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Determination of Synaptic Type ?

Paul Bush paul at phy.ucsf.edu
Mon Feb 5 14:25:38 EST 1996

In article <4eq6fn$2i6 at helios.herts.ac.uk>, AliStair G Rust <a.g.rust at herts.ac.uk> writes:
|> I appreciate that chemical synapses can be either inhibitory (Gray Type I) or
|> excitatory (Type II) and that studies on neuromuscular systems indicate that
|> during the course of development immature synapses preceed mature synapses.
|> However, how is the type of synapse determined ?  I've not found anything
|> specific in the literature I've read so I've got a few questions.
|> 1.  Is type 'genetically predetermined' so that say a postsynaptic cell always
|> makes excitatory connections with whatever it contacts ?  Or similarly is type
|> defined only by the pre-synaptic cell ?

Type is determined by the presynaptic cell. Excitatory cells (in the cortex this
means spiny cells) send out axons which always make excitatory synapses on the
neurons they contact. Similarly, inhibitory cells (smooth or sparsely spiny
cortical cells) always make the same type of synapse on their postsynaptic
targets. Note that while inhibitory neurons will always release GABA, the
inhibitory cortical neurotransmitter, the effect on the postsynaptic cell
(inhibitory or excitatory) depends on its membrane potential at the time.

|> 2.  Is it just the result of where the axon actually contacts its target cell ?
|> Kandel (Essentials in Neural Science and Behaviour 95) says that type can be
|> determined by whether the synapse is axo-dendritic, axo-axonic or axo-somatic.

In this context, Kandel means that we can infer whether the synapse is excitatory
or inhibitory from its position on the postsynaptic cell, not that the site of
synaptic contact developmentally determines the type of synapse. Thus, in
cortex, an axo-axonic synapse is inhibitory, an axo-somatic synapse is inhibitory
if on an excitatory cell and can be either if on an inhibitory cell. An
axo-dendritic synapse can be either excitatory or inhibitory. 

|> Is this the only way to tell ?

Examine the synapse at the EM (electron microscope) level. Inhibitory synapses
are symmetrical, meaning they have an equally dense staining layer on both the 
pre- and post-synaptic side of the synapse, and the presynaptic bouton contains 
flattened, eliptical vesicles (packets of neurotransmitter). Excitatory synapses 
are asymmetrical and have round vesicles. Inhibitory are type II, excitatory are
type I (opposite of what you said).

|> 3.  Does electrical activity / usage determine type ?  I'm envisaging that
|> immature synapses have a generic morphology which through the 'tuning' of the
|> ion channels, gives rise to the different types.

Excitatory/inhibitory is detemined by the presynaptic cell, as above. 'Tuning' of
the densities and types of ion channels on the postsynaptic side probably occurs
in response to electirical activity/usage, but this is most likely just changing
the strength of the synapse and not its sign.

|> Any comments would be most useful together with some pointers towards the right
|> kind of papers I should be reading.

I would recommend you first read an introductory textbook such as Kandel and
Schwartz's 'Principles of Neural Science'.

|> AliStair G. Rust 
|> <a.g.rust at herts.ac.uk>
|> Division of Computer Science,
|> University of Hertfordshire, Hatfield, Herts, AL10 9AB, UK


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