A sequence of events initiated by a signal that leads to a cellular response

Signal --> Message Transduced --> Desired Response

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Autocrine signals: "self", affect the same cell that releases them\

The signaling cell is the target

E.g. Prostaglandins that have a bunch of different functions like aiding in contraction and relaxation of muscles, dilation and constriction of blood vessels, and modulating inflammation.

Paracrine: diffuse to nearby cells 

E.g. The conduction of an electric signal (neurotransmitter) from a nerve cell to a muscle cell or another nerve cell

Juxtacrine: require direct contact between the signaling cell and the receiving cell

E.g.  Some growth factors signal this way that are critical for the development of cardiac and neural function

Many unicellular organisms (e.g. bacteria) signal via direct contact 

Hormones: signal travels to distant cells 

e.g. Ghrelin regulates hunger (increases hunger, leptin decreases it) 

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Receptors

Protein where the signal is received on the target cell 

Intracellular receptors 

Located inside a cell

Ligands for these signals are small and/or nonpolar so they can easily diffuse across the cell membrane to reach these receptors

Membrane receptors 

Located on surface of the cell 

Ligands for these signals are large and/or polar that cannot diffuse through the cell membrane 

Receptors are highly specific and three-dimensional, only certain ligands can bind specific receptors 

When a ligand binds to the receptor protein, the bond is noncovalent (therefore not strong) and is reversible

This is so the signal can be regulated more easily

Inhibitors can block the normal ligand to prevent communication

E.g. Caffeine 

Caffeine is a large, polar molecule that binds to receptors on nerve cells in the brain. Its structure is similar to adenosine, which binds to receptors after activity or stress 🡪 feelings of drowsiness. Caffeine binds to the same receptor, but does not activate it 🡪 a person remaining alert rather than feeling tired

The most common types of receptors are :

*Ion channel

*Protein kinase receptors: kinases phosphorylate molecules 

*G protein linked receptors: binds various ligands such as light-sensitive compounds, odors, pheromones, hormones, neurotransmitters 

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TRANSDUCTION

The passing along of the signal until the desired response is reached

Transduction can be short or long

Longer if a signal cascade of reactions is induced 

The passing along of the message usually happens by changing the shape of different proteins

*Phosphorylation (addition of phosphate groups) by kinases

*Dephosphorylation (removal of phosphate groups) by phosphotases 

Sometimes a second messenger is involved to stimulate signal transduction

Second messenger a small molecule that serves as an intermediate between the receptor and the cascade of responses after

Key for regulation

Distribute or amplify the initial signal received at the receptor

E.g. cAMP is a second messenger in our fight-or-flight response

http://learn.genetics.utah.edu/content/cells/cellcom/

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Essentially the epinephrine/adrenaline hormone activates a liver enzyme called glycogen phosphorylase that then catalyzes the breakdown of glycogen for quick energy when the body is in fight-or-flight

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Responses

A transduction pathway eventually triggers a response

The same signal can cause many different responses, such as:

*Opening of ion channels: by changing the balance of ion concentration inside and outside of the cell

*Alterations in gene expression: genes may be switched on (upregulated) or switched off (downregulated)

*Alterations of enzymatic activity

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