Detection of light Flashcards
Pupil Dilation function
-Pupil dilation is an example of a neural pathway
-Pupil dilation allows the eye to control how much light is hitting the retina
How does Pupil Dilation work
-Pupil Dilation is controlled by two sets of muscles in the eye
-Circular muscles contract to constrict the pupil
-Radial muscles contract to dilate the pupil
-These two sets of muscles work antagonistically, meaning when one contracts the other dilates and vice versa
Examples of the neural pathway occurring in bright light
Bright light (stimuli) — receptors in the eye— sensory neurone— CNS — motor neurone— circular muscles in the eye
-Circular muscles in the eye now contracts, constricting the pupil
-This limits the amount of light entering the pupil, preventing damage to the retina
Examples of the neural pathway occurring in low light
Low light— receptors in the eyes— sensory neurone— CNS— motor neurone— Radial muscles
-Radial muscles then constrict, dilating the pupil
-This maximises the amount of light entering the eye, improving vision
How does light enter the eyes + fovea
-Light enters the eyes through the pupils and is focused on a region of the retina called the fovea
-The fovea contains many light receptors (photoreceptors)
Rod Cells
-Rod cells are mostly found around the outer retina
-Rod cells are sensitive to light intensity, so can detect brightness and the presence of light
-Images generated from information just from rod cells are only in black and white
Cone Cells
-Cone cells are mostly grouped around the fovea (region of the retina)
-Cone cells are sensitive to different wavelengths of visible light
-They can be red-sensitive, green-sensitive or blue- sensitive
-The number of RGB sensitive cone cells stimulated determine the colours that are seen
How are action potentials generated by photoreceptors transmitted to the brain
-Action potentials generated by photoreceptors are transmitted to the brain via the optic nerve
-The optic nerve leaves from the back of the eye, which is known as the blind spot (has no photoreceptors)
How do photoreceptors generate an action potential
-Photoreceptors generate an action potential if stimulated by a bright enough light (rod cells) or a certain wavelength of light (cone cells)
Bleaching (rhodopsin as example)
-When light falls on light-sensitive pigments within photoreceptors, they become bleached
-For example, rod cells contain a light-sensitive pigment called rhodopsin
-When light falls on rhodopsin, it breaks into two parts: retinal and opsin
-The breaking apart of rhodopsin is known as bleaching
-The bleaching of a light-sensitive pigment causes a chemical change in the photoreceptors, and this results in the generation of a nerve impulse
-nerve impulse travels along a bipolar neurone to the optic nerve, which then carries information to the brain
what makes the action of rod cells different to that of other nerve cells
Action of rod cells in the dark
-In the dark, sodium ions are actively pumped out of rod cells, creating a concentration gradient
-Sodium cions diffuse back down the concentration gradient into the rod cell via sodium ion channels
-At this point, there is little charge difference between the inside and the outside of the rod cell (inside of rod cell is slightly more negative) meaning that the rod cell is depolarised
-The depolarised cell releases neurotransmitters across the synapse to a bipolar neurone
-The neurotransmitter inhibits the generation of an action potential, this means that a signal is not sent to the optic nerve
Action of rod cells in the light
Light bleaches rhodopsin, causing it to break down into retinal and opsin
-The bleaching of rhodopsin cause the sodium ion channels in the rod cell surface membrane to close, preventing the diffusion of sodium ion back into the cell
-Active transport is still occuring, so sosdium ions are being removed from the cell but cannot return
-The lack of positive ions entering the cell causes the interior of the cell to become increasingly negative, until it reaches a state of hyperpolarisation
-The hyperpolarised cell does not release inhibitory neurotransmitters
-This means that an action potential can be generated in neighboring bipolar neurones, meaning a nerve impulse is sent to the optic nerve
