BIO271 - Lecture 5 - Sensory Physiology: Photoreception

what is photoreception?

-ability to detect a small proportion of the electromagnetic spectrum from ultraviolet to near infrared

what do most electromagnetic waves not have?

-most of these do not travel through water

what are the only 2 things that can penetrate deeper water?

only:

1. visible light

2. very long wavelength of electromagnetic (EM) radiation

what do animals detect?

-they detect a narrow band of the EM spectrum in the visible range, which suggests the possibility that photoreceptors evolved in aquatic organisms

what can other vertebrates perceive?

-they can perceive different ranges of photoreception

what is the range of visible light that humans can detect?

-the range is from 350 nm - 750 nm

photoreceptors

-organs that range from single light-sensitive cells to complex, image forming eyes

what are the 2 major types of photoreceptors?

1. rhabdomeric photoreceptors

2. ciliary photoreceptors

rhabdomeric photoreceptors

-apical surface is covered with multiple out-foldings called microvillar projections

-found exclusively in invertebrates

ciliary photoreceptors

-have single, highly folded cilium; folds form disks that contain photopigments

-found in exclusively in vertebrates/mammals

eyespots

-are single cells or regions of a cell that contain photosensitive pigment, (ex. protist Euglena)

eyes

-are complex organs consisting of group of cells specialized for different functions

-include both multiple photoreceptor cells and separate pigment cells

-provide information such as light direction and contrast between light and dark

-some of these can form focused images

what are protist euglena?

-a unicellular, eukaryotic microorganism belonging to the kingdom Protista that inhabit freshwater and brackish environments

-have a unique mixotropic nature, possessing chlorophyll for photosynthesis (plant-like), while also being able to ingest nutrients (animal-like)

what are the 4 major types of animal eyes?

1. flat-sheet eyes

2. cup-shaped eyes

3. vesicular eyes

4. convex eyes

flat sheet eyes

-primitive retina

-provide some sense of light direction and intensity

-most often seen in larval forms or as accessory eyes in adults (some snails)

layer organization:

photoreceptor cells → pigment layer → primary afferent neurons

photoreceptor cells (flat sheet eyes)

-perceive light

pigment layer (flat sheet eyes)

-absorb light

primary afferent neurons (flat sheet eyes)

-sending info about the light source to the integrating center

cup-shaped eyes

-retinal sheet is folded to form a narrow aperture

-better discrimination of light direction and intensity

-seen in the Nautilus (mollusk)

-has a whole where light coming from different directions can be detected

layer organization: (cup-shaped)

hole → pigment layer → photoreceptor cells → afferent neurons

what animal has the most complex kind of cup-shaped eye?

-the Nautilus (mollusk)

vesicular eyes

-use a lens in the aperture to improve clarity and intensity

-lens refracts light and focuses it onto a single point on the retina

-found in some mollusks

-vertebrates have complex types of this eye with a lens that can generate a sharp, focused image

-provide the best resolution of images because it has a lens

layer organization:

lens → photoreceptor cells (retina) → afferent neurons

lens

-focuses the light on the retina

-provides directionality

-refracts light and focuses it onto a single point on the retina

-aka crystalline cone

convex eyes

-photoreceptors radiate outward forming a convex retina to perceive light

-present in annelids, mollusks, and arthropods

layer organization: (convex shape)

photoreceptor cells (retina) → afferent neurons

what are the most complex convex eyes?

-compound eyes are the most complex of this type

compound eyes

-most complex convex eyes found in insects and crustaceans

-consist of up to several thousand light detectors called ommatidia

-very effective at detecting movement

ommatidia

-light detectors

what does the ommatidium consist of?

in this orientation:

-a cornea

-a crystalline cone

-several rhabdomeric photoreceptors (retinular cells)

-retinular cells arrange radially with microvilli inward forming rhabdom

what do ommatidium give?

-they give a system where light is well funneled

what are the 2 types of compound eyes?

1. apposition compound eyes

2. super-position compound eyes

**two ways to form images

apposition compound eyes

-ommatidium operate independently; afferent neurons make interconnection to generate an integrated image from all ommatidia

-found in diurnal insects (→ lot of light present, has a 1:1 ratio)

(ex. dragonfly)

super-position compound eyes

-ommatidia work together to form a bright image on the retina

-function well in dim light

-found in nocturnal insects and crustaceans (→not a lot of light present, can signal to 1 sensory neuron, where there aren't a lot of photoreceptors available)

-being mapped onto 1 sensory neuron

(ex. crustaceans)

how do you improve an image?

-to improve an image, you want more ommatidium

what is insect vision like (compound eyes)?

-very blurry

-can see edges of objects but not a clear picture

vertebrate eye

-forms bright, focused images

what are the components of the anterior chamber in vertebrate eyes?

• sclera

• cornea

• iris

• pupil

• lens

• ciliary body

sclera

-the white of the eye

-protective part and where tears are formed

cornea

-transparent layer of sclera

iris

-two layers of pigmented smooth muscle

-regulates the size of the pupil

pupil

-opening in iris

-our apeture (we can change its size)

ciliary body

-muscles for changing lens shape (contains ciliary muscles)

what are the 2 cavities the eye is divided into?

1. aqueous humor

2. vitreous humor

what are the 2 cavities of the eye separated by?

-they are divided by the lens and ciliary body

aqueous humor

-watery fluid in the anterior chamber

vitreous humor

-gelatinous mass in the posterior chamber

what are the components of the posterior chamber in vertebrate eyes?

• retina

• choroid

• tapetum

retina

-contains photoreceptors

choroid

-vascular layer of eye

tapetum

-layer in the choroid of nocturnal animals that reflects light

blind spot

-where the optic nerve (behind the optic disk) exits the eye→ there are no rods and cones

what is the flow of light in the vertebrate eye?

flow:

light → cornea → aqueous humor → pupil → lens → vitreous humor → retina

refraction

-bending of light rays

-occurs when light waves pass at an oblique angle into two mediums of different densities

-consider the optical density of water vs air

how do concave lens refract light rays?

-they scatter light rays

how do convex lens refract light rays?

-they cause light rays to converge (creating a focal point from a focal length)

focal length/distance

-is the distance from the center of the lens to the focal point

-distance from a lens to its focal point

what are the 2 lens that undergo refraction of light?

1. concave lens

2. convex lens

focal point

-point at which light waves converge

what happens when light enters the eye?

-it is refracted by both the convex surface of the cornea and the convex surface of the lens

how is an image focused on the retina?

-an image focused on retina is upside down and reversed from left to right (so opposite from how we perceive it, we perceive an object in the right orientation)

in terrestrial vertebrates, how des refraction occur?

-in these types of vertebrates, most of the refraction occurs between the air and the cornea

image accommodation

-the brain is sending signals to the ciliary body to adjust the light

-is the process by which the eye adjusts the shape of the lens to keep objects in focus

-this is lost over time with age

what is the orientation of light rays from a distant object?

-the light rays from this distance are parallel when they strike the eye, and focal length is short

what is the orientation of light rays from a nearby object?

-the light rays from this distant are not parallel. The focal length increases and the image is not focused on the retina (its behind the retina)

what does the lens changing shape alter and help fix?

-it alters the focal length and helps bring the image of a nearby object into focus on the retina in the process of accommodation

what must incoming light rays do to produce a clear image?

-they must converge on the retina to produce this

what happens when the ciliary muscle is relaxed?

-when it is relaxed, the ligaments pull on and flatten the lens

-occurs when focusing on distant objects

-causes muscle to widen

what happens with the ciliary muscle contracts?

-it releases tension on the ligaments and the lens becomes more rounded and thicker

-occurs when focusing on a nearby object

-causes muscle to constrict

hyperopia

-aka far-sightedness

-occurs when the focal point falls behind the retina

myopia

-aka near-sightedness

-occurs when the focal point falls in front of the retina

how is hyperopia corrected?

-it is corrected with convex lens

-because it pre-converges light rays before they enter the eye, shifting the focus forward onto the retina for clearer near vision

how is myopia corrected?

-it is corrected with a concave lens

-because it diverges (spreads out) light rays before they enter the eye, shifting the focal point backward so it lands directly on the retina, rather than in front of it, which causes distant objects to appear blurry

the retina

-complicated circuit of cells

-arranged into several layers

-rods and cones are at the back and their tips face backwards

optic nerve

-axons of ganglion cells join together to form this

-exits the retina at the optic disk (“blind spot”)

horizontal cell

-important for lateral inhibition

-GABA releasing inhibitory cells

what is the orientation of the retina (+eye) in vertebrates?

order:

light → ganglion cells + (amacrine cells)→ bipolar cells + (horizontal cells) → photoreceptors (rods and cones) → pigment epithelium

macula lutea (ML)

-is part of the retina responsible for producing sharp visual images

what is the middle of the macula lutea (ML) called?

-it is called the fovea

fovea

-a small depression in the center of the ML where overlying bipolar and ganglion cells are pushed to the side

-contains only cones

-provides the sharpest images

-highest density of cones, no rods in the very center

rods

-sensitive to black and white images

-ciliary photoreceptor

-outer segment rod-shaped

-sensitive to very dim light

-has one type of photopigment

signal processing:

-principle of convergence – as many as 100 rods synapse with a single bipolar cell → many bipolar cells synapse with a ganglion cell

-large visual field because of its high amount

-fuzzy image

cones

-are less sensitive to contrasts of light

-help us perceive colour

-ciliary photoreceptor

-outer segment cone shaped

-sensitive to brighter light

-up to 3 types of photopigments in mammals

signal processing:

-have a 1:1 ratio

-One cone synapses with one bipolar cell which connects to one ganglion cell

-small visual field

-high resolution image

-concentrated in the fovea

where does light not need to pass?

-light does not need to pass through bipolar and ganglion cells, or blood vessels

age-related macular degeneration (AMD)

-a very common age-related disease

-can be due to the loss of the pigment epithelium or the abnormal development of blood vessels under the macula

(blood vessels can become leaky or cause inflammation which can lead to blindness)

-causes the loss of photoreceptors in the macula lutea

what are all mammalian photoreceptors?

-they are all ciliary photoreceptors

what are the 2 types of ciliary photoreceptors?

1. rods

2. cones

**epithelial cell type

what are the 3 structural components of photoreceptors?

1. outer segment

2. inner segment

3. synaptic terminal

outer segment

-series of disks containing photopigments

inner segment

-the cell body, which contains the nucleus

synaptic terminal

-makes connections with neurons in the retina

epithelial cell type

-cell that releases a neurotransmitter to a neuron, which can fire an AP, since it can not fire can AP

photopigments

-molecules that absorb energy from photons

-have 2 covalently bonded parts

what are the 2 covalently bonded parts of a photopigment?

1. chromophore

2. opsin

chromophore

-pigment that is a derivative of vitamin A, (ex. retinal)

opsin

-G-protein-coupled receptors

-a rod

what is the general process of photoreception?

1. chromophore absorbs energy from photon

2. chromophore changes shape

3. photoreceptor protein changes shape

4. signal transduction cascade

5. change in membrane potential (leads to a graded potential)

bleaching

-when activated retinal no longer bonds to opsin, thereby activating opsin

-called this once opsin is activated

what are the visual pigments in rods called?

-they are called rhodopsin

what are the visual pigments on cones called?

-they are called

• erythrolabe,

• cholorolabe

• cyanolabe

rhodopsin

-decomposes in presence of light

-triggers a complex series of reactions that initiate nerve impulses

-impulses travel along optic nerve

erythrolabe

-responds to red pigment

chlorolabe

-responds to green pigment

cyanolabe

-responds to blue pigment