NA+

Richard L. Hall rhall at webmail.uvi.edu
Wed Sep 20 12:21:46 EST 2000


More questionable still...the idea that neurons know.

A signal is either excitatory or inhibitory because of post synaptic 
responses.  Acetylcholine can be excitatory or inhibitory depending 
on receptor type and second messenger coupling mechanisms (Gi or Gs, 
etc.)

The release of NT by the presynaptic neuron can be modulated 
increasing or decreasing the amount of NT in the synaptic cleft.  The 
post synaptic membrane can up or down regulate receptor 
concentrations.  Thus it is possible to alter the strength of 
synaptic interactions.  It would be very unusual for these mechanisms 
alone to make an inhibitory synapse excitatory.

Glycine receptors on motor neurons of the spinal cord are inhibitory, 
suppressing motor activity as a consequence of increased chloride 
permeability.  Strychnine blocks glycine receptors reducing inward 
chloride currents thus moving the motor neuron closer to threshold. 
When startled, general spinal activity increases and motor activity 
is more readily stimulated, hyper-reflexia occurs, powerful muscles 
contract and may not relax.  You suffocate or break your neck and 
strangle.  Then you die.

It is the direction of current flow that dictates whether charge 
accumulates on the neural membrane.


rlh


>  > > 1) How does a neuron know whether to send excitatory or inhibitory
>  > > signals?.. I understand the idea that a neuron fires if the sum of the
>  > > signals IT receives is excitatory?..
>  > > 2) What is an example of an inhibitory neural process?
>  >
>  > My understanding is that it is the NT and receptor sites which determine
>  > whether an effect is inhibitory or excitatory.  In neurons the effect of a
>  > NT binding to its receptor is to allow an electrical current to flow
>through
>  > the membrane, which changes the electrical charge across the nerve
>membrane.
>  > Since neurons conduct electrical signals along their membranes, the effect
>  > of a particular type of receptor is either to excite this electrical
>  > activity or to inhibit it.
>
>Unless opinion has changed recently I believe the statement "allow an
>electrical current to flow"
>would not be acceptable to the neuroscience community.  Agreed that the
>nervous system
>models some aspects of electrical circuits but it does this through ions not
>the flow of electrons.
>
>Ron Blue
>http://turn.to/ai
>
>
>
>
>
>
>---

Richard L. Hall, Ph.D.
Comparative Animal Physiologist

University of the Virgin Islands
2 John Brewers Bay
St. Thomas, U.S.V.I. 00802

340-693-1386
340-693-1385 FAX

rhall at uvi.edu

"Live life on the edge...the view is always better"  rlh


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