Synaptic transmission is the process by which one neuron communicates with another neuron or a target cell (like a muscle or gland) across a synapse, which is the small gap between them. It is the fundamental mechanism for transmitting signals in the nervous system. Here's a detailed breakdown:
Presynaptic neuron: The neuron sending the signal.
Synaptic cleft: The tiny gap between neurons (about 20-40 nanometers wide).
Postsynaptic neuron (or cell): The neuron or target cell receiving the signal.
Synaptic vesicles: Tiny sacs in the presynaptic neuron containing neurotransmitters.
Neurotransmitters: Chemical messengers (e.g., glutamate, dopamine, acetylcholine).
An electrical signal (action potential) travels along the axon of the presynaptic neuron to its terminal.
The arrival of the action potential opens voltage-gated calcium channels in the presynaptic terminal.
Calcium ions ((Ca^{2+})) enter the neuron, triggering neurotransmitter release.
Synaptic vesicles fuse with the presynaptic membrane (exocytosis).
Neurotransmitters are released into the synaptic cleft.
Neurotransmitters diffuse across the cleft and bind to receptors on the postsynaptic membrane.
This binding can open ion channels (ligand-gated) or activate second-messenger pathways.
Depending on the neurotransmitter and receptor type:
Excitatory postsynaptic potential (EPSP): Depolarizes the postsynaptic membrane, making it more likely to fire an action potential.
Inhibitory postsynaptic potential (IPSP): Hyperpolarizes the postsynaptic membrane, making it less likely to fire.
Neurotransmitters are removed to stop the signal:
Reuptake into the presynaptic neuron.
Enzymatic breakdown (e.g., acetylcholinesterase breaking down acetylcholine).
Diffusion away from the synapse.
Chemical synapses: Use neurotransmitters; most common in the CNS.
Electrical synapses: Use gap junctions for direct ion flow; faster but less common.
In short: Synaptic transmission is the process of converting an electrical signal in one neuron into a chemical signal across a synapse and then back into an electrical signal in the next cell. It’s how the nervous system communicates.
If you want, I can also make a simple labeled diagram showing synaptic transmission—it really helps visualize the process. Do you want me to do that?
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Particular ions are distributed unequally across the cell membrane.
Concentrations of Na+ and Cl− are greater on the outside of the cell, while concentrations of K+ and organic anions, such as charged amino acids and proteins, are greater on the inside of the cell .
The organic ions are incapable of passing across the cell membrane.
This arrangement leads to an electrochemical gradient called the resting potential.
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State: The neuron is at rest (around –70 mV).
Ionic conditions:
Inside: High K⁺, low Na⁺.
Outside: High Na⁺, low K⁺.
Key channels:
The Na⁺/K⁺ ATPase pump maintains these gradients (3 Na⁺ out, 2 K⁺ in).
The membrane is more permeable to K⁺ due to leak channels → inside is negative.
action potential: sequence of ion movements across the neuronal (or muscle) cell membrane that generate and propagate the rapid electrical signal
Trigger: A stimulus depolarizes the membrane to reach threshold (around –55 mV).
Event:
Voltage-gated Na⁺ channels open.
Na⁺ rushes in (down its electrochemical gradient).
The membrane potential becomes positive (up to +30 to +40 mV).
Event:
Na⁺ channels inactivate (they close automatically after opening briefly).
Voltage-gated K⁺ channels open.
K⁺ exits the cell (outward current), restoring negative potential inside.
Event:
K⁺ channels remain open a bit longer → more K⁺ leaves than needed.
The membrane potential becomes more negative than resting (after-hyperpolarization).
Event:
Voltage-gated K⁺ channels close.
The Na⁺/K⁺ pump and K⁺ leak channels reestablish the resting ion gradients.
The membrane potential returns to –70 mV.
| Phase | Main Ion Movement | Channels Involved | Membrane Potential |
|---|---|---|---|
| Resting | K⁺ leak out | K⁺ leak channels | –70 mV |
| Depolarization | Na⁺ influx | Voltage-gated Na⁺ | +30 mV |
| Repolarization | K⁺ efflux | Voltage-gated K⁺ | Back to negative |
| Hyperpolarization | Continued K⁺ efflux | K⁺ channels (slow to close) | Below –70 mV |
| Return to Rest | Na⁺/K⁺ pump restores | Na⁺/K⁺ ATPase | –70 mV |
Would you like me to include a diagram or labeled graph of the action potential showing these ionic changes over time?