Action Potentials in Squid Axons

Action Potentials in Squid Axons In 1952, Hodgkin and Huxley published a series of four papers in the Journal of Physiology (London) reporting their experiments to investigate the underlying events of the action potential. In their final paper, they derived a series of equations that describe the relationship between sodium conductance (gNa+), potassium conductance (gK+) and the membrane potential in a squid axon following electrical stimulation. Hodgkin and Huxley were awarded the Nobel Prize for this work. In this practical, you will use a computer program based on the Hodgkin and Huxley equations to show what is happening to the membrane potential, gNa+ and gK+ during and after electrical stimulation. An example of the output from the program is illustrated in figure 1. It can be seen that the electrical stimulation depolarises the membrane. Once a depolarisation of 30mV has occurred, the conductance to sodium ions increases rapidly and the membrane potential rises to +20mV. The rise in gK+ is slower in onset an d lasts for longer than the increase in gNa+. The fall in gNa+ and the associated rise in gK+ returns the membrane potential towards the resting value. Methods and Results    Q1 and 2. Investigate the effects of varying stimulus amplitude and duration by running all the simulations shown in the matrix below in Table 1: Enter a ‘X’ in the Table 1 matrix for experiments that produce an action potential, and record the peak height, amplitude, latency and threshold of any action potentials in Table 2 overleaf. For experiments that fail to elicit an action potential, enter a ‘O’ in the matrix below, and record a value of à ¯Ã¢â‚¬Å¡Ã‚ ¥ (infinity) for the latency and ‘-‘ for the other parameters in the table overleaf.    Q3. Plot two graphs to show the relationship between: (i) Stimulus strength and latency and (ii) Stimulus duration and latency. How these graphs should be plotted is not immediately obvious, and information on how to complete thi s task will not be explicitly given! The optimal solution to the problem is for you to find, but the following points are provided for guidance: It is not legitimate to plot infinity on graphs It is not appropriate to extrapolate beyond data points It is not legitimate to plot average latencies. The graphs must be plotted so that every value of latency (except à ¯Ã¢â‚¬Å¡Ã‚ ¥) is represented. Use the blank sheet on the proforma, there is no need to use graph paper. Graph 1 : Stimulus strength and latency Remember you need to distinguish different stim durations in this graph Stimulus Duration (ms) Graph 2: Stimulus Duration and Latency Make sure you distinguish different strengths as well Stimulus Strength (à ¯Ã‚ Ã‚ ­A/cm2) These can be plotted accurately using excel for your submitted report. Experiments with dual stimuli Q4. Run a simulation with the following parameters to demonstrate the absolute refractory period:    Briefly describe the responses obtained in simulations A and B in the space below: For simulation A and B in stimulus 1 they had peak heights of +17mv therefore producing an action potential. In stimulus 2, there is very little depolarisation in simulation A as peak height is -92mv. This is lower than the resting potential, therefore showing that the neuron did not fully recover from stimulus1.