On Tue, 21 Feb 2006 15:51:48 +0100, SJM Guzman
<jose.guzman at medizin-uni-leipzig.de> wrote:
>>Questions for the experts in electrophysiology....
>>I have a couple of questions regarding homosynaptic/heterosynaptic
>transmission. Typically, postsynaptic currents or potentials (PSC or
>PSP) with a constant latency time (time between the stimulus artifact
>and the onset of the response) are refered as monosynaptic (better to
>say homosynaptic...?), whereas heterosynaptic responses don't have a
>constant latency time. What is the reason of that?
>>On the other hand, I would like to know, why the slope of the EPSP/C is
>used to evaluate the homosynaptic component of a heterosynaptic response
>(i.e field potentials)?
>>Thank you very much in advance for the answers.
>>Segundo J Guzman
I think you are confusing two dichotomies. There is monosynaptic vs.
multisynaptic referring to the number of sequential synapses in a
pathway and there is homosynaptic vs. heterosynaptic referring to the
way that synaptic inputs modulate onto a single cell. That is, if
cell A synapses directly on cell B, that is a monosynaptic connection.
If cell A synapses on cell C which then synapses on cell B, then from
A to B is disynaptic (or multisynaptic). I assume that your question
is about this subject. Heterosynaptic modulation occurs when cell A
modifies the way that cell B synapses on cell C.
In a monosynaptic synapse, an action potential in cell A produces a
synaptic potential in cell B and there is usually a fixed latency in
the synaptic potential, as you say. If the connection between A and B
is multisynaptic, though, mediated through cell C, then ordinarily
cell A would have to produce a synaptic potential in cell C strong
enough to elicit an action potential in C. The action potential in C
then produces a synaptic potential in B. There is a fixed latency
between the action potential in C and the synaptic potential in B (a
monosynaptic connection) but not between the action potential in A and
the complex process between the synaptic potential in C and the action
potential in C. If the synaptic potential in C is just barely at
threshold in cell C, it may take some time for the action potential to
be produced whereas if it is well above threshold, the action
potential will occur quickly. And if A is firing at a relatively high
frequency, it is unlikely that the action potentials in cell C will
follow one-for-one (unless you have a very unusual "relay" type of
connection) so that the synaptic potentials in B will not follow the
action potentials in A one-for-one. Of course, in a monosynaptic
connection, the synaptic potentials are always one-for-one with fixed
I am not sure what you mean by the slope of the EPSP/C. If you mean
the relation between end plate potential (as measured "normally", i.e.
during "current" clamp) and the end plate current as measured during
voltage clamp, that ratio is a measure of the membrane resistance.
Activation of nearby synapses (i.e. true heterosynaptic effects as
opposed to multisynaptic ones) can open membrane channels and reduce
the membrane resistance, causing a synaptic current to generate a
smaller synaptic potential. Is that what you mean?