NA+

zu_enlil at atdot.org zu_enlil at atdot.org
Thu Sep 21 03:04:32 EST 2000


On 20 Sep 2000 16:32:47 +0100, rcb5 at msn.com ("Ron Blue") wrote:

>> > 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?..

It doesn't. It sends a signal (depolarises), but it is the
neurotransmitter (NT) it releases, and the type/position of the
receptor that this NT binds with that determines the affect on the
receiving neuron.

>> > 2) What is an example of an inhibitory neural process?
>>
GABA, and the benzodiazepines (some of which occur naturally in the
brain).

>> My understanding is that it is the NT and receptor sites which determine
>> whether an effect is inhibitory or excitatory.  

There are many other factors. 

For example, GABA is mostly an inhibitory neurotransmitter, but even
simple things like the time of day can cause it to become
excitatory. In the suprachiasmatic nucleus (SN), GABA is normally
excitatory during the day, but inhibitory at night. The reason for
the difference is the variations in extracellular Chlorine (Cl-) ion
concentration during the course of a day. 

Chlorine is a negatively charged ion that slows neuron
depolarisation (firing). GABA receptors open Cl- channels to allow
this ion to enter a cell. However, during the day, the Cl-
concentration outside cells in the SN is less than that found inside
neurons, so opening Cl channels actually causes Cl to flow out from
SN neurons.  (Wagner et al, (1997) "GABA in the mammalian
superchiasmatic nucleus and its role in diurnal rhythmicity."
Nature, vol 387, p 173-7).

>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.

No, when one of neurotransmitters that directly produce excitation
or inhibition binds to its receptor an ion channel is opened
allowing the flow of electrically charged particles (ions) to enter
a cell. It is this that changes the charge gradient across the cell
membrane.

However, very few NTs have such a direct affect. The best known,
most wide spread being glutamate (mostly excitatory) and GABA
(mostly inhibitory).  Most of the other neurotransmitters mediate
excitation/inhibition by indirect means.

			Zu








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