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mV
mV
mV
AP
EPSP
IPSP
+40
Threshold
0
−55
−70
Threshold
−55
−70
−100
−70
t 0
t 0
t 0
1ms
(a) (b) (c)
Fig. 2.13. Electric potentials on a synapse: (a) presynaptic action potential; (b) excitatory
postsynaptic potential; (c) inhibitory postsynaptic potential (after [117]).
in the dendritic tree and less frequently at dendritic shafts. Inhibitory synapses often
contact the cell body, where they can have a strong effect on the graded potential
that reaches the axon hillock. Hence, they can mute a cell.
The synaptic efficacy, the amplification factor of a chemical synapse, can vary
greatly. It can be changed on a longer time scale by processes called long term
potentiation (LTP) and long term depression (LTD). These are believed to depend on
the relative timing of pre- and postsynaptic activity. If a presynaptic action potential
precedes a postsynaptic one, the synapse is strengthened, while it is weakened when
a postsynaptic spike occurs shortly before a presynaptic one.
In addition, transient modifications of synaptic efficacy exist, that lead to effects
of facilitation or depression of synapses by series of consecutive spikes. Thus, bursts
of action potentials can have a very different effect on the postsynaptic neuron than
regular spike trains. Furthermore, effects like gain control and dynamic linking of
neurons could be based on the transient modification of synaptic efficacy. This short-
term dynamics can be understood, for instance, in terms of models that contain a
fixed amount of a resource (e.g. neurotransmitter) which can be either available,
effective, or inactive.
2.6 Discussion
The top-down description of the human visual systems stops here, at the level of
synapses, although many interesting phenomena exist at deeper levels, like at the
level of channels or at the level of neurotransmitters. The reason for this is that it is
unlikely that specific low-level phenomena, like the generation of action potentials
by voltage sensitive channels, are decisive for our remarkable visual performance,
since they are common to all types of nervous systems.
For the remainder of this thesis, these levels serve as a substrate that produces
macroscopic effects, but they are not analyzed further. However, one should keep in
mind that these deeper levels exist and that subtle changes at the microscopic level,
like the increase of certain neurotransmitters after the consumption of drugs, can
have macroscopic effects, like visual hallucinations generated by feedback loops
with uncontrolled gains.
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