Physiology 2.6: Vision

what is the visible spectrum

• 400 to 750 m

  • capable of stimulating the receptors of the eye

what is a wavelength

• distance between successive peaks of electromagnetic radiation

eyes can sense both ____ and ____ of light

• wavelength

• intensity

optic nerve is considered to be the…..

• blind spot

  • no photoreceptors

parasympathetic stimulation of iris leads to…

• pupil constriction

  • less light entering

sympathetic stimulation of iris leads to…

• dilatation of pupil

  • increased light entering

what is this

• retina

  • contains photoreceptors

what is this

• optic nerve

  • axons of ganglion neurons in retina that carry visual information to brain

how does the eye focus

• lens and cornea

  • refract income light rays so they focus on the retina

where on the retina is there high focus

• fovea centrallis

  • depression on macula lutea

    • highest photoreceptor density

what is the fovea centralis

• greatest acuity image

  • highest photoreceptor density

    • depression in macula lutea

what is this

macula lutea

• depression in retina with highest photoreceptor density

describe refraction

• the bending of light rays as they pass from one transparent medium into another

  • the higher the density, the higher the refraction

visual information that reaches the retina is…

• reverse right to left and is upside down

describe accommodation of the eye

• eyes automatic ability to change the curvature and shape of the lens

  • helps focus on objects different distance

  • contraction or relaxation of ciliary muscles

what is responsible for the curvature of the lens

• ciliary muscle

  • applies tension to zonular fibers

if you want to see far-away objects, you need [shape] lens

• flat lens

  • less refraction

• action

  • more tension on zonular fibers

  • relaxation of ciliary muscle

if you want to see close up objects, you need [shape] lens

• thick lens

  • more refraction

• action

  • less tension on zonular fibers

  • contraction of ciliary muscle

describe myopia

• near-sightedness

  • can see nearsightedness

  • cannot see far

• eyeball too long

  • focus is in front of retina

what lens do you use to fix myopia

• concave/diverging lens

describe hyperopia

• farsightedness

  • can see far objects

  • cannot see near objects

• eyeball too short

  • focus is behind retina

how do you correct hyperopia

• convex/converging lens

what is astigmatism

• focusing programs in which there are irregularities in the curvature of the lens/cornea

  • multiple focal points

what is presbyopia

• normal, age-related loss of close-up vision

  • lens get stiffer

what is cataracts

• clouding of the lens

  • blurry vision

what is glaucoma

• impaired drainage of aqueous humor

  • increased intra-ocular pressure

  • compression of vascular layer below retina

    • damage to retina and blindness

describe age-related macular degeneration

• loss of visual information in center of visual field

  • damage to macula lutea

describe the histology of the retina

• light passes all the way back to the retina and hits photoreceptor cells

  • rods and cons

• passes through

  • horizontal cells

  • amacrine cells

describe rod photoreceptor cells

• respond to low light

  • non-colored vision

• have more photopigment

  • more sensitive

• none in central fovea

  • low focus

describe cone receptor cells

• respond to bright lights

  • colored vision

    • 3 types of cones with different photopigment (blue, green, red)

• less photopigments

• high density in fovea

  • high focus

describe the 3 types of cones

• red, green, and blue

  • detect different parts of the visible spectrum

• colored vision requires at least 2

in cone photoreceptors, each cone types responds to…

each range of wavelength

• for any given wavelength, the 3 cones are excited to different degrees

describe color blindness

• 1 or more cone mechanisms are lost

what is the purpose of horizontal and amacrine cells

• lateral integration between neurons and retina

  • regulate and fine-tune retinal signaling

describe the components of photoreceptors

• outer segment

  • membranous fold with photopigments

    • absorb light

• inner segment

  • basic cell machinery

• synaptic terminal

  • releases glutamate as neurotransmitter

what do photoreceptors release as neurotransmitter

glutamate

simple breakdown of photoreceptors in the dark

• photoreceptors depolarize

  • release glutamate

• bipolar cell activated

  • inhibits ganglion cell

    • no light is percieved

in visual transduction, glutamate activates…

• bipolar cells

  • activated bipolar cell inhibits ganglion cell

simple breakdown of photoreceptors in the light

• light causes photoreceptors to hyperpolarize

  • inhibited photoreceptor

  • inhibited bipolar cells

• ganglion cell activated

  • transmits signals to perceive

describe photoreceptor synaptic activity in the dark

• guanylyl cyclase converts GTP to cGMP

  • opens ligand-gated cation ion channels

    • influx of Na/Ca ions

    • depolarization

  • glutamate is released

describe photoreceptor synaptic activity in the lgiht

• when light strikes photoreceptor, pigment activates G-protein transducin

  • activates enzyme that decreases intracellular cGMP

    • hyperpolarization

  • glutamate release decreases

describe the 5 neuronal types in the retina and which generate APs vs generate graded potentials

• graded

  • photoreceptors

  • bipolar cells

  • horizontal cells

  • amacrine cells

• action potentials

  • ganglion cells

    • axons form the optic nerve

give me a brief description of the type of neuron cells in the retina

• photoreceptors

  • the rods/cones that detect light

• bipolar cells

  • bridges that pass signals from photoreceptors

• ganglion

  • send visual data to brain

• horizontal and amacrine cells

  • lateral processing and modulation of visual signals

what is the direct pathway for transmission of visual information through the retina

• photoreceptor → bipolar cells → ganglion cells → brain

  • light hits photoreceptor, bipolar cell activated

    • ganglion cell send image to brain

what is the indirect pathway for transmission of visual information through the retina

• direct pathway but with editors

  • includes the direct pathway and horizontal and amacrine cells

    • extra cells alter the signal horizontally

      • sharpen the edges and enhance the contrast

what is the purpose of the on and off pathway of the retina

• antagonistic arrangement acts as lateral inhibition

  • sharpens perception of edges and contrast

describe the on pathways

• glutamate is inhibitory for bipolar cells

  • light strikes

    • photoreceptor cell hyperpolarized… less release of glutamate

    • bipolar cell depolarized and releases excitatory NT

      • ganglion neuron is depolarized… increased AP firing

        • visual signal sent to brain

describe the off pathway

• glutamate is excitatory for bipolar cells

  • light strikes

    • photoreceptor cell is depolarized… less release of glutamate

      • bipolar cell hyperpolarizes and releases less excitatory NT

        • ganglion neuron is hyperpolarization… less AP firing

          • no visual signal sent to brain

What is the wavelength range of visible light.  Name two aspects of light that are distinguished by the eye.

• 400 to 750nm

• 2 aspects

  • color

    • determined by intensity of wavelength

  • brightness

    • determined by amplitude of light wave process by photoreceptors

describe this

iris

• regulates amount of light entering the eye

  • controls size of pupil

describe this

• pupil

  • lets light enter the eye

describe this

• retina

  • has photoreceptors that send visual signals to the brain

describe this

• optic nerve

  • bundle of ganglion axons that connects retina to the visual cortex of the brain

    • carries sensory and visual information

describe this

• optic disc

  • point where all ganglion cell axons gather and exit to form the optic nerve

    • blind spot because it has no photoreceptors

describe this

• lens

  • refracts light and focuses them directly onto retina

describe this

• cornea

  • focuses light and protects the eye

describe this

• macula lutea

  • pigmented area near center of retina

    • contains very high concentration of cone cells

      • responsible for color vision

        • exact center has fovea centralis

describe this

• fovea centralis

  • depression in the retina (in macula lutea)

    • highest concentration of cone photoreceptors

Briefly describe the focusing system of the eye. What is the quality of image imposed at fovea centralis and the macula lutea?

• the cornea and lens work to bend and refract incoming light rays directly onto the retina

  • focused light projects strait into macula and fovea

    • image is inverted and referred (brain processes it right-side up)