Inhibitory vs Excititory spike

r norman rsn_ at
Sat May 1 11:17:01 EST 2004

On 1 May 2004 08:52:36 -0700, nettron2000 at wrote:

>r norman <rsn_ at> wrote in message news:<r6s590hs7k98kfc34dlb15etoo0r2o2gdc at>...
>> On 30 Apr 2004 17:17:02 -0700, nettron2000 at wrote:
>> >would someone please explain the difference ? What im confused about
>> >is how both excititory and inhibitory spikes can occur in a spike
>> >train on an axon.How is this possible ? or is it ?
>> > I was under the impression that all action potentials were pretty
>> >much the same, according to the liturature.But then i read about
>> >IPSP's and EPSP's or is this something completely different ?
>> Yes, EPSPs and IPSPs are something completely different.
>> The spikes are action potentials running down an axon.  When the
>> action potential gets to the end, at the axon terminals, it causes the
>> release of synaptic transmitter from the presynaptic side of the
>> synapse.  Then, when the synaptic transmitter hits the membrane on the
>> postsynaptic side, it binds to membrane receptors and causes ion
>> channels to open.  Ions flow through the channels causing currents
>> (postsynaptic current, or PSC) which changes the voltage (potential)
>> on the postsynaptic cell, the postsynaptic potentials.  These can be
>> excitatory or inhibitory.
>> So the EPSP and IPSP is in a different cell and many steps removed
>> from the spikes.  However, someone might take a shortcut in explaining
>> what is happening and speak of an action potential in a presynaptic
>> cell that has an inhibitory synapse (and hence produces an IPSP) as an
>> inhibitory spike, or an action potential in a presynaptic cell that
>> forms an excitatory synapse as an excitatory spike.  But this would
>> have to be determined by context. There is no inherent excitatoriness
>> or inhibitoriness about an action potential.
>> Note: things are a lot more complicated than what I described in the
>> earlier paragraph.  There are cells that work without action
>> potentials and there are synapses on presynaptic terminals and
>> reciprocal synapses and metabotrophic receptors that produce far more
>> complex responses than postsynaptic potentials and all.  But that is
>> enough for a start.
>Thanks r norman for your very informative reply.Just to clarify one
>specific detail; when you mentioned " a different cell and many
>steps removed from the spikes.", did you mean to say in a different
>PART of the cell that the spikes are impinging on ?
>Thanks again, much appriciated.

No, I mean in a different cell.

The presynaptic cell has an axon and axon terminal ending on a
dendrite of the postsynaptic cell.  The action potential is in the
presynaptic axon.  The post-synaptic potential is in the dendrite of
the post-synaptic cell.

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