# Cell Communication and the Neuron > How cells send, receive, and respond to signals, and how a nerve cell passes a message across a synapse. Level: High school | Reading time: 12 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/cell-communication **Quick answer:** Cells communicate by releasing signal molecules that bind receptors on target cells, which turn the signal into a response. A neuron sends an electrical action potential along its axon, then releases neurotransmitters across the synapse to pass the message to the next cell. ## What you will learn - Describe the three stages of cell signaling: reception, transduction, and response. - Explain why only some cells respond to a given signal. - Describe how an action potential travels along an axon. - Outline the steps of chemical transmission at a synapse. ## Key takeaways - Only cells that have the matching receptor respond to a given signal. - Signaling has three steps: receptor, transducer and response. - An action potential is a wave of ion channels opening that travels along the axon without fading. - At the synapse, calcium entry makes vesicles release neurotransmitter, which crosses the gap and binds receptors on the next cell. ## Cells talk to each other A single cell in a multicellular body needs to know what its neighbors are doing. It must learn when to divide, when to release a hormone, when to contract, and when to die. It does so by exchanging chemical and electrical signals. Signals can travel in several ways: - **Direct contact.** Neighboring cells exchange small molecules through channels in their membranes (gap junctions in animals, and plasmodesmata in plants). - **Local signaling.** A cell releases molecules that act on cells nearby. - **Long-distance signaling.** Hormones are carried in the blood to targets across the body. - **Synaptic signaling.** Neurons release neurotransmitters across a tiny gap onto the next cell. ## Receptor, transducer, response Most signaling follows the same three-stage plan. 1. **Reception.** The signaling molecule, called a ligand, binds to a specific receptor protein, typically on the cell surface. The fit is like a key in a lock. 2. **Transduction.** The receptor changes shape and starts a chain of events inside the cell. This relay is called signal transduction. Often a single ligand sets off a cascade in which each step activates many molecules in the next, so a faint signal is amplified into a big response. 3. **Response.** The cell does something: opens an ion channel, turns a gene on or off, changes its shape, or starts to secrete. Only cells with the right receptor respond to a given ligand. That is how a hormone can travel through the whole body yet change only its target tissues. **Check your understanding:** What is the correct order of the stages of cell signaling? - A. Response, reception, transduction - B. Reception, transduction, response - C. Transduction, response, reception - D. Reception, response, transduction **Answer:** B. Reception, transduction, response A ligand is received by a receptor, the signal is transduced inside the cell, and the cell gives a response. Interactive 3D model: [White Blood Cell](https://cellnaut.com/studio/whiteBlood/granules) - Immune cells constantly sense chemical signals from infected tissue and respond by moving and releasing granule contents. Interactive 3D model: [Rod Photoreceptor](https://cellnaut.com/studio/rod/outerSegment) - A rod cell is a sensor: light striking rhodopsin in its outer segment starts the signal. ## The neuron: a cell built for signaling A dendrite is a branched extension that receives signals. The cell body (soma) combines them. If the combined input is strong enough, the neuron fires an electrical signal that runs along its axon to the far end. Interactive 3D model: [Neuron](https://cellnaut.com/studio/neuron/dendrites) - Dendrites receive thousands of inputs; the single axon carries the output. ## The action potential A resting neuron is electrically charged: the inside of the membrane is about 70 millivolts negative compared with the outside. This is the result of the sodium-potassium pump and of channels that let potassium leak out. When the neuron is stimulated past a threshold, sodium channels open. Sodium ions rush in and the inside briefly becomes positive. This swing is the action potential. It opens the neighboring channels, which open the next ones, and so the pulse travels down the axon like a row of falling dominoes. A moment later, potassium channels open and the membrane resets. An action potential is all-or-nothing: it has the same size no matter how strong the stimulus was. Many axons are wrapped in a fatty insulating sheath called myelin, which makes the pulse jump from gap to gap and speeds it to over 100 metres per second. ## Crossing the synapse An axon does not touch the next cell. At the synapse, a gap about 20 nm wide separates the two. The message must be translated to a chemical one to cross it. ![Stylized 3D illustration of a chemical synapse showing vesicles releasing neurotransmitters toward receptors on the next cell.](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/neuron-synapse-transmission-teaching.jpg) *A stylized chemical synapse: vesicles in the presynaptic terminal release neurotransmitter onto receptors on the next cell. Credit: Project-generated teaching illustration (AI-generated; simplified)* [Open the interactive, labelled version](https://cellnaut.com/topics/neuron-synapse-transmission) 1. The action potential reaches the axon terminal and opens calcium channels. Calcium ions flow in. 2. The calcium triggers vesicles full of neurotransmitter to fuse with the membrane and release their contents by exocytosis. 3. The neurotransmitter diffuses across the gap. 4. It binds to receptors on the next cell, which opens ion channels there. The next cell is either excited or inhibited. 5. The neurotransmitter is rapidly removed, either by enzymes that destroy it or by pumps that take it back up, so the signal ends cleanly. Many medicines and drugs act at this step. For example, some antidepressants slow the re-uptake of neurotransmitters so that they stay longer in the gap. > **Common misconception** > > **"A nerve signal is electricity flowing all the way from the brain to the muscle."** The electrical pulse is regenerated along every axon, and at most synapses it is converted into a chemical signal and back. Nerve signals are not like current in a wire. **Check your understanding:** What triggers synaptic vesicles to release neurotransmitter? - A. An influx of calcium ions - B. Sodium leaving the cell - C. DNA replication - D. A cell wall forming **Answer:** A. An influx of calcium ions When the action potential arrives, calcium channels open, and the incoming calcium makes the vesicles fuse with the membrane. **Check your understanding:** Why does a hormone in the blood affect only certain cells? - A. Only those cells have the matching receptor - B. The hormone is too big for other cells - C. Other cells have no membranes - D. Hormones only travel to the brain **Answer:** A. Only those cells have the matching receptor A signal needs a matching receptor. Cells without it do not respond even though the hormone passes by. ## Sources - [Biology 2e, Chapter 9: Cell Communication (OpenStax)](https://openstax.org/books/biology-2e/pages/9-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [NIH 3D: Neuron model](https://3d.nih.gov/entries/3DPX-015796/2) - See entry page and docs/ASSETS.md - [Project teaching illustration: neuron synapse transmission (AI-generated, stylized)](https://github.com/cclank/cell-architecture-studio/blob/main/docs/ASSETS.md) - MIT (project asset)