Interneurons and synaptic symmetry

Matthew Kirkcaldie m.kirkcaldie at
Mon May 3 03:15:11 EST 2004

In article <Xns94DEC4B89C385BilZ0rhotmailcom at>,
 BilZ0r <BilZ0r at> wrote:

> In a review paper I'm reading, there is constant reference to interneurons 
> of various sizes (i.e. small, medium and large), and asymmetric synapses.
> I was hoping someone could a) tell me if there is going to a general or 
> specific correlation between neurotransmitter type and the size of 
> interneuron

In general they all employ GABA, at least if you're speaking about the 
cerebral cortex.  In other parts of the CNS, glycine is used as well, 
and that may bear some relationship to the size of the neurons.  But 
most, if not all, neurons called "interneurons" are inhibitory via GABA 
or glycine.

>, b) what's the story with asymmetric synapses is

Well, at the electrophysiological level a synapse can be identified as 
inhibitory or excitatory, and immunohistochemistry can do much the same 
by inference.  At the electron microscopic level, unless you're using an 
EM-detectable immunolabelling technique, the chief distinction between 
synaptic types is the appearance of the presynaptic and postsynaptic 
membranes. Studies correlating electrophysiology and EM appearance have 
established the empirical relationship that a symmetric synapse (one 
with darkening of roughly equal thickness on the presynaptic and 
postsynaptic sides) is inhibitory (GABAergic in the cortex). If the 
postsynaptic membrane has a larger thickness of dark material (the 
post-synaptic density) then it's an asymmetric synapse, and likely to be 
excitatory (glutamatergic) in nature.

The post-synaptic density is thought to be a complex of membrane 
proteins, receptors, scaffold proteins and other complexes which go 
about the business of synaptic alteration, and the effects of synaptic 
transmission *other* than changes in membrane potential.

> c) if 
> there is a good text I can read for further readings/citations.

I seem to remember learning these things from a 1989 book called 
"Cortical Circuits"  by White.  But I'll bet there are newer references.  
It's a good book, however.

Hope that helps.



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