Health Basics  /  4 min read  /  Aug 5, 2026

How a decision in your head becomes a rep on the bar

BIO 102 — Human Physiology I: Cell, Muscle & Cardiorespiratory Systems

You decide to stand up out of the bottom of a squat, and you do. Between the decision and the movement, an electrical signal has to cross gaps it cannot jump electrically — dozens of times, in a few hundredths of a second.

How your body solves that problem explains a lot: why practice makes movement automatic, why you can move one arm without the other, and why the point where your set falls apart is almost never the muscle refusing to receive the message.

The gap the signal cannot jump

Neurons do not touch. Where one ends and the next begins there is a tiny space, and the electrical signal stops dead at the edge of it.

So the cell switches formats. When the action potential arrives at the terminal, voltage-gated calcium channels open and calcium rushes in. Calcium is the trigger: it causes small packets called vesicles to fuse with the membrane and dump neurotransmitter into the gap. That chemical drifts across, binds receptors on the far side, and starts a new electrical signal there.

Then it has to be cleared, fast — broken down or pulled back in — because a message that never ends is not a message. Clearance is as important as release, and a surprising number of medications work by adjusting it.

Every neuron is counting votes

The far side of a synapse does not receive orders. It receives opinions.

An excitatory input nudges the cell toward threshold. An inhibitory input pushes it away. A single neuron may be receiving thousands of these at once, and it adds them up two ways: temporal summation stacks signals arriving in quick succession from one source, and spatial summation stacks signals arriving simultaneously from different places.

Inhibition is not a malfunction. It is how you contract one muscle without contracting its opposite, how you keep from moving both legs when you step, and how a nervous system with billions of connections produces one clean action instead of a seizure.

The chemicals people talk about, and what they actually do

Acetylcholine is the one that talks to skeletal muscle. Glutamate is the main excitatory transmitter in the brain and GABA the main inhibitory one — most of your brain's conversation is those two. The monoamines — dopamine, serotonin, norepinephrine — are modulators. They set the tone of a system rather than issuing commands.

That distinction is where popular science goes wrong. The idea that depression is simply low serotonin, a "chemical imbalance" waiting to be topped up, is an oversimplification the research moved past years ago. A "dopamine detox" is not a physiological event. Exercise is genuinely associated with improvements in mood, and the mechanism is not a neurotransmitter refill — it is broader and less tidy than the marketing version.

Where nerve meets muscle

The final synapse gets its own name: the neuromuscular junction. The nerve terminal releases acetylcholine onto a specialized patch of muscle membrane called the motor end plate, receptors there open ion channels, and the muscle fiber fires its own action potential. An enzyme, acetylcholinesterase, immediately chops up the acetylcholine so the fiber can reset.

What is notable about this junction is its safety margin. In a healthy adult it releases far more transmitter than the minimum needed, so it essentially always works. Which means when your tenth rep fails, the handoff is not what failed. The limit sits upstream in the nervous system's drive or downstream inside the muscle itself — and both of those are trainable.

Not a gym problem. Sudden or unexplained muscle weakness, a drooping eyelid, double vision, or trouble swallowing or breathing needs immediate medical attention. Do not train through any of those and do not wait to see if they pass.