In Neurons and Signals a neuron passed a message to the next cell with a chemical across a synapse. That is one of several ways cells talk. Every cell in your body is constantly receiving signals, and most of them come from close neighbors, not from far away.
| Kind | How far | How it works | Example |
|---|---|---|---|
| Direct contact | Touching cells | Small molecules pass through channels joining two cells, or surface molecules on one cell bind receptors on the other | Gap junctions between heart muscle cells; plasmodesmata in plants; immune cells checking each other's surface proteins |
| Paracrine (local) | Nearby cells | A cell releases a signal that diffuses a short distance | Growth factors that tell nearby cells to divide |
| Synaptic (local) | Across a synapse | A neuron releases a neurotransmitter onto one target cell | Acetylcholine from a motor neuron to a muscle cell |
| Autocrine | The same cell | A cell responds to a signal it released itself | Some immune cells boosting their own activity |
| Endocrine (long distance) | Anywhere the blood reaches | Hormones travel in the blood (in plants, through vessels or the air) | Insulin from the pancreas; ethylene gas in ripening fruit |
Key idea
Cells signal by direct contact, locally (paracrine and synaptic signals) or over long distances (hormones). Whatever the distance, a signal only acts on a cell that has the right receptor.
Why it works
Why so many kinds? Distance sets speed and reach. A gap junction links two cells almost instantly, so heart muscle cells contract together. A neurotransmitter crosses a gap of a few tens of nanometers and is cleared in milliseconds, so the message is fast and private. A hormone takes seconds to minutes to circulate but reaches every tissue at once. Cells without the receptor simply ignore it, which is how one hormone can send different messages to different organs.
Worked examples
Worked example: Classify the signal
Cells in a healing wound release a protein that makes nearby skin cells divide. Which kind of signaling is this?
- The signal is released into the fluid around the cells, not carried by blood.
- It acts on nearby cells.
- Local signaling by diffusion is paracrine.
Answer: Paracrine signaling.
Worked example: One hormone, different responses
Epinephrine (adrenaline) circulates in the blood. It makes liver cells release glucose and makes heart muscle beat faster, but has no effect on some other cells. Why?
- Epinephrine is an endocrine signal: it reaches all tissues.
- Only cells with epinephrine receptors respond.
- Liver and heart cells have receptors linked to different pathways inside, so their responses differ.
Answer: A response depends on the target cell's receptors and on the pathways behind them, not just on the hormone.
Common mistake
Thinking only hormones count as signals. Neighboring cells signal all the time, by contact and by paracrine signals, and a nerve signal crosses each synapse as a local chemical signal. A check: ask how the signal travels. Through the blood: endocrine. Across a short space or a synapse: local. Through a junction or by touching: direct contact.
Bacteria signal too. Many release small molecules and sense how much has built up, which tells them how crowded they are. This quorum sensing lets a whole population switch behaviors at once.