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| 07 | Electrical Synapse Overview.pdf [native] [pdf in chunks] | 1 MB | 5 | ℹ️ | ⬇️ |
instagram-1793127205135.jpg (248.02 kB . 1322x1194 - viewed 27 times)The short answer: a neuron typically needs its membrane potential to depolarise to around −55 mV to trigger an action potential, but the exact threshold varies with cell type and its current state.
The key is that we're talking about voltage across the cell membrane, not voltage applied externally.
Resting potential
Typical baseline for a neuron.
Approximate firing threshold
Voltage-gated sodium channels can initiate a regenerative action potential.
Action-potential peak
A typical overshoot during the spike, before repolarisation.
Suppose neuron A is at −50 mV while neuron B is at −70 mV. There is a 20 mV voltage difference.
Current can flow through their gap junction, tending to bring their membrane potentials closer together. If enough depolarising current reaches neuron B, it might reach its firing threshold.
But the 20 mV difference does not mean neuron B automatically receives a 20 mV increase. The amount depends on junctional conductance, membrane resistance, capacitance and other inputs.
For example, neuron B might depolarise from −70 mV to −65 mV, a 5 mV change, rather than reaching −50 mV.
If by motivate the cell you mean make it more likely to fire, a neuron doesn't necessarily need to reach threshold. Even a small depolarisation of 1–5 mV can meaningfully change its firing probability, depending on how close it already is to threshold and what other inputs it receives.
So there are two different quantities:
Are you interested specifically in how much voltage passes through an electrical synapse to influence the neighbouring neuron, or how much voltage a neuron needs to fire in the first place?
An electrical synapse is a direct electrical connection between two cells, allowing ionic current to pass from one cell to another through specialised intercellular channels. Unlike the familiar chemical synapse, it doesn't need to release neurotransmitters into a synaptic cleft to transmit its signal.
The remarkable thing is that electrical synapses can make groups of neurons behave less like independent processing units and more like a coordinated, electrically coupled network.
The Electrical Synapse (Gap Junction) – Developing Expertise in Neuroscience
There are two major forms of communication between neurons:
Both operate in the nervous system, and they can coexist in the same neural circuit.
PPT - Chemical and Electrical Synapses PowerPoint Presentation, free download - ID:5928223
The essential structure is the gap junction.
A gap junction consists of aligned channel structures in the membranes of two adjacent cells. In vertebrates, these channels are assembled from proteins called connexins.
Here's the construction:
Ions can then move directly between the cells. Depending on the channel, some small molecules and intracellular signalling substances can pass as well.
This isn't a synaptic vesicle releasing a chemical into extracellular space. It's a direct intercellular pathway.
Primarily, electrical current carried by ions such as sodium, potassium and other mobile ions, depending on the local electrochemical conditions.
The direction and magnitude of current depend on the voltage difference between the coupled cells and the junction's conductance.
A simplified relationship is:
\[ I_{\mathrm{junction}}=g_{\mathrm{j}}(V_1-V_2) \]
Where:
The greater the voltage difference, the greater the current, assuming conductance remains constant.
That current changes the receiving cell's membrane voltage. If enough depolarisation occurs, the receiving neuron may fire an action potential.
Importantly, electrical coupling doesn't automatically mean that every signal causes the second neuron to fire. The effect depends on coupling strength, membrane properties, timing and the activity of the surrounding network.
| Property | Electrical | Chemical |
|---|---|---|
| Connection | Direct channel between cells | Cells separated by a synaptic cleft |
| Signal transfer | Ionic current | Neurotransmitter and receptors |
| Speed | Extremely fast | Usually slower |
| Direction | Often bidirectional | Usually one-way |
| Signal amplification | Limited at the junction itself | Can be substantial |
| Flexibility | Modifiable, but often less versatile | Extensive receptor and release mechanisms |
| Information transfer | Voltage changes; some small molecules | Many types of chemical signals |
| Typical role | Coupling, timing, coordination | Excitation, inhibition, modulation, learning |
These are generalisations, not absolute rules. Electrical synapses can be directional, regulated and plastic; chemical synapses can be extraordinarily fast.
NCBI Bookshelf+1
The most important distinction is this: chemical synapses can transform a signal through receptors and biochemical cascades, whereas electrical synapses directly couple the electrical states of cells.
Electrical synapses aren't simply wires that copy one neuron's firing into another.
In a typical electrical junction, current can flow in either direction. If neuron A becomes more positive than neuron B, current flows according to the voltage difference and the junction's conductance. If the voltage relationship reverses, current can reverse too.
This allows a network to share electrical activity without a permanently designated sender and receiver.
Some electrical junctions conduct more strongly in one direction than the other. This is called rectification.
The result is a connection that can favour A-to-B transmission over B-to-A transmission, despite the cells being physically connected.
Electrical coupling also filters signals. Slow voltage changes may spread effectively, while rapid fluctuations can be attenuated because of membrane capacitance, electrical resistance and the geometry of the coupled cells.
That means the junction can transmit some components of a signal more effectively than others.
A further surprise is that coupling can sometimes have an inhibitory effect. If a neuron receives an excitatory voltage change but shares current with a less-excited neighbour, some of that current flows away. The receiving cell's voltage change becomes smaller.
Thus, an electrical synapse can promote excitation, reduce excitation or influence firing synchrony, depending on the circuit.
PubMed+1
A. Synchronise neuronal populations
Coupled neurons can bring their voltage fluctuations and firing times into closer alignment. This helps coordinate groups of cells that need to act together, including networks involved in rhythmic activity and motor coordination.
Frontiers | Inhibitory Circuits in Cortical Layer 5
B. Coordinate inhibitory interneurons
Some GABA-releasing interneurons are electrically coupled. Their electrical coupling helps coordinate their activity, which can synchronise the chemical inhibition they deliver to other neurons. This influences population rhythms, including gamma-frequency activity in certain circuits.
C. Process sensory information
Electrical synapses in the retina help distribute visual signals through particular pathways. Their effects can improve signal-to-noise relationships and influence how the visual system represents changing light levels and spatial patterns.
Frontiers | Central Mechanism Controlling Pubertal Onset in Mammals: A Triggering Role of Kisspeptin
D. Coordinate neuroendocrine activity
Electrical coupling between certain hormone-related neurons can help coordinate their firing and support appropriately timed bursts of hormone release.
These roles are established across different neural systems, although the precise contribution of electrical coupling varies by cell type and circuit.
PubMed+2
In the mammalian brain, the best-known neuronal gap-junction protein is connexin 36 (Cx36).
Connexin 36 is particularly important for electrical coupling between many classes of neurons, including certain cortical, hippocampal and thalamic interneurons. Disrupting Cx36 can substantially reduce coupling in these circuits.
PubMed+1
A few molecular details matter:
This last point is particularly important. The functional strength of an electrical synapse isn't determined solely by the number of gap-junction channels. The receiving cell's own electrical properties determine how much its voltage changes in response to incoming current.
PubMed+1
One of the older assumptions was that electrical synapses were relatively rigid connections: useful for fast communication, but less adaptable than chemical synapses.
That picture is incomplete.
Electrical coupling can change through several mechanisms:
These processes can alter how strongly neurons influence one another and, consequently, how a circuit operates.
PubMed+1
Yes, in the broad physiological sense: electrical coupling is modifiable, and those modifications can alter circuit function.
But we need to distinguish two things:
The first is well established. The second is much more complicated. Electrical synapses can influence the activity of memory-related circuits, but we cannot simply conclude that a stronger electrical junction stores a particular memory.
This is where things get particularly interesting.
A chemical synapse can change its strength, receptor complement and biochemical signalling in response to activity. Such changes are central to well-established models of learning and memory.
Electrical synapses introduce another dimension: they change how activity propagates through groups of neurons.
Imagine three coupled neurons receiving different inputs.
Their electrical connections redistribute current. Depending on the network's properties, this may help synchronise responses, suppress a weak signal, enhance the impact of coincident inputs or change which neurons reach firing threshold.
In that sense, electrical coupling can change the collective computation performed by a neural network.
PubMed+1
However, electrical coupling doesn't necessarily improve information processing in every situation. Sharing current can also blur differences between neurons, attenuate a signal or synchronise cells that would otherwise respond independently.
There is an important distinction here.
Electrical synapses could help coordinate neurons that participate in a memory-related network. Their activity might influence when neurons fire together, how signals propagate and how chemical synapses are subsequently modified.
But there is no established evidence that electrical synapses generally act as direct links between stored memories, or that memories are encoded as discrete objects passed through gap junctions.
A useful working model is:
These mechanisms work together rather than representing competing explanations of cognition.
These two mechanisms are easy to confuse.
Mª José Mas Salguero (@MasTwitts) on X
Electrical synapse
Cells are directly connected by gap-junction channels. Ionic current can pass between their interiors.
Cognition is an emergent property - PMC
Ephaptic coupling
Cells influence one another through extracellular electrical fields and currents, without needing a direct gap-junction connection.
Both are forms of electrical interaction, but only the first is an electrical synapse in the strict gap-junction sense.
This matters because neural tissue is an electrically active medium. Electrical interactions need not always occur through neurotransmitters or direct cellular channels.
Gap junctions also occur in many non-neuronal tissues. They are important in the heart, where electrical coupling helps coordinate the spread of excitation between cardiac muscle cells, and in other tissues where cells exchange ions and small signalling molecules.
The proteins and physiological roles differ across tissues. In the nervous system, neuronal gap junctions are particularly interesting because they combine direct electrical coupling with the complex, nonlinear behaviour of excitable cells.
There is also an evolutionary distinction: vertebrate gap junctions are principally built from connexins, whereas invertebrate gap junctions generally use proteins called innexins. The broader strategy of direct intercellular coupling is ancient.
American Physiological Society+1
Their capabilities are impressive, but their limitations are just as informative.
They do not transmit every aspect of a neuron's state. They couple electrical potentials and permit the passage of certain small substances; they don't copy a complete neural state, including every receptor configuration, protein concentration or biochemical process.
They do not automatically create synchrony. Coupling tends to reduce voltage differences, but membrane properties and circuit architecture can produce more complicated outcomes, including asynchronous firing.
They are not necessarily faster in every meaningful sense. The junctional transfer itself has virtually no chemical-release delay, but the complete neural response still depends on action-potential propagation, membrane charging and the receiving cell's firing threshold.
They are not equivalent to direct axonal connections. An axon carries electrical signals along a neuron's own membrane. An electrical synapse transfers current between separate cells.
These distinctions are important when constructing models of neural computation.
NCBI Bookshelf+1
Several questions remain active areas of investigation:
The central research problem is no longer merely whether electrical synapses exist. It is understanding what computation a particular electrically coupled circuit performs, and how that computation changes when its coupling changes.
For further reading, three useful reviews are:
An electrical synapse is more than a fast alternative to chemical neurotransmission. It is a mechanism for coupling the dynamics of individual cells into the dynamics of a larger network.
That distinction matters. A network's behaviour depends not just on what each neuron computes independently, but on how the neurons influence one another, share current, align their timing and interact with their chemical synapses.
If you're interested in systems-level neuroscience, I would focus on three things: electrical coupling, network synchronisation and the way coupling changes the computation performed by a population of neurons. Together, they offer a powerful framework for investigating how local cellular behaviour produces coordinated neural activity.
An electrical synapse is a direct electrical connection between two cells, allowing ionic current to pass from one cell to another through specialised intercellular channels. Unlike the familiar chemical synapse, it doesn't need to release neurotransmitters into a synaptic cleft to transmit its signal.
The remarkable thing is that electrical synapses can make groups of neurons behave less like independent processing units and more like a coordinated, electrically coupled network.
The Electrical Synapse (Gap Junction) – Developing Expertise in Neuroscience
7
There are two major forms of communication between neurons:
Both operate in the nervous system, and they can coexist in the same neural circuit.
PPT - Chemical and Electrical Synapses PowerPoint Presentation, free download - ID:5928223
The essential structure is the gap junction.
A gap junction consists of aligned channel structures in the membranes of two adjacent cells. In vertebrates, these channels are assembled from proteins called connexins.
Here's the construction:
Ions can then move directly between the cells. Depending on the channel, some small molecules and intracellular signalling substances can pass as well.
This isn't a synaptic vesicle releasing a chemical into extracellular space. It's a direct intercellular pathway.
Primarily, electrical current carried by ions such as sodium, potassium and other mobile ions, depending on the local electrochemical conditions.
The direction and magnitude of current depend on the voltage difference between the coupled cells and the junction's conductance.
A simplified relationship is:
\[ I_{\mathrm{junction}}=g_{\mathrm{j}}(V_1-V_2) \]
Where:
The greater the voltage difference, the greater the current, assuming conductance remains constant.
That current changes the receiving cell's membrane voltage. If enough depolarisation occurs, the receiving neuron may fire an action potential.
Importantly, electrical coupling doesn't automatically mean that every signal causes the second neuron to fire. The effect depends on coupling strength, membrane properties, timing and the activity of the surrounding network.
| Property | Electrical | Chemical |
|---|---|---|
| Connection | Direct channel between cells | Cells separated by a synaptic cleft |
| Signal transfer | Ionic current | Neurotransmitter and receptors |
| Speed | Extremely fast | Usually slower |
| Direction | Often bidirectional | Usually one-way |
| Signal amplification | Limited at the junction itself | Can be substantial |
| Flexibility | Modifiable, but often less versatile | Extensive receptor and release mechanisms |
| Information transfer | Voltage changes; some small molecules | Many types of chemical signals |
| Typical role | Coupling, timing, coordination | Excitation, inhibition, modulation, learning |
These are generalisations, not absolute rules. Electrical synapses can be directional, regulated and plastic; chemical synapses can be extraordinarily fast.
NCBI Bookshelf+1
The most important distinction is this: chemical synapses can transform a signal through receptors and biochemical cascades, whereas electrical synapses directly couple the electrical states of cells.
Electrical synapses aren't simply wires that copy one neuron's firing into another.
In a typical electrical junction, current can flow in either direction. If neuron A becomes more positive than neuron B, current flows according to the voltage difference and the junction's conductance. If the voltage relationship reverses, current can reverse too.
This allows a network to share electrical activity without a permanently designated sender and receiver.
Some electrical junctions conduct more strongly in one direction than the other. This is called rectification.
The result is a connection that can favour A-to-B transmission over B-to-A transmission, despite the cells being physically connected.
Electrical coupling also filters signals. Slow voltage changes may spread effectively, while rapid fluctuations can be attenuated because of membrane capacitance, electrical resistance and the geometry of the coupled cells.
That means the junction can transmit some components of a signal more effectively than others.
A further surprise is that coupling can sometimes have an inhibitory effect. If a neuron receives an excitatory voltage change but shares current with a less-excited neighbour, some of that current flows away. The receiving cell's voltage change becomes smaller.
Thus, an electrical synapse can promote excitation, reduce excitation or influence firing synchrony, depending on the circuit.
PubMed+1
A. Synchronise neuronal populations
Coupled neurons can bring their voltage fluctuations and firing times into closer alignment. This helps coordinate groups of cells that need to act together, including networks involved in rhythmic activity and motor coordination.
Frontiers | Inhibitory Circuits in Cortical Layer 5
B. Coordinate inhibitory interneurons
Some GABA-releasing interneurons are electrically coupled. Their electrical coupling helps coordinate their activity, which can synchronise the chemical inhibition they deliver to other neurons. This influences population rhythms, including gamma-frequency activity in certain circuits.
C. Process sensory information
Electrical synapses in the retina help distribute visual signals through particular pathways. Their effects can improve signal-to-noise relationships and influence how the visual system represents changing light levels and spatial patterns.
Frontiers | Central Mechanism Controlling Pubertal Onset in Mammals: A Triggering Role of Kisspeptin
D. Coordinate neuroendocrine activity
Electrical coupling between certain hormone-related neurons can help coordinate their firing and support appropriately timed bursts of hormone release.
These roles are established across different neural systems, although the precise contribution of electrical coupling varies by cell type and circuit.
PubMed+2
In the mammalian brain, the best-known neuronal gap-junction protein is connexin 36 (Cx36).
Connexin 36 is particularly important for electrical coupling between many classes of neurons, including certain cortical, hippocampal and thalamic interneurons. Disrupting Cx36 can substantially reduce coupling in these circuits.
PubMed+1
A few molecular details matter:
This last point is particularly important. The functional strength of an electrical synapse isn't determined solely by the number of gap-junction channels. The receiving cell's own electrical properties determine how much its voltage changes in response to incoming current.
PubMed+1
One of the older assumptions was that electrical synapses were relatively rigid connections: useful for fast communication, but less adaptable than chemical synapses.
That picture is incomplete.
Electrical coupling can change through several mechanisms:
These processes can alter how strongly neurons influence one another and, consequently, how a circuit operates.
PubMed+1
Yes, in the broad physiological sense: electrical coupling is modifiable, and those modifications can alter circuit function.
But we need to distinguish two things:
The first is well established. The second is much more complicated. Electrical synapses can influence the activity of memory-related circuits, but we cannot simply conclude that a stronger electrical junction stores a particular memory.
This is where things get particularly interesting.
A chemical synapse can change its strength, receptor complement and biochemical signalling in response to activity. Such changes are central to well-established models of learning and memory.
Electrical synapses introduce another dimension: they change how activity propagates through groups of neurons.
Imagine three coupled neurons receiving different inputs.
Their electrical connections redistribute current. Depending on the network's properties, this may help synchronise responses, suppress a weak signal, enhance the impact of coincident inputs or change which neurons reach firing threshold.
In that sense, electrical coupling can change the collective computation performed by a neural network.
PubMed+1
However, electrical coupling doesn't necessarily improve information processing in every situation. Sharing current can also blur differences between neurons, attenuate a signal or synchronise cells that would otherwise respond independently.
There is an important distinction here.
Electrical synapses could help coordinate neurons that participate in a memory-related network. Their activity might influence when neurons fire together, how signals propagate and how chemical synapses are subsequently modified.
But there is no established evidence that electrical synapses generally act as direct links between stored memories, or that memories are encoded as discrete objects passed through gap junctions.
A useful working model is:
These mechanisms work together rather than representing competing explanations of cognition.
These two mechanisms are easy to confuse.
Mª José Mas Salguero (@MasTwitts) on X
Electrical synapse
Cells are directly connected by gap-junction channels. Ionic current can pass between their interiors.
Cognition is an emergent property - PMC
Ephaptic coupling
Cells influence one another through extracellular electrical fields and currents, without needing a direct gap-junction connection.
Both are forms of electrical interaction, but only the first is an electrical synapse in the strict gap-junction sense.
This matters because neural tissue is an electrically active medium. Electrical interactions need not always occur through neurotransmitters or direct cellular channels.
Gap junctions also occur in many non-neuronal tissues. They are important in the heart, where electrical coupling helps coordinate the spread of excitation between cardiac muscle cells, and in other tissues where cells exchange ions and small signalling molecules.
The proteins and physiological roles differ across tissues. In the nervous system, neuronal gap junctions are particularly interesting because they combine direct electrical coupling with the complex, nonlinear behaviour of excitable cells.
There is also an evolutionary distinction: vertebrate gap junctions are principally built from connexins, whereas invertebrate gap junctions generally use proteins called innexins. The broader strategy of direct intercellular coupling is ancient.
American Physiological Society+1
Their capabilities are impressive, but their limitations are just as informative.
They do not transmit every aspect of a neuron's state. They couple electrical potentials and permit the passage of certain small substances; they don't copy a complete neural state, including every receptor configuration, protein concentration or biochemical process.
They do not automatically create synchrony. Coupling tends to reduce voltage differences, but membrane properties and circuit architecture can produce more complicated outcomes, including asynchronous firing.
They are not necessarily faster in every meaningful sense. The junctional transfer itself has virtually no chemical-release delay, but the complete neural response still depends on action-potential propagation, membrane charging and the receiving cell's firing threshold.
They are not equivalent to direct axonal connections. An axon carries electrical signals along a neuron's own membrane. An electrical synapse transfers current between separate cells.
These distinctions are important when constructing models of neural computation.
NCBI Bookshelf+1
Several questions remain active areas of investigation:
The central research problem is no longer merely whether electrical synapses exist. It is understanding what computation a particular electrically coupled circuit performs, and how that computation changes when its coupling changes.
For further reading, three useful reviews are:
An electrical synapse is more than a fast alternative to chemical neurotransmission. It is a mechanism for coupling the dynamics of individual cells into the dynamics of a larger network.
That distinction matters. A network's behaviour depends not just on what each neuron computes independently, but on how the neurons influence one another, share current, align their timing and interact with their chemical synapses.
If you're interested in systems-level neuroscience, I would focus on three things: electrical coupling, network synchronisation and the way coupling changes the computation performed by a population of neurons. Together, they offer a powerful framework for investigating how local cellular behaviour produces coordinated neural activity.
Polish_20261028_033002685.jpg (230.41 kB . 1080x1248 - viewed 29 times)Catecholamines are the body's fast chemical messengers: dopamine, norepinephrine, and epinephrine. This real focuses on dopamine.
All three are built from the amino acid tyrosine. Tyrosine hydroxylase makes L-dopa, the rate-limiting step. Dopamine decarboxylase turns it into dopamine. Dopamine beta hydroxylase then makes norepinephrine, and in the adrenal medulla, PNM T adds a methyl group to make epinephrine.
Neurons pack dopamine into vesicles with the transporter VMAT two. Calcium triggers its release. Dopamine then binds D1-like receptors, which raise cyclic AMP, or D2-like receptors, which lower it. The signal ends mainly by uptake through the dopamine transporter.
What remains is broken down by the enzymes MOP and COMT. Dopamine is metabolized to dopac, then homovanilic acid. Norepinephrine and epinephrine end up as VMAs and metanephrines. These appear in urine and help diagnose tumors like fiocromositaoma and neuroblastoma.
In the brain, dopamine runs four main pathways: nigrostriatal for movement, mesolimbic for motivation and reward, mesocortical for cognition, and tuberine ventubular, which inhibits prolactin.
Norepinephrine and epinephrine drive fight or flight. Heart rate rises, vessels constrict, airways open, and more glucose enters the blood.
Lose dopamine neurons in the substantia nigra and Parkinson's disease follows. Treatment gives L-dopa with carbamodo,pa or blocks MOP and COMT. Cocaine blocks the transporter, amphetamine reverses it, and antipsychotics block D2 receptors.
Dopamine is not just pleasure; it is movement, motivation, and learning.
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