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PubMed1995

The effects of clamping and reoxygenation on repopulation during fractionated irradiation.

PURPOSE: Clamping of solid tumors is often used during radiobiological experiments to eliminate reoxygenation and hypoxia as confounding variables. The appropriateness of this procedure has been questioned recently. METHODS AND MATERIALS: This article describes experiments designed to examine the tumor surviving fraction (TSF) during fractionated treatment (up to 9 x 2 Gy, every 6 h or 24 h) given under normal, unclamped conditions (aerobic), and to compare it to TSF values for treatment given under clamped conditions (anoxic). The dose per fraction for the clamped procedure was 5.5 Gy, approximately equivalent to the unclamped doses of 2 Gy (based on an assumed OER of 2.75). RESULTS: Overall, the survival in the unclamped experiments was greater than in the clamped experiments. When the contribution of repopulation to this phenomenon was analyzed, no significant difference was found for four (KHT-C, KHT-LP1, RIF-1, B16F1) of five transplantable murine tumor cell lines used. However, for the SCC-VII squamous cell carcinoma, following small numbers of fractions there appeared to be more repopulation during the aerobic treatment schedule. Reoxygenation during the treatment was also analyzed, and the results indicate that, even with 24 h between doses, reoxygenation was insufficient to prevent an increasing hypoxic fraction during the treatment, which can explain the enhanced survival seen for the aerobic vs. the anoxic irradiation conditions. CONCLUSION: With the fractionation schedules used in this study, there was no evidence for a difference in repopulation kinetics when the treatment was given to clamped (anoxic) or unclamped (aerobic) tumors. In all five tumors studied, reoxygenation between the 2 Gy fractions was insufficient to prevent hypoxia from affecting the response of the tumors to the treatment.

Animals↗
PubMed1989

Effects of nifedipine on the hemodynamic response to clamping and declamping of the abdominal aorta in dogs.

Clamping and declamping of the infrarenal abdominal aorta may adversely affect cardiovascular function, particularly in the presence of heart disease. This effect may be further altered by drugs used in the treatment of symptomatic coronary artery disease. The effect of nifedipine on the hemodynamic response to aortic clamping and declamping was determined in 12 dogs anesthetized with 50% nitrous oxide and 0.6% end-tidal isoflurane and monitored with aortic, left ventricular (LV), and thermodilution pulmonary artery catheters. Six dogs received a nifedipine bolus of 100 micrograms/kg followed by an infusion of 4 micrograms/kg/min. Six dogs did not receive any nifedipine and served as controls. Before clamping, nifedipine produced immediate decreases in arterial pressure, systemic vascular resistance (SVR), and LV dP/dt, and a modest increase in cardiac output (CO). During aortic clamping, nifedipine-treated dogs demonstrated marked increases in heart rate (HR), dP/dt, and CO while maintaining a low SVR. There were no significant changes upon declamping. The nifedipine-treated animals maintained a high CO and low SVR. Thus, nifedipine greatly altered the hemodynamic responses to aortic clamping and declamping. Awareness of these alterations is important when caring for patients being treated with nifedipine who are undergoing aortic surgery.

Analysis of Variance↗
PubMed1990

Comparison of bretylium and lidocaine in the prevention of ventricular fibrillation after aortic cross-clamp release in coronary artery bypass surgery.

The authors compared bretylium and lidocaine for reducing the incidence and persistence of ventricular fibrillation following aortic cross-clamp release performed during coronary artery bypass surgery. Thirty-three adult patients scheduled for elective bypass surgery were randomly assigned in a double-blind fashion to receive a bolus of bretylium, 10 mg/kg, lidocaine, 2 mg/kg, or saline, in equal volumes prior to the release of the aortic cross-clamp. Coronary artery bypass surgery was conducted using standard cardiopulmonary bypass (CPB) procedures with systemic cooling to 24 degrees to 28 degrees C. Temperature, arterial blood gases, and electrolytes were recorded. After clamp release, the first electrical rhythm was noted. Abnormal rhythms (ventricular fibrillation) were allowed to persist for 1 to 2 minutes, and if spontaneous conversion to a supraventricular rhythm did not occur, defibrillation with internal DC countershocks was applied. Patients were compared with respect to occurrence of ventricular fibrillation, need for DC countershocks, antiarrhythmic drugs, and inotropic support. There was no significant difference among the groups with respect to age, sex, preoperative medications, past medical histories, ejection fractions, average number of bypasses, cross-clamp times, or temperatures during bypass. The incidence of ventricular fibrillation after aortic cross-clamp removal was: saline 91%, lidocaine 64% (P less than 0.01), and bretylium 36% (P less than 0.01). The number of countershocks required to defibrillate, while lower in the bretylium group, did not reach statistical significance. After cardiopulmonary bypass, cardiac output and systemic vascular resistance were comparable. Bretylium warrants further study in this setting.

Aged↗
PubMed2004

Identification of specific amino acid residues in the E. coli beta processivity clamp involved in interactions with DNA polymerase III, UmuD and UmuD'.

Variants of a pentapeptide sequence (QL[S/F]LF), referred to as the eubacterial clamp-binding motif, appear to be required for certain proteins to bind specifically to the Escherichia coli beta sliding clamp, apparently by making contact with a hydrophobic pocket located at the base of the C-terminal tail of each beta protomer. Although both UmuC (DNA pol V) and the alpha catalytic subunit of DNA polymerase III (pol III) each bear a reasonable match to this motif, which appears to be required for their respective interactions with the clamp, neither UmuD not UmuD' do. As part of an ongoing effort to understand how interactions involving the different E. coli umuDC gene products and components of DNA polymerase III help to coordinate DNA replication with a DNA damage checkpoint control and translesion DNA synthesis (TLS) following DNA damage, we characterized the surfaces on beta important for its interactions with the two forms of the umuD gene product. We also characterized the surface of beta important for its interaction with the alpha catalytic subunit of pol III. Our results indicate that although UmuD, UmuD' and alpha share some common contacts with beta, each also makes unique contacts with the clamp. These findings suggest that differential interactions of UmuD and UmuD' with beta impose a DNA damage-responsive conditionality on how beta interacts with the translesion DNA polymerase UmuC. This is formally analogous to how post-translational modification of the eukaryotic PCNA clamp influences mutagenesis. We discuss the implications of our findings in terms of how E. coli might coordinate the actions of the umuDC gene products with those of pol III, as well as for how organisms in general might manage the actions of their multiple DNA polymerases.

Amino Acids↗
PubMedDate not supplied

Dial 9-1-1 for DNA damage: the Rad9-Hus1-Rad1 (9-1-1) clamp complex.

Genotoxic stress activates checkpoint signaling pathways that block cell cycle progression, trigger apoptosis, and regulate DNA repair. Studies in yeast and humans have shown that Rad9, Hus1, Rad1, and Rad17 play key roles in checkpoint activation. Three of these proteins-Rad9, Hus1, and Rad1-interact in a heterotrimeric complex (dubbed the 9-1-1 complex), which resembles a PCNA-like sliding clamp, whereas Rad17 is part of a clamp-loading complex that is related to the PCNA clamp loader, replication factor-C (RFC). In response to genotoxic damage, the 9-1-1 complex is loaded around DNA by the Rad17-containing clamp loader. The DNA-bound 9-1-1 complex then facilitates ATR-mediated phosphorylation and activation of Chk1, a protein kinase that regulates S-phase progression, G2/M arrest, and replication fork stabilization. In addition to its role in checkpoint activation, accumulating evidence suggests that the 9-1-1 complex also participates in DNA repair. Taken together, these findings suggest that the 9-1-1 clamp is a multifunctional complex that is loaded onto DNA at sites of damage, where it coordinates checkpoint activation and DNA repair.

Animals↗
PubMed2005

DNA polymerase clamp loaders and DNA recognition.

Clamp loaders are heteropentameric ATPase assemblies that load sliding clamps onto DNA and are critical for processive DNA replication. The DNA targets for clamp loading are double-stranded/single-stranded junctions with recessed 3' ends (primer-template junctions). Here, we briefly review the crystal structures of clamp loader complexes and the insights they have provided into the mechanism of the clamp loading process.

Adenosine Triphosphatases↗
PubMed2004

The clamp-loader-helicase interaction in Bacillus. Atomic force microscopy reveals the structural organisation of the DnaB-tau complex in Bacillus.

The clamp-loader-helicase interaction is an important feature of the replisome. Although significant biochemical and structural work has been carried out on the clamp-loader-clamp-DNA polymerase alpha interactions in Escherichia coli, the clamp-loader-helicase interaction is poorly understood by comparison. The tau subunit of the clamp-loader mediates the interaction with DnaB. We have recently characterised this interaction in the Bacillus system and established a tau(5)-DnaB(6) stoichiometry. Here, we have obtained atomic force microscopy images of the tau-DnaB complex that reveal the first structural insight into its architecture. We show that despite the reported absence of the shorter gamma version in Bacillus, tau has a domain organisation similar to its E.coli counterpart and possesses an equivalent C-terminal domain that interacts with DnaB. The interaction interface of DnaB is also localised in its C-terminal domain. The combined data contribute towards our understanding of the bacterial replisome.

Adenosine Triphosphatases↗
PubMed2006

Clamping the supra-celiac aorta can effectively increase the success rate of orthotopic rat liver transplantation by increasing the tolerable time of the anhepatic phase.

BACKGROUND: Although using a Kamada model with a cuff approach for vascular anastomosis shortens the anhepatic time in orthotopic rat liver transplantation (ORLT), cardiac arrest is still often encountered during the anhepatic phase. We hypothesize that low cardiac output is the reason for this. In this study, we used the supra-celiac aorta clamp (SCAC) method during the anhepatic phase in ORLT to evaluate the success rate. MATERIALS AND METHODS: Spraque-Dawley rats were used in this the study. First, we tested the time between the clamping of the supra-hepatic inferior vena cava (IVC) clamped and cardiac arrest in between two groups according to whether SCAC was performed (n = 40) or not (n = 20). Clamping of the supra-hepatic IVC was performed 30 s or 1 min after SCAC (n = 20 for each) in the SCAC group. Second, we performed the Kamada method for ORLT in two groups according to whether SCAC was performed (n = 20, 1-min SCAC) or not (n = 20). Successful ORLT was defined as a recipient rat surviving more then 5 days. RESULTS: The mean time between IVC clamping and cardiac arrest was longer in the group of 1-min SCAC group than in the 30-s SCAC group or without SCAC (19.8 versus 11.45 versus 3.87 min, respectively, P < 0.001). The success rate of ORLT was higher in the SCAC group than in the group without SCAC (85% versus 15%, respectively, P < 0.001). CONCLUSIONS: SCAC can effectively increase the success rate of ORLT by increasing the tolerable time during the anhepatic phase. The reason for this is the avoidance of hypovolumic cardiac arrest during the anhepatic phase.

Animals↗
PubMed2005

Sharp liver transection versus clamp crushing technique in liver resections: a prospective study.

BACKGROUND: Parenchymal liver transection constitutes an important phase of liver resection. Serious intraoperative bleeding, together with injuries to vital structures of the liver remnant, can occur during this stage. A method of sharp liver parenchymal transection with scalpel is compared in a prospective randomized manner with the widely used clamp crushing technique. METHODS: Patients scheduled for hepatectomy under selective hepatic vascular exclusion (N = 82) were allocated randomly to either the sharp transection group (n = 41) or the clamp crushing group (n = 41). Warm ischemic time, blood loss and transfusions, postoperative morbidity and mortality, and tumor-free margins were recorded in both groups and analyzed. RESULTS: When the sharp transection group was compared with the clamp crushing group, the two groups were similar in warm ischemic time (median 36 vs 34 minutes), total operative time (median 205 vs 211 minutes), intraoperative blood loss (median 500 vs 460 mL), blood transfusion requirements (median value 0 in both groups), and overall complication rate (44% vs 39%). However, sharp transection yielded better tumor-free margins compared with the clamp crushing technique (12 +/- 1.4 mm vs 8 +/- 1.5 mm, mean +/- SD, P < .05). CONCLUSION: Sharp liver parenchymal transection with a scalpel is equally safe in terms of blood loss and mortality compared with the clamp crushing method. Although it is a technically demanding method, requiring selective hepatic vascular occlusion, it may be recommended when the tumor-free margins are anticipated to be narrow.

Adolescent↗
PubMed2004

Enhanced renal cryoablation with hilar clamping and intrarenal cooling in a porcine model.

OBJECTIVES: To evaluate the effects of renal vascular control and intrarenal cooling on the size of renal lesions attainable with a 3.4-mm cryoprobe. METHODS: Three groups of pigs underwent unilateral laparoscopic renal cryoablation with a 3.4-mm cryoprobe inserted to a depth of 1 cm. An 8-minute double-freeze cycle was used. One week later, an acute contralateral cryolesion was created before killing the animal. In group 1 (n = 6), bilateral cryolesions were created without hilar clamping or intrarenal cooling. In group 2 (n = 6), the cryolesions were created after hilar clamping alone. In group 3 (n = 6), the cryolesions were created after both hilar clamping and application of intrarenal cooling with saline ice-slush infused into the renal pelvis. After nephrectomy, the gross diameters were determined for each cryolesion. The mean diameters of the zones of complete and partial necrosis were determined by histopathologic examination. RESULTS: In group 3, the cortex cooled from 36.9 degrees C to a mean of 24.8 degrees C. Acutely, no statistically significant difference was found between the lesions produced with clamping alone (37.6 mm) and intrarenal cooling (40.4 mm); however, both were significantly larger than the control cryolesions (28.7 mm). At 1 week, the area of complete necrosis produced with intrarenal cooling (34.3 mm) was significantly larger than the areas of necrosis produced by clamping alone (27.8 mm) or conventional cryoablation (23.9 mm; alpha = 0.05, Tukey's honestly significantly different [HSD] test). CONCLUSIONS: Enhanced cryolesion necrosis was achieved with intrarenal cooling with a 3.4-mm cryoprobe. Intrarenal cooling may be a valuable adjunct to cryoablation in selected cases.

Animals↗
PubMed1999

[Selective use of vascular clamps in major hepatectomy].

OBJECTIVE: To report the results of a selective use of vascular occlusions in major hepatectomies according to the size and location of the hepatic lesion. BACKGROUND: Total vascular exclusion (TVE) and portal triad clamping (PTC) ensure efficient hemostatic effect but lead to warm ischemia of the liver. Lobar vascular occlusion (LVO) avoids warm ischemia of the remnant liver but could result in increased blood loss. PATIENTS AND METHODS: Sixty consecutive major hepatectomies were studied. TVE was applied in 22 patients with large lesions (= 10 cm) or lesions with connections to the major hepatic veins or inferior vena cava. PTC (n = 15) and LVO (n = 23) were applied in remaining cases. RESULTS: Clamping method was efficient in 87%, 93% and 100% for LVO, PTC and TVE, respectively. Median blood transfusions were 0.3 and 2 units for LVO, PTC and TVE, respectively. Postoperative aminotransferase peak value was significantly lower after LVO than after PTC or TVE, while those peaks were not statistically different with these latter two methods. Postoperative prothrombin time fall value was identical in the three groups. Mortality was 3.3% (2/60) and was not influenced by the type of clamping, but both deaths and most complications occurred in patients with abnormal underlying liver parenchyma. CONCLUSION: Provided that adequate techniques are used, the need for blood transfusions is more dependent on the characteristics of the resected tumor than on the type of clamping used. Total vascular exclusion does not create more ischemic injury to the liver than portal triad clamping and it should be recommended for the resection of large or strategically located tumors. Other tumors can be resected in more than 80% of the cases with LVO, thus avoiding ischemia to the remnant liver. With the control of hemorrhage, pathology of underlying liver parenchyma has emerged as the main prognostic factor in major liver resections.

Blood Loss, Surgical↗
PubMed2000

Capture of particulate emboli during cardiac procedures in which aortic cross-clamp is used. International Council of Emboli Management Study Group.

BACKGROUND: Previous studies have shown that atheroemboli are associated with neurologic complications following cardiac operation. Additionally, it has been demonstrated that embolization is closely related to the application and removal of ascending aortic cross-clamps. METHODS: A prospective registry of 304 patients was initiated to describe patient selection, procedures, particulate capture, and clinical outcomes after intraaortic filtration in patients undergoing cardiac surgical procedures with cardiopulmonary bypass and median sternotomy. Prior to the removal of the cross-clamp, the intraaortic filter (EMBOL-X Inc, Mountain View, CA) was deployed and left in place until the patient was weaned from extracorporeal circulation. Upon removal, filters were fixed in formalin and shipped to a core laboratory for examination. RESULTS: Mean patient age was 68 years (range 25 to 88 years), 40% had ascending aortic calcification, 59% of the procedures were coronary artery bypass grafting (CABG), 20% were valve replacement or repair, and 12% were a combination of CABG and valve surgery. Aortic cross-clamps were used in nearly all cases (302 of 304 patients); partial clamps were used in 84% of the CABG procedures. To date, 243 filters have been examined. Sixty-two percent of the filters analyzed revealed fibrous atheroma, 2% grumous atheroma, and 6% epiaortic debris. Platelet and fibrin strands were found in 52% of the filters and 22% contained evidence of thrombus or red blood cells. CONCLUSIONS: These findings from the International Council of Emboli Management Registry confirm that particulates are released during cardiac surgical procedures using the aortic cross-clamp. Continued observational and randomized studies are necessary to confirm the clinical relevance of particulate extraction.

Adult↗
PubMed1997

Effect of spinal cord preconditioning on paraplegia during cross-clamping of the thoracic aorta.

BACKGROUND: Paraplegia is a devastating complication of operations for thoracic or thoracoabdominal aneurysms. Preconditioning the brain with sublethal ischemia induces resistance to subsequent ordinarily lethal ischemia (ischemic tolerance). We investigated whether ischemic tolerance could be induced by preconditioning canine spinal cord. The role of heat-shock proteins (HSP) in this process was investigated. METHODS: In experiment 1, the preconditioning group (n = 6) had aortic cross-clamping for 20 minutes, whereas controls (n = 6) had no cross-clamping. After 48 hours the aorta was cross-clamped for 60 minutes in both groups. Neurologic examination was performed 24 hours later and the spinal cord was studied for immunohistochemically. In experiment 2, either 48 hours after 20 minutes of clamping or after sham operation (n = 4), HSP were investigated immunohistochemically. RESULTS: In experiment 1, 3 of 6 controls became paraplegic but none of the 6 preconditioning group dogs became paraplegic. The HSP appeared on sections from all 6 PC dogs and 3 control dogs that did not exhibit paraplegia. In experiment 2, HSP were present in clamped animals but could not be detected after sham operation. CONCLUSIONS: Ischemic tolerance was induced by preconditioning the canine spinal cord, in which HSP are believed to be involved.

Animals↗
PubMed1997

Traumatic aortic transections: eight-year experience with the "clamp-sew" technique.

BACKGROUND: Because traumatic aortic transection is associated with high mortality rates, great debate exists about the appropriate operative technique for treatment of patients who have acute traumatic aortic transection. METHODS: To determine the safety and efficacy of the "clamp-sew" method, we retrospectively reviewed our 8-year experience treating 75 patients who had aortic injuries secondary to blunt trauma. Seventy-one of these patients were treated surgically. The clamp-sew method was used in all of these operations. RESULTS: Aortic cross-clamp time averaged 24 minutes (range, 14 to 36 minutes), with 4/71 having times in excess of 30 minutes. One patient (clamp time, 28 minutes) became paraplegic. Significant associated injuries were seen in 51/75 patients (48/71 patients with operation), including intrathoracic (35 patients), orthopedic (28 patients), intraabdominal (24 patients), and central nervous system (17 patients) damage. No patient died within 24 hours of operation. Overall 30-day mortality was 12% (9/75), with 7/9 having two or more aforementioned associated injuries. Of these 7, 5 had central nervous system injuries. Two of 9 died within 30 days without two or more associated injuries: 1 Jehovah's Witness of low hemoglobin, and 1 patient of sepsis. CONCLUSIONS: Although any of several maneuvers may be appropriate in managing traumatic aortic injuries, the simple "clamp-sew" technique is a safe and effective method for the treatment of traumatic aortic transections.

Adolescent↗
PubMed2002

Clamped-filament elongation model for actin-based motors.

Although actin-based motility drives cell crawling and intracellular locomotion of organelles and certain pathogens, the underlying mechanism of force generation remains a mystery. Recent experiments demonstrated that Listeria exhibit episodes of 5.4-nm stepwise motion corresponding to the periodicity of the actin filament subunits, and extremely small positional fluctuations during the intermittent pauses [S. C. Kuo and J. L. McGrath. 2000. Nature. 407:1026-1029]. These findings suggest that motile bacteria remain firmly bound to actin filament ends as they elongate, a behavior that appears to rule out previous models for actin-based motility. We propose and analyze a new mechanochemical model (called the "Lock, Load & Fire" mechanism) for force generation by means of affinity-modulated, clamped-filament elongation. During the locking step, the filament's terminal ATP-containing subunit binds tightly to a clamp situated on the surface of a motile object; in the loading step, actin.ATP monomer(s) bind to the filament end, an event that triggers the firing step, wherein ATP hydrolysis on the clamped subunit attenuates the filament's affinity for the clamp. This last step initiates translocation of the new ATP-containing terminus to the clamp, whereupon another cycle begins anew. This model explains how surface-tethered filaments can grow while exerting flexural or tensile force on the motile surface. Moreover, stochastic simulations of the model reproduce the signature motions of Listeria. This elongation motor, which we term actoclampin, exploits actin's intrinsic ATPase activity to provide a simple, high-fidelity enzymatic reaction cycle for force production that does not require elongating filaments to dissociate from the motile surface. This mechanism may operate whenever actin polymerization is called upon to generate the forces that drive cell crawling or intracellular organelle motility.

Actins↗
PubMed1969

The N-shaped current-potential characteristic in frog skin. I. Time development during step voltage clamp.

A fast (10 musec) voltage-clamp system similar to that used on nerve axons was applied across the frog skin. An electrical analog is used to obtain the electrical parameters and to estimate the time (300 musec) required to voltage clamp the excitable membrane layer in the skin. The speed of the clamp allows observation of the early development in time of an N-shaped current-potential (I-V) relation. The isochronal I-V curves constructed from step clamp data show the beginning of a negative slope in about 250 musec after successively applied step changes in skin potential (> 200 mv). Subsequently, the negative slope reaches a quasi-steady state interval (0.4-1.5 msec) and then decays and disappears in the next 20 msec. The negative slope I-V characteristic is only found in skins which exhibit spike generation under current clamp.

Animals↗
PubMed1974

An assessment of the double sucrose-gap voltage clamp technique as applied to frog atrial muscle.

The homogeneity of voltage clamp control in small bundles of frog atrial tissue under double sucrose-gap voltage clamp conditions was assessed by intracellular microelectrode potential measurements from cells in the test node region. The microelectrode potential measurements demonstrated that (1) good voltage control of the impaled cell existed in the absence of the excitatory inward currents (e.g., during small depolarizing clamp pulses of 10-15 mV), (2) voltage control of the impaled cell was lost during either the fast or slow excitatory inward currents, and (3) voltage control of the impaled cell was regained following the inward excitatory currents. Under nonvoltage clamp conditions the transgap recorded action potential had a magnitude and waveform similar to the intracellular microelectrode recorded action potentials from cells in the test node. Transgap impedance measured with a sine-wave voltage of 1,000 Hz was about 63% of that measured either by a sine-wave voltage of 10 Hz or by an action potential method used to determine the longitudinal resistance through the sucrose-gap region. The action potential data in conjunction with the impedance data indicate that the extracellular resistance (R(s)) through the sucrose gap is very large with respect to the longitudinal intracellular resistance (R(i)); the frequency dependence of the transgap impedance suggests that at least part of the intracellular resistance is paralleled by a capacitance. The severe loss of spatial voltage control during the excitatory inward current raises serious doubts concerning the use of the double sucrose-gap technique to voltage clamp frog atrial muscle.

Action Potentials↗
PubMed1975

Axon voltage-clamp simulations. I. Methods and tests.

This is the first in a series of four papers in which we present the numerical simulation of the application of the voltage clamp technique to excitable cells. In this paper we describe the application of the Crank-Nicolson (1947) method for the solution of the parabolic partial differential equations that describe a cylindrical cell in which the ionic conductances are functions of voltage and time (Hodgkin and Huxley, 1952). This method is compared with other methods in terms of accuracy and speed of solution for a propagated action potential. In addition, differential equations representing a simple voltage-clamp electronic circuit are presented. Using the voltage clamp circuit equations, we simulate the voltage clamp of a single isopotential membrane patch and show how the parameters of the circuit affect the transient response of the patch to a step change in the control potential. The stimulation methods presented in this series of papers allow the evaluation of voltage clamp control of an excitable cell or a syncytium of excitable cells. To the extent that membrane parameters and geometrical factors can be determined, the methods presented here provide solutions for the voltage profile as a function of time.

Action Potentials↗