The Neuron Fires
A neuron carries information as an all-or-none electrical impulse and passes it to the next cell by releasing neurotransmitters across the synaptic gap. · 12 min
Every idea in this course — a memory forming, a fear rising, a decision made — is carried by cells passing signals to one another. There are roughly eighty-six billion of them in your brain, and one at a time they do something surprisingly simple. This folio is about that single event: how one nerve cell takes in a signal, decides whether to fire, and hands the message on.
Guess before you learn
A neuron only fires once the signals reaching it cross a threshold. Suppose it is already firing from a threshold-level push, and now you push much harder — well past threshold. What happens to the electrical impulse the neuron sends?
The impulse is all-or-none: once threshold is crossed, every spike is the same size. A stronger stimulus makes the neuron fire more frequently, not more forcefully. Keep your pencil mark — 'same size, faster rate' is the whole logic of how a neuron signals how strong something is.
9–12
3–5
A brain cell is called a neuron. It waits for a signal. When enough signals arrive, it fires — always the same strong pulse, never halfway. Then it hands the message to the next neuron across a tiny gap.
6–8
A neuron is a cell built to carry signals. Messages come in through its dendrites and add up in the cell body. If the total crosses a threshold, the neuron fires an electrical impulse down its axon. That impulse is all-or-none: it happens at full strength or not at all — a bigger input cannot make a bigger impulse, only more frequent ones. At the axon's end, the impulse releases neurotransmitters, chemicals that cross a narrow gap, the synapse, to the next neuron.
9–12
A neuron holds a small voltage across its membrane while at rest. Inputs gathered at the dendrites nudge that voltage; if they push it past a threshold, the neuron fires an action potential — a brief electrical spike that runs the length of the axon. The spike obeys the all-or-none law: its size does not depend on how far past threshold the input went. Stimulus strength is coded by how often the neuron fires, not how large each impulse is.
The signal is electrical within a neuron but chemical between neurons. When the action potential reaches the axon terminals, it triggers the release of neurotransmitters into the synaptic gap. These molecules bind receptors on the next neuron's dendrites, nudging its voltage up or down. The message is rebuilt, cell by cell, as an alternation of electrical impulse and chemical relay.
K–2
Your brain is made of tiny cells that talk to each other. One cell gets a little zap. If the zap is big enough, it sends a zap to the next cell. Zap by zap, a message travels.
Undergrad
At rest the membrane sits near −70 mV, maintained by ion pumps and selective permeability. Depolarizing input that reaches threshold, roughly −55 mV, opens voltage-gated sodium channels; sodium rushes in, the potential shoots toward +40 mV, and the change propagates as the action potential. Potassium efflux then repolarizes the membrane, briefly overshooting into hyperpolarization before rest is restored.
The all-or-none property follows from the channel behavior: once threshold is crossed the spike runs to completion independent of the triggering stimulus, so information is rate-coded. At the synapse the electrical signal becomes chemical — vesicular neurotransmitter release, receptor binding, and postsynaptic potentials that are excitatory or inhibitory — then cleared by reuptake or breakdown, readying the synapse for the next impulse.
Postgrad
The action potential is the canonical excitable-system phenomenon: the Hodgkin–Huxley formalism models it as coupled voltage- and time-dependent conductances, principally sodium and potassium, whose nonlinear feedback yields a threshold, a stereotyped spike, and a refractory period. All-or-none firing is the signature of a system driven across an unstable fixed point; suprathreshold input alters spike timing and rate, not amplitude.
Synaptic transmission converts this near-digital event back into a graded analog signal: presynaptic depolarization gates calcium entry, vesicles fuse and release quanta of transmitter, and postsynaptic receptors produce graded potentials summed across space and time at the axon hillock. Much of the nervous system's computation lives in this analog integration, with the all-or-none spike serving as the noise-tolerant unit of long-distance transmission.
all-or-none
A neuron either fires a full-strength impulse or none at all — there is no partial spike. How strong a signal is gets coded by how often it fires.
Now follow the signal itself. The move that matters is that it changes form twice: it arrives as chemistry, becomes an electrical spike inside the cell, then turns back into chemistry to cross to the next neuron. Trace it once, step by step, before you look at the graph of what the voltage is doing while it happens.
Trace one signal from neuron to neuron — the steps fade as you master them
At the dendrites
The firing threshold
An all-or-none action potential
Neurotransmitters released across the synapse
Receptors on its dendrites
Why is this true?
Why does the body bother turning an electrical signal into a chemical one at the synapse, only to turn it back into electricity?
The chemical gap gives the brain flexibility. A neurotransmitter can excite the next cell or inhibit it, and different transmitters and receptors let one connection be tuned, strengthened, or blocked — which pure wiring could not do. The synapse is where signals get combined and adjusted, not just passed along.
That is the unit the whole nervous system is built from: receive, sum, cross a threshold, fire once at full strength, and pass a chemical message on. Billions of these events, layered and timed, become perception and memory. The next folio moves up a level — from the single cell to whole regions of the brain, and what we learned about them from injuries that took specific abilities away.
Practice — new ink and old, interleaved
1.Close the page. Name the four rules that protect people inside a study.
Informed consent, minimized harm, the right to withdraw, and debriefing.
How close were you? Grade yourself honestly — it sets your review date.
2.Halfway through a stressful task, a participant says she wants to stop. The researcher needs ten more minutes of data. What must happen?
3.Without looking: name the three things a neuron does, from input to output.
It receives signals at the dendrites, fires an all-or-none action potential down the axon if they cross threshold, and releases neurotransmitters to the next neuron.
How close were you? Grade yourself honestly — it sets your review date.
4.Match each school to how it gathered its evidence.
5.In one sentence, explain why the signal is called 'electrical within a neuron but chemical between neurons.'
6.A study gives one group a caffeine pill and another a look-alike sugar pill, then times both on a puzzle. What is the dependent variable?
7.From the ethics folio: a brain study will briefly deceive volunteers about its purpose. What must the researchers do afterward?
8.From the experiment folio: a study finds people with faster reaction times have more of a certain neurotransmitter. Can it conclude the transmitter causes faster reactions?
9.You touch something mildly warm, then something scalding. How does a temperature-sensing neuron most likely signal the difference?