ionic strength, and changes in the properties of the plasma membrane or cytoskeleton (the latter being a form of mechanotransduction). These changes are detected by proteins known as osmosensors or osmoreceptors. In humans, the best characterised osmosensors are transient receptor potential channels present in the primary cilium of human cells. In yeast, the HOG pathway has been extensively characterised. Temperature The sensing of temperature in cells is known as thermoception and is primarily mediated by transient receptor potential channels. Additionally, animal cells contain a conserved mechanism to prevent high temperatures from causing cellular damage, the heat-shock response. Such response is triggered when high temperatures cause the dissociation of inactive HSF1 from complexes with heat shock proteins Hsp40/Hsp70 and Hsp90. With help from the ncRNA hsr1, HSF1 then trimerizes, becoming active and upregulating the expression of its target genes. Many other thermosensory mechanisms exist in both prokaryotes and eukaryotes. Light In mammals, light controls the sense of sight and the circadian clock by activating light-sensitive proteins in photoreceptor cells in the eye's retina. In the case of vision, light is detected by rhodopsin in rod and cone cells. In the case of the circadian clock, a different photopigment, melanopsin, is responsible for detecting light in intrinsically photosensitive retinal ganglion cells. Receptors Receptors can be roughly divided into two major classes: intracellular and extracellular receptors. Extracellular receptors Extracellular receptors are integral transmembrane proteins and make up most receptors. They span the plasma membrane of the cell, with one part of the receptor on the outside of the cell and the other on the inside. Signal transduction occurs as a result of a ligand binding to the outside region of the receptor (the ligand does not pass through the membrane). Ligand-receptor binding induces a change in the conformation of the inside part of the receptor, a process sometimes called "receptor activation". This results in either the activation of an enzyme domain of the receptor or the exposure of a binding site for other intracellular signaling proteins within the cell, eventually propagating the signal through the cytoplasm. In eukaryotic cells, most intracellular proteins activated by a ligand/receptor interaction possess an enzymatic activity; examples include tyrosine kinase and phosphatases. Often such enzymes are covalently linked to the receptor. Some of them create second messengers such as cyclic AMP and IP3, the latter controlling the release of intracellular calcium stores into the cytoplasm. Other activated proteins