Nerve simulations
Biology 303 Comparative Animal Physiology
I. The resting potential and extracellular Potassium concentration.
Summary: Analyze the variation of the neuron resting potential as you change the extracellular potassium concentration. Draw a semi-log plot of potential vs. log ion concentration. At high concentrations of K, do the data obey the Nernst equation? At normal concentrations of K, do the data obey the Nernst equation? If not, what equation do they obey?
Procedure:
(1) Start the "current-clamp" program, CCWin.
(2) Open the file "RestPot.ccs"
(3) "Run" a trace by selecting the menu entry Run/Begin or pressing Ctrl+B.
(4) Change external Potassium concentration (mM [K]o) by selecting the menu entry Parameters/Ions or pressing Ctrl+D and changing the value. Then click the OK button or press Enter to accept the new value.
(5) "Overlay" a new trace by selection the menu entry Run/Overlay or pressing Ctrl+Y.
(6) Repeat steps 4 and 5 until you have traces collected for 2, 5, 10, 20, 50, 100, and 200 mM [K]o.
(7) Prepare a table of final resting potential vs concentration for each case.
(8) Make a “proper” plot of resting potential vs. log [K]o.
Report: Make a “proper” graph of resting potential vs. log [K]o. Answer Qs: At high concentrations of K, do the data obey the Nernst equation? At normal concentrations of K, do the data obey the Nernst equation? If not, what equation do they obey?
II. Subthreshold responses
Summary: Observe the graded response to sub-threshold stimulus current, noting proportional relation between stimulus and response, even with both positive and negative stimuli. Notice the effect of the membrane capacitance on the data.
Procedure:
(1) Start the "current-clamp" program, CCWin.
(2) Open the file "Graded.ccs"
(3) "Run" a trace with Run/Begin or Ctrl+B
(4) Change the stimulus strength (injected current) with the menu selection Parameters/Protocol (or Ctrl+E). Try values between -1.5 and +1.5 nA. DO NOT MAKE INJECTED CURRENT GREATER THAN 1.5!
(5) Note that Run/Begin (Ctrl+B) clears the screen before stimulating the neuron. Run/Overlay (Ctrl+Y) draws the new response on top of the older ones so you can see the difference.
(6) Note that the response is "graded", that is, proportional to the stimulus. The curved rise and fall are due to the capacitance of cell. The neuron is an R-C circuit!
(7) Now try a stimulus of +1.7 nA. Is there a difference in the response?
Report: Answer Q: Is there a difference in the response at a stimulus of +1.7 nA? If so, why is it different?
III. The threshold and the strength-duration relation
Summary: Measure the stimulus amplitude threshold at various values of stimulus duration and plot a strength-duration curve. Observe the "active response" that occur just at threshold.
Procedure:
(1) Start the "current-clamp" program, CCWin.
(2) Open the file "Active.ccs"
(3) "Run" a trace with Run/Begin or Ctrl+B
(4) Change the stimulus strength (injected current) with the menu selection Parameters/Protocol (or Ctrl+E). Try values between 5 and 8 nA.
(5) Note that Run/Begin (Ctrl+B) clears the screen before stimulating the neuron. Run/Overlay (Ctrl+Y) draws the new response on top of the older ones so you can see the difference.
(6) Make sure you have some stimuli below threshold, some well above threshold, and some just barely around threshold. Are all action potentials exactly alike? What does "all-or-none" response mean? Can you see the "local active response" that occurs if the stimulus is almost at threshold?
(7) Set the stimulus duration to 2 msec (increase the "Offset of step" from 6 to 7 msec) and determine the stimulus current threshold necessary to evoke an action potential.
(8) Repeat the threshold measurement for stimuli of 3, 4, and 5 msec.
(9) Make a “proper” plot of stimulus strength vs. stimulus duration.
Report: Make a “proper” graph of stimulus strength vs. stimulus duration. Answer Qs: Are all action potentials exactly alike? What does "all-or-none" response mean? Can you see the "local active response" that occurs if the stimulus is almost at threshold?
IV. The effect of Na concentration on the action potential.
Summary: Observe the action potential at various levels of extracellular Na+ concentration. How does decreasing [Na+]o change the response?
Procedure
(1) Start the "current-clamp" program, CCWin.
(2) Open the file "AP-vs-Na.ccs"
(3) "Run" a trace with Run/Begin or Ctrl+B
(4) Change the extracellular Na+ concentration (mM [Na]o) using the menu selection Parameters/Ions (or Ctrl+D). Try values of 200, 100, 50, 20, and 10 mM. Show an overlay of the responses to all these concentrations. How does a decrease in extracellular Na+ change the action potential?
Report: Answer Q: How does a decrease in extracellular Na+ change the action potential?
V. Repetitive activity to sustained stimulation
Summary: Observe the repetitive response produced by sustained stimulation. Plot frequency of response vs. stimulus amplitude.
Procedure:
(1) Start the “current-clamp” program, CCWin.
(2) Open the file “Repet.ccs”
(3) “Run” a trace with Run/Begin or Ctrl+B
(4) Under Paramters, select Protocol (or Cntl+E) and adjust the offset of step from 100 to 110 so that the total stimulus pulse is 100 msec (going from 10 to 110 msec).
(5) Measure the frequency of action potentials. Note that the duration of the stimulus pulse is 100 msec, so 5 action potential during this time is a frequency of 50/second.
(6) Vary the stimulus amplitude from just barely above threshold to about 5 nA. Note that at higher stimulus intensities, the action potentials are so frequent that they fail because of the absolute refractory period. Collect a table of frequency vs. stimulus strength showing a good range of frequencies from around 10/second to over 200/second.
(7) Make a “proper” plot of frequency vs log strength.
Report: Make a “proper” graph of frequency vs log strength.