Neurobiological Background - Hierarchical Neural Networks for Image Interpretation - page 28

old, a wave of activity is generated and actively propagated towards the axon ter-
minals. Thereafter, the neuron becomes insensitive to stimuli during a refractory
period of some milliseconds. Propagation is based on voltage sensitive channels
in the axon's membrane. For fast transmission, some axons are covered by myelin
sheaths, interrupted by nodes of Ranvier. Here, the action potential jumps from node
to node, where it is regenerated. The axon terminates in many synapses that make
contacts with other cells.
Only some neurons, that have no axons or only very short axons, use the graded
potential directly for neurotransmitter release at synapses. They can be found, for
instance, in the retina and in higher areas of invertebrates. Although the graded
potential contains more information than the all-or-nothing signal of an action po-
tential [87], it is used for local communication only since it decays exponentially
when conducted over longer distances. In contrast, the action potential is regener-
ated and thus is not lost. Action potentials have a uniform spike-like shape with a
duration of 1ms. The frequency of sending action potentials and the exact timing of
these potentials relative to each other and relative to the spikes of other cells or to
other sources of reference, such as subthreshold oscillations or stimulus onset, may
contain information.
Neurons come in many different shapes as they form specific networks with
other neurons. Depending on their task, they collect information from many other
neurons in a specific way and distribute their action potential to a specific set of other
cells. Although neurons have been modeled as simple leaky integrators with a sin-
gle compartment, it is more and more appreciated that more complex computation
is done in the dendritic tree than passive conductance of postsynaptic potentials. For
example, it has been shown that neighboring synapses can influence each other e.g.
in a multiplicative fashion. Furthermore, active spots have been localized in den-
drites, where membrane potentials are amplified. Finally, information also travels
backwards into the dendritic tree when a neuron is spiking. This may influence the
response to the following presynaptic spikes and also be a substrate for modification
of synaptic efficacy.

2.5 Synapses

While neurons communicate internally by means of electric potentials, communi-
cation between neurons is mediated by synapses. Two types of synapses exist: elec-
trical and chemical.
Electrical synapses couple the membranes of two cells directly. Small ions pass
through gap-junction channels in both directions between the cells. Electrical trans-
mission is graded and occurs even when the currents in the presynaptic cell are
below the threshold for an action potential. This communication is very fast, but un-
specific and not flexible. It is used, for instance, to make electrically connected cells
fire in synchrony. Gap-junctions play also a role in glia cells, where Ca 2+ waves

travel through networks of astrocytes.