Direct Demonstration of a Growth-Induced Water Potential Gradient

When transpiration is negligible, water potentials in growing tissues are less than those in mature tissues and have been predicted to form gradients that move water into the enlarging cells. To determine directly whether the gradients exist, we measured water potentials along the radius of stems of intact soybean (Glycine max [L.] Merr.) seedlings growing in vermiculite in a water-saturated atmosphere. The measurements were made in individual cells by first determining the turgor with a miniature pressure probe, then determining the osmotic potential of solution from the same cell, and finally summing the two potentials. The osmotic potentials were corrected for sample mixing in the probe. The measurements were checked with a thermocouple psychrometer that gave average tissue water potentials. In the elongating region, the water potential was highest near the xylem and lowest near the epidermis and in the center of the pith. In the basal, more mature region of the same stems, water potentials were near zero next to the xylem and throughout the tissue. These basal potentials reflected mostly the potential of the xylem, which extended into the elongating tissues. Thus, the high basal potential confirmed the high potential near the xylem in the elongating tissues. The psychrometer measurements for each tissue gave average potentials that agreed with the average of the cell potentials from the pressure probe. We conclude that a radial gradient was present in the elongating region that formed a water potential field in three dimensions around the xylem and that confirmed the predictions of Molz and Boyer (F.J. Molz and J.S. Boyer [1978] Plant Physiol 62: 423-429).     

Spontaneous Action Potential Initiation and Propagation in Regenerating Cell Segments of Acetabularia mediterranea

Thavarungkul, P., Lertsithichai, S. and Sherlock, R. A. 1987.Spontaneous action potential initiation and propagation in regeneratingcell segments of Acetabularia mediterranea.—J. exp. Bot.38: 1541–1556. The spontaneous action potentials which occur during the regenerationof anucleate isolated stalk segments (ISS's) of the unicellularalga Acetabulana mediterranea have been studied using a novelnon-invasive technique. This involved measurement of spatialsamples of the time dependent potential in the external medium(sea water) and an inverse transformation to give the currentdistribution at the cell surface. The initiation region andsubsequent propagation (if any) of each action potential wasthen analysed from the computed transmembrane currents. Theresults showed that the occurrences of the spontaneous actionpotentials followed a rhythm which had a period of approximately24–30 h. These action potentials initiated more frequentlyat the ends of the ISS's than in the middle region. Our resultsshowed no firm correlation between the regions of action potentialinitiation and the site of the regenerating apex. Generallyboth propagating and non-propagating action potentials wereobserved in the same ISS, the ratio of which seems to dependon the length of the cell. The analysed initiation sites andthe propagating behaviour of the spontaneous action potentialgenerated by the ISS's during the regeneration process do notprovide any clear support to the symmetry breaking role of thepropagating action potentials as proposed by some workers. Key words: Acetabularia, regeneration, action potential     

Repolarization gradients in the intact heart: transmural or apico-basal?

Controversies regarding the genesis of the T wave in the electrocardiogram and the role of midmural M cells in the intact heart include: In normal, intact canine and human hearts there is no significant transmural gradient in repolarization times. The T wave results primarily from apico-basal differences in repolarization times. Also, in the intact heart there is no midmural region of prolonged action potential duration. This contrasts with isolated preparations, such as the wedge preparation or myocardial slices or disaggregated myocytes in which M cells, with action potentials longer than those of endocardial and epicardial myocardium, can be found. This disparity in action potential duration probably results from partial uncoupling of myocardial cells in the regions where measurements are made, e.g., the cut surface of a wedge preparation. In regions of a wedge where cellular coupling is normal, or in isolated myocardial bundles or sheets, no evidence for M cells is detected. In some wedge preparations, a drug-induced large transmural repolarization gradient, involving M cells, can lead to Torsade de Pointes, possibly caused by so-called phase two reentry. In contrast, when a gradient of repolarization times of more than 100?ms was created in intact hearts, no evidence for reentry was found and no spontaneous arrhythmias occurred. In conclusion, in the intact heart, M cells appear not to contribute to repolarization gradients and arrhythmias. Furthermore, no significant repolarization gradients between endocardium and epicardium exist. The T wave in the body surface electrocardiogram is caused by apico-basal and anterior-posterior differences in repolarization times.     

Initiation and abolition of the action potential of a single node of Ranvier

1. Using single node preparations of the bull frog or the toad, observations were made on the variation of the voltage across the nodal membrane under various experimental conditions. 2. The time constant of the variation in the membrane voltage caused by a long subthreshold rectangular pulse was of the order of 0.1 msec. 3. The action potential was initiated when the potential inside the node was raised stimulating pulses above a threshold level of approximately 15 mv. for a node in normal Ringer; it was greater in a relatively refractory node and in a partially narcotized node. 4. The variation of the membrane voltage caused by long stimulating pulses of subrheobasic strengths was in general proportional to the strength of the applied pulse. A non-linear behavior of the membrane voltage was observed with barely subthreshold stimulating pulses. 5. The early portion of the action potential of a node was not modified by a direct current which was strong enough to produce measurable potential changes (IR drops) across the resting membrane. 6. A strong pulse of inward current applied to the node during activity abolished the portion of the action potential following the pulse in all-or-none manner. 7. There was no refractory period after a response abolished in its early phase. Following a response abolished later, the recovery in the spike height started from the level of the action potential at the time of abolition. 8. Initiation and abolition of action potentials at a single node are interpreted as "transitions" between the two "equilibrium potential levels" at the node.     

Simple models of stimulation of neurones in the brain by electric fields

The excitation of pyramidal cells in the motor cortex, produced by electric fields generated by distant electrodes or by electromagnetic induction, has been modelled. Linear, steady-state models of myelinated axons capture most of the geometrical aspects of neurone activation in electric fields. Some non-linear features can be approximated. Models with a proximal sealed-end and distal infinite axon, or of finite length, are both serviceable. Surface anodal stimulation produces hyperpolarisation of the proximal axon (closest to the anode) and depolarisation in the distal axon. The point of maximum depolarisation can be influenced by the location of the cathode (greater separation of anode and cathode causes more distal depolarisation). Axon bends can produce very localised depolarisation. Cathodal stimulation may be less effective than anodal as a result of anodal block of conduction of action potentials in the distal axon. The latencies of responses to anodal stimulation, recorded in the distal axon, will decrease as the stimulus strength is increased and the point of action potential initiation moves distally node by node. Larger jumps in latency will be produced when the point of action potential initiation moves from one axon bend to another.     

Cell water potential,osmotic potential,and turgor in the epidermis and mesophyll of transpiring leaves

Water potential, osmotic potential and turgor measurements obtained by using a cell pressure probe together with a nanoliter osmometer were compared with measurements obtained with an isopiestic psychrometer. Both types of measurements were conducted in the mature region of Tradescantia virginiana L. leaves under non-transpiring conditions in the dark, and gave similar values of all potentials. This finding indicates that the pressure probe and the osmometer provide accurate measurements of turgor, osmotic potentials and water potentials. Because the pressure probe does not require long equilibration times and can measure turgor of single cells in intact plants, the pressure probe together with the osmometer was used to determine in-situ cell water potentials, osmotic potentials and turgor of epidermal and mesophyll cells of transpiring leaves as functions of stomatal aperture and xylem water potential. When the xylem water potential was-0.1 MPa, the stomatal aperture was at its maximum, but turgor of both epidermal and mesophyll cells was relatively low. As the xylem water potential decreased, the stomatal aperture became gradually smaller, whereas turgor of both epidermal and mesophyll cells first increased and afterward decreased. Water potentials of the mesophyll cells were always lower than those of the epidermal cells. These findings indicate that evaporation of water is mainly occurring from mesophyll cells and that peristomatal transpiration could be less important than it has been proposed previously, although peristomatal transpiration may be directly related to regulation of turgor in the guard cells.     

Propagation of action potentials in squid giant axons. Repetitive firing at regions of membrane inhomogeneities

Effects of reduction in potassium conductance on impulse conduction were studied in squid giant axons. Internal perfusion of axons with tetraethylammonium (TEA) ions reduces G K and causes the duration of action potential to be increased up to 300 ms. This prolongation of action potentials does not change their conduction velocity. The shape of these propagating action potentials is similar to membrane action potentials in TEA. Axons with regions of differing membrane potassium conductances are obtained by perfusing the axon trunk and one of its two main branches with TEA after the second branch has been filled with normal perfusing solution. Although the latter is initially free of TEA, this ion diffuses in slowly. Up until a large amount of TEA has diffused into the second branch, action potentials in the two branches have very different durations. During this period, membrane regions with prolonged action potentials are a source of depolarizing current for the other, and repetitive activity may be initiated at transitional regions. After a single stimulus in either axon region, interactions between action potentials of different durations usually led to rebound, or a short burst, of action potentials. Complex interactions between two axon regions whose action potentials have different durations resembles electric activity recorded during some cardiac arrhythmias.     

Factors and processes involved in membrane potential build-up in yeast: diS-C3(3) assay.

No methods are currently available for fully reliable monitoring of membrane potential changes in suspensions of walled cells such as yeast. Our method using the Nernstian cyanine probe diS-C3(3) monitors even relatively fast changes in membrane potential delta psi by recording the shifts of probe fluorescence maximum lambda max consequent on delta psi-dependent probe uptake into, or exit from, the cells. Both increased [K+]out and decreased pHout, but not external NaCl or choline chloride depolarise the membrane. The major ion species contributing to the diS-C3(3)-reported membrane potential in S. cerevisiae are thus K+ and H+, whereas Na+ and Cl- do not perceptibly contribute to measured delta psi. The strongly pHout-dependent depolarisation caused by the protonophores CCCP and FCCP, lack of effect of the respiratory chain inhibitors rotenone and HQNO on the delta psi, as well as results obtained with a respiration-deficient rho- mutant show that the major component of the diS-C3(3)-reported membrane potential is the delta psi formed on the plasma membrane while mitochondrial potential forms a minor part of the delta psi. Its role may be reflected in the slight depolarisation caused by the F1F0-ATPase inhibitor azide in both rho- mutant and wildtype cells. Blocking the plasma membrane H(+)-ATPase with the DMM-11 inhibitor showed that the enzyme participates in delta psi build-up both in the absence and in the presence of added glucose. Pore-forming agents such as nystatin cause a fast probe entry into the cells signifying membrane damage and extensive binding of the probe to cell constituents reflecting obviously disruption of ionic balance in permeabilised cells. In damaged cells the probe therefore no longer reports on membrane potential but on loss of membrane integrity. The delta psi-independent probe entry signalling membrane damage can be distinguished from the potential-dependent diS-C3(3) uptake into intact cells by being insensitive to the depolarising action of CCCP.     

Mathematical models of action potentials in the periphery and center of the rabbit sinoatrial node

Mathematical models of the action potential in the periphery and center of the rabbit sinoatrial (SA) node have been developed on the basis of published experimental data. Simulated action potentials are consistent with those recorded experimentally: the model-generated peripheral action potential has a more negative takeoff potential, faster upstroke, more positive peak value, prominent phase 1 repolarization, greater amplitude, shorter duration, and more negative maximum diastolic potential than the model-generated central action potential. In addition, the model peripheral cell shows faster pacemaking. The models behave qualitatively the same as tissue from the periphery and center of the SA node in response to block of tetrodotoxin-sensitive Na(+) current, L- and T-type Ca(2+) currents, 4-aminopyridine-sensitive transient outward current, rapid and slow delayed rectifying K(+) currents, and hyperpolarization-activated current. A one-dimensional model of a string of SA node tissue, incorporating regional heterogeneity, coupled to a string of atrial tissue has been constructed to simulate the behavior of the intact SA node. In the one-dimensional model, the spontaneous action potential initiated in the center propagates to the periphery at approximately 0.06 m/s and then into the atrial muscle at 0.62 m/s.     

Electrical properties of motoneurons in the spinal cord of rat embryos

Electrical properties of immature motoneurons were studied in vitro using isolated segments of spinal cords of rat embryos aged 14-21 days of gestation. Stable resting potentials and evoked synaptic potentials were recorded for more than 9 hr, indicating that motoneurons remain viable for many hours. Motoneurons are electrically excitable at 14 days of gestation and from the onset of excitability the action potentials are Na+-dependent but slow rising long-duration Ca2+-dependent action potentials can be evoked if K+ conductance is reduced. Thus, during embryonic development the regenerative potential inward current is Na+-and Ca2+-dependent. During motoneurons' differentiation there are some changes in their electrical properties: resting membrane potential increases, input resistance decreases, input capacitance increases, threshold for action potential decreases, and maximum rate of rise of action potential increases. Afferent motoneuron contacts are formed at 16-18 days of gestation when excitatory synaptic potentials can first be evoked in response to dorsal root stimulation. The changes in input capacitance and threshold for action potential occur at the onset of functional afferent motoneuron contacts, but it is not known whether these changes are autonomous or are influenced by the newly formed sensory inputs.     

Nicotine-induced and depolarisation-induced glutamate release from hippocampus mossy fibre synaptosomes: two distinct mechanisms

Hippocampus mossy fibre terminals activate CA3 pyramidal neurons via two distinct mechanisms, both quantal and glutamatergic: (i) rapid excitatory transmission in response to afferent action potentials and (ii) delayed and prolonged release following nicotinic receptor activation. These processes were analysed here using rat hippocampus mossy fibres synaptosomes. The relationships between synaptosome depolarisation and glutamate release were established in response to high-KCl and gramicidin challenges. Half-maximal release corresponded to a 52 mV depolarisation step. KCl-induced release was accompanied by transient dissipation of the proton gradient across synaptic vesicle membrane. Nicotine elicited a substantial glutamate release from mossy fibre synaptosomes (EC₅₀ 3.14 μM; V _max 12.01 ± 2.1 nmol glutamate/mg protein; Hill's coefficient 0.99). However, nicotine-induced glutamate release was not accompanied by any change in the membrane potential or in the vesicular proton gradient. The effects of acetylcholine (200 μM) were similar to those of nicotine (25 μM). Nicotinic α7 receptors were evidenced by immuno-cytochemistry on the mossy fibre synaptosome plasma membrane. Therefore, the same terminals can release glutamate in response to two distinct stimuli: (i) rapid neurotransmission involving depolarisation-induced activation of voltage-gated Ca²⁺ channels and (ii) a slower nicotinic activation which does not involve depolarisation or dissipation of the vesicular proton gradient.     

Action potentials and chemosensitive conductances in the dendrites of olfactory neurons suggest new features for odor transduction

Odors affect the excitability of an olfactory neuron by altering membrane conductances at the ciliated end of a single, long dendrite. One mechanism to increase the sensitivity of olfactory neurons to odorants would be for their dendrites to support action potentials. We show for the first time that isolated olfactory dendrites from the mudpuppy Necturus maculosus contain a high density of voltage-activated Na+ channels and produce Na-dependent action potentials in response to depolarizing current pulses. Furthermore, all required steps in the transduction process beginning with odor detection and culminating with action potential initiation occur in the ciliated dendrite. We have previously shown that odors can modulate Cl- and K+ conductances in intact olfactory neurons, producing both excitation and inhibition. Here we show that both conductances are also present in the isolated, ciliated dendrite near the site of odor binding, that they are modulated by odors, and that they affect neuronal excitability. Voltage- activated Cl- currents blocked by 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid and niflumic acid were found at greater than five times higher average density in the ciliated dendrite than in the soma, whereas voltage-activated K+ currents inhibited by intracellular Cs+ were distributed on average more uniformly throughout the cell. When ciliated, chemosensitive dendrites were stimulated with the odorant taurine, the responses were similar to those seen in intact cells: Cl- currents were increased in some dendrites, whereas in others Cl- or K+ currents were decreased, and responses washed out during whole-cell recording. The Cl- equilibrium potential for intact neurons bathed in physiological saline was found to be -45 mV using an on-cell voltage- ramp protocol and delayed application of channel blockers. We postulate that transduction of some odors is caused by second messenger-mediated modulation of the resting membrane conductance (as opposed to a specialized generator conductance) in the cilia or apical region of the dendrite, and show how this could alter the firing frequency of olfactory neurons.     

Outer acrosomal membrane of guinea pig spermatozoa: Isolation and structural characterization

We describe a protocol to isolate a highly enriched fraction of outer acrosomal membrane from guinea pig spermatozoa and present new data on the ultrastructure of this membrane domain. Cauda epididymal spermatozoa were suspended into a low ionic strength buffer and subjected to brief homogenization; this stripped the plasma membrane from the spermatozoa and severed the acrosomal apical segment from the spermatozoon. The crescent-shaped apical segments retained the outer acrosomal membrane and specific components of the acrosomal matrix. Enriched fractions of apical segments were isolated on discontinuous sucrose gradients and the outer acrosomal membrane purified by subsequent centrifugation onto Percoll density gradients. The isolated outer acrosomal membrane did not form vesicles, but instead rolled up into spiral sheets. Both thin section and negatively stained specimens revealed a paracrystalline arrangement of filaments associated with the luminal surface of the membrane. The isolated outer acrosomal membrane revealed a limited number of polypeptides by SDS-PAGE, and the polypeptide pattern was distinct from the plasma membrane fraction. The isolated acrosomal membranes possessed no oubain sensitive Na+, K+-ATPase activity, whereas about 20% of the ATPase activity of the plasma membrane enriched fraction was inhibited by oubain. The potential function of the structural differentiations of the outer acrosomal membrane in the membrane fusion events of the acrosome reaction is discussed.     

Changes in membrane potential during calcium ion influx and efflux across the mitochondrial membrane.

1. A depolarisation of the membrane of rat liver mitochondria, as measured with the safranine method, is seen during Ca2+ uptake. The depolarisation is followed by a slow repolarisation, the rate of which can be increased by the addition of EGTA or phosphate. 2. Plots relating the initial rate of calcium ion (Ca2+) uptake and the decrease in membrane potential (delta psi) to the Ca2+ concentration show a half-maximal change at less than 10 micron Ca2+ and a saturation above 20 micron Ca2+. 3. Plots relating the initial rate of Ca2+ uptake to delta psi are linear. 4. Addition of Ca2+ chelators, nitriloacetate or EGTA, to deenergized mitochondria equilibrated with Ca2+ causes a polarisation of the mitochondrial membrane due to a diffusion potential created by electrogenic Ca2+ efflux. 5. If the extent of the response induced by different nitriloacetate concentrations is plotted against the expected membrane potential a linear plot is obtained up to 70 mV with a slope corresponding to two-times the extent of the response induced by valinomycin in the presence of different potassium ion gradients. This suggests that the Ca2+ ion is transferred across the membrane with one net positive charge in present conditions.     

Single-molecule investigation of the interactions between reconstituted planar lipid membranes and an analogue of the HP(2-20) antimicrobial peptide

In this study, we employed electrophysiology experiments carried out at the single-molecule level to study the mechanism of action of the HPA3 peptide, an analogue of the linear antimicrobial peptide, HP(2–20), isolated from the N-terminal region of the Helicobacter pylori ribosomal protein. Amplitude analysis of currents fluctuations induced by HPA3 peptide at various potentials in zwitterionic lipid membranes reveal the existence of reproducible conductive states in the stochastic behavior of such events, which directly supports the existence of transmembrane pores induced the peptide. From our data recorded both at the single-pore and macroscopic levels, we propose that the HPA3 pore formation is electrophoretically facilitated by trans-negative transmembrane potentials, and HPA3 peptides translocate into the trans monolayers after forming the pores. We present evidence according to which the decrease in the membrane dipole potential of a reconstituted lipid membranes leads to an augmentation of the membrane activity of HPA3 peptides, and propose that a lower electric dipole field of the interfacial region of the membrane caused by phloretin facilitates the surface-bound HPA3 peptides to break free from one leaflet of the membrane, insert into the membrane and contribute to pore formation spanning the entire thickness of the membrane.     

Pressure probe and isopiestic psychrometer measure similar turgor

Turgor measured with a miniature pressure probe was compared to that measured with an isopiestic thermocouple psychrometer in mature regions of soybean (Glycine max [L.] Merr.) stems. The probe measured turgor directly in cells of intact stems whereas the psychrometer measured the water potential and osmotic potential of excised stem segments and turgor was calculated by difference. When care was taken to prevent dehydration when working with the pressure probe, and diffusive resistance and dilution errors with the psychrometer, both methods gave similar values of turgor whether the plants were dehydrating or rehydrating. This finding, together with the previously demonstrated similarity in turgor measured with the isopiestic psychrometer and a pressure chamber, indicates that the pressure probe provides accurate measurements of turgor despite the need to penetrate the cell. On the other hand, it suggests that as long as precautions are taken to obtain accurate values for the water potential and osmotic potential, turgor can be determined by isopiestic psychrometry in tissues not accessible to the pressure probe for physical reasons.     

Comparison between gradient-dependent hydraulic conductivities of roots using the root pressure probe: the role of pressure propagations and implications for the relative roles of parallel radial pathways

Hydrostatic pressure relaxations with the root pressure probe are commonly used for measuring the hydraulic conductivity (Lp(r)) of roots. We compared the Lp(r) of roots from species with different root hydraulic properties (Lupinus angustifolius L. 'Merrit', Lupinus luteus L. 'Wodjil', Triticum aestivum L. 'Kulin' and Zea mays L. 'Pacific DK 477') using pressure relaxations, a pressure clamp and osmotic gradients to induce water flow across the root. Only the pressure clamp measures water flow under steady-state conditions. Lp(r) determined by pressure relaxations was two- to threefold greater than Lp(r) from pressure clamps and was independent of the direction of water flow. Lp(r) (pressure clamp) was two- to fourfold higher than for Lp(r) (osmotic) for all species except Triticum aestivum where Lp(r) (pressure clamp) and Lp(r) (osmotic) were not significantly different. A novel technique was developed to measure the propagation of pressure through roots to investigate the cause of the differences in Lp(r). Root segments were connected between two pressure probes so that when root pressure (P(r)) was manipulated by one probe, the other probe recorded changes in P(r). Pressure relaxations did not induce the expected kinetics in pressure in the probe at the other end of the root when axial hydraulic conductance, and probe and root capacitances were accounted for. An electric circuit model of the root was constructed that included an additional capacitance in the root loaded by a series of resistances. This accounted for the double exponential kinetics for intact roots in pressure relaxation experiments as well as the reduced response observed with the double probe experiments. Although there were potential errors with all the techniques, we considered that the measurement of Lp(r) using the pressure clamp was the most unambiguous for small pressure changes, and provided that sufficient time was allowed for pressure propagation through the root. The differences in Lp(r) from different methods of measurement have implications for the models describing water transport through roots and the potential role of aquaporins.     

Measurement of diffusion potentials in liposomes. Origin and properties of the threshold level in the oxonol VI response

A model is presented for the response of the membrane potential probe oxonol VI on diffusion potentials in liposomes. In this model the dependence of the probe response on the initial ion gradient is explained in terms of internal volume, internal ion concentration, membrane capacity and initial membrane potential. It is found that in the presence of an initial membrane potential (positive outside) there is a threshold value of the ion gradient needed for a probe response, which increases when the internal volume or the internal ion concentration decrease. The model is confirmed by experiments with liposomes of different sizes and internal KCl concentrations, prepared from asolectin or lipids isolated from the thermophilic cyanobacterium Synechococcus 6716. The significance of the model for threshold values observed in other energy-dependent phenomena is discussed.     

Proton flux in large unilamellar vesicles in response to membrane potentials and pH gradients.

The transport of protons across liposomes composed of phosphatidylcholine in response to electrical potentials or pH gradients has been investigated. The results support three major conclusions. The first of these concerns the need for reliable measurements of electrical potentials and pH gradients. It is shown that the potential probe tetraphenylphosphonium and the pH probe methylamine provide accurate and self consistent measures of electrical potentials and pH gradients respectively in these systems. Second, it is shown by two independent techniques that the pH gradients induced in response to valinomycin and potassium dependent electrical potentials are significantly smaller than would be expected for electrochemical equilibrium. The pH gradients observed are stable over an 8 h time course and are sensitive to the ionic composition of the buffers employed, where the presence of external sodium results in the smallest induced pH gradients. These results are discussed in terms of current models of proton conductance across membranes. In a final area of investigation, it is shown that valinomycin and carbonyl cyanide m-chlorophenyl hydrazone (CCCP) can transport sodium ions in a synergistic manner.     

Effects of Temperature on the Generator and Action Potentials of a Sense Organ

Charge transfer through the receptor membrane of the nonmyelinated ending of Pacinian corpuscles is markedly affected by temperature. The rate of rise and the amplitude of the generator potential in response to a constant mechanical stimulus increase with temperature coefficients of 2.5 and 2.0 respectively. The duration of the falling phase, presumably a purely passive component, and the rise time of the generator potential are but little affected by temperature. The following interpretation is offered: Mechanical stimulation causes the conductance of the receptor membrane to increase and ions to flow along their electrochemical gradients. An energy barrier of about 16,000 cal/mole limits the conductance change. The latter increases, thus, steeply with temperature, causing both the rate of rise and the intensity of the generator current to increase. The membrane of the adjacent Ranvier node behaves in a distinctly different manner. The amplitude of the nodal action potential is little changed over a wide range of temperature, while the durations of its rising and falling phases increase markedly. The electrical threshold of the nodal membrane is rather constant between 40 and 12°C. Below 12°C the threshold rises, and the mechanically elicited generator current fails to meet the threshold requirements of the first node. Cold block of nerve impulse initiation then ensues, although the receptor membrane still continues to produce generator potentials in response to mechanical stimulation.