Two examples are given here to represent the general properties of receptors: the Pacinian corpuscle and the rods and cones in the eye.
1. The Pacinian corpuscle
Named by its Italian discoverer Filippo Pacini (si splendido uomo!*), it is a 1 mm diameter skin receptor which enables us to perceive pressure and vibration. This is the receptor responsible for our awareness off smooth vs rough surfaces, shallow vs intense tactile sensation, etc.
How does this seemingly magical process take place?
The central capsule is surrounded by lamellae. At the heart of the capsule lies the sensory nerve tube. Throughout the corpuscle you can see capillaries branching out.
It’s easy: when you tap you finger on a surface, for example, the pressure exerted upon the lamellae of the Pacinian corpuscles in your finger skin makes them bend and exert that pressure back onto the sensory neuron in the capsule.
This physically squishes out Na+ ions as the plasma membrane is deformed, which creates a generator potential. If this potential passes a threshold, an action potential is triggered.
The greater the stimulus, the higher the nerve impulse frequency. A maintained stimulus such as having a hat on won’t generate multiple impulses, whereas a tiny insect repeatedly disturbing the lamellae will prove much more troublesome.
The Pacinian corpuscle is a good representative of receptors’ specificity i.e. the response only takes place in the presence of specific stimuli such as pressure.
2. Rods and Cones
They are the two main specialised eye photon receptors and differ in their distribution and sensitivity.
Rods are extremely sensitive in low light and can perceive as few as 6 photons, while cones sense more abundant light and therefore can identify different colours.
Have a look above: this works in the same way as the Pacinian corpuscle in the sense that action potentials are generated by polarised membranes as a result of light entering the eye and causing a chemical reaction in the respective cells.
Note the cone cell is connected to a single neurone while a few rod cells share the same neurone. Why could this be? When very little light enters the eye, it isn’t sufficient to generate an action potential per rod cell. So the cumulative light from a few rods creates a response great enough to trigger an action potential in one neurone.
The downside of this arrangement for rod cells is that visual acuity, that is the ability to tell two points apart, decreases.