head where the information is really received. A general description of the
glutamatergic synapse includes a presynaptic button facing through a cleft with
postsynaptic spine. The area of the presynaptic button, opposed to the postsynaptic
spine, contains vesicles filled of glutamate and is called the active zone (AZ). A
number of vesicle ranging 10–20 are anchored to the presynaptic membrane by the
SNARE complex (soluble NSF attachment proteins) which is a protein complex
docking the vesicles ready to be released [13]. The arrival of a presynaptic spike
activates the fusion and the pore formation of a vesicle activated by the SNARE
complex following the Ca
2+ influx (see, e.g., [13]). The first step of the transfer of
the single elementary bit of information is then the release of a vesicle of glutamate
regulated by the SNARE complex following the arrival of a presynaptic spike. If
we consider the spike as the elementary bit of the neuronal information carried,
then we can consider the EPSP as the elementary bit of the synaptically coded
information.
The presynaptic surface, containing the docked vesicles, is separated from the
postsynaptic one by a distance (cleft) of $20 nm. The synaptic cleft is a volume
where the molecules of glutamate, released by the presynaptic vesicle, diffuse by
Brownian motion [14]. The arrival of the presynaptic spike, thanks to the Ca
2+ and
the SNARE complex, induces the formation of a pore between a vesicle and the
presynaptic membrane. This pore is the path followed by the glutamate molecules
to transit from the vesicle to the synaptic cleft.
If we assume a generic horizontal section, the diameter of a cortical or hippocampal glutamatergic synapse ranges 0.2–1 μm [15–18]. Assuming an AZ of circular
space and the cleft of $20 nm, we get a volume of cylindrical space which many
authors use to study the synaptic transmission by a computer modeling approach
[14, 16, 19]. Not all the synaptic “cylinder” is free for the diffusion of glutamate.
The AZ covers only a part of the whole synapse (mean radius 0.11 μm), while the
surrounding part is occupied by fibrils which anchor the pre- and postsynaptic
neuron [20–22].
At the postsynaptic side, two types of glutamate receptors are colocalized in an
area which is almost of the same size of the AZ and is considered as of circular shape
too (lower part of the cylinder) [22, 23]. This area is called postsynaptic density
(PSD) and contains α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid sensitive receptors (AMPA receptors) and N-methyl-d-aspartate sensitive receptors
(NMDA receptors) which contains sites to bind glutamate molecules (but also for
the glycine which is necessary for the synaptic activity). These two types of receptors have different roles in the transfer of synaptic information which we will
discuss later in details. Both types are tetramers composed of a dimer of dimers
[24]. As we will see later, the dimeric composition of the receptor plays an important role in shaping the postsynaptic response.
Apparently, the information transfer process is very simple in principle. The
arrival of a presynaptic spike produces the fusion of a vesicle with the release of
glutamate which activate postsynaptic receptors producing a depolarizing current
(EPSC) which causes a variation of the postsynaptic membrane potential called
EPSP which, diffusing through the dendritic branches, contribute at the soma, to
the generation of the postsynaptic spike. However, any of the passage from the
presynaptic to the postsynaptic side undergoes to a series of rearrangement of the
information which makes the whole process extremely complex both to study and
to interpret. In Figure 1 a schematic representation of the information flow by
synaptic transmission is presented.
Essentially, the different modulation systems produce a sort of complex
nonlinear variability of the postsynaptic response. Variability of the EPSP is caused
94
Advances in Neural Signal Processing
glutamatergic synapse includes a presynaptic button facing through a cleft with
postsynaptic spine. The area of the presynaptic button, opposed to the postsynaptic
spine, contains vesicles filled of glutamate and is called the active zone (AZ). A
number of vesicle ranging 10–20 are anchored to the presynaptic membrane by the
SNARE complex (soluble NSF attachment proteins) which is a protein complex
docking the vesicles ready to be released [13]. The arrival of a presynaptic spike
activates the fusion and the pore formation of a vesicle activated by the SNARE
complex following the Ca
2+ influx (see, e.g., [13]). The first step of the transfer of
the single elementary bit of information is then the release of a vesicle of glutamate
regulated by the SNARE complex following the arrival of a presynaptic spike. If
we consider the spike as the elementary bit of the neuronal information carried,
then we can consider the EPSP as the elementary bit of the synaptically coded
information.
The presynaptic surface, containing the docked vesicles, is separated from the
postsynaptic one by a distance (cleft) of $20 nm. The synaptic cleft is a volume
where the molecules of glutamate, released by the presynaptic vesicle, diffuse by
Brownian motion [14]. The arrival of the presynaptic spike, thanks to the Ca
2+ and
the SNARE complex, induces the formation of a pore between a vesicle and the
presynaptic membrane. This pore is the path followed by the glutamate molecules
to transit from the vesicle to the synaptic cleft.
If we assume a generic horizontal section, the diameter of a cortical or hippocampal glutamatergic synapse ranges 0.2–1 μm [15–18]. Assuming an AZ of circular
space and the cleft of $20 nm, we get a volume of cylindrical space which many
authors use to study the synaptic transmission by a computer modeling approach
[14, 16, 19]. Not all the synaptic “cylinder” is free for the diffusion of glutamate.
The AZ covers only a part of the whole synapse (mean radius 0.11 μm), while the
surrounding part is occupied by fibrils which anchor the pre- and postsynaptic
neuron [20–22].
At the postsynaptic side, two types of glutamate receptors are colocalized in an
area which is almost of the same size of the AZ and is considered as of circular shape
too (lower part of the cylinder) [22, 23]. This area is called postsynaptic density
(PSD) and contains α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid sensitive receptors (AMPA receptors) and N-methyl-d-aspartate sensitive receptors
(NMDA receptors) which contains sites to bind glutamate molecules (but also for
the glycine which is necessary for the synaptic activity). These two types of receptors have different roles in the transfer of synaptic information which we will
discuss later in details. Both types are tetramers composed of a dimer of dimers
[24]. As we will see later, the dimeric composition of the receptor plays an important role in shaping the postsynaptic response.
Apparently, the information transfer process is very simple in principle. The
arrival of a presynaptic spike produces the fusion of a vesicle with the release of
glutamate which activate postsynaptic receptors producing a depolarizing current
(EPSC) which causes a variation of the postsynaptic membrane potential called
EPSP which, diffusing through the dendritic branches, contribute at the soma, to
the generation of the postsynaptic spike. However, any of the passage from the
presynaptic to the postsynaptic side undergoes to a series of rearrangement of the
information which makes the whole process extremely complex both to study and
to interpret. In Figure 1 a schematic representation of the information flow by
synaptic transmission is presented.
Essentially, the different modulation systems produce a sort of complex
nonlinear variability of the postsynaptic response. Variability of the EPSP is caused
94
Advances in Neural Signal Processing