Biophysics Laboratory Techniques. The Axon Guide to Electrophysiology and Biophysics Laboratory Techniques . Electrode Test. .. current-voltage relations of ion channels and how they are affected by the properties and concentrations of. If there's no literal injection of charge/current into the cell, how can the cell do Patch-Clamp Electrophysiology Patch clamping is voltage clamp but cleverly adapted so that you only need .. Pure human serum had little effect on the hERG channel waveform or the current–voltage relationship when compared to PSS. is no current! • Without conductance, there is no current. • Ohm's law: – linear relationship between current and voltage. – not universal, especially not in living.
Neurophysiologist study this V-I curve to understand the relationship between current and voltage to model biophysical properties of a nerve cell.
The flow of current across ion channels is measured using patch clamp technique. Patch clamp technique usually used to study the voltage dependence of a particular channel. V-I curves are used to calculate the reverse potential of a membrane when all the ion channels are open and the reverse potential caused by a particular type of ion channel can also be estimated by the application of ion channel blockers. For example TTX is used to record voltage dependent sodium current.
Electrically excitable cells neuron has basically two types of ion channels, voltage gated and non-voltage gated. Non-voltage gated ion channels are always open and this category of channels are mainly responsible for maintaining the resting potential.
Tunnel diodes and Gunn diodes are examples of components that have negative resistance. Devices which have hysteresis ; that is, in which the current-voltage relation depends not only on the present applied input but also on the past history of inputs, have I—V curves consisting of families of closed loops. Each branch of the loop is marked with a direction represented by an arrow.
Examples of devices with hysteresis include iron-core inductors and transformersthyristors such as SCRs and DIACsand gas-discharge tubes such as neon lights. I—V curve similar to a tunnel diode characteristic curve.
VBO is the breakover voltage. Memristor I—V curve, showing a pinched hysteresis Gunn diode I—V curve, showing negative differential resistance with hysteresis notice arrows In electrophysiology[ edit ] An approximation of the potassium and sodium ion components of a so-called "whole cell" I—V curve of a neuron.
While I—V curves are applicable to any electrical system, they find wide use in the field of biological electricity, particularly in the sub-field of electrophysiology. In this case, the voltage refers to the voltage across a biological membrane, a membrane potentialand the current is the flow of charged ions through channels in this membrane.
The current is determined by the conductances of these channels. In the case of ionic current across biological membranes, currents are measured from inside to outside.
Understanding the cell as an electrical circuit
That is, positive currents, known as "outward current", corresponding to positively charged ions crossing a cell membrane from the inside to the outside, or a negatively charged ion crossing from the outside to the inside. Similarly, currents with a negative value are referred to as "inward current", corresponding to positively charged ions crossing a cell membrane from the outside to the inside, or a negatively charged ion crossing from inside to outside.
The figure to the right shows an V—I curve that is more relevant to the currents in excitable biological membranes such as a neuronal axon. The blue line shows the V—I relationship for the potassium ion. Note that it is linear, indicating no voltage-dependent gating of the potassium ion channel.
The yellow line shows the V—I relationship for the sodium ion.
Membrane capacitance, CM Because the membrane is an electrical insulator separating opposing charges inside and outside the cell, the cell membrane not only has a resistance but also a membrane capacitance. Therefore, to change the membrane voltage, it is necessary to charge the capacitance.
Current-voltage characteristic — Hodgkin Huxley Tutorial documentation
The applied charge Q divided by the membrane capacitance CM gives the membrane voltage Vm: We can see that for a given amount of applied charge, the smaller the membrane capacitance, the larger the membrane voltage change. Such a circuit of parallel resistance R and capacitance C is known as an RC circuit. RC circuits are commonly used in electronics as basic filters to select particular input frequency ranges.
Similarly, the cell membrane acts as a filter on current or voltage injected into the cell. Basic schematic of the electrical properties of a plasma membrane. A circuit diagram showing the membrane capacitance and membrane resistance in parallel to each other. Traces showing a voltage step top and the resulting current response bottom for a simple plasma membrane being voltage clamped.
Initially, a square shaped voltage step leads to an instantaneous jump in current the initial peak. This current then decreases exponentially falling flank to reach a steady state. Contrary, when the voltage step is reversed, we observe a large instantaneous current of the opposite direction that decreases exponentially until it reaches steady state again.
Controlling the membrane voltage and measuring the resulting current in this way constitutes a basic voltage clamp experiment. How do the properties of the electrode and cell membrane influence the shape of the current curve see Figure 1B?
Initially, the entire current charges the membrane capacitance with no current flowing across the membrane resistance. As the membrane capacitance becomes more and more charged, an increasing fraction of the injected current flows across the membrane resistance. Once the capacitance is fully charged the system reaches steady state and the entire current flows across the membrane resistance.
Current–voltage characteristic - Wikipedia
The values of membrane capacitance and membrane resistance determine how quickly this steady state is reached: Determining the state of a recording We can use the above relationships to monitor various stages of a whole cell recording. To do so, we apply a small voltage pulse at the electrode, the so-called test pulse.
By observing the shape and amplitude of the current response to the test pulse Figure 2, right columnwe gain lots of useful information about the recording electrode and the cell. Importantly, many of the concepts apply to other forms of electrophysiological recording as well. The pipette is indicated on the left. Recording electrode in the bath Entering the bath is the first stage in a recording: By definition, the voltage between the recording electrode and the reference electrode is zero.