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There are many other types of
synapses that can have more complex behavior. For example in the
bullfrog sympathetic ganglion, synaptic cotransmission occurs in which
a cocktail of several neurotransmitters is released which then bind to
several different receptor types. With such complex synapses, it is
possible to get an initial brief depolarization followed by
hyperpolarization and then by a long lasting low amplitude
depolarization.
Many cortical neurons have AMPA synapses which depress. That is, when
given repeated stimuli, the synapse produces less and less
transmitter. We can readily model this by adding a desensitized state
to the standard two-state model for the synapse. Instead of the open
state going directly back to the closed state, we allow there to be an
intermediate state (desensitized) which then returns to the closed
state.
![\begin{displaymath}
C \stackrel{\alpha [T]}{\rightharpoonup} O \end{displaymath}](img66.gif)
![\begin{displaymath}
O \stackrel{\beta}{\rightharpoonup} X \end{displaymath}](img67.gif)
![\begin{displaymath}
X \stackrel{\beta_2}{\rightharpoonup} C\end{displaymath}](img68.gif)
Since C+O+X=1 (that is the probability of being any of these three
states is 1) we eliminate the equation for the closed states and are
left with the following pair of equations:
![\begin{displaymath}
\frac{ds}{dt} = \alpha [T] (1-s-x) -\beta s \end{displaymath}](img69.gif)
![\begin{displaymath}
\frac{dx}{dt} = \beta s -\beta_2 x\end{displaymath}](img70.gif)
The slower is
, the longer the synapse remains in the
desensitized state, x.
Next: Playing around with synapses
Up: Synapses
Previous: Gap junctions.
G. Bard Ermentrout
2/12/1998