protruding from the shaft of the dendritic branches. When activated they produce
the so-called excitatory postsynaptic current (EPSC) which is a current which
depolarizes the membrane (increases V m ) producing the so-called excitatory postsynaptic potential (EPSP). The regulatory effect of the GABAergic (mainly GABA A
type) synapses is to repolarize V m by the so-called inhibitory postsynaptic current
(IPSC) producing the opposite effect on the membrane voltage and generating the
inhibitory postsynaptic potential (IPSP).
The integration of the activity of these large amounts of inputs at the soma of the
neuron determines the spiking behavior of the neuron (coding). Considering that
on the average a neuron makes a single synapse to another neuron, each neuron
receives contacts from thousands of neurons each of which try to send the information it carries. However, several neurons of a given area (sending area) can give
each a single contact to the same neuron (receiving neuron). If the many neurons of
the sending areas are excited by a stimulus, the integration of the synaptic responses
on the receiving neuron will produce the postsynaptic representation of the stimulus. For example, several neurons of the dentate gyrus can input on the same neuron
of one of the subfields of hippocampus (CA1 or CA3) [7]. Moreover, such an input
interacts with the inputs coming from other neurons located on different areas
(e.g., from the entorhinal cortex in the example given before [7]).
If we only look to this short and incomplete representation of the problem of the
information management by a single neuron, it becomes clear how and why the
correspondence between the sequence of spikes and the code it generates is very
variable such that the correlation between the inputs and the code generated is
unpredictable and appears random.
In this framework, a great amount of complexity depends on the mechanisms
which regulate the transmission of a single bit (spike) information to each synapse.
The synaptic activity is greatly influenced by many factors [8]. First of all, the way
glutamatergic synapse contributes to the postsynaptic neuronal code depends
strongly on the biophysical properties of the dendrite where the synapse is located
and on the path from its location to the soma. The electrical signal generated at a
synapse attenuates with distance according to the cable properties of the dendritic
path which changes along the arborization depending mainly on the dendritic size
[9, 10]. The attenuation with distance is of exponential type [9, 10]. Usually, the
higher input impedance of the branches more far from the soma seems to help the
diffusion of the far signals producing EPSP with higher amplitude, a phenomenon
which some authors consider as a sort of “synaptic democracy” [11, 12].
To give an idea of some of the basic mechanisms involved in the modulation of
the synaptic information transferred to a neuron, in the following we will briefly
remember, in a simplified way, the basic mechanism of the synaptic transmission
with a particular attention to those processes which participate to the modulation of
the signal. A part of the ability to transmit and modulate the information depends
directly on the synaptic structure. For this reason we will first describe a general
glutamatergic synapses and after the pre- and postsynaptic mechanisms influencing
the modulation of the information carried by a single bit of synaptic information
(the EPSP). The modulatory effect on a sequence of elementary bits (a “word”) will
also be considered, and a final discussion will summarize the effects of the different
modulatory systems.
2. Synaptic structure and mechanisms
A classical glutamatergic synapse is located on the top of a spine of the dendritic
tree. The spine is composed of a neck, protruding from the dendritic shaft, and a
93
Information Processing and Synaptic Transmission
DOI: http://dx.doi.org/10.5772/intechopen.88405
the so-called excitatory postsynaptic current (EPSC) which is a current which
depolarizes the membrane (increases V m ) producing the so-called excitatory postsynaptic potential (EPSP). The regulatory effect of the GABAergic (mainly GABA A
type) synapses is to repolarize V m by the so-called inhibitory postsynaptic current
(IPSC) producing the opposite effect on the membrane voltage and generating the
inhibitory postsynaptic potential (IPSP).
The integration of the activity of these large amounts of inputs at the soma of the
neuron determines the spiking behavior of the neuron (coding). Considering that
on the average a neuron makes a single synapse to another neuron, each neuron
receives contacts from thousands of neurons each of which try to send the information it carries. However, several neurons of a given area (sending area) can give
each a single contact to the same neuron (receiving neuron). If the many neurons of
the sending areas are excited by a stimulus, the integration of the synaptic responses
on the receiving neuron will produce the postsynaptic representation of the stimulus. For example, several neurons of the dentate gyrus can input on the same neuron
of one of the subfields of hippocampus (CA1 or CA3) [7]. Moreover, such an input
interacts with the inputs coming from other neurons located on different areas
(e.g., from the entorhinal cortex in the example given before [7]).
If we only look to this short and incomplete representation of the problem of the
information management by a single neuron, it becomes clear how and why the
correspondence between the sequence of spikes and the code it generates is very
variable such that the correlation between the inputs and the code generated is
unpredictable and appears random.
In this framework, a great amount of complexity depends on the mechanisms
which regulate the transmission of a single bit (spike) information to each synapse.
The synaptic activity is greatly influenced by many factors [8]. First of all, the way
glutamatergic synapse contributes to the postsynaptic neuronal code depends
strongly on the biophysical properties of the dendrite where the synapse is located
and on the path from its location to the soma. The electrical signal generated at a
synapse attenuates with distance according to the cable properties of the dendritic
path which changes along the arborization depending mainly on the dendritic size
[9, 10]. The attenuation with distance is of exponential type [9, 10]. Usually, the
higher input impedance of the branches more far from the soma seems to help the
diffusion of the far signals producing EPSP with higher amplitude, a phenomenon
which some authors consider as a sort of “synaptic democracy” [11, 12].
To give an idea of some of the basic mechanisms involved in the modulation of
the synaptic information transferred to a neuron, in the following we will briefly
remember, in a simplified way, the basic mechanism of the synaptic transmission
with a particular attention to those processes which participate to the modulation of
the signal. A part of the ability to transmit and modulate the information depends
directly on the synaptic structure. For this reason we will first describe a general
glutamatergic synapses and after the pre- and postsynaptic mechanisms influencing
the modulation of the information carried by a single bit of synaptic information
(the EPSP). The modulatory effect on a sequence of elementary bits (a “word”) will
also be considered, and a final discussion will summarize the effects of the different
modulatory systems.
2. Synaptic structure and mechanisms
A classical glutamatergic synapse is located on the top of a spine of the dendritic
tree. The spine is composed of a neck, protruding from the dendritic shaft, and a
93
Information Processing and Synaptic Transmission
DOI: http://dx.doi.org/10.5772/intechopen.88405