Cells are smart. Cells receive and process signals that originate outside their borders in order to respond to changes in their environment. Cells aren’t just targets, they send messages as well as receive them.
What kind of signals do cells receive?
Most cell signals are in the form of chemicals, such as hormones, neurotransmitters, growth factors, and extracellular matrix components. Extracellular matrix (ECM) is a collection of extracellular molecules secreted by cells that provides structural and biochemical support to the surrounding cells.
Many cells also respond to mechanical or physical signals. A well-known example of this would be the maintenance of blood sugar levels in our body. When blood sugar levels increase, the pancreas releases insulin to signal to other cells that they need to uptake glucose, which reduces glucose in the blood. Skin sensory cells respond to pressure of touch, similar sensory cells in the ear react to the movement of sound waves. In order to maintain a consistent flow of blood through the heart, specialized cells in the vascular system can detect changes in blood pressure and send signals to affect appropriate adjustments.
How Do Cells Recognize Signals?
Within the cell membrane are proteins that act as receptors that bind signaling molecules. Receptors are specific to corresponding signaling molecules. In other words, insulin receptors bind insulin, serotonin receptors bind serotonin, and the vitamin D receptor (VDR) binds with hormonally active vitamin D, 1,25(OH)2D3.
How Do Cells Respond to Signals?
Once a receptor protein receives a signal, it undergoes a conformational change, resulting in a series of biochemical reactions within the cell. These intracellular signaling pathways, also called signal transduction cascades, typically amplify the message, producing multiple intracellular signals for every one receptor that is bound.
How Do Signals Affect Cell Function?
The signals initiate transfer or uptake of phosphate groups, which cause a conformational change in enzymes, either activating or inhibiting the enzyme activity. Then, when appropriate, protein phosphatases remove the phosphate groups from the enzymes, thereby reversing the effect on enzymatic activity.
Cells typically receive signals in chemical form via various signaling molecules. When a signaling molecule joins with an appropriate receptor on a cell surface, this binding triggers a chain of events that not only carries the signal to the cell interior, but amplifies it as well. Cells can also send signaling molecules to other cells. Some of these chemical signals, such as neurotransmitters, travel only a short distance, while others must go much farther to reach their targets.