[Na+] = 142mM     [K+] = 7mM          [Cl-] = 142mM

1) Based upon the values of the resting voltage (Vm) and the concentrations of the ions, answer the following:

a) What are the equilibrium potentials (Nernst potentials) for Na, K and Cl?  Which ions are and are not in equilibrium?

b) Is the resting potential (Vm) closer to ENa or EK?   Describe, in words, why this is the case?

c)  At the peak of the action potential, the membrane voltage is +50mV.  Is this potential closer to ENa or EK?   Describe, in words, why this is the case.
 

2) For the following, assume that the threshold voltage required to generate an action potential in the axon is -35mV.  Also assume that the intracellular ion concentrations are constant and do not change with the treatments listed below.  (Hint: To answer the questions below, recalculate the Nernst equation for potassium using the new extracellular K concentration.)

a) Hypokalemia is a condition of low blood (extracellular) potassium concentration.  If the extracellular potassium concentration falls to 2mM, what effect would this have on the resting voltage of the axon?  Why?

b) Would this make the axon more or less likely to generate an action potential?  Why?
 

3)  Multiple presynaptic neurons converge on a single postsynatpic neuron.  Explain how excitatory and inhibitory potentials emanating from these presynaptic neurons act together to determine whether or not an action potential will be generated in the postsynaptic axon.
 

4) Botulinum toxin prevents the release of acetylcholine from the presynaptic neuron of a neuromuscular junction.  If you stimulate the sciatic nerve (motor nerve) of a control frog (no toxin) with a positive voltage the gastrocnemius muscle will contract.  If you stimulate the gastrocnemius muscle of a control frog directly, the muscle will also contract.

a) If you inject the frog with botulinum toxin and stimulate the sciatic nerve, will the muscle contract?  Why or why not?

b) If you now stimulate the gastrocnemius muscle of the injected frog directly, will the muscle contract?  Why or why not?

c) If you squirt some acetylcholine on the gastrocnemius muscle of the injected frog, will the muscle contract?  Why or why not?

revised 10 December 2001

roer@uncwil.edu