Spark and Signal: How Neurons Fire and Talk

Hi, I'm Atlas. Think of a neuron as a tiny biological wire with a chemical relay station at the end. When a neuron is resting, it actively pumps ions so the inside is slightly negative compared to the outside — around minus seventy millivolts. We call this the resting membrane potential, and it is like a charged battery waiting to be used. When a strong enough stimulus arrives and pushes the voltage past a threshold, voltage-gated sodium channels snap open. Positive sodium ions rush in, and the inside suddenly flips to positive. That spike is depolarization, the rising part of the action potential. Almost immediately, sodium channels close and potassium channels open, letting potassium leave so the inside returns to negative. That recovery is repolarization. This electrical event travels down the axon without weakening, because each patch re-triggers the next, like dominoes. Now here is the key relay: at most synapses — called chemical synapses — the electrical signal cannot cross the gap. There are special synapses called electrical synapses, or gap junctions, where the action potential does pass directly between cells, but those are the exception. In the far more common chemical synapse, the arriving action potential causes voltage-gated calcium channels to open at the axon terminal. Calcium ions flood in, and that calcium influx is the trigger that pushes neurotransmitter-filled vesicles to fuse with the membrane and release their contents into the synaptic gap. Those neurotransmitters drift across and bind to receptors on the next cell, which can start a brand-new action potential there. So the message is electrical within a neuron and chemical between neurons at a chemical synapse. If you ever feel lost, follow the order: rest, threshold, depolarize, repolarize, calcium in, then neurotransmitters released across the synapse.