What is Signal
Signal is a message those are given to a cell through extracellular or intracellular signalling mechanisms by which target cell response accordingly.
On the basis of cell Signalling molecules, the extracellular cell Signalling are of various types-
Endocrine :
The signal molecules moves to a distantly located target cell therefore signalling molecules move through blood vascular system. Eg. Hormonal signalling
Paracrine :
Signalling molecules secreted in vicinity of target cell. Eg. Neurotransmitter
Autocrine :
signalling molecules acts on its own derived cell
Eg. T cell in immune response
Juxtacrine :
Signalling molecules not secreted but cell to cell physical contact is required eg. Notch signalling and cathedrin signalling.
Signal molecules:
These are heterogeneous compounds
These are of two types membrane bound and secretory molecules.
Membrane bound signal molecules mediate contact dependent signalling. Mostly, signal molecules secreted by signalling cells. Secreted extracellular signalling molecules further categories into three categories: Autocrine, endocrine and paracrine signal molecules.
Extracellular signal molecules are very specific for target cell and These are mostly hydrophilic and bound to the cell surface receptors of target cell. Small lypophilic molecules diffuse across the plasma membrane and interact with intracellular receptors. Eg. Arachidonic acid derived forms of eicosanoids ( prostaglandins, prostacyclin, thromboxanes and leukotrienes) formed from phospholipids.
Secondary messengers:
These are low molecular weight signalling molecules eg. cAMP, cGMP, diacylglycerol (DAG), IP3, phosphoinositides and calcium.
Receptors:
Chemically proteins or glycoprotein
1. Intracellular receptors:
located in cytosol or nucleus
For steroid hormones, thyroid hormones, vitaminD
2. Cell surface receptors :
all hydrophilic and some lypophilic molecules bound to them.
Act as signal as signal transducer to produce intracellular messenger.