Title: How the eye works 06 July 2018 Learning question: How do rod cells respond to light? Homework: study!

Learning Objectives Candidates should be able to: (d) describe, with the aid of diagrams, how a sensory receptor converts a stimulus into nerve impulses with reference to the rod cell in the retina; (e) outline assessment of receptor activity through routine eye tests (with reference to visual acuity, colour vision and response of pupil); (f) outline the use of blink/iris reflex tests to indicate levels of consciousness.

Key words Rods Cones Retina Bibolar cell Ganglion Synapses Rhodopsin Dendrites Axons Polarisation Hyperpolarisation Generator potentials

How a rod cell responds to a light stimulus Rods and cones act as transducers – they convert light energy into electrical energy to the optic nerve Electrical signals are transmitted via action potentials At rest (resting potential), -40mV are maintained across the surface of the of the rod cell membrane by the sodium-potassium pump (3Na+ out for every 2 K+ in)

How a rod cell responds to a light stimulus Constant flow of ions in and out of rod cell membrane creates a circulating electrical current Open channels in outer segment of rod allows Na ions to pass in through cell membrane Open channels in inner segment allow K ions to pass through cell membrane Results in constant flux of Na ions in and K ions out of the rod cells (passively) Na/K pump works to pump Na out and K into the rod cell at the same time

How a rod cell responds to a light stimulus Rod cells form synapses with bipolar cells Synaptic bulb (endings) release neurotransmitter glutamate, which diffuses across the synaptic cleft Glutamate prevents generator potentials from being created in bipolar cells

Light and Rhodopsin Rhodopsin, found in rod cells exists in two forms due to bonding in carbon 11: 11-cis-retinal (bent shape) All-trans-retinal (straight shape) Light causes this molecule to straighten and change shape This results in Na channels closing and local current stopping Cell becomes less negative inside (hyperpolarised) light

Light and Rhodopsin Hyperpolarisation when All-trans-retinal is formed causes the rod cell to stop secreting glutamate No inhibition of bipolar cell Depolarisation of bipolar cell, resulting in generator potentials being produced. Generator potentials transmitted across the synapse to ganglion cells and from here an action potential is carried along an optic nerve to the brain

Bleaching Rod cells can become “bleached out” – you can experience this when you walk from a sunny area to a dark room Your vision takes time to adjust due to enzymes regenerating rhodopsin from trans to cis The change in the opsin protein causes bleaching light

Eye tests and receptor activity Visual acuity means clearness of vision Snellen chart tests for this Reading letters 1 inch at 20 feet is regarded as 20/20 vision 20/40 acuity means that the patient can read words or letters at 20 feet, whereas someone with 20/20 vision could read the same letters at 40 feet

Pupil response test The swinging flashlight pupil test is used to compare your pupils' response to light. The lights in the room will be dimmed, and you will again be asked to look at a distant object. Your doctor will "swing" the light rhythmically from one eye to the other, noting the response of each pupil. Your pupils should constrict or stay the same size when the light is shone on them. Dilating pupils may alert your doctor to a possible optic nerve problem.

Blink response and levels of consciousness Blink reflex is the rapid closing of the eyelid when something threatening approaches the eye. The blink reflex is one of the last to be lost as unconsciousness deepens If this is not present, this indicates the person is in a coma Eye opening indicates the arousal mechanism of the brain is active Eye opening may be spontaneous or triggered by speech, painful stimuli, or may not happen at all 