Receptors: MDCAT Biology notes
Receptors MDCAT notes: types of sensory receptors, chemoreceptors, mechanoreceptors, Meissner and Pacinian corpuscles, rods, cones and blind spot.
What receptors do
A receptor is a structure that detects a stimulus, meaning a change in the internal or external environment. It converts the stimulus into nerve impulses, which sensory neurons carry to the central nervous system. Receptors can be single cells, nerve endings, or parts of complex sense organs such as the eye and ear. Each receptor type responds best to one kind of stimulus.
Classification by stimulus
| Receptor type | Stimulus | Examples |
|---|---|---|
| Chemoreceptors | Chemicals | Taste buds (tongue), olfactory (smell) receptors in the nose |
| Mechanoreceptors | Touch, pressure, sound, stretch | Meissner's and Pacinian corpuscles; hair cells of the ear |
| Photoreceptors | Light | Rods and cones of the retina |
| Thermoreceptors | Heat and cold | Temperature nerve endings in the skin |
| Nociceptors | Harmful stimuli (pain) | Free (bare) nerve endings |
Skin receptors
Human skin contains several receptors, each sensing a particular stimulus:
- Meissner's corpuscles:touch (light pressure). They lie just below the epidermis in the dermal papillae and are plentiful in fingertips and lips.
- Pacinian corpuscles:deep pressure. They lie deep in the dermis and have a layered, onion-like capsule.
- Free nerve endings: pain, and also heat and cold.
Red and white "blood corpuscles" are blood cells, not receptors. This is a common distractor.
Photoreceptors of the eye
The retina has two types of photoreceptor cells:
| Rods | Cones |
|---|---|
| Sensitive to dim light, for night vision | Work in bright light |
| Black-and-white vision | Colour vision |
| Pigment: rhodopsin; vitamin A deficiency causes night blindness | If some cones cannot distinguish colours, the result is colour blindness |
The blind spot (optic disc) is where the optic nerve leaves the eye. It has no rods and no cones, so an image falling there is not seen. The fovea has the most cones and gives the sharpest vision.
Worked examples
- A receptor fires when sugar touches the tongue. The stimulus is a chemical, so it is a chemoreceptor.
- A person feels a firm squeeze on the arm but not a light brush. The deep receptor responding is a Pacinian corpuscle.
- A patient cannot see well at dusk. The rods are failing, which fits night blindness.
Common MDCAT traps
- Taste and smell are both chemoreceptors, not mechanoreceptors.
- Meissner's is touch; Pacinian is deep pressure. Do not swap them.
- The blind spot lacks both rods and cones, not just one type.
- Rods handle dim light and cones handle colour.
- Nociceptors detect pain, not light or chemicals.
Quick revision
- Receptors turn stimuli into nerve impulses.
- Taste buds are chemoreceptors.
- Pacinian corpuscles sense deep pressure.
- Meissner's corpuscles sense touch.
- Colour blindness is a cone defect.