Re: What significance is the fact that paramecium are neg.galvanotropic?

Jill:
You asked a toughie.
This is a trick I use each time I teach freshman microbial biology; it is 
pretty neat to see thousands of tiny cells all swimming either to the left 
or right (as I switch polarity) at my whim.   (...maybe this is the dark 
side of science...the arbitrary control of nature).  
But seriously, I have had a hard time explaining galvanotaxis, since it is 
a complex story.  Simply put, all the cilia have a calcium-activated 
reverse gear; when a cell bumps into something, calcium channels open, the 
cell depolarizes (the voltage normally present across the outer membrane 
leaks away, as ions rush through open channels to even out the across-
membrane charges), and the ciliary machinery switch gears, causing the 
cilium to flops into a new pattern of wiggling: the cell backs up fast.  

Now you need to know the surprising fact that not all the cilia are 
swimming forward during forward swimming; some are moving in reverse mode.  
When the number of posterior cilia that are swimming forward exceeds the 
anterior cilia that are in reverse mode, the cell moves forward.  The 
voltage gradient induces more cilia to remain in the forward mode...when 
the cell is oriented towards the negative pole (cathode), the boundary 
between anterior and posterior cilia is moved forward, so the posterior 
cilia propell the cell inexorably in that direction.   They are powerless 
to change direction.  If they do point in a new direction, an imbalance 
forms in the number of forward-beating cilia one one side of the cell, and 
this causes a torque that twists the cell in the right direction (towards 
the cathode, or negative pole).

Confused?  Join the club.

What is the significance of this?  Galvanotaxis probably has no natural 
significance, since only cells near a electric eel or catfish will 
encounter a voltage field in ntaure, but galvanotaxis provides a neat 
illustration of the use many ciliates have as models of neuronal activity; 
in fact, some ciliates have been dubbed “swimming neurons”.   They provide 
models (simplified examples) of how our very own nerve cells communicate, 
via voltage gated channels (cells can tell whether they are stimulates or 
not by the voltage gradient across their outer membrane; when they are 
stimulated, the depolarize, meaning ions like potassium and chloride rush 
through pores or channels that can open when the voltage exceeds a certain 
level, i.e. they are "voltage gated").

Ciliates are the only creatures that can show obvious behavioral changes 
when their cell membrane voltages change, although, as I mentioned, the 
molecular processes are the same as those that allow us to think (and I 
apologize if I have not thought clearly enough to help you understand 
this).

Cheers,
Dean Jacobson