Lecture notes for Wednesday, April 6, 2016



Which kinds of differentiated cells have "resting potentials"?

= Negative voltage inside their plasma membranes, caused by > 20 times higher concentrations of potassium ions inside their plasma membranes compared with the potassium concentration in the surrounding water, combined with their plasma membranes being permeable to potassium ions.

Outward leakage of about a millionth of this difference in potassium concentration creates a positive voltage outside, and this voltage pulls back any more potassium ions from leaking out.

Action potentials are positive feed-back "voltage-gated" leakage of sodium and calcium ions into cells.

1) Nerve cells use resting potentials and acting potentials to transmit signals, by stimulating secretion of synaptic vesicles.

2) Muscle cells use resting potentials and acting potentials to synchronize contraction.

3) Heart muscle cells use resting potentials and acting potentials to time contractions, and to cause this spontaneous contraction to occur in waves.

4) Oocytes use resting potentials and acting potentials as their fast block to polyspermy.

Notice the irony that depolarization increases fusion of cortical granules to the plasma membrane, but inhibits the fusion of sperm plasma membranes to the oocyte plasma membrane!

5) Some pigment cells in the skins of zebra fish use resting potentials and acting potentials to stimulate and inhibit each other in ways that (somehow I do not yet understand) produce alternating white & black horizontal lines.

6) Paramecia and other ciliate protozoa use action potentials (= quick depolarizations) to cause temporary increases in calcium ion concentrations in their cytoplasm, to reverse direction of cilia power strokes. By this means, paramecia back up when they bump into an obstacle.

[and remember the video I made of ciliate chemotaxis? Do you think they must depolarize whenever an attractant concentration depolarizes?]

Maybe depolarization also can cause reversal of direction of other kinds of cell locomotion? For example, contact inhibition of crawling by tissue cells could result from cell-cell touching causing one or both of them to depolarize, combined with depolarization stimulating cells to crawl in an opposite direction.

7) Venus fly traps use changes in resting potentials to control decreases in osmotic pressure, which is how they close their trap-like leaves.

8) Maybe many other cell types also use resting potentials and acting potentials to control many different kinds of changes in cell properties (that need to occur faster than diffusion).

9) ??? If the sodium pumps of most body cells continually use as much as 20-40% of their ATP energy, doesn't that strongly imply that their resting potentials have some major use?

10) ??? Please invent some possible uses? Voltages are suspected of helping regeneration.

What kinds of evidence would suggest that a given cell type used resting and/or action potentials? (How to test whether cells are controlling some process using changes in voltage differences between the cytoplasm and outside their plasma membrane?

    A) Create a ~100 millivolt voltage gradient in the tissue culture medium surrounding cells.

    Observe whether cells' behaviors change in any way.

    B) Raise potassium concentrations in the cells' surrounding water.

    C) Poison cells' sodium pumps.

    D) Ionophore chemicals (VERY poisonous, even if you touch a tiny drop!) that allow calcium (or certain other ions) to leak through membranes

    E) Use microelectrodes inserted into cells to force increases or decreases of trans-membrane voltages; and see if the cells do something consistent in reverse.

    F) Use fluorescent dyes to look for spatial and/or time changes in calcium or other ion concentrations in cytoplasm.


Some evidence supports the idea that electric fields may control bone formation and osteoporosis.

In that case, the voltage has been assumed to be caused by piezoelectricity.

[Many crystals (quartz, sucrose, but not calcium phosphate) create electrical voltages when compressed, stretched or twisted.] These voltages can be many volts, but are very brief, with tiny current.

Applying an external voltage to a piezoelectric crystal will cause it to elongate or contract. Sonar pingers work that way; also sonicators and quartz watches.

Pierre Curie figured out these relations between symmetries of locations of charged ions, voltage generation by changing crystal's shapes & sizes, etc.

This probably was the first example of "Curie's Principle.


Many orthopedic researchers have confirmed that voltages are generated (temporarily) by compressing bone, cartilage, and collagen. They say this must be piezoelectricity. I think it is really caused by electroosmosis. (Are they unfair to regard me as pedantic?)

If we get a voltage by squeezing tissues, who cares whether this is caused by crystal symmetry or by dissolved positive counter-ions.

What are your thoughts?

This might lead to a cure for osteoporosis.


Optional reading, NOT required: Harris et al. 1990 paper on galvanotaxis.
Some of the illustrations shown in the lecture came from this paper.



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