Electrophysiological Studies of the Antrum Muscle Fibers of the Guinea Pig Stomach

The membrane potentials of single smooth muscle fibers of various regions of the stomach were measured, and do not differ from those measured in intestinal muscle. Spontaneous slow waves with superimposed spikes could be recorded from the longitudinal and circular muscle of the antrum. The development of tension was preceded by spikes but often tension appeared only when the slow waves were generated. Contracture in high K solution developed at a critical membrane potential of -42 mv. MnCl₂ blocked the spike generation, then lowered the amplitude of the slow wave. On the other hand, withdrawal of Na⁺, or addition of atropine and tetrodotoxin inhibited the generation of most of the slow waves but a spike could still be elicited by electrical stimulation. Prostigmine enhanced and prolonged the slow wave; acetylcholine depolarized the membrane without change in the frequency of the slow waves. Chronaxie for the spike generation in the longitudinal muscle of the antrum was 30 msec and conduction velocity was 1.2 cm/sec. The time constant of the foot of the propagated spike was 28 msec. The space constants measured from the longitudinal and circular muscles of the antrum were 1.1 mm and 1.4 mm, respectively.     

The electrical and mechanical activity of the esophageal cell of Ascaris lumbricoides

The esophagus of Ascaris is a syncytial muscle organ of tubular shape in which the myofibrils are arranged radially between the lumen and the external surface. A resting potential of almost 40 mv (cytoplasm negative) is maintained by the extracellular organic anions (volatile fatty acids) found in the perienteric fluid. Replacement of these anions by Cl- ions results in a large depolarization. The resting potential is also decreased when the external pH is lowered. The leading phase of the action potential with a positive overshoot of about 18 mv elicits contraction of the myofibrils, development of negative pressure within the lumen, and suction of liquid and food particles. The mechanical energy stored in the elastic components of the cell is released when the myofibrils relax, thus injecting the contents of the lumen into the intestine. A fast and synchronous relaxation is elicited by a regenerative membrane polarization, a negative spike with a peak value of up to 108 mv produced by an increase in the permeability of the membrane to K+ ions. Cells completely depolarized in "chloride" saline are still able to generate such large potassium spikes.     

Current- and Voltage-Clamped Studies on Myxicola Giant Axons : Effect of tetrodotoxin

A new dissection procedure for preparing Myxicola giant axons for observation under voltage clamp is described. Preparation time is generally 40–45 min. 65–70% of the preparations attempted may be brought through the entire procedure, including insertion of the long internal electrode, and support an initial action potential amplitude of 100 mv or greater. Mean values for axon diameter, resting membrane potential, action potential amplitude, maximum peak inward transient current, and resting membrane resistance are 560 µ, —66.5 mv, 112 mv, 0.87 ma/cm² and 1.22 KΩ cm ² respectively. Cut branches do not seem to be a problem in this preparation. Behavior under voltage clamp is reasonably stable over several hours. Reductions in maximum inward transient current of 10% and in steady-state current of 5–10% are expected in the absence of any particular treatment. Tetrodotoxin blocks the action potential and both the inward and outward transient current, but has no effect on either the resting membrane potential or the steady-state current. This selective action of tetrodotoxin on the transient current is taken as an indication that this current component is probably carried by Na.     

Augmentation and suppression of action potentials by estradiol in the myometrium of pregnant rat.

The purpose of this study was to investigate the actions of estradiol on spontaneous and evoked action potentials in the isolated longitudinal smooth muscle cells of the pregnant rat. Single cells were obtained by enzymatic digestion from pregnant rat longitudinal myometrium. Action potentials and currents were recorded by whole-cell current-clamp and voltage-clamp methods, respectively. The acute effects of 17beta-estradiol on action potentials and inward and outward currents were investigated. The following results were obtained. The average resting membrane potential of single myometrial cells was -54 mV (n = 40). In many cells, an electrical stimulation evoked a membrane depolarization, and action potentials were superimposed on the depolarization. In some cells, spontaneous action potentials were observed. Estradiol (30 microM) slightly depolarized the membrane (ca. 5 mV) and attenuated the generation of action potentials by reducing the frequency and amplitude of the spikes. Afterhyperpolarization was also attenuated by estradiol (30 microM). On the other hand, in 5 of 35 cells, estradiol increased the first spike amplitude and action potential duration, while frequency of the spike generation and afterhyperpolarization were inhibited. In voltage-clamped muscle cells, estradiol inhibited both inward and outward currents. Acute inhibition or augmentation of spike generation by estradiol is due to the balance of inhibition of inward and outward currents. Inhibition of both currents also prevented afterhyperpolarization, causing potential-dependent block of Ca spikes.     

Mechanisms of direct and neural excitability in electroplaques of electric eel

1. Current flow outward through the caudal, reactive membrane of the cell causes direct stimulation of the electroplaque. The electrical response in denervated as well as in normal preparations recorded with internal microelectrodes is first local and graded with the intensity of the stimulus. When membrane depolarization reaches about 40 mv. a propagated, all-or-nothing spike develops. 2. Measured with internal microelectrodes the resting potential is 73 mv. and the spike 126 mv. The latter lasts about 2 msec. and is propagated at approximately 1 M.P.S. 3. The latency of the response decreases nearly to zero with strong direct stimulation and the entire cell may be activated nearly synchronously. 4. Current flow inward through the caudal membrane of the cell does not excite the latter directly, but activation of the innervated cell takes place through stimulation of the nerve terminals. This causes a response which has a latency of not less than 1.0 msec. and up to 2.4 msec. 5. The activity evoked by indirect stimulation or by a neural volley includes a prefatory potential which has properties different from the local response. This is a postsynaptic potential since it also develops in the excitable membrane which produces the local response and spike. 6. On stimulation of a nerve trunk the postsynaptic potential is produced everywhere in the caudal membrane, but is largest at the outer (skin) end of the cell. The spike is initiated in this region and is propagated at a slightly higher rate than is the directly elicited response. Strong neural stimulation can excite the entire cell to simultaneous discharge. 7. The postsynaptic potential caused by neural or indirect stimulation may be elicited while the cell is absolutely refractory to direct excitation. 8. The postsynaptic potential is not depressed by anodal, or enhanced by cathodal polarization. 9. It is therefore concluded that the postsynaptic potential represents a membrane response which is not electrically excitable. Neural activation of this therefore probably involves a chemical transmitter. 10. The nature of the transmitter is discussed and it is concluded that this is not closely related to acetylcholine. 11. Paired homosynaptic excitation discloses facilitation which is not present when the conditioning stimulus is direct or through a different nerve trunk. These results may be interpreted in the light of the existence of a neurally caused chemical transmitter or alternatively as due to presynaptic potentiation. 12. The electrically excitable system of the electroplaque has two components. In the normal cell a graded reaction of the membrane develops with increasing strength of stimulation until a critical level of depolarization, which is about 40 mv. 13. At this stage a regenerative explosive reaction of the membrane takes place which produces the all-or-nothing spike and propagation. 14. During early relative refractoriness or after poisoning with some drugs (eserine, etc.) the regenerative process is lost. The membrane response then may continue as a graded process, increasing proportionally to the stimulus strength. Although this pathway is capable of producing the full membrane potential the response is not propagated. 15. Propagation returns when the cell recovers its regenerative reaction and the all-or-nothing response is elicited. 16. Excitable tissues may be classified into three categories. The axon is everywhere electrically excitable. The skeletal muscle fiber is electrically excitable everywhere except at a restricted region (the end plate) which is only neurally or chemically excitable. The electroplaque of the eel, and probably also cells of the nervous system have neurally and electrically excitable membrane components intermingled. The electroplaques of Raia and probably also of Torpedo as well as frog muscle fibers of the "slow" system have membranes which are primarily neurally and chemically excitable. Existence of a category of invertebrate muscle fibers with graded electrical excitability is also considered. 17. In the eel electroplaque and also probably in the cells of neurons, tests of the mode of neural activation carried out by direct or antidromic stimulation cannot reveal the neurally and chemically activated component. The data of such tests though they appear to prove electrical transmission are therefore inadequate for the detection and study of the chemically initiated process.     

Evidence for an intrinsic control of myometrial contractile periodicity in sheep during pregnancy.

Reduction in concentration of prostaglandins in plasma by administration of sodium meclofenamate to pregnant sheep failed to alter the frequency or duration of electromyographic activity bursts or the response to oxytocin of myometrial tissue transplanted to the omentum. However, a significant (P < 0.05) delay (8.6 +/- 3.8 versus 1.3 +/- 0.3 min) in the myometrial response to oxytocin was observed when the hormone was administered 1 min after a spontaneous burst of electromyographic activity compared with 15 min after a burst, indicating a period of refractoriness. Similarly, the myometrial threshold for electrical stimulation was higher at 10-25% of the interval between contractions than close to the expected time of the next contraction. Stimulation of the myometrium at intervals of 30 s revealed a cycling of the electrical stimulation threshold: significantly higher voltages were required to elicit responses between spontaneous bursts of electromyographic activity (18.0 +/- 2.2 V) than during bursts (11.3 +/- 1.6 V). In contrast, there was no voltage differential in animals close to labour (< 24 h). These data provide no evidence to support a role for prostaglandins in the generation of contractions during pregnancy, but suggest that periodicity of contractions is associated with inherent changes in myometrial responsiveness to stimulation, which could occur as a result of a cycling of the resting membrane potential.     

Patterns of electrical propagation in the intact pregnant guinea pig uterus

Previous studies have reported on propagation of individual spikes in isolated segments of the pregnant uterus, but there is no information on patterns of spike propagation in the intact organ. There is also no information on propagation of myometrial burst. The aim of this study was to record, at high resolution, patterns of propagation of electrical activities in the pregnant uterus. Sixteen timed-pregnant guinea pigs were euthanized at term, and their uteruses isolated. Fetuses were removed and replaced by an equal amount of Tyrode. A 240-electrode array was positioned at various locations along the organ, all signals were recorded simultaneously, and the electrical propagations were reconstructed. In the intact pregnant uterus at term, spikes propagated with high velocity in longitudinal (6.8 +/- 2.4 cm/s) and slower velocity in circular direction (2.8 +/- 1.0 cm/s; P < 0.01). Direction of propagation and frequency of activity were highly variable but showed similar patterns at the ovary or cervical end and along the anterior, posterior, and antimesometrial borders. Along mesometrium, spike propagation was sparse and fractionated. Migration of burst (0.6 +/- 0.4 cm/s) was significantly much slower than that of individual spikes (P < 0.001). Initial burst activity was located at variable locations along the ovarial end of the antimesometrial border, while the latest excitation occurred at the cervical end (1.2 +/- 0.9 min). In conclusion, high resolution electrical mapping of the intact pregnant uterus reveals fundamental properties in spatial and temporal patterns of spike and burst propagation that determine the contraction of the organ.     

Coherence resonance for neuronal bursting with spike undershoot

Although the bursting patterns with spike undershoot are involved with the achievement of physiological or cognitive functions of brain with synaptic noise, noise induced-coherence resonance (CR) from resting state or subthreshold oscillations instead of bursting has been widely identified to play positive roles in information process. Instead, in the present paper, CR characterized by the increase firstly and then decease of peak value of power spectrum of spike trains is evoked from a bursting pattern with spike undershoot, which means that the minimal membrane potential within burst is lower than that of the subthreshold oscillations between bursts, while CR cannot be evoked from the bursting pattern without spike undershoot. With bifurcations and fast-slow variable dissection method, the bursting patterns with and without spike undershoot are classified into “Sub-Hopf/Fold” bursting and “Fold/Homoclinic” bursting, respectively. For the bursting with spike undershoot, the trajectory of the subthreshold oscillations is very close to that of the spikes within burst. Therefore, noise can induce more spikes from the subthreshold oscillations and modulate the bursting regularity, which leads to the appearance of CR. For the bursting pattern without spike undershoot, the trajectory of the quiescent state is not close to that of the spikes within burst, and noise cannot induce spikes from the quiescent state between bursts, which is cause for non-CR. The result provides a novel case of CR phenomenon and extends the scopes of CR concept, presents that noise can enhance rather than suppress information of the bursting patterns with spike undershoot, which are helpful for understanding the dynamics and the potential physiological or cognitive functions of the nerve fiber or brain neurons with such bursting patterns.     

The Initiation of Spike Potential in Barnacle Muscle Fibers under Low Intracellular Ca++

Electrical properties of the muscle fiber membrane were studied in the barnacle, Balanus nubilus Darw. by using intracellular electrode techniques. A depolarization of the membrane does not usually produce an all-or-none spike potential in the normal muscle fiber even though a mechanical response is elicited. The intracellular injection of Ca⁺⁺-binding agents (K₂SO₄ and K salt of EDTA solution, K₃ citrate solution, etc.) renders the fiber capable of initiating all-or-none spikes. The overshoot of such a spike potential increases with increasing external Ca concentration, the increment for a tenfold increase in Ca concentration being about 29 mv. The threshold membrane potential for the spike and also for the K conductance increase shifts to more positive membrane potentials with increasing [Ca⁺⁺]_out. The removal of Na ions from the external medium does not change the configuration of the spike potential. In the absence of Ca⁺⁺ in the external medium, the spike potential is restored by Ba⁺⁺ and Sr⁺⁺ but not by Mg⁺⁺. The overshoot of the spike potential increases with increasing [Ba⁺⁺]_out or [Sr⁺⁺]_out. The Ca influx through the membrane of the fiber treated with K₂SO₄ and EDTA was examined with Ca⁴⁵. The influx was 14 pmol per sec. per cm² for the resting membrane and 35 to 85 pmol per cm² for one spike. From these results it is concluded that the spike potential of the barnacle muscle fiber results from the permeability increase of the membrane to Ca⁺⁺ (Ba⁺⁺ or Sr⁺⁺).     

The Spread of Excitation among Neurons in the Heart Ganglion of the Stomatopod, Squilla oratoria

Neurons in the heart ganglion of the mantis shrimp (a stomatopod crustacean) are functionally tightly linked together. The extracellular action potential from the whole trunk very often shows a complex form, but the response is all-or-none to the applied stimulus, indicating that the excitation in one neuron spreads very rapidly to all others. Application of isotonic MgCl₂ solution or repetitive stimulation sometimes separates the spike into its components. The resting potential of the soma membrane is 50 to 60 mv. External stimulation elicits a spike of 60 to 80 mv amplitude with a step on its rising phase. Hyperpolarization reveals one more inflection on the rising phase. These inflections divide the soma action potential into three parts, A₁, A₂, and B spikes in that order from the foot. The B spike disappears on increasing the hyperpolarization, but A₁ and A₂ remain, indicating that B originates from the soma membrane, whereas A₁ and A₂ originate from the two axons of the bipolar cell. Thus the impulse invades the soma from two directions, one from the stimulated side, the other from the other side via the "parallel axons" and the "side-connections;" the latter are presumed to interconnect the axons. When the parallel axons are cut, conduction takes place across the soma with a greatly reduced safety factor and a prolonged conduction time. Neuron-to-neuron transmission takes place in either direction.     

Vagal and sympathetic effects on the pacemaker fibers in the sinus venosus of the heart

1. Action potentials from sinus venosus and auricle fibers of spontaneously beating frog hearts have been recorded with intracellular electrodes. 2. Sinus fibers show a slow depolarization, the pacemaker potential, during diastole. The amplitude of this potential varies in different parts of the sinus. In some fibers the membrane potential falls by 11 to 15 mv. during diastole and the transition to the upstroke of the action potential is comparatively gradual. In other regions the depolarization develops more slowly and the action potential takes off more abruptly from a higher membrane potential. It is proposed that the fibers showing the largest fall in membrane potential during diastole are the pacemaker fibers of the heart, and that the rest of the preparation is excited by conduction. In auricle fibers the membrane potential is constant during diastole. 3. The maximum diastolic membrane potential and the overshoot of the action potential vary inversely with the amplitude of the pacemaker potential. The highest values were measured in auricle fibers. 4. Stimulation of vagi suppresses the pacemaker potentials. While the heart is arrested the membrane potential of the sinus fibers rises to a level above the maximum diastolic value reached in previous beats. In 28 experiments vagal stimulation increased the membrane potential from an average maximal diastolic value of 55 mv. to a "resting" level of 65.4 mv. The biggest vagal polarization was 23 mv. 5. In contrast to the sinus fibers vagal inhibition does not change the diastolic membrane potential of frog auricle fibers. 6. Vagal stimulation greatly accelerates the repolarization of the action potential and reduces its amplitude. These changes were seen both in the sinus and in auricle fibers stimulated by direct shocks during vagal arrest. 7. The conduction velocity in the sinus venosus of the tortoise is reduced by vagal stimulation. Block of conduction often occurs. 8. In the frog sinus venosus sympathetic stimulation increases the rate of rise of the pacemaker potential, accelerating the beat. The threshold remains unchanged. The rate of rise of the upstroke and the amplitude of the overshoot are increased. 9. The analogies between the vagal inhibition of the heart and the nervous inhibition of other preparations are discussed.     

Calcium and action potentials of bullfrog sympathetic ganglion cells

Bullfrog sympathetic ganglion cells were capable of producing action potentials (Ca spikes) in an isotonic (84 mM) CaCl2 solution. The peak level of Ca spikes showed an approximately 30 mv increase with a 10-fold increase in the Ca concentration. Na as well as Ca ions were capable of acting as charge carriers during the production of action potentials in a solution containing relatively high Ca and relatively low Na ions. A decrease in the external Ca concentration depressed the maximum rate of rise at a fixed resting potential level, and increased the maximum rate of rise of the Na spikes at a high resting potential level at which Na inactivation was completely depressed. Compared to Na spikes, Ca spikes were less sensitive to TTX and procaine. Ganglion cells were also capable of producing action potentials (Sr spikes) in an isotonic SrCl₂ solution and prolonged action potentials in an isotonic BaCl₂ solution, but these cells were rendered inexcitable in an isotonic MgCl₂ solution. The peak level of the Sr spikes was dependent on the external Sr concentration and was insensitive to both TTX and procaine. Sr ions, like Ca ions, reduced Na inactivation during the resting state, and depressed the maximum rate of rise of the Na spikes at a high resting potential level. It was concluded that Ca (and Sr) ions exert dual actions on the membrane; namely, regulating the Na permeability and acting as charge carriers during the active state of the membrane.     

Action of tetanus toxin on cholinergic neuroblastoma X glioma hybrid cells: selective blockade of Ca spikes

We examined the effect of tetanus toxin on clonal neuroblastoma X glioma hybrid cells, NG108-15, by intracellular microelectrode studies of passive membrane electrical properties and action potentials generated under various conditions. Binding of tetanus toxin to the surface of the cells was demonstrated by indirect immunofluorescent staining but no morphological alteration was observed in tetanus toxin-treated cells under a phase contrast microscope. These is no significant difference between the tetanus toxin-treated and untreated cells in their passive electrical membrane properties, i.e. resting membrane potentials, input resistances, time constants and input capacities. Cells in 120 mM Na+, 2 mM Ca2+ salt solution showed Na spikes, and cells in high Ca2+ (30 mM), Na+-free salt solution showed Ca spikes in response to depolarizing current pulses. While the Na spike was not affected by tetanus toxin, the Ca spike was blocked by the toxin. The minimum dose of tetanus toxin for maximum suppression of the peak potential level of the Ca spike was 250 ng/ml. Addition of tetraethyl ammonium (TEA) to extracellular fluid enhanced the Ca spike in untreated cells. In toxin-treated cells, TEA did not alter the effect of tetanus toxin on the Ca spike. Blockade of the Ca spike by tetanus toxin could be detected even at low extracellular Ca2+ concentration (10 mM) by adding TEA to the extracellular fluid and adjusting the membrane potential to a steady hyperpolarized level (-80 mV) to ensure optimal and uniform electrical responses. The usefulness of NG108-15 hybrid cells for in vitro investigations on the mechanism of action of tetanus toxin was discussed.     

After-Potentials and Large Depolarizations of Single Nodes of Ranvier Treated with Veratridine

Potential differences between normal nodes of Ranvier (single fiber from the sciatic nerve of the frog, air-gap method) and a node exposed to 1 to 2.5 x 10^-6 gm veratridine per ml were measured. Negative after-potentials occurred immediately after application of the alkaloid when spike configuration and resting potential were virtually unchanged. The after-potentials decreased in magnitude and their time constant increased as the resting membrane was depolarized either by outward currents or by a train of impulses. Increase of (Na)_o markedly increased the amplitude of the after-potential. After prolonged application of veratridine or with higher concentrations, a large slow depolarization (rate of potential change about 7 mv per second) could be triggered by a train of impulses or even a single spike. This depolarization could promptly be terminated by withdrawing Na. It is concluded that, once the nodal membrane has become permeable to Na (as during a spike), veratridine prevents the normal return of P_Na to its resting value.     

三叉神经中脑核神经元膜的电学特性

用大鼠脑干脑片,给三叉神经中脑核79个神经元作了细胞内记录,测算了20个神经元膜的电学特性:静息电位-60.3±5.6mV;输入阻抗为10.5±5.4MΩ;时间常数1.3±0.5ms。电刺激可诱发动作电位,测算32个神经元的有关参数:阈电位-50—-55mV;波幅69.5±6.1mV;超射11.9±3.6mV;波宽0.8±0.2ms。TTX(0.3μmol/L)或无钠使之消失。通以长时程矩形波电流可引起200—250Hz的2—15个重复放电,但在通电停止前终止,TEA或4-AP可延长放电。膜电位-60—-55mV时在动作电位之后可看到阈下电位波动,它不受TTX的影响,无钙时消失,TEA或4-AP使波幅增大。静息电位去极化可使45个神经元中的40个发生外向整流作用,并被TEA,4-AP或无钙抑制,超极化则发生内向整流作用,Cs或无钠抑制之。灌流液中加入各种钾通道阻断药时神经元的稳态I-V曲线发生相应变化,提示I_(DR),l_A,I_(K(Ca))及I_Q可能都与静息时的膜电导有关。     

Internally perfused Myxicola giant axons showing long-term survival.

An improved method for internally perfusing the Myxicola giant axon based on removing the axoplasm by dispersing it in KCl-KF salt solutions is described. Proteolytic enzymes are not introduced. With this improved method perfused preparations show long-term stability of their electrical properties and the ability to generate action potentials for many hours. Mean initial values for resting membrane potential, action potential amplitude, and peak inward current were -68 mV, 118 mV, and 3.62 mA/cm2, respectively. Mean resting membrane resistance was 75% of that in intact axons. In one series of voltage clamp experiments, perfused preparations remained excitable for a mean period of 5 1/2 h, but this period could exceed 10 h. 4 min are needed for exchange of internal solutions. At least 50 mM KF is required both in the axoplasm liquefying solution and in the standard perfusate to obtain stable preparations.     

An active electrical response in fibroblasts

L cells have a resting potential of about -16 mv (internal negative) at 37°C in Dulbecco''s modified Eagle''s medium containing 10% fetal calf serum and a potassium concentration of 5.4 mM. Membrane resistivity is about 20,000 Ωcm² when the surface filopodia described by others are taken into account. Mechanical and electrical stimuli can evoke an active response from mouse L cells, cells of the 3T3 line, and normal fibroblasts which we have termed hyperpolarizing activation or the H.A. response. This consists of a prolonged (3–5 sec) increase in the membrane permeability by a factor of 2–10 with a parallel increase in membrane potential to about -50 mv. The reversal potential for the H.A. response is -80 mv. The resting cells are depolarized to about -12 mv when the external medium contains 27 mM potassium, and the potential reached at the peak of the H.A. response is about -30 mv. The reversal potential for the H.A. response is about -40 mv in 27 mM external potassium. This effect of potassium ions on the reversal potential of the H.A. response leads us to conclude that the response represents an increase in membrane permeability, predominantly to potassium, by at least a factor of five. This increase must be greater than 20-fold if previous measurements of the ratio of potassium permeability to chloride permeability in L cells are valid for the preparation used in the present study.     

Effects of temperan a central synapse between identified motor neurons in the locust

Summary Changing the temperature from 10–40 °C modifies the transmission at an established monosynaptic connection between the fast extensor tibiae (FETi) and flexor tibiae motor neurons in the metathoracic ganglion of the locustSchistocerca gregaria (Forskål). Striking changes occur to the shape of the spikes, to membrane resistance, to the synaptic delay, and to the evoked synaptic potentials.In the presynaptic FETi motor neuron, raising the temperature reduces the amplitude of an antidromic spike recorded in the soma by a factor of 10 (40 mV to 4 mV), reduces the time taken to reach peak amplitude by 5 (3.5 to 0.7 ms) and decreases the duration at half maximum amplitude by 0.5. The conduction velocity of the spike in the axon is increased by 50% from 10 °C to 40 °C. Orthodromic spikes are affected by temperature in a similar way to the antidromic spikes.The membrane resistance of both pre- and postsynaptic motor neurons falls as the temperature is raised. The membrane resistance of FETi falls by a factor of 4 (about 4 M at 10 °C to 1 M at 40 °C). A contributory component to this fall could be the increase in the frequency of synaptic potentials generated as a result of inputs from other neurons. No temperature dependence could be demonstrated on the voltage threshold relative to resting potential for evoking orthodromic spikes, but because the resistance changes, the current needed to achieve this voltage must be increased at higher temperatures.The latency measured from the peak of the spike in the soma of FETi to the start of the EPSP in the soma of a flexor motor neuron decreases by a factor of 20 (10 ms at 10 °C to 0.5 ms at 40 °C).In a postsynaptic flexor tibiae motor neuron, the amplitude of the evoked synaptic potential increases by a factor of 3.4 (5 mV to 17 mV), its duration at half maximum amplitude decreases by 3 (7 ms at 12 °C to 2.3 ms at 32 °C) and its rate of rise increases by 3. An increased likelihood that spikes will occur in the flexor contributes to the enhanced amplitude of the compound EPSP at temperatures above 20 °C.Abbreviation FETi fast extensor tibiae motor neuron     

Calcium dependence of electrical and mechanical activity in rat ileum examined by the sucrose-gap technique

1. Single sucrose gap recordings showed that spontaneous action potentials of rat ileal smooth muscle consisted of slow waves and superimposed spikes which generated rhythmic contractions. As external potassium was raised, the resting potential progressively depolarized.2. Calcium-free salines inhibited spontaneous mechanical activity and inhibited the plateau phase of the action potential, but spontaneous spike depolarizations persisted.3. Verapamil, nifedipine and diltiazem all inhibited spontaneous mechanical activity and the plateau phase of the action potential, while in addition diltiazem augmented spike amplitude.4. Mn ions also inhibited mechanical activity and the action potential plateau, without affecting spike activity while the calcium ionophore A23187 enhanced both mechanical and electrical activity with a pronounced effect on spike amplitude.5. These results are consistent with the view that the plateau phase of the ileal smooth muscle action potential is dependent upon an influx of extracellular calcium possibly through voltage dependent slow calcium channels.     

Electrical properties of neurons recorded from the rat supraoptic nucleus in vitro

The electrical properties of neurons in the supraoptic nucleus (so.n.) have been studied in the hypothalamic slice preparation by intracellular and extracellular recording techniques, with Lucifer Yellow CH dye injection to mark the recording site as being the so.n. Intracellular recordings from so.n. neurons revealed them to have an average membrane potential of -67 +/- 0.8 mV (mean +/- s.e.m.), membrane resistance of 145 +/- 9 M omega with linear current-voltage relations from 40 mV in the hyperpolarizing direction to the level of spike threshold in the depolarizing direction. Average cell time constant was 14 +/- 2.2 ms. So.n. action potentials ranged in amplitude from 55 to 95 mV, with a mean of 76 +/- 2 mV, and a spike width of 2.6 +/- 0.5 ms at 30% of maximal spike height. Both single spikes and trains of spikes were followed by a strong, long-lasting hyperpolarization with a decay fitted by a single exponential having a time constant of 8.6 +/- 1.8 ms. Action potentials could be blocked by 10(-6) M tetrodotoxin. Spontaneously active so.n. neurons were characterized by synaptic input in the form of excitatory and inhibitory postsynaptic potentials, the latter being apparently blocked when 4 M KCl electrodes were used. Both forms of synaptic activity were blocked by application of divalent cations such as Mg2+, Mn2+ or Co2+. 74% of so.n. neurons fired spontaneously at rates exceeding 0.1 spikes per second, with a mean for all cells of 2.9 +/- 0.2 s-1. Of these cells, 21% fired slowly and continuously at 0.1 - 1.0 s-1, 45% fired continuously at greater than 1 Hz, and the remaining 34% fired phasically in bursts of activity followed by silence or low frequency firing. Spontaneously firing phasic cells showed a mean burst length of 16.7 +/- 4.5 s and a silent period of 28.2 +/- 4.2 s. Intracellular recordings revealed the presence of slow variations in membrane potential which modified the neuron's proximity to spike threshold, and controlled phasic firing. Variations in synaptic input were not observed to influence firing in phasic cells.