2000..Nobel Prize in Medicine and Physiology.

Arvid Carlsson

Paul Greengard

Eric R. Kandel

The Nobel Prize in Physiology or Medicine 2000 was awarded jointly to Arvid Carlsson, Paul Greengard and Eric R. Kandel "for their discoveries concerning signal transduction in the nervous system".

A signal transduction in biology, is a cellular mechanism. It converts a stimulus into a specific cellular response.^[1] Signal transduction starts with a chemical or physical signal to a receptor, and ends with a change in cell function.

Receptors are in the cell membrane, with part of the receptor outside and part inside the cell.  The chemical signal binds to the outer portion of the receptor, changing its shape. This causes another signal inside the cell.  Some chemical messengers, such as testosterone, can pass through the cell membrane, and bind directly to receptors in the cytoplasm or nucleus.

Sometimes there is a cascade of signals within the cell. With each step of the cascade, the signal can be amplified, so a small signal can result in a large response.^[1] Eventually, the signal creates a change in the cell, either in the expression of the DNA in the nucleus or in the activity of enzymes in the cytoplasm.

Most often, ordered sequences of biochemical reactions inside the cell are involved. These are carried out by enzymes and linked through second messengers. So a "second messenger pathway" is produced. These things usually happen quickly, sometimes very quickly. They may last from milliseconds (in the case ofion flux) to days for gene expression.

The number of proteins and other molecules that take part increases during the process. So a 'signal cascade' develops and a relatively small stimulus may cause a large response.

In bacteria and other single-cell organisms, the transduction processes a cell has limits the number of ways it can respond to its environment. In multicellular organisms, lots of different signal transduction processes are used to coordinate the behavior of individual cells. By this means the function of the organism as a whole is organized. The more complex the organism, the more complex the repertoire of signal transduction processes the organism must possess.

Thus, sensing of both the external and internal environment at the cellular level, relies on signal transduction. Many disease processes such as diabetes, heart disease, autoimmunity and cancer arise from defects in signal transduction pathways. This highlights the critical importance of signal transduction to biology and medicine.^[2]