3. Receptors

Receptors

  • specific - they only detect on particular stimulus

  • Many different types, some are cells, some are proteins on cell surface membranes

How receptor cells communicate

  • through the nervous system

  • When the NS is in its resting state (not being stimulated), there’s a difference in charge between the inside and outside of the cell

  • This is generated by ion pumps and ion channels. This means there’s a voltage across the membrane, known as a potential difference

  • The potential difference when a cell is at rest is called its resting potential

  • When a stimulus is detected the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell, altering the potential difference

  • The change in the potential difference due to a stimulus is called the generator potential

  • A bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger change in potential difference, so a bigger generator potential is produced

  • If the generator potential is big enough it’ll trigger an action potential = an electrical impulse along a neurone

  • This is only triggered if the generator potential reaches the threshold

  • The strength of the stimulus is measured by the frequency of action potential

  • If the stimulus is too weak, there’s generator potential wont reach the threshold, so there’s no action potential

Pacinian corpuscles

  • pressure receptors in your skin. Mechanoreceptors = detect mechanical stimuli

  • Contain a sensory nerve ending. This is wrapped In loads of layers of connective tissue called lamellae

  • When a pacinian corpuscle is stimulated the lamellae are deformed and press on the sensory nerve ending

  • This causes the sensory neurones cell membrane to stretch, deforming the stretch-mediated sodium ion channels

  • The channels open and sodium ions diffuse into the cell, creating a generator potential wont reach

  • If the generator potential reaches the threshold, it triggers an action potential

Photoreceptors

  • light receptors in the eye

  • Light enters through the pupil, the amount of light that enters is controlled by the muscles of the iris

  • Light rays are focused by the lens onto the retina, which lines the inside of the eye

  • The retina contains photoreceptors that detect light (Fovea contains alot)

  • Nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve (which is a bundle of neurones)

  • Where the optic nerve leaves the eye is called the blind spot, there aren’t any photoreceptor cells so its not sensitive to light

Photoreceptors convert light into an electrical impulse

  • light enters the eye, hits the photoreceptors and it absorbed by light-sensitive optical pigments

  • Light bleaches the pigments, causing a chemical change and altering the membrane permeability to sodium ions

  • A generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone

  • Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain

2 types of photoreceptors — rods and cones

Rods

  • mainly found in the peripheral parts of the retina

  • Different optical pigments make them sensitive to certain wavelengths of light

  • Only give information in black and white (monochromatic vision)

Cones

  • packed together in the fovea

  • Different optical pigments make them sensitive to certain wavelengths of light

  • Give information in colour (trichromatic vision)

  • 3 types of cones each containing a different optical pigment: red, green and blue-sensitive. Stimulated in different proportions to see different colours

Sensitivity

  • rods are very sensitive to light because many rods join one neurone. So many weak generator potentials combine to reach the threshold and trigger an action potential

  • Cones are less sensitive to light because one cone joins one neurone. So it takes more light to reach the threshold and trigger an action potential

Visual acuity

  • rods give low visual acuity because many rods join the same neurone, meaning light from two points close together can’t be told apart

  • Cones give higher visual acuity because cones are close together and one cone joins one neurone. Because separate action potential will go to the brain from each cone, so you can distinguish two points that are close together as separate