Learning question : How do rod cells respond to light?

1. Title: How the eye works 04 January 2020 Learning question: How do rod cells respond to light? Homework: study!

2. 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.

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

4. 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)

5. 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

6. How a rod cell responds to a light stimulus • Rod cells form synapseswith bipolar cells • Synaptic bulb (endings) release neurotransmitter glutamate, which diffuses across the synaptic cleft • Glutamate prevents generator potentialsfrom being created in bipolar cells

7. 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

8. 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

10. 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 opsinprotein causes bleaching light

12. 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

13. 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.

14. 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 