Detection of light by mammals C/W 18/10/2016 Describe the structure of the human retina Explain the role of rhodopsin in initiating action potentials Explain how the distribution of human rod and cone cells maintain vision in different light intensities Engage – Outline on the white boards what happens during synaptic transmission – add simple annotations.

Recap synaptic transmission 18/10/2016

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How do sensory receptors work? The following process is common in one form or another to most sensory receptors: Stimulus  local change in permeability  generator potential  action potential In sense organs such as the eye, several receptor cells will often synapse with a single sensory neurone. If the generator potential from an individual receptor cell is insufficient to set up a synapse, the potentials from several may add together or summate and trigger an action potential. Frequency of action potential helps to determine strength 18/10/2016

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The Human Eye Sensitive to light 400 – 700nm. Other species sensitive to different wavelengths of light Demo dissection or students complete 18/10/2016

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The role of the retina The retina contains over a hundred million light- sensitive cells (photoreceptors) Rods and cones Rods – Greyscale vision only, low light intensities or at night. Very sensitive to light. Spread evenly, except at ????? where there are none. Several synapse with same sensory neurone. Cones – Tightly packed together in the fovea. Around 6 million. Bright light and colour. Each cone usually has its own sensory neurone. 18/10/2016

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How do rods and cones work? Rhodopsin is the visual pigment – made from vitamin A. Photon of light causes Rhodopsin to break up into opsin and retinal. We call this bleaching. When rhodopsin is bleached it triggers that causes sodium ion channels to close. Interior becomes more negative than usual. This is hyperpolarisation is known as the generator potential and depends on the amount of light hitting the rod. If stimulus is large enough, NTs are released into the synapse with bipolar cell. This causes an action potential in the sensory neurone. All sensory neurones leave the eye at the same point to form the optic nerve leading to the brain. 18/10/2016

Nerve impulses The concentration of sodium ions, potassium ions and other charged particles outside the axon is different from that inside the axon and this is the basis of the nerve impulse. The axon membrane is relatively impermeable to sodium ions, but quite freely permeable to potassium ions. 18/10/2016

The resting potential An axon is said to be ‘at rest when it is not conducting a nerve impulse. The extracellular concentration of ions is greater than the concentration in the axons cytoplasm. This gradient is created by a sodium potassium pump. The pump uses ATP to move sodium ions out of the axon and potassium ions into the axon. 18/10/2016

K+ concentration gradient Na+ concentration gradient In very simple terms. -70mV potential difference, no net movement of K+. Outside Cell (Extracellular) High Na+ concentration Low K+ concentration Positive (relative to inside of cell) K+ concentration gradient Na+ concentration gradient CELL MEMBRANE Inside Cell (intracellular) Low Na+ concentration High K+ concentration Negative (relative to inside of cell) 18/10/2016

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Exam Questions 18/10/2016

Synapses Wherever two neurones meet they are linked by a synapse. Every cell in the central nervous system is covered with synaptic knobs from other cells. A synapse is effectively a gap between two cells. The arrival of an impulse at the synaptic knob increases the permeability of the presynaptic membrane to Calcium ions as calcium ion channels open up. This causes synaptic vesicles, which contain a transmitter substance or neurotransmitter, to move to the presynaptic membrane. The vesicles then fuse with the membrane and release the neurotransmitter molecules into the synaptic cleft. These molecules diffuse across the gap and attach to specific protein receptor sites on the sodium channels of the post-synaptic membrane. This causes sodium channels to open. Sodium ions then flood into the post-synaptic neuron causing a change in the potential difference across the membrane and an excitatory post-synaptic potential (EPSP). 18/10/2016