Your eyes could, in theory, spot a single candle thirty miles away
Physiologists like to express the faintest signal each sense can catch in everyday terms. For vision, the classic figure is one candle 30 miles off on a dark night, once your eyes have adjusted. For touch, it is a bee's wing dropped onto your cheek from a centimetre above.
These benchmarks describe absolute thresholds, the smallest stimulus a receptor will respond to. Smell registers a single drop of perfume spread through a six-room house. Taste picks up one drop of bitter quinine sulfate in 250 gallons of water. Hearing catches a watch ticking 20 feet away in otherwise total silence. The exact numbers shift with the body part being touched or the substance being smelled, but the images convey how sensitive the equipment is.
In physiology a stimulus is any change inside or outside the body that an organism can detect; the word comes from the Latin for a goad, the pointed stick used to drive cattle. Receptors turn such changes into electrical signals through a step called transduction. Some face outward, such as light-sensitive cells in the retina, while others monitor the inside: stretch receptors in the carotid arteries track blood pressure, and when they sense less stretching, the body tightens its blood vessels and speeds the heart to compensate.
Pain has its own wiring. Nerve endings called nociceptors come in two main kinds. A-fibres are insulated with myelin and carry quick, sharp pain, while unmyelinated C-fibres conduct more slowly and deliver the dull, burning ache that follows. That difference explains why a stubbed toe hurts in two distinct waves.
The senses hide some neat mechanics. Only about 2 percent of the airborne compounds you inhale reach the smell receptors high in the nose. Taste cells are replaced roughly every ten days. In the inner ear, tiny crystals of calcium carbonate rest on a gel above sensory hair cells; tilt your head or accelerate and the crystals shift, bending the hairs and telling the brain which way is down.
Source: Stimulus (physiology)
