Midterm #2 Flashcards
Sensation
Refers to how cells of the nervous system detect stimuli in the environment (such as light, sound, heat, etc.), and how they transducer (convert) these signals into a change in membrane potential and neurotransmitter release.
Perception
Refers to the conscious experience and interpretation of sensory information
Sensory neurons
Specialized cells that detect a specific category of physical events, such as: the presence of specific molecules (smell, taste, nausea, pain), the presence or absence of physical pressure
(touch, stretch, vibration), the temperature (heat, cold, pain), the pH of a liquid (sour taste, suffocation, pain), electromagnetic radiation (vision).
Sensory Transduction
Sensory neurons have specialized receptors that transduce sensory stimuli into a change in membrane potential. Come in all shapes and sizes. Many sensory neurons do not have axons or action potentials, but they all release neurotransmitter. Sensory neurons that do not have action potentials release neurotransmitter in a graded fashion, dependent on their membrane potential. The more depolarized they are, the more neurotransmitter they release.
Photoreceptor cells
The sensory neurons responsible for vision. These cells transduce the electromagnetic energy of visible light into a change in membrane potential, which affects how much neurotransmitter they release. These cells do not have action potentials.
Opsins
Light-sensitive proteins. The opsins in photoreceptor cells are metabotropic receptors. They are only sensitive to light because they bind a molecule of retinal, which changes shape in response to light. The change in the shape of retinal is what activates this metabotropic receptor.
Retinal
Small molecule (synthesized from vitamin A) that attaches to the opsin proteins in the photoreceptor cells in our eyes. The retinal molecule is technically what absorbs the electromagnetic energy of visible light that allows us to see.
The two configurations of the retinal molecule
When retinal absorbs a wavelength of visible light, it activates the opsin protein (a metabotropic receptor). This launches an intracellular G protein signalling cascade that changes the membrane potential of the photoreceptor cell, affecting how much neurotransmitter it releases.
4 types of photoreceptor cells that contribute to vision and their metabotropic opsin protein
- Red cone cells express the red cone opsin.
- Green cone cells express the green cone opsin.
- Blue cone cells express the blue cone opsin
- Rod cells express the rhodopsin opsin. (The last to evolve; 100 times more sensitive to light than cone cells).
Cone Photoreceptors: Trichromatic Coding
Blue cone opsins are most sensitive to short wavelengths of light. Green cone opsins are most sensitive to medium wavelengths of light. Red cone opsins are most sensitive to long wavelengths of light.
Color perception is a function of the relative rates of activity across the three types of cone cells (i.e. colors are discriminated by the ration of activity across these cells.)
Trichromatic Coding
The key consideration for our brain for identifying color is how much each type of cone cell is activated relative to its maximum level. Each color of the rainbow corresponds to a particular pattern of activation across the three types of cones cells.
How is yellow light created
When red and green light bulbs are so close together that our eyes can’t differentiate them, the color looks yellow to us. Green light (530nm) activates the green cone opsin more than the red cone opsin. Red light (680nm) activates the red cone opsin more than the green cone opsin. The combination of red and green light causes the red and green cone opsin to be activated at similar amounts, which is what happens with yellow light (580nm).
Additive Color (Light)
Primary colours of light: Green, Red, Blue. Combine to make yellow, cyan, magenta. Sunlight is white light, since it contains an equal mixture of all the colors.
Our perception of light and colour has three dimensions to it
- Brightness - intensity (luminance, amount)
- Saturation - purity (in terms of wavelength mixture)
- Hue - dominant wavelength (colour)
Perceptual Dimensions of colour & light
If brightness is zero, the image is completely black. Hue and saturation have no impact without brightness. If there is (bright) light, the next question is whether the light is saturated with a particular wavelength (colour). If saturation is 0% then you are in the middle of the colour cone where there is an equal contribution from all visible wavelengths. An image with 0% saturation is grayscale (black and white), because all wavelengths are present in equal amounts. If saturation is >0%, the hue indicates the colour that the light is saturated with.
Protanopia
Absence of the red cone opsin (1% of males). People with this condition have trouble distinguishing colours in the green-yellow-red spectrum. Visual acuity is normal because red cone cells switch to using the green cone opsin. Simple mutations of the red cone opsin price less pronounced deficits in colour vision.
Deuteranopia
Absence of the green cone poison (1% of males). People with this condition have trouble distinguishing colours in the green-yellow-red spectrum. Visual acuity is normal because green cone cells switch to using the red cone opsin. Simple mutations of the green cone opsin (6% of males) produce less pronounced deficits in colour vision.
Tritanopia
Absence of the blue cone opsin (1% of the population). Blue cone cells do not compensate for this in any way, but since the blue cone opsin is no that sensitive to light anyway, visual acuity is not noticeably affected).
Visible light
Refers to electromagnetic energy that has a wavelength between 380 and 760 nm. We detect this light using four kinds of photoreceptor cells ( 1 rod & 4 cone cells).
The cornea
The outer, front layer of the eye. It focuses incoming light a fixed amount.
The conjunctiva
Is a mucous membranes that line the eyelid.
The sclera
Is opaque and does not permit entry of light.
The iris
A ring of muscle. The contraction and relaxation of this muscle determine the size of the pupil, which determines how much light will enter the eye.
The lens
Consists of several transparent layers. We change the shape of this lens to focus near versus far, a process known as accommodation.