Membrane Potentials Made Easy:

A Review of ZOOL 211 Coverage

While transmembrane potentials will not be discussed in lecture, an understanding of their causes and consequences will be expected throughout this semester. You should immediately review your notes from ZOOL 211 or equivalent course (a prerequisite for this course!) and then consider the following questions and the NEUROSYS simulation program referred to later in this manual.

1. Consider sodium, calcium, potassium, and chloride ions. How are they distributed across the plasma membrane of a typical mammalian cell? How large and in what directions (In ---> Out or Out ---> In) are their concentration gradients?

2. Use the Nernst equation to calculate the equilibrium potentials of each of the following ions at 37oC:

ION [ION]inside [ION]outside

a. Na+ 10 mM 100 mM

b. K+ 120 12 mM

c. Ca++ 1.3 µM 13 mM

d. Cl- 4 mM 40 mM

If this were a real mammalian cell, the equilibrium potential for which ion would most closely approximate Vm ? Why (explain!)?

3. Refer to the ions above in #2.

a. List three changes in permeabilities (not concentrations!) that would result in depolarization.

b. List three changes in permeabilities (not concentrations!) that would result in hyperpolarization.

c. List three changes in concentration that would result in hyperpolarization.

d. List three changes in concentration that would result in depolarization.

 

4.

a. Explain completely the ionic events responsible for an action potential in an excitable cell (nerve or muscle).

 

b. Define voltage-gated sodium channel, channel inactivation, refractory period, conductance, graded potentials, and conduction velocity.

 

 

5. #1. Dr. Harley Schmedlap, noted physiologist and hero to all 422 students, has discovered yet another interesting little beast while wandering through the Zoo-Psych Building. He has taken the following measurements from the beast's nerve cells and surrounding interstitial fluids. He has asked you, his trusty lab partner, to complete his study before sending the results to the Journal of Irreproducible Results.

Ion [Ion]in [Ion]out EIon Rel. Perm.

G+ 200 mM 20 mM ________ 10

P++ 1 100 ________ 0

C- 10 __________ -61.5 mV 0.1

R+ 4 400 ________ 0.03

______________________________________________________________________

A. Please supply the missing data by placing the requested values (and signs where appropriate) in the blanks in the table above.

B. Please utilize the Nernst equation to give an approximation of the Vm.

Don't forget the sign!

__________________mV

C. If during an action potential, the Relative Permeability of the nerve membrane to P++ increases to a value of 400, what value will the membrane potential approach?

(Don't forget the sign!)

__________________mV

D. If only G+ and C- were permeable, would you observe a net movement or diffusion of C- across the membrane? Why or why not?

 

 

 

E. List two changes in membrane permeability that would result in an EPSP at the dendrites of the nerves and then two that would yield an IPSP:

EPSP: IPSP:

1. 1.

2. 2.

F. If Schmedlap observes that these axons are unmyelinated and small in diameter, what could you infer about the conduction velocity of these axons? Explain!

 

 

 

#1 (Continued)

G. Please write the equation that would accurately predict the membrane potential (Vm) for these neurons.

 

 

 

6. Finally, you are required to enter the neurosys program and modify the membrane properties of the neuron to make it spontaneously active, i.e., a pacemaker. (See complete instructions in item #2 under the Membrane Characteristics section of the Neurosys description late in this volume.




ANSWERS