How does a neuron send a signal?
A neuron sends a signal electrically along its own axon, as an action potential that flips the charge across the membrane, and chemically to the next cell, by releasing neurotransmitters across the synapse. The action potential is all-or-none; a stronger stimulus makes the neuron fire more often, not harder.
Inside the neuron: the action potential
At rest, the inside of a neuron is about 70 millivolts more negative than the outside (the resting potential, about -70 mV). The sodium-potassium pump keeps it that way by moving sodium out and potassium in, so sodium is concentrated outside and potassium inside.
- Signals arriving at the dendrites and cell body nudge the voltage up. If it reaches threshold (around -55 mV) at the axon hillock, the neuron fires.
- Depolarization: voltage-gated sodium channels open, sodium rushes in, and the inside briefly becomes positive.
- Repolarization: sodium channels close and potassium channels open, so potassium flows out and the inside turns negative again.
- A brief overshoot below rest (hyperpolarization) and a refractory period stop the signal from running backward.
Each patch of membrane triggers the next, so the impulse travels down the axon. Myelin, a fatty sheath, insulates the axon, and the signal jumps between the gaps (nodes of Ranvier), which is much faster.
Between neurons: the synapse
When the action potential reaches the axon terminal, calcium flows in and vesicles release neurotransmitter into the synaptic cleft. The molecules bind receptors on the next neuron. Some make it more likely to fire (excitatory), others less likely (inhibitory). The receiving neuron adds up all its inputs, and only if the total reaches threshold does it fire.
The signal then has to stop. Neurotransmitter is taken back up by the sending neuron (reuptake) or broken down by enzymes. SSRIs, a class of antidepressants, work by slowing the reuptake of serotonin, so it stays in the cleft longer.
Common exam mistakes
- Saying a stronger stimulus makes a bigger action potential. It is all-or-none; intensity is coded by firing rate and by how many neurons fire.
- Calling the whole process electrical. It is electrical within a neuron and chemical across the synapse.
- Mixing up agonists and antagonists. An agonist mimics or boosts a neurotransmitter's effect; an antagonist blocks it.
- Thinking the neurotransmitter enters the next cell. It binds receptors on the outside.
These ideas sit at the start of AP Psychology and come back in how memory works. For the electrical basics, see voltage vs current.
See it in 3D
This is easier to follow when you can turn the neuron around. Ask Isotoon "How does a neuron send a signal?" for a short video of ions crossing the membrane and the wave moving down the axon. On Pro, the 3D interactive lets you step from resting potential to the synapse and tap each part for a note. More on the psychology page.
Questions
- What does all-or-none mean for a neuron?
- Once the voltage at the axon hillock reaches threshold, the neuron fires a full action potential of the same size every time. If threshold is not reached, it does not fire at all. There is no half-strength action potential.
- Why does myelin make signals faster?
- Myelin insulates the axon so the action potential only has to be regenerated at the small gaps between myelin segments, the nodes of Ranvier. The signal skips from node to node, called saltatory conduction, instead of crossing every patch of membrane.