Cell signalling pathways
Both unicellular and multicellular organisms are able to communicate using cell signalling pathways. These enable them to act upon signals from the environment and also send signals to other cells. Cell signalling is an important aspect of controlling cell behaviour. There are a vast range of signals that stimulate cell signalling. These can be physical signals which include heat, light and pressure, but are more commonly chemical signals such as hormones and neurotransmitters.
Cells can transmit signals over short and long distances using both local and distant methods. Local methods include gap junctions, paracrine signalling and contact-dependent signalling. Gap junctions is the closest form of cell signalling allowing molecules to pass directly from cell-to-cell. Contact-dependent signalling requires cell membranes to be in direct contact with each other so they can transmit signals locally within their environment. Paracrine signalling involves a cell releasing a signal which is detected by cells in immediate proximity, often in the same tissue. One form of paracrine signalling is autocrine signalling. Autocrine signalling refers to the instance where a cell releases a signal, detects this signal and responds to this same signal.
Distant methods inculde synaptic and endocrine signalling. Synaptic signalling invovles neurons transmitting signals along their axon resulting in the release of neurotransmitters across the synaptic cleft to other neurons. Endocrine signalling involves the secretion of hormones into the blood stream.