Vision 2 - transduction Flashcards
what is visible light
an electromagnetic wave
what is the definition of phototransduction
the conversion of light energy to an electrochemical response by photoreceptors
what are the photoreceptors in the eye
rods and cones on the retina
how is an action potential generated in the eye
the phototransduced rods and cones activate optic nerve neurons - fire an action potential
what is the structure of a rod
outer - contains stacks of discs/lamellar which have photoreceptors
inner - nucleus, axon then synaptic terminal
what is the structure of a cone
outer - contains stacks of discs/lamellar which have photoreceptors
inner - cell organelles
what is the structure of the lamellae
made from a cell membrane with integrated visual pigments
- in rods = rhodopsin
- in cones = cone opsins S, M and L
what visual pigments are in rods
rhodopsin
what visual pigments are in cones
cone opsins S, M and L
what comprises the visual pigment rhodopsin
opsin and 11-cis-retinal
what is a chromophore
an atom or group whose presence is responsible for the colour of a compound - contains 11-cis-retinal - formed from dietary vitamin A
what is 11-cis-retinal
the light sensitive component of visual pigments
how does bleaching of the visual pigment occur
chromophore rests in opsin
- light falls on it
- 11-cis-retinal is isomerised to all-trans-retinal
- all-trans-retinal cannot fit into opsin
- rhodopsin splits
- results in bleaching of visual pigment
how does bleaching of the visual pigment result in phototransduction
the phototransduction cascade
what role does vitamin A play in the visual pigment
visual pigment regeneration
what is different about photoreceptor cells in relation to other cells in the body
when at rest (in the dark) kept in a depolarised state by open Na+/Ca+ channels
what occurs in the phototransduction cascade
photon of light hits rhodopsin on rods - activated rhodopsin causes transducer GDP to covert to transducer GTP - causes Phosphodiesterase (PDE) to convert to activated PDE - causes cGMP to convert to GMP - CLOSES Na+ CHANNELS - relative hyper polarisation of photoreceptor cell - hyperpolarisation transmitted by a flux of Ca+ ions to the synapse - Stimulates retinal cell
how does visual pigment regeneration occur
rhodopsin splits to form all-trans-retinal (AT-Rol) - needs to be regenerated
IN PIGMENT EPITHELIAL CELL:
- AT-Rol (with dietary it A from liver - converted to 11-cis-rol - converted to 11-cis-ral (coversions occur when retinal esters synthesised)
IN PHOTORECEPTOR:
- 11-cis-ral is now called 11-cis-retinal - joins with opsin to form more rhodopsin
what are the consequences of vitamin A deficiency
vision affected - e.g. night blindness, abnormal conjunctive and corneal epithelium
when can vitamin A deficiency occur
any condition that might affect vit A absorption, malnutrition, malabsorption syndromes e.g. coeliac disease
what are signs of vitamin A deficiency
bitots spots in conjunctiva - can be first indication
corneal ulceration
corneal melting - can lead to future corneal opacification
