Question: What is a cell signaling?
Cell Signaling is related to biology, Cells should communicate between themselves and one another in order to adjust and respond to the environment. These cells receive the signals and implement an action plan.
The signals are usually chemical in nature and include hormones, growth factors and neurotransmitters. Cells have receptors on their surface which on binding to the chemical messenger initiate the response. There are several receptors each with its own function and structure. These on activation initiate the cascade of the signalling pathways.
Question: What is a receptor protein and what does it do?
These receptors are broadly classified into
-Enzyme-linked receptors and
-Ion channel receptors.
These receptors interact with the extracellular signals as well as the intracellular molecules and effect only the cell physiology. This is because the cell signaling molecules are usually macro in size and cannot pass through the membrane.
Question :How do cells communicate with each other?
Signaling mechanisms are broadly classified into four types: paracrine, endocrine, autocrine, and direct signaling.
1. Paracrine signaling occurs between the local cells which elicit quick responses and last for short duration. In other words, it is a form of signaling where the target cell is very near to the signal emitting cell. As such these signals are also called as paracrine signals.
Example: Neurotranmission which takes between the axons. The signals usually move very fastly within the nerve cells and are propagated by electrical impulses. These impulses travel via the synapse very quickly to the next nerve ending.
2. Endocrine signaling occurs between the distant cells and is usually mediated by hormones released from specific cells which travel to the target cells and produces a slow and long-lasting response. The signals originate from endocrine cells. Endocrine cells are found from the thyroid gland and the pituitary glands. In this form of signaling the response is slow as the signal is diluted to a great extent. This is in contrast to the paracrine signaling where the signal is highly concentrated as it is very near to the target.
3. Autocrine signals are produced by the signaling cells which can also bind to the drug or ligand that is released. Here the signaling cell and target cell both are the same. This usually occurs during the early developmental stages to avoid improper development and function. This form of signaling aids in regulating the inflammatory response and pain.
4. Direct signaling occurs by transferring the signaling molecules across the gap junctions of the neighbouring cells.
Figure 1: Figure showing the signaling mechanisms. A: Autocrine; B: Signaling across gap junctions; C: Paracrine; D: Endocrine.
Cell Signaling can be explained by three stages.
-Reception: where a signal is detected by the receptor and activated to do the pharmacological action. Membrane receptors like G Protein-Coupled Receptors and Tyrosine Kinase receptors aid in this function
-Transduction: Here the activated receptor gets changed structurally affecting the physiology of the receptor. As such the cascade of signaling events gets initiated.
-Response: and this eventually leads to cellular response (phosphorylation).
Question: What is a cell signaling pathway?
Cell responds to a particular ligand when it comes and binds to its receptor. For instance GABA on binding to the GABA receptors can activate the ion channel and as such allows the negatively charged chloride ions to move into the nerve cell. But this is not a single step mechanism but usually triggers a set of mechanisms which is collectively called as signaling pathway. There are several signaling pathways which help in transmitting the message or information within and between the cells. Some of them are Cyclic AMP signalling pathway, Cyclic ADP-ribose (cADPR) signalling and nicotinic acid–adenine dinucleotide phosphate (NAADP) signalling pathway, Inositol 1,4,5-trisphosphate (InsP3)/Ca2+ signaling cassette, Multipurpose inositol polyphosphate signaling pathway, PtdIns 3-kinase signaling, Nitric oxide (NO)/cyclic GMP signaling pathway, Mitogen-activated protein kinase (MAPK) signaling, Sphingomyelin signaling pathway, Hedgehog signaling pathway.
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