Neurobiological Background - Hierarchical Neural Networks for Image Interpretation - page 29

Fig. 2.12. Synaptic transmission at chemical synapse. Presynaptic depolarization leads to the
influx of Ca 2+ ions through voltage gated channels. Vesicles merge with the membrane and
release neurotransmitters into the synaptic cleft. These diffuse to receptors that open or close
channels in the postsynaptic membrane. Changed ion flow modifies the postsynaptic potential
(adapted from [117]).
Chemical synapses allow for more specific communication between neurons
since they separate the potentials of the presynaptic and postsynaptic cells by the
synaptic cleft. Communication is unidirectional from the presynaptic to the postsy-
naptic cell, as illustrated in Figure 2.12.
When an action potential arrives at a synaptic terminal, voltage gated channels in
the presynaptic membrane are opened and Ca 2+ ions flow into the cell. This causes
vesicles containing neurotransmitters to fuse with the membrane at specific docking
sites. The neurotransmitters are released and diffuse through the synaptic cleft. They
bind to corresponding receptors on the postsynaptic membrane that open or close ion
channels. The modified ion flux now changes the postsynaptic membrane potential.
Neurotransmitters act either directly or indirectly on ion channels that regulate
current flow across membranes. Direct gating is mediated by ionotropic receptors
that are an integral part of the same macromolecule which forms the ion channel.
The resulting postsynaptic potentials last only for few milliseconds. Indirect gat-
ing is mediated by activation of metabotropic receptors that are distinct from the
channels. Here, channel activity is modulated through a second messenger cascade.
These effects last for seconds to minutes and are believed to play a major role in
adaptation and learning.
The postsynaptic response can be either excitatory or inhibitory, depending on
the type of the presynaptic cell. Figure 2.13 shows a presynaptic action potential
along with an excitatory (EPSP) and an inhibitory postsynaptic potential (IPSP).
The EPSP depolarizes the cell from its resting potential of about − 70 mV and brings
it closer towards the firing threshold of − 55 mV. In contrast, the IPSP hyperpolarizes
the cell beyond its resting potential. Excitatory synapses are mostly located at spines