Intracellular electrodes are used for recording the transmembrane voltage. The stimulating voltage injects current into the cell. This results in a voltage across the membrane. If the voltage due to the stimulus is sufficiently large, and crosses threshold an action potential results.
The action potential can propagate along the axon. This may be observed by placing several recording electrodes along the length of a long axon. The recorded membrane potential will show the action potential travelling under each electrode.
The Hodgkin-Huxley model of the cell membrane considers an electrical model as shown below. It shows the principal conduits for charge transfer across the membrane, namely the membrane’s potassium conductance (gK), sodium conductance (gNa), chloride conductance (gCl) and the sodium-potassium pump. The membrane also exhibits capacitive property indicated as “C” in the equivalent circuit.
To understand the HH model the following reduced model is useful.
When a current is injected across the cell membrane, a potential difference results. This causes the voltage sensitive channels (the potassium and sodium conductances) to open. The resulting currents cause further potential changes. These channel related currents and potential changes are self-limiting and cause the channels to close again, restoring the resting state. Meanwhile, the adjacent regions of the membrane are also affected by the potential changes and their channels are also induced to open. Thus the action potential propagates.
In the HH model, we can apply a synthetic stimulus current to the cell membrane, and observe the calculated conductance, current and potential changes.
The threshold of stimulus, where an action potential is initiated depends on both the stimulus strength and duration.
Aim: To study the propagating action potential as simulated by the enhanced model based on the Hodgkin-Huxley model. To determine the strength-duration curve of the simulated cell membrane.
1. Propagating Action Potential
2. Hodgkin-Huxley model of depolarization
Procedure:
The screen will show a moving surface. This is the action potential changing in time and space.
On the top are two scroll bars with which you can control the stimulus amplitude (PA = pulse amplitude) and the stimulus duration (PW= pulse width).
Next to these is a button with which a single stimulus can be given. Pressing this button will deliver a pulse of current to the membrane. If the stimulus is sufficiently strong an action potential will be generated. If the stimulus is not strong enough then a subthreshold excitation will be produced.
Next to the stimulus button is a “Hold” button which can be used to freeze the display for closer examination.
On the left is a scroll bar which can be used to observe the membrane in detail at one point along the length of the axon. Move the observation point to two different places and measure the time of the action potential at these two places. Use this to calculate the propagation velocity.
Vary the stimulus PA and PW and determine the threshold curve (the strength-duration curve).
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Pulse width( ms) |
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PA(mA) |
0.01 |
0.02 |
0.03 |
0.04 |
0.06 |
0.08 |
0.11 |
0.14 |
0.17 |
0.2 |
0.24 |
0.3 |
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Threshold+ |
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Draw the strength duration threshold curve for this nueron. Also make rough sketches of the sodium and pottasium conductances during the action potential.