23 Cell Signaling

Andrea Bierema

Learning Objectives

Students will be able to:

  • Describe how ligands and receptors are involved in cell signaling.
  • Describe the different types of cell signaling.
  • Explain how neurons (nerve cells) communicate with each other.
  • Identify how drugs influence dopamine signaling.

Cell Signaling Overview

Long green lines with red blotches and blue blotches that are twice the width of the green lines.
An image of normal brain tissue from the thalamus. In red are the receptors for orexin. Orexins are peptide hormones that help keep us awake and alert, and may also stimulate the appetite. They are produced in the hypothalamus and act through their receptors located in the nuclei (shown in blue) of cells in many different parts of the brain. The green stain highlights the neurofilaments (filaments found in nerve cells). A drug called orexin-RA-1 that blocks the action of these receptors is currently under development as a sleeping pill. It may also have the effect of reducing the appetite.

Cells “communicate” with each other via signaling. That is, one cell either directly connects or- more often- sends signaling molecules (which are often proteins) to another cell (or sometimes even back to itself), which triggers a response. Cell signaling influences a variety of cell functions, including the synthesis of specific proteins and cell division.

Not all cells can “catch” a particular signal. In order to detect the signal, the cell must have the right receptor for that signal. When a signaling molecule binds to its receptor, it alters the shape or activity of the receptor, triggering a change inside of the cell. Signaling molecules are often called ligands, a general term for molecules that bind specifically to other molecules (such as receptors).

Image of three cells. Sending cell has small circles coming out of it labeled as ligand. The next cell is the target cell and it has a something in the cell membrane labeled as a receptor. Arrows pointing within the cell are labeled as "response!". The third cell is labeled "non-target cell," it does not have anything in the cell and it is labeled "no receptor for ligand."
A sending cell produces ligand (signaling molecules) that bind to a receptor of the target cell, which elicits a response. Non-target cells do not have a receptor for that specific ligand and so do not respond.
The message carried by a ligand is often relayed through a chain of chemical messengers inside the cell. Ultimately, it leads to a change in the cell, such as protein synthesis.
A cell has a receptor connected on the cell's membrane. A ligand outside of the cell binds to the outside part of the receptor. Inside of the cell, arrows point down to "red blobs" and then to a "response." The inside of the cell is labeled "chemical messengers relay the signal.
Once a ligand (a signaling molecule) binds to a receptor, the receptor relays the signal to other molecules (such as proteins) in the cell. This chain of events causes a response in the cell.


Click here to see the entire image that this exercise was based on.

Forms of Signaling

There are different ways in which one cell sends another cell a signal. The video below summarizes these types of signals.


Complete this mini-quiz on the main types of cell signaling after watching the video above.

Neuron Communication

One unique example of cell signaling is synaptic signaling, in which a nerve cell (called a “neuron”) transmits a signal to another neuron. This process is named for the synapse, the junction between two nerve cells where signal transmission occurs.

When the sending neuron fires, an electrical impulse moves rapidly through the cell, traveling down a long, fiber-like extension called an axon. When the impulse reaches the synapse, it triggers the release of neurotransmitters, which quickly cross the small gap between the neurons. When the neurotransmitters arrive at the receiving cell, they bind to receptors and cause a chemical change inside of the cell (often, opening ion channels and changing the electrical potential across the membrane. The video below illustrates this process.


The original image came from this Khan Academy article.


Neurons and Drug Use

One neurotransmitter commonly discussed is dopamine. This molecule is naturally produced in the body and the release of it causes “feel good” sensations. Drugs can change the dopamine pathway, such as preventing it from releasing from the receptors.

To learn how different drugs impact this pathway, see the animated infographic below (press play to see the animations).

Press play on the above infographic to view animations of cell signaling. This animated infographic was produced by American Addiction Centers. Learn more about the project and find an accessible version on their page: https://drugabuse.com/featured/the-science-behind-addiction/


This chapter is a modified derivative of “Introduction to Cell Signaling.” by Khan Academy, CC BY-NC-SA 4.0. Download the original article for free at https://www.khanacademy.org/science/biology/cell-signaling/mechanisms-of-cell-signaling/a/introduction-to-cell-signaling