Why sort receptors by speed
There are thousands of receptors, but pharmacologists tame them by sorting almost all into just four superfamilies. The cleanest way to tell them apart is to ask one question: *once the ligand binds, how long until something happens?* The answer ranges from a few thousandths of a second to several hours, and that timescale tells you the family.
FAMILY SPEED EXAMPLE SIGNAL Ligand-gated ion channel ~milliseconds nerve & muscle firing GPCR (G-protein-coupled) ~seconds most hormones, smell, vision Kinase-linked receptor ~minutes-hours insulin, growth factors Nuclear receptor ~hours steroids, thyroid hormone
The fast two: channels and GPCRs
The fastest is the ligand-gated ion channel. The receptor *is* a pore through the membrane. When the ligand binds, the pore snaps open and ions rush through within milliseconds — this is how nerves and muscles fire. The nicotinic receptor for acetylcholine is the classic example.
Next is the GPCR — the G-protein-coupled receptor, by far the biggest family and the target of roughly a third of all medicines. It does not open a pore itself. Instead, binding makes it nudge a partner inside the cell, which sets off a relay of second messengers. This indirect relay takes seconds and amplifies the signal enormously. We devote the next guide entirely to it.
The slow two: kinases and nuclear receptors
The kinase-linked receptor sits in the membrane like the others, but binding switches on an enzyme built into its own tail — a kinase, which tags other proteins with phosphate and so reprograms the cell over minutes to hours. The insulin receptor and the receptors for growth factors work this way, which is why their drugs change how a cell grows and uses fuel rather than firing it instantly.
Slowest of all is the nuclear receptor. It does not sit in the membrane at all; it floats inside the cell. Its ligands must be greasy enough to slip across the membrane on their own — steroid hormones, thyroid hormone, vitamin D. Once bound, the receptor travels to the DNA and turns genes on or off. Because the cell must then build new proteins, the effect takes hours and lingers long after the drug is gone.