2. Electrical synapses are specialized sites where gap junction channels bridge
the plasma membrane of two neurons. They permit the direct transmission
of electrical signals (electrical coupling) between cells
Electrical synapses are present throughout the central nervous system and
have been studied specifically in the neocortex, hippocampus, thalamus.
Impulse is transmitted in both direction
Formed at a narrow gap between the pre- and postsynaptic neurons known
as a gap junction.
Gap junction funnels are composed of two hemi-channels called connexons
in vertebrates, one contributed by each cell at the synapse.
Electrical Synapse:
3. GABAergic neurons:
GABAergic neurons are inhibitory neurons of the nervous system that play a
vital role in neural circuitry and activity.
They are so named due to their release of the neurotransmitter gamma-
aminobutyric acid (GABA), and occupy different areas of the brain.
GABAergic neurons exhibit heterogeneous morphology and physiological
properties and represent only 10–30% of the total neuronal population in the
neocortex
GABAergic neurons could form neocortical networks whose activity is
shaped by connections among the GABAergic cells
4. GABAergic neurons form networks in the neocortex:
Neocortical GABAergic neurons are heterogeneous.
Distinguished according to criteria including morphology, action potential patterns, expression of peptides,
expression of calcium binding proteins, and synaptic properties
6. Properties of electrical synapses in the neocortex
Selectivity
Strength and kinetic
properties
Dynamic properties
Electrical coupling was only found between GABAergic neurons whereas pyramidal cells were not coupled
It is important that the strength of electrical coupling is sufficient to have an impact on the electrical activity of coupled
cells
7. A network of FS cells interconnected via electrical and GABAergic synapses could
promote synchronous firing
8. Discussion and future directions:
The high specificity of electrical synapses suggests that there are mechanisms enabling inhibitory neurons
to identify and select different cell types in the neocortex.
The cellular and molecular mechanisms that underlie the construction of these networks are unknown.
Connexin-based gap junctions have important functional roles. eg, Elimination of Cx36-containing gap
junctions between suprachiasmatic nucleus neurons impairs the diurnal cycle in mice
The diversity of transmitter receptors and voltage-dependent channels expressed by different neocortical
interneurons suggests that the action of specific networks can be selectively modulated by
neurotransmitters.
Establish the anatomical organization of these networks, which could help us to understand the functional
structure of the neocortex
Editor's Notes
Neocortical neurons are either excitatory, releasing glutamate, or inhibitory, releasing GABA; the axons of both glutamatergic and GABAergic neurons make extensive local connections.
However, recent studies using paired recordings from neocortical neurons identified morphologically and physiologically have revealed that electrical synapses are present exclusively between GABAergic interneurons. author propose that the remarkable selectivity and the properties of electrical synapses define multiple functional networks of distinct GABAergic interneurons in the neocortex. These networks could have different roles in coordinating cortical activity
This heterogeneity suggests that different types of GABAergic cells might have different functional roles in the cerebral cortex.
FS cells include neocortical basket cells and are characterized by expression of parvalbumin. They are highly interconnected via both electrical and GABAergic synapses in both juvenile and adult rodent neocortex
Low threshold spiking (LTS) cells, which typically contain the peptide somatostatin and include Martinotti cells, rarely have GABA-mediated interconnections but are highly interconnected by electrical synapses
CB1-IS(cannabinoid receptor) cells represent only small fraction of neocortical neurons but finding electrical synapses in a pair of CB1-IS cells is high (90%)
To date, electrical synapses have been demonstrated using paired recordings in at least five identified classes of neocortical GABAergic neurons, in all layers of the neocortex
The general finding is that the probability of electrical coupling is high, O50%, if both cells belong to the same class, but !5% if the two cells belong to different classes. The high rate of specific connectivity via electrical synapses between same-type interneurons suggests that electrical synapses define distinct networks of GABAergic cells embedded in the cerebral cortex
Five networks of distinct electrically coupled GABAergic interneurons in the cerebral cortex (numbers indicate cortical layers). Jagged lines between interneurons represent electrical coupling. Circles represent synapses onto other interneurons of the same class or onto pyramidal cells (black). Labels below the cell bodies of FS, MB and LTS cells are chemical markers of these interneurons.
First, electrical coupling was only found between GABAergic neurons – pyramidal cells were not coupled (Figure 1a,b). Second, electrical synapses were found almost exclusively between GABAergic cells of the same class.
Cell-type-specific electrical coupling between GABAergic interneurons. (a) (i) Infrared video-micrograph showing a pair of electrically coupled FS cells recorded simultaneously using patch electrodes (ii) Positive or negative current injections in cell 1 resulted in a simultaneous depolarization or hyperpolarization of both cells. (b) (i) Infrared video-micrograph of two pyramidal neurons (Pyr) that were not electrically coupled. (ii) Injection of current in cell 1 did not affect the membrane voltage of cell 2. (c) Simultaneous recording from a pair of electrically coupled CB1-IS cells depolarized until they fired at a frequency of w7 Hz. The traces illustrate two single trials. Diamonds indicate spikes occurring in both cells within a window of %5 ms. The DC coupling coefficient (i.e. the ratio of the membrane potential of the two cells during an injection of current into one of the cells) was 9.8%. Inset, a presynaptic spike (red) and the corresponding postsynaptic spikelet (blue). The traces are averages of 45 trials. (d) Cross-correlogram of the firing activity of the cells shown in (c). Note The Increase Of Firing Probability Near 0 Ms, Indicating That The Spiking Of Both Cells Was Synchronized
How a network of FS cells interconnected via electrical and GABAergic synapses could promote synchronous firing. (a) When a network of FS 9fast spiking) cells (green) receives coherent excitatory inputs (red lines), the firing of FS cells will be promoted by signaling via electrical synapses (jagged lines). (b) If the same number of excitatory inputs arrives in as a non-coherent wave, the firing of the FS cells will inhibit each other via their mutual GABAergic synapses (black circles) and transmission of after-hyperpolarizing potentials through electrical synapses, and the global firing of the network and output to pyramidal cells (red cells) will be diminished
In physics, two wave sources are perfectly coherent if they have a constant phase difference and the same frequency, and the same waveform. Non coherent means random and changing beheivour.
