QUESTIONS

1. The resting membrane potential of a cell is:   - _____mV.  Explain the negative sign.
    
2. When the membrane potential reaches ______mV (threshold), the depolarization phages begins.  The repolarization phase begins at _______mV.
    
3. A nerve fiber is frequently compared to an undersea telephone cable.  Explain the concept of the length constant.
    
4. What is the relationship between fiber size & conduction speed?
    
5. What cells produce myelin?
    
6. Explain saltatory conduction.
    
7. What is a "compound action potential"?
   
   
   
   
    

ANSWERS

1. The resting membrane potentialis -70 mV.  The value of 70 mV is determined by the difference in net charge between the outside & the inside of the cell membrane.   The negative charge is designated because we are using the outside of the cell as our reference point & there is more net charge on the inside of the cell.  Hence - 70 mV.
   
   
   
   
    
2. When the membrane potential reaches  -60 mV (threshold), the depolarization phages begins.  The repolarization phase begins at  +35 mV.
   
   
   
   
    
3. Signal is attenuated going from point A to point B as your phone call goes accross the Atlantic. This is because of conductor resistence & loss of signal through the insulation.  In a nerve fiber loss is from ion leakage & electrical resistence of the axoplasm.  This signal loss is proportional to the cable/fiber length.
   
   The lenght constant is the length the cable travels to be attenuated to .37 of its original size. This decay process is called electrotonic decay & a rather complex math equation describes the length constant.
   
   When a region of the nerve is stimulated & an action potential is triggered, the ion changes occuring in the action potential are adequate to bring an area of the same axon to threshold 2 length constants ahead of the action potential.
   
   
   
   
    
4. The aforementioned complex equation involves the radius of the nerve fiber.  Bottom line is the size of the nerve determines the speed of propagation.  (4x increase in size corresponds to double velocity.)  Myelin serves to increase this radius.
   
   (Note, the aforementioned equation also involves resistence.  Myeline increases resistance between nodes of ranvier & effectively increases the length constant up to 100x)
   
   
   
   
    
5. Myelin is produced by Schwann cells also known as Oligodendroglia.
   
   
   
   
    
6. A myelinated nerve fiber is only myelinated segmentally.  In between myelinated segments are unmyelinated "nodes."  These are the nodes of Ranvier and it is at these locations that action potentials occur (this is the only place where voltage gaited Na+ channels open).  When an action potential is generated at one node, it jumps from node to node rather than spreading slowly through the axon.
   
   
   
   
    
7. Nerve fibers are divided into A, B & C .  Mammals don't have B fibers so we'll forget about those. A fibers are myelinated & C fibers aren't.  A nerve will have many nerve types & sizes &, as you'd expect, conduction velocities differ in all these fibers. When action potentials are measured, you get a whole bunch of peaks & this collection of peaks is the compound action potential.