convergence times

[nmg@skivs.ski.org]

Two cautionary notes on Sejnowski's latest message:

1)  I presume that the 10 msec estimate for cycle time comes from
typical values of membrane time constant.  However, this estimate
might be substantially off under physiological conditions.  Under
these conditions, cell membranes might be constantly bathed in
neurotransmitters, which by increasing membrane conductance,
reduce the cell's time constants.  Moreover, voltage-dependent  ionic
channels in dendritic trees might considerably reduce electrotonic
conductance times.  Hence, it would not be surprising if the
contribution of electrotonic condution to "cycle time" is significantly
smaller than 1 msec.

2) It is true that conventional dendrodendritic synapses appear to be
essentially  inexistent in the neocortex.  But one must remember that
this statement relates to vesicle-containing synapses identified with
electron-microscopy methods.  In the retina, non-vesicular synapses
have been observed. Although, once again, the retina and the cortex
have several differences, it seems that it would be hard to rule out
the existence of such non-vesicular synapses in the cortex based on 
available data.  And if one may speculate, then why not to have this
type of synapse being dendrodendritic.  Anyway, my point  is that it
might be too soon to state that action potentials mediate all cortical
local synaptic circuits.

Norberto