A. Photoreceptors contain pigment that absorb light and transduce it into electrical signals
B. Photoreceptors are eye spots and eyes. Eye spots may be seen in invertebrates and useful for finding food. Eyespots are also seen in some vertebrates, such as the iguana, but is known for diurnal cycles (seeking light) and not for vision.
C. Localization of photoreceptors in small spot/sheet provides directional information
(Photoreception is a special sense, not a general sense. Special senses just have receptors in one location, such as vision. General senses have receptors all over the body, such as pain or touch.)
II. Light -most animals lack the ability to detect other electromagnetic waves except light which can be described in terms of wavelength and frequency
III. Eyes differ amongst animals – plates in saltwater & fresh water snails; cups in the Nautilus (shelled squid-like animal); camera with a lens (vertebrates); compound eye of arthropods (insects…)
IV. Optics –light waves diverge from their source and then the cornea and lens focus (converge) the light waves on the retina at the focal point.
A. Light Refraction– the bending of light waves; refraction occurs at the cornea and lens; the lens can change shape thus affecting refraction
B. Light entering the eye is regulated by the iris and the pupil – iris is a smooth muscle regulated by ANS the determines the size of the pupil. Sympathetic stimulation dilates the pupil and parasympathetic stimulation constricts the pupil.
C. Photoreceptors are located in the retina
1. Outer pigmented layer –this layer contains phagocytes and captures stray light waves
2. Inner sensory layer with photoreceptors & other neural components; containing macula lutea with the fovea centralis
3. Optic disk– blind spot where optic nerve exits eye; lacks photoreceptors
4. Photoreceptors are rods and cones
a. Outer segment is rich in photopigments that are folded to form disks
b. Inner segment contains nuclei and mitochondria
c. Photoreceptors then synapse with one or more bipolar cells which in turn synapse with ganglion cells
d. Cones are concentrated in fovea and fewer in number; and rods are more peripheral and very numerous. For example, humans have about 6 million cones and 120 million rods.
e. Cones show very little convergence and are not very sensitive to light; acuity is great
f. Rods show great convergence, are very sensitive to light but have poor acuity
g. Rods, the most numerous photoreceptors, contain opsin and retinal (needs vitamin A for synthesis) combined into a photopigment called rhodopsin
A. Photoreceptor outer segment has chemically regulated sodium channels –in darkness the sodium channels are open and the membrane potential is about -40 mV. Sodium ions move inward on the outer segment and potassium ions move outward in the inner segment resulting in a resting membrane potential of -40 mV. The sodium channels stay open by intracellular cGMP.
B. In the dark, the synaptic end of the photoreceptor (synapsing w/ bipolar cell) releases glutamate which is an inhibitory neurotransmitter resulting in postsynaptic hyperpolarization.
C. In the light, the chemical configuration of retinal (embedded in the “opsin” portion of the photopigment) changes from a cis to a trans configuration
D. In the trans configuration, a conformational change occurs in opsin that then activates transducin (a G protein)
E. Activation of transducin (a G protein) results in the activation of cGMP phosphodiesterase that breaks down cGMP to 5′-GMP. This results in the closing of the Na + channels and the photoreceptor V m goes to a more polarized value (more hyperpolarized)
F. This decreases the release of glutamate and allows the bipolar cell to depolarize.
G. Bipolar cells in turn stimulate ganglion cells to generate AP
H. Recovery – Bleaching is the breakdown of rhodopsin into opsin and retinal after absorbing a photon and changing retinal to the trans form. Energy is required to change back to the original form of rhodopsin & takes several minutes. This is the basis for light-dark adaptation, in addition to pupil changes. Breakdown of rhodopsin is quick, but production is slow.
I. Color vision – different types of cones have different types of opsins sensitive to different wavelengths of light
J. Membrane potentials hyperpolarize in vertebrate photoreceptors and depolarize in invertebrate photoreceptors in response to light.