Definition
Hormones are chemical messengers secreted by glands directly into the bloodstream, where they circulate and act on distant tissues that carry the matching receptors. Unlike a neuron's millisecond signal across a synapse, a hormone's influence unfolds over a slower timescale — minutes, hours, sometimes days — and reaches every cell its receptors permit.
In Robert Sapolsky's framing, hormones rarely create behavior outright. Instead they tilt the odds: they raise or lower the probability that the brain responds to a given situation in a particular way. Hormones set a backdrop against which the environment acts.
Why it matters
How it works
A triggering signal — often from the brain's hypothalamus — prompts a gland to release a hormone. The molecule travels through blood until it meets a cell with a receptor shaped to bind it. Steroid hormones can slip into the cell and alter gene expression directly; peptide hormones dock at the cell surface and launch internal cascades. Either way, the target cell's behavior shifts, and feedback loops eventually damp the signal back down.