Resting membrane potential
Ion gradients and selective membrane permeability create a voltage difference across the membrane. The sodium–potassium pump helps maintain these gradients over time; it does not produce each action-potential upstroke.
How does a neuron send a message?
Follow a signal from ion gradients and incoming synaptic activity to an action potential, then across a chemical synapse to a receiving cell. Each step depends on cell type and context.
EXPLORE THE SIGNAL
From membrane voltage to a synaptic message
Select a phase to connect the voltage trace with the channel events that shape it. Values and timing are illustrative, not a universal recording from a specific neuron.
AT A TYPICAL CHEMICAL SYNAPSE
Electrical signal in one cell; chemical message across the cleft
- Action potential arrivesPresynaptic terminal depolarizes
- Ca²⁺ channels openCalcium enters
- Vesicle fusionTransmitter released
- Synaptic cleftTransmitter diffuses
- Receptor binds signalResponse depends on the target cell
Signal termination varies: reuptake, enzymatic breakdown, diffusion, and uptake by nearby cells can all contribute, depending on the transmitter and synapse.
AstrocyteA neuron maintains ion gradients
The cytoplasm and extracellular fluid have different ion concentrations. Selective channels, transporters, and pumps maintain those conditions, giving ions electrochemical gradients that can do work when channels open.
Inputs are graded and can add together
Synaptic inputs can produce local voltage changes that vary in size and decay with distance. Their timing and location matter: excitatory and inhibitory inputs can combine near the axon initial segment, and the balance is not a simple count of signals.
Threshold starts a regenerative action potential
When depolarization reaches threshold in an excitable region, voltage-gated sodium channels open. Sodium entry drives further depolarization and opens additional channels, producing the rapid rising phase shown in the model above.
Channel states bring voltage back down
Sodium-channel inactivation and increased potassium conductance produce repolarization; lingering potassium conductance can cause a brief after-hyperpolarization. The sodium–potassium pump restores and maintains concentration gradients over time rather than directly repolarizing each individual spike.
The spike propagates along the axon
Local current from an active membrane region depolarizes the next region. Refractory membrane behind the spike helps limit backward re-excitation. In many myelinated axons, current spreads farther and action potentials are regenerated at nodes of Ranvier; conduction speed depends on axon properties and myelination.
A typical synapse converts electrical to chemical signaling
When an action potential reaches a presynaptic terminal, depolarization can open voltage-gated calcium channels. Calcium entry promotes synaptic-vesicle fusion and neurotransmitter release; transmitter diffuses across the cleft and binds receptors on a target cell.
Receptors shape the response; clearance ends the message
A transmitter's effect depends on A protein that detects a signal, such as a hormone, and helps a cell respond. type, target cell, ion gradients, and signaling state. Transmitter action can end through reuptake, enzymatic breakdown, diffusion, or uptake by nearby cells; different transmitters use different combinations of these routes.
PAUSE AND CHECK
Can you follow the signal?
What is the sodium–potassium pump's main role in this lesson's model?
At a typical chemical synapse, how does a presynaptic action potential influence the next cell?
Does one excitatory synaptic input always trigger an action potential?
INSPECT THE SOURCES