Mammalian Light Detection & Retina Structure

Detection of Light by Mammals

• Light enters the eye and is focused on the retina.
• Receptor cells in the retina convert light into electrical signals.
• Optic nerve carries impulses to the brain.
• Light changes shape to trans-retinal, triggers impulses in retina.
• Iris controls amount of light entering eye.
• Retina converts light energy into electrical impulses.
• Optic Nerve: sensory neurone carries impulses from receptor to brain.
• Pupil: allows light to enter eye

Structure of the Human Retina

• Light goes from back of the eye to rod surface.
• Rods and cones are at back of retina, behind irises.
• Ganglion cell axons bundle down and run down to optic nerve, making blind spot.
• Rods and cones have synaptic connections.

Rods

• High sensitivity.
• Low visual acuity: many rod synapse with 1 neurone (retinal convergence, so image interpretation / retinal convergence, so image interpretation / retinal convergence, so image requires lower intensity stimulus to exceed threshold value).
• Works only at low light intensities/ contain rhodopsin.
• Rod and cone cells contain optical pigments that absorb light and break them down.
• Rod cell is evenly distributed around periphery but NOT in central fovea.
• Retina contains 120 million rods and 6 million cones.
• Rods and cones release neurotransmitter called glutamate (in the eye, glutamate in rods is inhibitory).
• In the dark, rods release glutamate, that binds to receptor on bipolar neurone cell membrane, inhibits bipolar neurones by IPSP, bipolar neurone stops firing so doesn't release excitatory neurotransmitter, so ganglion cell doesn't fire, no generation of AP in brain, no light.
• To generate an AP in the brain, glutamate release is stopped/reduced.
• Rhodopsin = only cis retinal + opsin (optical pigment).
• Rhodopsin breaks down into opsin and trans retinal, when rhodopsin absorb light (rhodopsin is bleached).
• Opsin binds to Na+ channel on rod cell surface membrane, shuts Na+ channel.
• Sodium potassium pump continue to work, Na+ is actively transported out of cell.
• The rod cell membrane hyperpolarises.
• Glutamate isn't released from rod.
• AP occur in bipolar neurone, as enough Na+ diffuse in, so enough depolarisation exceeds threshold value.
• AP occur at ganglion cell, AP to brain so we perceive stimulus.
• In the light, rod is hyperpolarised.
• In the dark, rod is polarised (opposite with normal neurones).
• Less inhibitory transmitter glutamate is released.
• Retina contains rod cells and bipolar neurones, contains large numbers of mitochondria, and releases glutamate an inhibitory neurotransmitter.
• In a question about explaining role of rhodopsin in a generation of a nerve impulse in a cell, explain that Rhodopsin breaks down into opsin and retinal, Na+ close, hyperpolarisation, less glutamate is released.
• Rhodopsin bleached, forms opsin and retinal, Na+ close, rod cell becomes hyperpolarised, less released/stopping glutamate.

Cones

• Low sensitivity.
• High visual acuity: 1 cone synapse with 1 neurone.
• Works only at high light intensities.
• Cones contain iodopsin (not in spec).
• Graph seen in exams, 'O' is in the middle.
• Found in the fovea where focal point in GCSE (cones).
• Humans are trichromats (3 different cones).
• E.g. red/green/blue cones that respond to red/green/blue light, overlap and can respond to yellow as green and blue both activate.
• Colour blindness is generally sex linked through the absence of cone cells.

Distribution of Human Rod and Cone Cells

• The distribution of human rod and cone cells dictate vision in different light intensities.

Control of Heart Rate in Mammals

• The autonomic nervous system controls heart rate.
• It divides into sympathetic and parasympathetic systems.
• The sympathetic system increases heart rate, and the parasympathetic returns heart rate.
• There is a cardiac centre in the medulla oblongata.
• Individual neurones are stimulated to release noradrenaline at the SAN from the sympathetic nerve.
• The parasympathetic nerve stimulates acetylcholine release at the SAN.
• Receptors via the cardiac control centre in the medulla oblongata, via sinoatrial node hormone released.
• Baroreceptors detect changes in blood pressure in aorta and carotid arteries.
• Contractions measure right at the aorta to measure highest pressure caused by heart.
• Chemoreceptors detect changes in pH due to carbon dioxide concentration, located in carotid arteries, aorta and brain.
• Stress increases nerve impulses freq in sympathetic nervous system, releases adrenaline which Affects target cells in cardiac centre and SAN, increase heart rate and prepares body for increased activity for fight or flight

Responses to Blood Pressure and Chemistry

• Increased blood pressure (driven by respiratory demand) is detected by baroreceptors in the carotid artery and aorta.
• More impulses are sent to the cardioinhibitory centre in the medulla oblongata.
• The parasympathetic neurone sends more impulses to the SAN (sinoatrial node) via the vagus nerve.
• Decreased blood pressure is detected by baroreceptors in the carotid artery and aorta.
• Baroreceptors send more impulse to cardioacceleratory centre in medulla oblongata.
• Sympathetic neurone sends more impulse to SAN.
• Stimulates acetylcholine release, SAN depolarises less frequently, heart stroke/v/rate decreases, blood pressure decreases.
• Stimulates noradrenaline release, SAN increases depolarisation, heart stroke/v/rate increases, blood pressure increases.
• High carbon dioxide/ low pH is detected by chemoreceptors in the carotid artery and aorta.
• Chemoreceptors send more impulse to cardioacceleratory centre in medulla oblongata, affects medulla oblongata.
• Sympathetic neurone sends more impulse to SAN (noradrenaline).
• More heart rate increases, increase blood flow to lungs, gas exchange increases, remove more carbon dioxide from blood.
• Low carbon dioxide/ high pH is detected by chemoreceptors in the carotid artery and aorta.
• Increase in heart rate decreases blood flow to the lungs, gas exchange decreases, remove less carbon dioxide from blood.
• When stressed, sympathetic nerve stimulates adrenal medulla to release adrenaline carried around body in blood.
• Binds to receptors in target organs inc SAN, stimulates CCC and increase impulse in sympathetic neurones supplying heart + has direct effect on SAN (increase heart rate) and supplies xtra oxygen and glucose if needed to fight or flight.
• Remember that adrenaline results in increased nerve impulses in sympathetic nervous system, adrenaline is released and circulates in blood.

Urea Production and Removal

• Excess amino acids are deaminated in the liver to produce ammonia (toxic).
• Ammonia enters the ornithine cycle: ammonia + carbon dioxide -> urea (less toxic) (MS).
• Urea is less toxic, can be safely transported to kidneys for excretion.
• Remaining part of aa is used in respiration/converted to lipid (storage).
• Urea is transported in blood to kidneys.
• Ultrafiltration occurs in glomerulus.
• High hydrostatic pressure in glomerulus forces small molecules like water, urea, salts, and glucose, out of capillary into Bowman's capsule against the osmotic gradient.
• Filtrate containing urea enters kidney tubules for further processing.
• Blood cells and large proteins remain in blood due to their big size.

Selective Reabsorption in the Proximal Tubule (PT)

• In PT, over 80% filtrate is reabsorbed. Glucose, amino acids, and vitamins are fully selectively reabsorbed by active transport (MS) back to blood. Sodium is actively transported; chloride and water by osmosis.
• microvilli increase SA for reabsorption.
• Mitochondria provide energy in ATP form for active transport.
• Folded basal membrane provides a large SA for reabsorption.

The Loop of Henle (LofH) as a Counter-Current Multiplier

• The role is to decrease water potential in medulla to conserve water in bloodstream.
• LofH creates conc gradient in interstitial fluid, allowing water to be reabsorbed from filtrate
• With sodium and chloride actively transported out of LofH, the water potential is reduced in the bloodstream actively transports salts into interstitial fluid. There the interstitial fluid has high salt concentration, and water potential decreases, so water is drawn out of descending limb and collecting duct.
• As water potential of filtrate decreases throughout the medulla, the counter-current system maintains this gradient, allowing kidney to produce concentrated urine to conserve.
• Longer LofH give bigger the counter current exchange mechanism, even saltier medulla, even more water being reabsorbed out of collecting duct & conservation of more water.
• The Loop of Henle acts as a counter-current multiplier.
• With sodium/chloride ions moved out of ascending limb by active transport.
• The ascending limb is impermeable to water which lowers water potential in medulla.

Role of ADH

• Water osmosis out of osmoreceptors in hypothalamus cause them to shrink, triggers hypothalamus to produce more ADH.
• ADH forms hormone-receptor complex on cell surface membrane in collecting duct triggering cAMP activation and triggers cellular processes that increase water reabsorption, urine more concentrated.
• When blood plasma concentration is lower (higher water potential), it is detected by {osmoreceptors) in hypothalamus, less ADH released by pituitary gland, collecting duct is less permeable, no {reabsorption) of water.

Blood Water Content

• Higher water content in blood results in the release of less ADH, which leads to less permeable cells in the collecting duct.
• Low water content results in activates phosphorylase, more ADH is released, collecting duct cell more permeable to water.
• The receptor binds to receptors and vesicles with aquaporins on membrane fuse with cell surface membrane.

Kangaroo Rats

• Kangaroos generate up to 90% of water needs through metabolic processes and never drink water
• Kangaroos produce extremely concentrated urine (over 6000 mOsmol/kgH2O vs. ~1400 in humans).
• Adaptations include More juxtamedullary nephrons & Long LofH.

Endotherms vs. Ectotherms

• Endotherms produce heat through metabolic processes that keep body temperature constant (higher than ambient temperature).
• Ectotherms depend on external environment, and fluctuates with environment.
• Mammals and birds are examples of endotherms.
• Fish and reptiles are examples of ectotherms.

Adaptations of Endotherms and Ectotherms

• Endotherm Adaptations: Higher metabolic rate (5x higher than ectotherms of same size), build shelters, sweating, vasodilation.
• Ectotherm Adaptations: Basking in sun, moving to shade, pressing against warm surfaces, skin areas of maximise heat absorption.

Thermoregulation in Endotherms

• Behaviour allows for regulation of temperature with clothing, shelter, activity levels and moving towards warmer/cooler areas.
• Vasodilation increases blood flow, allowing heat to escape; vasoconstriction to conserve heat.
• Sweat production evaporates to cool the body.
• Hair erector pili muscles contract and relax to reduce or increase insulation.

Role of The Autonomic Nervous System (Thermoregulation)

• Hypothalamus acts as body's thermostat.
• Thermoreceptors in skin and hypothalamus detect temperature changes.
• Negative feedback loops maintain homeostasis by adjusting physiological responses to temp changes. thermostat sends signals via autonomic nervous system.
• Receptors: cold receptors and warm.

Effectors

• Includes sphincters to reduce hair, increase muscle contraction, to generate more sweat.

Ecosystems

• Ecosystem: interactions of living and non-living factors