current builds up and contributes to the accelerating burst of spikes.

Interaction of oscillatory processes: The effects of subthreshold A.C. current on sinoatrial nodal rhythm

In this study of interactions occurring within the heart, isolated superfused strips of rabbit atria, containing the sinoatrial (s.a.) node, were subjected to sinusoidal subthreshold current pulses of varied frequencies and intensities. A.C. current from an R.C. oscillator was applied through a Grass stimulation isolation unit, push-pull connection, and non-polarizing (Ag-AgCl-KCl-Tyrode Agar-Agar) electrodes. A Grass polygraph and tachometer were used to record the applied pulses and nodal firing rates; simultaneous magnetic tape recordings were obtained and used for data analysis. Suction electrode recordings and oscilloscope displays were used to determine how the cyclic impulses affected cellular activity. The s.a. nodal rhythm was modified by subthreshold A.C. current applications; when frequencies were low, firing rates of the node were modulated by the A.C. and mean rates were reduced. As frequencies were progressively increased, slow waxings and wanings in heart rate were produced. These periodic fluctuations were not readily correlated with either the A.C. frequency nor the intrinsic nodal rate, but were representative of the difference between the two. As applied current frequency neared the pacemaker's intrinsic rate, a synchronization occurred and the discharges locked in at a specific phase of the applied current alternation. This synchronization maintained during slight further increases in A.C. frequency but above a critical rate this relationship broke down and the waxing and wanings in frequency of pacemaker rate again developed. It was concluded that pacemaker action of the s.a. node is effected by integration of cellular activity through electron coupling.

Circadian and other rhythmic activity of neurones in the ventromedial nuclei and lateral hypothalamic area.

1. The frequency of firing was simultaneously recorded from single neurones of the ventromedial nuclei (VMN) and the lateral hypothalamic area (LHA) in urethane anaesthetized rats for many hours. 2. There were circadian changes of VMN and LHA neurone activity. The pattern of this circadian rhythm is as follows: throughout the day LHA neurones show higher activity than that of VMN, as indicated by higher frequency and more fluctuations in their rates of firing. In late afternoon the discharge rate of LHA neurones increases further, showing oscillations of short duration. In the early evening hours LHA neurone activity gradually goes down, as the VMN neurones become active. Throughout the night, VMN neurones are more active than those of LHA, just the opposite of the day period. In early morning hours VMN neurones gradually become quiet, while LHA neurones begin to show activity. 3. Superimposed on the circadian rhythm, at certain periods of the day, VMN and LHA neurones showed short duration oscillations in rate of firing, roughly every 7-15 sec and every 3-5 min. 4. Activities in neurones of the VMN and LHA were reciprocally related; a decrease in firing rate of one was associated with an increase in the other. This phenomenon was shown clearly by analysis of auto- and cross-correlation functions of firing patterns of VMN and LHA neurones. 5. The effects of stimulations of the prefrontal cortex and splanchnic afferents on VMN and LHA neurones depended on the basic firing frequency, thus they varied with the time of day. Definite relationships exist between basic firing frequency of a cell and the magnitude of changes evoked by these stimuli. Reactions of VMN and LHA neurones were the opposite in most instances. Septal stimulations (at more than 10/sec) always produced inhibition of LHA neurone activity. 6. Intravenous injection of glucose inhibited LHA neurones and accelerated firing of VMN cells. This was true during the day period as well as at night when background activities of VMN and LHA neurones were different from that of the day. 7. Stimulation of the septal area with subthreshold pulses at a low rate (1-0.3/sec) suppressed or altered oscillations in firing frequency of LHA neurones. Severance of connection between LHA and structures caudal thereto had no effect on LHA neurone firing rates or rhythms. Sections between the septal area and LHA, however, abolished or greatly altered the oscillatory rhythms of LHA cell activity, although spontaneous discharges continued at a somewhat lower rate for periods of hours. 8. Stimulation of suprachiasmatic nuclei with weak intensity and low frequency also changed oscillatory fluctuations in firing of LHA neurones. 9. Possible origins of circadian rhythm and oscillations of short duration in firing pattern of VMN and LHA neurones were discussed.

Electrical properties of frog motoneurons in the in situ spinal cord.

Electrical properties of the spinal motoneurons of Rana temporaria and R. esculenta were investigated in the in situ spinal cord at 20-22 degrees C by means of intracellular recording and current injection. Input resistance values depended on the method of measurement in a given cell but were generally inversely related to axon conduction velocity. The membrane-potential response to a subthreshold current pulse was composed of at least two exponentials with mean time constants of 2.5 and 20 ms. The membrance potential reached by the peak of a spike depended on the mode of spike initiation and membrane potential. Preceding a suprathreshold depolarization by a hyperpolarizing pulse could delay and eliminate spike initiation, similar to effects reported in certain invertebrate neurons. Antidromic invasion frequently failed in motoneurons of normal resting potential. Antidromic spike components (m,IS, SD) were similar to those of cat motoneurons. The delayed depolarization and the long afterhyperpolarization following an antidromic spike had many properties in common with the analogous afterpotentials of cat motoneurons. The reversal potential of the short afterhyperpolarization occurring immediately after the spike varied with resting potential and could not be used to determine potassium equilibrium potential. Sustained rhythmic firing could be evoked by continuous synaptic drive or long pulses of injected current. The plot of firing rate versus current strength had a substantial linear region. Both steady firing and adaptation properties varied markedly with motoneuron input resistance.

Charge movement and mechanical repriming in skeletal muscle.

1. Muscles were placed in a solution which depolarized the membrane to -30 to -20 mV so that mechanical activation was made refractory. Mechanical repriming and the recovery of voltage dependent charge movement were studied using a voltage clamp technique. 2. Mechanical repriming was investigated by determining the duration of a hyperpolarizing pulse required to elicit a just-visible contraction for various post-pulse potentials. As the post-pulse potential was made more positive shorter repriming times were required to produce a threshold contraction. The relationship approached a minimum repriming time for very positive post-pulse potentials. 3. These results suggest that hyperpolarization gradually removes some component of the activation mechanism from a refractory state and that the effectiveness of the amount which has recovered depends on the post-pulse potential. A quantitative explanation is given using a simple model in which the essential component is assumed to be the charge movement process. 4. The rate of repriming contraction is voltage dependent; at -160 mV the rate is about twice that at -120 mV. Between 4 and 10 degrees C the rate has a Q10 of about 9. 5. Recovery of charge movement was studied using a repriming duration less than that required to produce a threshold contraction. The observed charge movement increased linearly with repriming time, consistent with the approximately linear initial segment of a slow exponential recovery process. Extrapolation of the recovery curve indicated that 2-5 n/CmuF of charge is reprimed in the time necessary to reprime a threshold contraction. 6. The charge which recovers during a subthreshold repriming pulse is distributed according to membrane potential in the same way as a fully reprimed charge. 7. These results are consistent with the hypothesis that voltage dependent charge movement is an intermediate step in excitation-contraction coupling. 8. The characteristics of a second type of charge movement are also described.

A study of hypothalmic neurosecretory cells of bullfrogs in vitro.

Neurosecretory cells of preoptic nuclei of bullfrogs were studied in isolated hypothalamo-hypophysial preparations under constant perfusion with oxygenated Ringer solution at 15-17 degress C. Antidromic potentials were recorded following stimuli applied to the posterior lobe of the pituitary or the stalk. 2. Intracellularly and extracellularly recorded potentials resembled those obtained in vivo from neurosecretory cells of the mammalian hypothalamus. They were unique in that the antidromic potential had a long duration (10-20 msec) and a distinct notch on the rising phase (between A and B spikes). The conduction velocity of the stalk fibres in vitro at this temperature was 0-1--0-2 m/sec. 3. When two successive stimuli were given to the posterior lobe or to the stalk separated by intervals of between 30 and 65 msec, the test (second) response showed a longer delay of the B spike. This delay between the A and B components was as long as 10 msec. Further shortening of stimulus intervals produced block of B spikes in the test response. A complete separation of A and B spikes occur spontaneously in a few instances. 4. Evidence indicated that inhibitory recurrent axon collaterals play a role in the control of bullfrog neurosecretory cells. Antidromic potentials were inhibited by a 'conditioning' stimulus for as long as 300-400 msec, even when the stimulus did not evoke an antidromic potential. 5. It was found that in addition to the inhibitory interaction there is a facilitatory recurrent axon collateral system which operates within the nuclei. The evidence for this is: (1) stimulation of the posterior lobe, with single subthreshold pulses evoked an action potential if preceded by another stimulus of subthreshold or just threshold intensity. The durations of such facilitatory effects were found to be 20--400 msec; (2) a single pulse given to the posterior lobe did occasionally evoke two spikes from neurosecretory cells; the second spike which occurred 15-30 msec after the first had the characteristics of a trans-synaptically produced potential; (3) gradual changes in the intensity of stimuli applied to the neural lobe produced a sudden shift in latencies ranging between 15 and 30 msec. The potentials having long latency also showed characteristics of those transsynaptically excited. In addition, an increase in excitability of neurosecretory cells by antidromic stimulation was confirmed by using orthodromically induced action potentials in in vivo studies. Possible functional significance of inhibitory and excitatory recurrent collateral system in neurosecretory cells was discussed. 6. Two to threefold increase in NaCl concentration of a perfusate slightly increased the latency and refractory period of antidromic potential but not the shape of the potential. Norepinephrine added to a perfusate (1 mug/ml). augmented the separation of A and B spikes of the antidromic potential. Acetylcholine at a concentration of 1 mug/ml. did not have an appreciable effect on the antidromic potential.

Contractile activation in frog skeletal muscle.

Contractile activation was studied in frog single muscle fibers treated with tetrodotoxin to block action potentials. The membrane potential in a short segment of the fiber was controlled with a two-electrode voltage clamp, and the contractile response of superficial myofibrils in this segment was observed microscopically. The strength-duration relation for contractile activation was similar to that reported by Adrian, Chandler, and Hodgkin (1969); at 3.9 degrees C, the contraction threshold was -44 mV for long depolarizing pulses (100-ms) and increased to +64 mV for 2-ms depolarizations. Hyperpolarizing postpulses shifted the threshold for 2-ms pulses to more positive values, and a similar, but smaller, effect was produced by hyperpolarizing prepulses. The decay of excitability following subthreshold pulses showed two apparently distinct components; at 3.9 degrees C, excitability fell to 50% of its initial value within 4 ms, while the subsequent decline of excitability proceeded with a half-time of about 20 ms.

After effects of spikes in cockroach giant axons.

AbstractCockroach abdominal giant axons (AGF) demonstrate a period of hyperexcitability following a spike; namely, an ineffective, subthreshold stimulus becomes effective if it occurs 5 to 100 msec after a spike. This phenomenon appears not to be synaptically mediated since it is not affected by high Mg2+ low Ca2+. It is likely that the normally subthreshold pulses summate with the negative after potential of a preceding spike. Hyperexcitability was found in the fine terminals of the AGF in the head and in the last abdominal and thoracic ganglia. It does not occur in the five smaller abdominal ganglia or connectives. The hyperexcitability of the AGF may be functional. Sensory inputs from one cercal nerve, when activating the AGF, increase the sensitivity to following inputs from the same or the second cercal nerve for periods much longer than the EPSP (100–120 msec).

The kinetics of mechanical activation in frog muscle.

1. The kinetics of mechanical activation were examined in muscle fibres of the frog's sartorius muscle, using a voltage clamp to control membrane potential, tetrodotoxin to eliminate electrical activity and microscopic observations to determine the mechanical threshold.2. The strength-duration curve was determined over a range of membrane potentials varying between -52 mV (rheobase) and +90 mV. At 4 degrees C the critical duration was about 11 msec at -30 mV, 4 msec at 0 mV and 2 msec at +40 mV.3. For pulses where V > -10 mV the threshold criterion at 4 degrees C was that the ;area above -30 mV' must exceed about 120 mV msec.4. The effect of a brief subthreshold pulse declines with a time constant of about 3 msec at -100 mV and about 8 msec at -85 mV at 4 degrees C.5. Although the strength-duration curve is well fitted by assuming a first-order mechanism in which the rate of release of activator increases with membrane potential, other experiments show that the over-all mechanism is probably second order in time.6. A short pulse must be at least 50% threshold if it is to give a visible contraction when added to a long pulse which is just below rheobase.7. Delayed rectification was conspicuous with medium or long pulses which were just below the mechanical threshold, but short pulses could give contraction without turning on any appreciable potassium conductance.8. The Appendix extends Falk's (1968) treatment of the charging of the tubular system under a voltage clamp.

Subthreshold changes at the site of initiation of spreading cortical depression by mechanical stimuli

1. 1. The effect of the local application of potassium chloride in concentrations which are subthreshold for the initiation of spreading depression on the cerebral cortex of rats is summed with the effect of subthreshold mechanical stimuli. The percentage of occurrence of SD increases with subthreshold values of both kinds of stimuli. 2. 2. The effect of a subthreshold mechanical pulse is also summed with the effect of application of 2,4-dinitrophenol in subthreshold concentrations. The time effect of DNP is, however, longer than of potassium chloride. 3. 3. Two successive subthreshold mechanical stimuli or a volley of the latter applied at intervals longer than 200 msec do not lead to a summation and initiation of SD. On the contrary an inhibition of suprathreshold mechanical pulse follows a previously applied subthreshold one. This inhibition reaches its maximum (50%) for a testing stimulus 2 mm above threshold applied in the 15 sec after a conditioning stimulus — 1 mm below threshold. A return to normal values is attained after 2 min. 4. 4. Evoked potentials at the site of application of a subthreshold mechanical stimulus are reduced approximately to 25 per cent of their original amplitude when a subthreshold stimulus 2 mm below the threshold is used. The recovery of the amplitude to 50 per cent of the initial value is approximately three times faster than in the case of SD. 5. 5. The lowering of the evoked potentials is limited only to the region of stimulus application and shows a steep spatial decrement. Already 3 mm from the edge of the affected area no reduction of evoked potentials can be observed. 6. 6. The mechanism of the summation of subthreshold stimuli as well as the relative refractoriness following subthreshold mechanical stimulus is discussed.