Signal Transduction and Second Messengers

Gap Junctions

Connexin proteins create ion-carrying tunnels called connexons that control rapid ion passage between cells.

Hormones

Extracellular messengers carried by blood that regulate various physiological processes.

Neurohormones

Chemical messengers released from a neuron into the blood, acting similarly to hormones.

Autocrine

cell releases a messenger that binds to own receptors and shuts off the signal.

Paracrine

cell releases a messenger that binds to nearby target cells and turns on the signal.

Cytokine

A local mediator that promotes cell growth.

Nitric oxide

A local mediator that causes relaxation of blood vessels (dilation).

Eicosanoids

Molecules released after stimuli that act as local mediators.

Lipophilic messengers

Messengers that like water, can diffuse across membranes, and are released upon synthesis.

Lipophobic messengers

Messengers that do not like water, require exocytosis to be released, and are stored in vesicles.

Receptor affinity

The strength of the interaction between a messenger and its receptor; higher affinity receptors take priority.

Upregulation

The process by which a cell increases the number of receptors in response to messenger deficiency.

Downregulation

The process by which a cell decreases its number of receptors in response to excess messengers.

Signal transduction pathway

The sequence of events that occurs between receptor activation and the cell's response.

Channel-linked receptors

Simplest receptors where a messenger binds and opens a channel, such as Ca++ channels.

Enzyme-linked receptors

Receptors that, upon messenger binding, activate an attached enzyme to catalyze a chemical reaction.

G protein-linked receptors

The largest category of receptors, which bind to G proteins and can either open or close ion channels or communicate indirectly through second messengers.

what is the sequence of the transduction pathway for a first messenger system

Extracellular (first) messenger binds to receptor  —> Open or close channel

What is the sequence of the signal transduction pathway for a second messenger system?

1. Reception (first messenger —> receptor)

2. Activation of G proteins

3. adenylate cyclase

4. hundreds of cAMP

5. Activate Kinase A

6. phosphorylates hundreds of proteins