Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
6.5 Neurons and Synapses Understanding: - Neurons transmit electrical impulses - The myelination of nerve fibers allows for salutatory conduction - Neurons pump sodium and potassium ions across their membranes to generate a resting potential - An action potential consists of depolarisation and repolarisation of the neurons - Propagation of nerve impulses is the result of local crrents that cause each successive part of the axon to reach the threshold potential - Synapses are junctions between neurons and between neurons and receptor on effector cells - When pre-synaptic neurons are depolarised they release a neurotransmitter into the synapse - A nerve impulse is only initiated if the threshold potential is reached Applications: - Secretion and reabsorption of acetylcholine by neurons at synapses - Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors Nature of science: - Cooperation and collaboration between groups of scientists: biologists are contributing to research into memory and learning. Skills: - Analysis of oscilloscope traces showing resting potentials and action potentials. Communication in the body Two systems: 1. Endocrine system 2. Nervous system Make a brief summary of the organs involved in each and what each organ does. Endocrine Endocrine Consists of glands, pancreas and reproductive organs Releases hormones Nervous Nervous Consists of nerve cells called neurons Central and peripheral 85 billion neurons in humans Neuron Label the following: Cell body Axon Dendrite Axon terminal Nucleus Myelin sheath Neuron Non-myelinated Nerve fibres are cylindrical Plasma membrane enclosing cytoplasm 1um diameter Nerve impulse 1 metre/second Myelinated Sheath Many layers of phospholipids bilayers Schwann cells deposit myelin when it grows round the axon Node of Ranvier = gap between myelin deposited by adjacent Schwann cells. Myelinated Sheath Nerve impulse jumps from one node of Ranvier to the next. Saltatory conduction 100 metres/second Non Myelinated Myelinated Non Myelinated Myelinated No myelin layers Schwann Cells make myelin that wraps round axon Impulse is 1m/s Impulse jumps between Nodes of Ranvier 100m/s Multiple Sclerosis What are the symptoms? What causes it? Is there a cure? Resting Potential (-70 mV) No transmission = resting potential There IS a potential difference across the membrane Due to an imbalance of positive and negative charges across the membrane Resting Potential (-70 mV) Continuous sodium potassium pumps transfer sodium and potassium ions across the membrane 3 sodium ions out, 2 potassium ions in Concentration gradients created for both Resting Potential (-70 mV) Membrane is 50 times more permeable to potassium ions Potassium ions leak back across the membrane faster than sodium ions Sodium concentration gradient very steep Resting Potential (-70 mV) Overall there is a charge imbalance Negative charge on the inside Positive charge on the outside Action Potential Rapid change in membrane potential 1. Depolarisation – negative to positive 2. Repolarisation – positive to negative (inside membrane of axon) Depolarisation (+30mV) Due to opening of sodium channels Allows sodium ions to diffuse into neuron down concentration gradient Reverses the charge imbalance Inside becomes positive Repolarisation (-70mV) Rapidly after depolarisation Sodium channels close Potassium channels open Potassium ions diffuse out of ion down their concentration gradient Inside of cell becomes negative again Resting Potential (-70 mV) Returns to resting potential before another impulse can be sent Sodium potassium pump moving sodium ions out, and potassium ions in Label the diagrams Summarise what happens during each stage Include the following on the diagrams: 1. Label pumps/proteins 2. What are pumps/proteins doing at each point 3. K+ and Na+ movement 4. Overall charges inside and outside 5. The potential inside the neuron (+30/-70mV) Action Potential Depolarisation Sodium channels Sodium ions Potassium channels Potassium ions Charge change inside neuron Overall charge in neuron after Repolarisation Action Potential Depolarisation Repolarisation Sodium channels Open Close Sodium ions Stay inside neuron Potassium channels Potassium ions Diffuse into neuron Closed Stay inside neuron Charge change inside neuron Negative to positive Diffuse out of neuron Positive to negative Overall charge in neuron after Positive (+30mV) Negative (-70mV) Open Nerve impulses Start at one end of a neuron Propagated along axon to other end of neuron Only move in one direction as they are only initiated at one end of the neuron Ion movements that depolarise one part of the neuron trigger depolarisation of the neighbouring part Local Currents The propagation of an action potential along the axon is due to sodium ion movements Sodium ion concentration increases inside the axon during depolarisation Sodium ions then diffuse inside the axon to the area that has not yet been depolarised The same happens outside of the axon, however sodium ions move from the area that has not been depolarised to the area that has Local Currents This causes the membrane potential in the unpolarised area to rise from -70mV to -50mV The Sodium channels are voltage-gated and open when a membrane potential of -50mV has been reached This is known as the threshold potential Oscilloscope traces Membrane potentials can be measured Displayed in an oscilloscope Time on x-axis Membrane potential on y-axis Resting potential = -70mV Local currents = rise to -50mV Action potential = narrow spike to +30mV Returns to resting potential = -70mV Oscilloscope traces Membrane potentials can be measured Displayed in an oscilloscope Time on x-axis Membrane potential on y-axis Resting potential = -70mV Local currents = rise to -50mV Action potential = narrow spike to +30mV Returns to resting potential = -70mV Synapses Junctions between cells in the nervous system Chemicals (neurotransmitters) send signals across synapses Synaptic Transmission 1. Nerve impulse propagated along pre-synaptic neuron until it reaches the end of the neuron and the pre-synaptic membrane Synaptic Transmission 2. Depolarisation of pre-synaptic membrane causes calcium ions (Ca2+) to diffuse through channels in the membrane into the neuron Synaptic Transmission 3. Influx of Ca2+ causes vesicles containing neurotransmitter to move to the pre-synaptic membrane and fuse with it Synaptic Transmission 4. Neurotransmitter is released into the synaptic cleft by exocytosis Synaptic Transmission 5. Neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post synaptic membrane Synaptic Transmission 6. The binding of the neurotransmitter to the receptors causes adjacent sodium channels to open Synaptic Transmission 7. Sodium ions diffuse down their concentration gradient into the post-synaptic neuron, causing it to reach its threshold potential Synaptic Transmission 8. An action potential is triggered in the postsynaptic membrane and is propagated along the neuron Synaptic Transmission 9. The neurotransmitter is rapidly broken down and removed from the synaptic cleft Acetylcholine Used as a neurotransmitter at many synapses Produced from choline (diet) and acetyl (aerobic respiration) Loaded into vesicles and released into synaptic cleft Binding sites are specific Acetylcholinesterase Breaks acetylcholine into chlorine and acetate Choline reabsorbed back into pre-synaptic neuron where it is converted back into a neurotransmitter Threshold Potentials Action potential generated only when threshold potential reached Only at this potential do voltage gated sodium channels open causing depolarisation Threshold Potentials At a synapse – the amount of neurotransmitter secreted may not be enough to cause the threshold potential to be reached in the post-synaptic membrane It will then not depolarise Sodium ions now in post-synaptic neuron are pumped back out again and resting potential is resumed Post-synaptic neurons can have many pre-synaptic neurons Research Neonicotinoids - What are they? - What do they do? - Advantages? - Arguments against?