Electromagnetic radiation
Exists in waves which are long radio waves and short X rays
Light
- An electromagnetic radiation with a wavelength between 400-700 nanometers
- Different wavelengths = different colours
Wavelengths of light
Refraction- the bending of light as it passes through a transparent object ie a prism splitting white light into different wavelengths (reflections)
Tetrachromats
Ancestral creatures that have four kinds of cone cells to see different colours; red, green, blue and ultraviolet
Dichromats
Modern creatures that lost two cone cells and have difficulty seeing red and green at night
Trichromats
Species that have re-developed the ability to see red and orange
Phototransduction
The process by which light energy produces graded receptor potentials
Photoreceptors
- Modified neurons that have their ends inserted into the pigmented layer of the retina
- Vulnerable to damage as they can be destroyed by intense light
Photopigment
- Visual pigment molecules that change shape when they absorb energy from photons of light
- Embedded in stacked discs of rod and cone cells
- Vision is repaired every 24 hrs as the tip of the rod/cone cells are phagocytized
- Made by a combination of retinal and 4 other opsin proteins to determine what colours of light are being absorbed
Rods
- Contain one type of pigment (rhodopsin) to perceive vision in one colour
- More sensitive to light than cones
- 92 million rods in the retina
Cones
- Each cone has one of three pigments to detect different colours
- Require brighter light to activate
- Direct connection to the optic nerve making visual image sharper
Optic Nerve
- Formed by ganglion cell axons
Visual signal direction: - Rods/cones -> bipolar neurons
- Bipolar neurons -> ganglion cells
Threshold depolarization
The amount of depolarization required to activate voltage-gated ion channels to allow Na+ arrival
What potential is an all-or-none phenomena
Action potentials as they either happen completely (threshold stimulus) or not at all (sub-threshold stimulus)
What can produce graded potentials
Changes in membrane potential from sensory to neurotransmitter receptors
Graded Potentials
Brief changes in polarization near the dendrite of the neuron
- Depolarization through the action potential threshold can cause voltage-gated ion channels to open = Action potentials travelling along the neuronal axon. Can be found throughout the sensory system (rods and cones are an exception)
Resting membrane potential changes
- Depolarization: the inside of the membrane is more positive. Increases the probability of producing nerve impulses
- Hyperpolarization: the inside of the membrane becomes more negatively charged. Decreases the probability of producing nerve impulse
What happens in the dark?
Photoreceptor cells are slightly depolarized, releasing an inhibitory neurotransmitter to the bipolar cell stopping it from stimulating the ganglion cell to fire
What happens in the light?
- Light bleaches the pigment which hyperpolarizes the photoreceptor and it stops releasing inhibitory neurotransmitters to the bipolar cell
- The bipolar cell can now depolarize/release an excitatory neurotransmitter to the ganglion cell, sending an action potential along the optic nerve
Light adaptation
- Occurs when we move from darkness to bright light
- First, both rods and cones are strongly stimulated
- Then, glare is produced by large amounts of pigments broken down instantaneously
- Pupils become narrower to reduce the amount of light reaching the retina
- After 5-10 minutes, visual sharpness improves as the rod system turns off and retinal sensitivity decreases
Dark adaptation
- Occurs when we go from a bright area to a dark one
- Bright light has bleached rod pigments, has shut off and requires time to reactivate
- The pupils dilate to maximize the amount of light reaching the retina
- Rhodopsin accumulates in the dark, which allows retinal sensitivity to increase
- At about 20-30 minutes, peak sensitivity is reached
Visual perception
- Ganglion cells merge in the back of the eyeball = the optic nerve which crosses at the optic chiasma = the optic tracts which are connected to the thalamus
- Optic radiations project to the primary visual cortex in the occipital lobes
Visual processing
Retinal ganglia: Simplify and condense information. Look for particular things like sharp changes in colour and brightness
Lateral geniculate nucleus: Integrates visual information to emphasize cone vision and begin processing depth perception
Primary visual cortex
- Maps retinal information onto the occipital lobe for further processing for contrast, colour, shape and movement
- Ventral “what” stream goes to the temporal lobes for memory, and the limbic system for emotions
- Dorsal “where/how” stream goes tot he occipital and parietal lobes to let you recognize what you’re looking at and how you can interact with it
