Photoreceptors (A-level Biology)
Photoreceptors
Light Receptors in the Eye
Receptors, which are specialised cells, help to make up the sense organs to detect stimuli.
The retinas of the eye contain photoreceptors that detect light. They contain different light-sensitive optical pigments, so they are sensitive to different wavelengths of light.
There are two types of photoreceptors in the retina: cones and rods.
Light rays enter through the pupil and are focused onto the retina by the lens.
The photoreceptors in the retina produce impulses that travel through the optic nerve to the brain.
Conversion of Light into Electrical Impulses
- The light is absorbed by light-sensitive optical pigments in the photoreceptors.
- This causes a chemical change that increases the permeability of the membrane to Na+ ions.
- Na+ ions rush into the membrane. This stimulates a generator potential, which sends the impulse along a bipolar nerve to the brain.
Cone Cells
Cone cells are located in the centre of the retina in an area called the fovea. There are around 6 million of these cells which help to produce the clearest image possible.
- Cone cells give information in colour. There are three types: red-sensitive receptors, green-sensitive receptors and blue-sensitive receptors. Different intensities of light in these pigments allow for colour to be seen.
- Cone cells only detect high light intensity. This is because each cone cell connects to only one neurone, so it takes higher intensities of light to trigger a generator potential.
- They have high visual acuity. Visual acuity is the ability to tell apart two points that are close together. Cone cells only connect to one other neurone, so each individual stimuli can be distinguished.
Rod cells
Rod cells are located in the periphery of the retina (the outer region), giving peripheral vision, and are more abundant than cone cells, approximating 120 million cells.
- Rod cells absorb light energy. A light-sensitive pigment, rhodopsin, in contained in rod cells. It absorbs light energy to split into cis-retinal and cis-opsin, in a process known as bleaching.
- Rod cells are monochromatic. This means that they only give information in black and white.
- Rod cells can detect both high and low light intensity. This is because three rod cells are connected to one bipolar cell, so many weaker signals can combine to trigger a generator potential.
- Rod cells have low visual acuity. Many rod cells are connected to one neurone, which means the stimuli merge together, making points harder to differentiate.
How Rod Cells Work
- The membrane is depolarised. Active transport pumps sodium ions through open sodium ion channels, making the cell slightly more negative on the inside, causing a depolarisation.
- Glutamate in rod cells stop information being passed to the brain. Glutamate, a neurotransmitter, stops neurones connecting the rod cells to the optic nerve (creating an ISPS), inhibiting information reaching the brain.
- The membrane becomes hyper-polarised. After bleaching, the sodium ion channels close, not affecting the active transport of sodium ions, as cis-opsin binds to the membranes.
- An action potential forms. The neurotransmitter isn’t released into the synaptic cleft creating an action potential, getting transported to the brain through the optic nerve.
Overview
The table below gives an overview of where the photoreceptors are located and what their function is.
Photoreceptors are specialized cells in the eye that detect light and convert it into neural signals that can be processed by the brain.
The two types of photoreceptors in the eye are rods and cones.
Rods are specialized photoreceptors that are sensitive to low levels of light and are responsible for night vision. Cones, on the other hand, are specialized photoreceptors that are responsible for color vision and visual acuity in bright light.
Photoreceptors work by absorbing photons of light and triggering chemical and electrical changes in the cell, which then triggers a nerve impulse that is sent to the brain.
The process by which photoreceptors convert light into a neural signal is known as phototransduction. This process involves the activation of a photopigment in the photoreceptor, which triggers a cascade of chemical reactions that result in the release of a neurotransmitter.
The neurotransmitter plays a critical role in phototransduction as it helps to transmit the neural signal from the photoreceptor to the bipolar cells in the retina, which then pass the signal on to the ganglion cells and eventually to the brain.
When there is too much light in the eye, it can cause the photoreceptors to become overstimulated, leading to visual discomfort and even temporary vision loss in extreme cases.
The pigment epithelium is a layer of cells in the eye that helps to support and protect the photoreceptors, as well as to recycle the photopigments that are used in phototransduction.
Photoreceptor cells are replenished through a process known as neurogenesis, where new photoreceptor cells are generated from stem cells in the retina.
Some disorders related to photoreceptors include age-related macular degeneration, retinitis pigmentosa, and night blindness. These conditions can result in a loss of visual function and can ultimately lead to vision loss if left untreated.
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