The physiological function of a cell and cellular communication are outcomes of a range of extrinsic signals, intracellular signaling pathways, and cellular responses. No two cell types express the same repertoire of signaling components. Receptors are highly selective for their cognate ligands, but once activated, they can alter multiple cellular processes such as DNA transcription, protein synthesis, and metabolic activity.
Graded and Abrupt Responses
Some signaling systems generate smoothly graded responses that vary proportionately with the signal concentration. For example, complexes made up of steroid hormone and its nuclear receptor bind to specific steroid hormone-responsive genes and cause increase or decrease in the mRNA production in proportion to the hormone concentration.
Some responses can also occur in an all-or-none (discontinuous) manner, where they are undetectable below the threshold concentration and reach a maximum level as soon as they cross the threshold. Such responses are irreversible. For example, at the neuromuscular junction, the activation of acetylcholine receptor-dependent ion channels causes Na+ influx into the muscle cells, depolarizing a part of the plasma membrane. The depolarization leads to the opening of the voltage-gated Na+ channels in the same membrane region, causing a higher influx of Na+ and further membrane depolarization. As additional Na+ channels open, the depolarization exceeds the threshold, and an action potential propagates to the entire muscle membrane.
Fast and Slow responses
Response timing is the time a cell takes to respond to an extracellular signal that varies significantly between different signaling systems. Certain cellular responses are highly rapid. For example, ion channels can open up to cause a change in the membrane potential within milliseconds of receiving the signal. Other responses, such as protein synthesis or metabolic alterations, can take several minutes. Some responses can also take hours and days, for example, cell division and programmed cell death.
The speed of any signaling response mainly depends on the nature of the intracellular signaling molecules involved in the response. For example, if proteins already present in the cell only need to be modified for activation/ deactivation (such as by phosphorylation or binding of GTP), then the response would occur rapidly. If changes in gene expression have to occur in response to a signal (such as during embryonic development), the response can take several minutes or hours. Responses that occur at a fast pace are transient and can be rapidly reversed. Whereas responses that occur at a slow pace mostly generate long-term or permanent cellular changes.
