• Spike backpropagation
• Active synaptic inputs on the dendritic tree
3. Discussion
This short overview was aimed to stress how the information transferred among
synapses and its elaboration undergo to many regulation systems which involve
structural, functional, and cooperative processes. By identifying the EPSP as the
elementary bit of the information transferred by a single synapse, we have outlined
some of the pre- and postsynaptic sources of variability.
In general the word “variability” can be used with two meanings. It can be
attributed either to something which vary in an unpredictable way, or it can mean
the possibility to change following specific actions. This is especially true for the
causes of variability of the EPSP. EPSP variability can be due (a) to stochastic
processes [8, 32–34] or (b) to specific systems of regulation which operate at
different levels of the synaptic transmission (intrasynaptic or extrasynaptic). About
the stochastic variability, we cannot say too much. If a process occurs randomly, we
can only try to understand its effects observing the responses and trying to explain
the phenomena by a plausible model which (statistically) describes the natural
event. The big problem in this respect is to identify if this type of system depends
really on stochastic processes or if stochasticity is apparent because the lack of the
full information needed to characterize the processes. From the most top point of
view, almost all the causes of EPSP variability described herein can appear of
stochastic type [8, 32–34], but we cannot definitively exclude that the apparent
stochasticity is due to our incomplete understanding of all the underlying mechanisms and/or to the lack of knowledge of all the steps underlying the process. Just to
give an example, if we consider the response variability depending on the number
of molecules in the vesicle, its position on the AZ (eccentricity), and its variable
release probability, [8, 17, 27–29, 32–34] we can assume a stochastic origin of the
presynaptic factors of the synaptic response variability. However, the mechanism of
the vesicle opening is under the control of the SNARE complex which is intimately
connected to the vesicle and is the responsible for the Ca
2+ dependent pore opening.
This complex can have different configurations depending on the state of the
neuron (see, e.g., [13]). We cannot exclude that a more complete understanding of
the SNARE complex functionality could permit the definition of a relationship
between the information passed by the synapse and the characteristics (position
and number of molecules) of the released vesicle. This is only a possibility for one of
the many regulatory factors involved in the synaptic response modulation, and their
discussion is not in the goal of the present chapter. The important point that we
want to stress is to outline the large variability of the EPSP and that this variability is
controlled by many different systems. Variability, then, in the context of this
chapter, has to be intended as the ability to be modulated (“tuning”) of the system.
The tuning of the information to transfer is not only due to the pre- and
postsynaptic neuron. The activity of thousands of synapses inputting on a neuron
produces waves of potential into the dendritic tree which directly influence the
characteristics of the information transferred by each single synapse [65, 66].
Even two single synapses, closely located on a dendritic branch, influence each
other. The synapse which fires first, in fact, by changing the membrane potential
influences the response of the synapse firing later if the time interval between the
two events is compatible with the decay time of the first event [30].
101
Information Processing and Synaptic Transmission
DOI: http://dx.doi.org/10.5772/intechopen.88405
• Active synaptic inputs on the dendritic tree
3. Discussion
This short overview was aimed to stress how the information transferred among
synapses and its elaboration undergo to many regulation systems which involve
structural, functional, and cooperative processes. By identifying the EPSP as the
elementary bit of the information transferred by a single synapse, we have outlined
some of the pre- and postsynaptic sources of variability.
In general the word “variability” can be used with two meanings. It can be
attributed either to something which vary in an unpredictable way, or it can mean
the possibility to change following specific actions. This is especially true for the
causes of variability of the EPSP. EPSP variability can be due (a) to stochastic
processes [8, 32–34] or (b) to specific systems of regulation which operate at
different levels of the synaptic transmission (intrasynaptic or extrasynaptic). About
the stochastic variability, we cannot say too much. If a process occurs randomly, we
can only try to understand its effects observing the responses and trying to explain
the phenomena by a plausible model which (statistically) describes the natural
event. The big problem in this respect is to identify if this type of system depends
really on stochastic processes or if stochasticity is apparent because the lack of the
full information needed to characterize the processes. From the most top point of
view, almost all the causes of EPSP variability described herein can appear of
stochastic type [8, 32–34], but we cannot definitively exclude that the apparent
stochasticity is due to our incomplete understanding of all the underlying mechanisms and/or to the lack of knowledge of all the steps underlying the process. Just to
give an example, if we consider the response variability depending on the number
of molecules in the vesicle, its position on the AZ (eccentricity), and its variable
release probability, [8, 17, 27–29, 32–34] we can assume a stochastic origin of the
presynaptic factors of the synaptic response variability. However, the mechanism of
the vesicle opening is under the control of the SNARE complex which is intimately
connected to the vesicle and is the responsible for the Ca
2+ dependent pore opening.
This complex can have different configurations depending on the state of the
neuron (see, e.g., [13]). We cannot exclude that a more complete understanding of
the SNARE complex functionality could permit the definition of a relationship
between the information passed by the synapse and the characteristics (position
and number of molecules) of the released vesicle. This is only a possibility for one of
the many regulatory factors involved in the synaptic response modulation, and their
discussion is not in the goal of the present chapter. The important point that we
want to stress is to outline the large variability of the EPSP and that this variability is
controlled by many different systems. Variability, then, in the context of this
chapter, has to be intended as the ability to be modulated (“tuning”) of the system.
The tuning of the information to transfer is not only due to the pre- and
postsynaptic neuron. The activity of thousands of synapses inputting on a neuron
produces waves of potential into the dendritic tree which directly influence the
characteristics of the information transferred by each single synapse [65, 66].
Even two single synapses, closely located on a dendritic branch, influence each
other. The synapse which fires first, in fact, by changing the membrane potential
influences the response of the synapse firing later if the time interval between the
two events is compatible with the decay time of the first event [30].
101
Information Processing and Synaptic Transmission
DOI: http://dx.doi.org/10.5772/intechopen.88405