Communication involves transmitting and receiving information
A SIGNALING cell sends a signal and is received by a TARGET cell
[ S ignal molecules can induce different responses in their target cells e.g. acetylcholine : causes cardiac muscle to relax, but skeletal muscle to contract ]
If a change in the form of a signal is required, it is called a SIGNAL TRANSDUCTION
Analogy: Faxing a letter – conversion of a printed form of information into an electronic form – back into a printed form ANIMATION
Communication Systems Signal molecules in the plasma membrane of the signal cell interact with membrane bound receptors on the target cell. These signals are therefore restricted to cells which are in direct contact CONTACT DEPENDENT Nerve cells or neurones elicit responses by the release of a neurotransmitter at synapses. Can signal over very long distances via a network of nerve cells. Very fast signalling e.g. GABA (Gamma-Amino-Butyric-Acid – an inhibitory neurotransmitter) NEURONAL Secretion of a local mediator. This affects cells in the immediate area of the signalling cell e.g. Histamine PARACRINE Secretion of a hormone into the bloodstream for dispersal. The signalling cell and the target cell can be far apart. Very slow method e.g. Insulin, Adrenaline ENDOCRINE
Some small hydrophobic molecules can cross the plasma membrane and enter the cell by diffusion
Best known classes are the STEROID hormones e.g. cortisol & testosterone and the THYROID hormones e.g. thyroxine
The hormones can diffuse across the plasma membrane and bind to receptor proteins that are located either in the cytosol or in the nucleus itself
They work by activating GENE REGULATORY PROTEINS in the cell, which stimulate transcription of particular sets of genes in the nucleus
The mode of action of cortisol: Cortisol is a steroid hormone that is released in the body in response to physical or psychological stress. The secretion of cortisol induces energy-directing processes for the purpose of providing the brain with sufficient energy sources that prepare an individual to deal with stressors. In addition to its role as a so-called "stress hormone", cortisol plays many key roles in almost every physiologic system. Regulation of blood pressure, cardiovascular function, carbohydrate metabolism, and immune function are among the best known functions of cortisol.
A neurotransmitter ( e.g. acetylcholine , noradrenaline ) binds to this type of receptor, altering its conformation to open or close a channel (often through or near the receptor) to the flow of Na2+, K+, Ca2+, or Cl- ions across the membrane.
Driven by their electrochemical gradient ( i.e. one side of the membrane has numerous ions, while the other side has few) the ions rush into or out of the cell, creating a change in the membrane potential due to the positive or negative nature of the ions.
This flow of ions through the channel can trigger a nerve impulse, or alternatively stop one from occurring.
On binding the signal, the G-protein is activated by the binding of GTP
This activated protein diffuses away from the receptor protein site and activates its target protein
This may be an ion-channel protein or an enzyme such as adenylate cyclase or phospholipase C These enzymes catalyse the formation of small molecules known as secondary messengers which trigger the intracellular response to the original signal transduction event to the cell surface.
The cyclic AMP (cAMP) signal transduction pathway
Signals can be of many different types and can act either by diffusing across the plasma membrane (such as STEROID HORMONES e.g. testosterone and NITRIC OXIDE ) or by interacting with a receptor protein on the cell surface
The variety of signals, receptors and responses means that the system of signal reception and transduction can generate very specific effects in different types of cell
The response of a cell to a signal can involve ion flow, activation of specific proteins, or changes in gene expression
These effects can be short-lived, as in the case of the generation of an action potential, or they may be permanent alterations that control the developmental fate of the cell
It is therefore clear that the idea of a cell as a self-contained unit is in fact very far from the reality of the situation - cells are constantly engaged in the exchange of information in the form of molecular signals and it is this that enables cells in multicellular systems to function in an integrated way.