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Effects of Ion Channels in Membrane Biophysics (Remote trigger)
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Objectives:

 

  1. To get an overview of the ion channels role in regulating sodium and potassium currents which play an important role in action potential generation.
  2. To understand the generation of sodium and potassium currents with respect to the action potential generation with the help of remotely triggered equipment.
  3. To study the effects of drugs like TEA or TTX on the ion channels.

 

TEA: Tetra ethyl ammonium which is known to block the K+ permeability but doesn't affect the Na+ permeability.

TTX: Tetrodoxin is known to block the Na+ permeability but doesn't affect the K+ permeability.

 

Theory:

 

How do sodium and potassium currents regulate the action potential generation? And what is the effect of drugs like TEA or TTX play ion channels? These questions are addressed in this exercise.

 

We hope you have gone through the Hodgkin-Huxley simulator in experiment 2 and passive analog neuron model. Here you will study how to use a different approach of understanding the action potential generation along with sodium and potassium currents generation using a hardware equivalent of a neuron. In addition to that, the role of TEA or TTX on the ion channels is also demonstrated using equivalent hardware circuit.

 

Membrane potential is the difference in voltage (or electrical potential difference) between the interior and exterior of a cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with many diverse protein embedded in it. Proteins have low specific resistivities and comprise the protein channels through which charged ions flow across the membrane. Lipids, on the other hand, have high specific resistivities. The purely bilipid part of the membrane is essentially a very thin insulator, separating the relatively conducting electrolytes inside and outside the cell. At rest neurons maintain a constant voltage differential across their membrane called the resting potential, Vm also called the equilibrium potential. This potential arises from the fact that.

 

  • Neuronal membranes are semi permeable, allowing only certain ions to pass from one side to the other (mostly sodium (Na), Potassium (K), and calcium (Ca)).
  • Neurons actively maintain a concentration gradient across the membrane of those same ions.
  • The ions involved carry Potassium (K), for instance, has a higher concentration inside the cell and carries a positive charge. Diffusive forces drive K out of the cell. The subsequent loss of positive ions leads to a net negative charge inside the membrane. The resulting electrical force attracts positive ions, including those attached to K, back into the cell.

 

To get an overview of the ion channels role in regulating sodium and potassium currents which play an important role in action potential generation.

 

The Opening and Closing of Channels Changes Neuronal Permeability During the Action Potential:

 

  • The opening and closing of voltage-gated channels changes the cell's permeability to sodium and potassium during the action potential.
  • Sodium permeability increases rapidly during the rising phase of the action potential.
  • Sodium permeability decreases rapidly during repolarization.
  • Potassium permeability is greatest during repolarization.
  • Potassium permeability is decreasing slowly during hyperpolarization.

 

Now let's see the simultaneous changes in sodium and potassium permeability during the action potential. The rapid increase in sodium permeability is responsible for the rising phase of the action potential. The rapid decrease in sodium permeability and simultaneous increase in potassium permeability is responsible for the repolarization of the cell. The slow decline in potassium permeability is responsible for the hyperpolarization.

 

Ion Channel Activity During the Action Potential: A Summary

 

During an action potential, voltage-gated sodium channels first open rapidly, then inactivate, then reset to the closed state. Voltage-gated potassium channels open andc close more slowly.

 

  • Rest. Voltage-gated sodium and potassium channels are closed when the neuron is at rest.
  • Depolarization. Voltage-gated sodium channels open rapidly, resulting in movement of sodium into the cell. This causes depolarization.
  • Initiation of Repolarization. Voltage-gated sodium channels begin to inactivate and voltage-gated potassium channels begin to open. This initiates repolarization.
  • Repolarization. Voltage-gated sodium channels continue to inactivate, then reset to the closed state. Potassium channels continue to open. This results in a net movement of positive charge out of the cell, repolarizing the cell.
  • Hyperpolarization. Some voltage-gated potassium channels remain open, resulting in movement of potassium out of the cell. This hyperpolarizes the cell.

We have seen that sodium moves into the neuron and potassium moves out during an action potential. However, the amount of sodium and potassium that moves across the membrane during the action potential is very small compared to the bulk concentration of sodium and potassium. Therefore, the concentration gradient for each ion remains essentially unchanged.

 

 

 

 

 

The Absolute Refractory Period:

 

Just after the neuron has generated an action potential, it cannot generate another one. Many sodium channels are inactive and will not open, no matter what voltage is applied to the membrane. Most potassium channels are open. This period is called the absolute refractory period. The neuron cannot generate an action potential because sodium cannot move in through inactive channels and because potassium continues to move out through open voltage-gated channels. A neuron cannot generate an action potential during the absolute refractory period.

 

The Relative Refractory Period:

 

Immediately after the absolute refractory period, the cell can generate an action potential, but only if it is depolarized to a value more positive than normal threshold. This is true because some sodium channels are still inactive and some potassium channels are still open. This is called the relative refractory period. The cell has to be depolarized to a more positive membrane potential than normal threshold to open enough sodium channels to begin the positive feedback loop. The lengths of the absolute and relative refractory periods are important because they determine how fast neurons can generate action potentials.

 

The neuron is a cell with electric activity. It is based on the idea that neuronal activity can be completely described by the flow of different currents associated with the neuron's membrane. The membrane of the cell has an electric potential Vm called membrane potential and is assumed equal at all points of the membrane. The presence of such an electric potential at the membrane of the neuron is the result of the charges balancing between the internal and external environment of the cell. Several types of ions of either positive or negative charge are present outside and inside the cell, and the difference between inner and the outer concentration of the different ion species produces the polarization of the membrane. The membrane potential is measured in Volts (V). The electric activity of a neuron is due to the continuous exchanges of electric currents or charges with other neurons.

 

To understand the generation of sodium and potassium currents with respect to the action potential generation with the help of remotely triggered equipment.

 

Hardware neuron model can provide real time processing. By going through the  circuit dynamics one can understand both biological as well as physiological behaviors of neuron.

 

We have designed an analog neuron model using Resistors, transistors, capacitors and externally input voltage. These all are some basic electronic components which will make analog neuron to behave like normal neuron.

 

  • Resistance represents the difficulty a particle experiences while moving in a medium. It is measured in ohms. The inverse of resistance is conductance. Conductance is the ease at which a particle can move through a medium. It is measured in Siemens. Because they are inversely related, high conductance are correlated to low resistance, and vice versa. It is important to note that generally speaking resistance and conduction in the neuron are dealing with the ability of ions to cross the membrane. Thus it often referred to as membrane resistance or membrane conductance. As such, when the majority of ion channels are closed, few ions cross the membrane, and membrane resistance is said to be high.
  • The capacitor is a passive electronic components consisting of pair of conductors separated by an insulator. The cell membrane is also said to act as a capacitor, and has a property known as capacitance. A capacitor consists of two conducting regions separated by an insulator. A capacitor works by accumulating a charge on one of the conducting surfaces. As this charge builds, it creates an electric field that pushes like charges on the other side of the insulator away. This causes an induced current known as a capacitive current. It is important to realize that there is no current between the conducting surfaces of the capacitor. Capacitance may be defined two ways as:
  1. An ability to store and separate charge or.
  2. As the quantity of charge required to create a given potential difference between two conductors.
  3. Thus given a set number of charges on each side of the membrane, a higher capacitance results in a lower potential difference. In a cellular sense, increased capacitance requires a greater ion concentration difference across the membrane.

  • Transistor is an active semiconductor device commonly used to amplify (strengthen) or switch electronic signal. Here we are using 3 transistors, two NPN and one PNP transistor. Transistor has mainly three terminals. Emitter (E), Base (B) and Collector(C). Transistor T1 and T3 are NPN transistor and T2 is a PNP transistor. For an NPN transistor collector voltage is more positive than emitter. So current flows from collector to emitter. For a PNP transistor emitter voltage is more positive than collector. So current flows from emitter to collector.

We have added one diode at the base of T1 to eliminate the bias voltage of T2. Strictly speaking it limits the fast inward current to a short burst.

 

Here we give an input excitation to the cell membrane as square wave form of amplitude 2Volt peak to peak (Vpp), since we want to obtain the output as pulse wave form. A square wave resembles to an impulse wave form in shape when pulse width is low. Here R1 represent a variable resistor which represent the membrane resistance and is inversely proportional to membrane conductance. By varying this R1 membrane conductance can be changed considerably i.e., when membrane resistance (R1) decreases the membrane conductance increases making flow of signals easier. Cm is the membrane capacitance. In any cell membrane there is a charge separation across the cell. The seperation of charge by a insulator causes a capacitive effect on the cell. This effect is modelled as membrane capacitance. If there is only the resistor when the input voltage is applied, then voltage will be changed to steady state value, hence we are using a capacitor Cm along with it which resist this change. When the applied input makes the membrane capacitance to change above threshold value, then only voltage gated sodium channels open. The membrane potential is measured with respect to ground.


When the input excitation is given the membrane capacitance Cm begins to charge, when the voltage across the capacitor reaches more than cut in voltage of transistor T1, the transistor turns on and the current flows from collector to emitter. Then the base voltage of transistor T2 becomes less and T2 also turns on and current flows from emitter to collector i.e., Na channel is on and INa begins to flow inwardly. The energy for it provided by an electrical gradient of Na+ across the membrane, here it is modelled as ENa.


The threshold value of potassium channels is modeled as transistor base emitter cut in voltage. The sodium current charges the capacitor C1. When the voltage across the capacitor C1 reaches more than cut in voltage of transistor T3, the transistor turns on and the potassium current flows from collector to emitter outwardly i.e., K channels on. Thus the depolarising phase of an action potential.

 

By this time membrane capacitor Cm becomes fully charged and begins to discharge i.e., when the capacitor voltage drops transistor T1 turn off, consequently T2. Then sodium current stops its flow i.e., sodium channel closes. As a result capacitor C1 begins to discharge and transistor T3 turn which leads potassium current to stop flow. Thus the repolarising phase of an action potential.

 

 

 To study the effects of drugs like TEA or TTX on the ion channels.

 

 

 

 

  

Some chemical agents can selectively block voltage-dependent membrane channels. Tetrodoxin (TTX), which comes from the Japanese puffer fish, blocks the voltage-dependent changes in Na+ permeability, but has no effect on the voltage-dependent changes in K+ permeability. This observation indicates that the Na+ and K+ channels are unique; one of these can be selectively blocked and not affect the other. Another agent, tetraethylammonium (TEA), has no effect on the voltage-dependent changes in Na+ permeability, but it completely abolishes the voltage-dependent changes in K+ permeability.

 

To get an overview picture of how the voltage-gated ionic channels regulate their mechanism (opening and closing), we study it by controlling the sodium and potassium channels. We can disable the sodium channel in electrical circuit shown above by disconnecting certain components in order to visualize the biological effect caused by TTX on voltage-gated Na+ channels i.e., no sodium current flow. And to visualize the effects caused by TEA on voltage-gate K+ channels, we disconnect certain components in order to show the analogous effect of TEA i.e., no flow of potassium current. For this we make the emitter resistance of transistor T2 high and base resistance of transistor T3 high. In an electronic analog neuron the sodium and potassium current have coupled effects in the membrane potential so to disable membrane potential we disable both sodium and potassium current. The signal that can be seen when you disable ionic current is the charging and discharging of membrane capacitance. That is the voltage present in a cell membrane is at its normal state.   

 

 

 

 

Cite this Simulator:

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