darush at pacbell.net
Mon Feb 24 02:27:08 EST 1997
Sanaz Tabarestani wrote:
I'm a second year medical student and I have a question about
the mechanism of presynaptic inhibition .I'm wondering if anyone
can help me.
I've read about two different mechanisms and I don't know which
one is correct.Both of them are on GABA .
The first one is that the GABA acts on GABAa receptors to
the conductance to cl(-) ,but as the boutons actively accumulate
cl(-) the result is an efflux of cl(-) and depolarization of
the terminal and partial inactivation of the voltage dependent
Na(+) mechanism, so the amplitude of any action potential that
arrives at the axon terminal will reduce.
The second one is that the GABA opens anion channels ,allowing
large numbers of chloride ions to diffuse into the terminal
fibril.The negative charges of these ions cancel much of the
excitatory effect of the positively charged sodium ions that
eneter the terminal fibril when an action potential arrives.
Thanks in advance for your help,
Reply from Darush Pessaran:
The inhibitory transmitter substance GABA has the specific effect of
opening anion channels, allowing large numbers of chloride ions to
diffuse into the terminal fibril. The negative charges of these ions
cancel much of the excitatory effect of the positively charged sodium
ions that enter the terminal fibrils when an action potential arrives.
Therefore,the action potential itself in these terminal fibrils becomes
greatly reduced, thus also reducing the degree of excitation of the
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