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Sensitivity- Rods are very sensitive to light (fire action potentials in dim light). This because many rods join one neurone, so many weak generator potentials combine to reahc the threshold and trigger an action potential. cones are less sensitive than rods ( they only fire action potentials in brought light), because one cone joins one neurone, so it takes more light to reach the threshold and trigger an action potential.

Visual acuity- the ability to tell apart 2 close objects Rods give low acuity as many rods are joined to the same neurone, which means light from 2 objects close together can't be told apart. Cones give high visual acuity, as they are close together and one cone joins to one neurone. When light from 2 points hits 2 cones, 2 action potentials go to the brain- so you can distinguish 2 points that are close together as 2 separate points

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2 types of photoreceptor- rods and cones. Rods are mainly found in the peripheral parts of the retina, and cones are mainly found in the fovea. Rods only give black and white, while cones give colour. There are 3 types of cone; red-sensitive; green-sensitive; and blue-sensitive. They're stimulated in different proportions so you see different colours

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Light enters the eyes and hits the receptors, it is absorbed by light sensitive pigments. light bleaches the pigments, causing a chemical change and altering the membrane permeability to sodium. A generator potential is created and if it reaches the threshold, a nerve impulse, a nerve impulse is sent along a bipolar neurone. Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.

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Detect light in the eyes. It enters the eye through the pupil, and the amount of light that enters is controlled by muscles in the ris. Light rays are focused by the lens on the retina, which lines the inside of the eye. The retina contains the photoreceptive cells. The fovea have lots of photoreceptors. Nerve impulses from the photoreceptors 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 is called the blind spot- as there are no photo receptive cells.

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mechanoreceptors. They detect mechanical stimuli, pressure and vibration. In the skin. They contain the end of a sensory neurone, a sensory nerve ending. The sensory nerve ending is wrapped in loads of layers of connective tissue called lamellae.

Whe it is stiumlated the lamellae are deformed and press on the sensroy nerve ending. This causes deformation of stretch-mediated sodium channels in the sensorys neurones cell emrbane. The sodium ion chamnnels open and sodium ions diffuse into the cell, creating a generator potential. If it reaches the threshold, it triggers an action potensial.

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If the generator potential is big enough it will trigger an action potential- an electrical impulse along a neurone. An electrical impulse is only triggered if the generator potential reahces the threshold level. If the stimulus is too weak the generator potential wont reach the threshold level, so there's no action potential.

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When a stimulus is detcted, the cell membrane is excited and becomes more permeable , allowing more ions in and out of the cell- altering the potential difference. The change in potential difference due to a stimulus is called the generator potential. A bigger stimulus excites the membrane more, causing a bigger chance in potential difference- so a bigger generator potential is produced.

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When a nervous system receptor is at rest, there's a difference of charge between the inside and the outide of the cell- inside is -/ve charged relative to the outiside of the cell. This means theres a voltage across the membrane. Or a potential difference. This is its resting potential. It is generated by ion pumps and ion channels.

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They are specific to 1 stimulus. Some are cells, like photoreceptors, and connected to the nervous system. Some are proteins on the cell surface membrane, such as glucose receprotrs in the cell surface membrane of pancreatic cells. Receptors in the nervous system convert the energy of the stimulus into the electrical energy used by the neurones.