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 depolarization and repolarization of the neurons Propagation of nerve impulses is the result of local currents 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 depolarized 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: Endocrine system Nervous system Role of system Organs involved What does each organ do?

Endocrine Nervous

Endocrine Consists of glands, pancreas and reproductive organs Releases hormones

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 Understanding: Neurons transmit electrical impulses

Neuron

No myelin sheath Nerve impulse 1 metre/second Non- Myelinated No myelin sheath Nerve impulse 1 metre/second Understanding: The myelination of nerve fibers allows for salutatory conduction

Myelination Many layers of phospholipids bilayers Schwann cells deposit myelin when they grows round the axon Node of Ranvier = gap between myelin deposited by adjacent Schwann cells. Understanding: The myelination of nerve fibers allows for salutatory conduction

Myelination Nerve impulse jumps from one node of Ranvier to the next. Saltatory conduction 100 metres/second Understanding: The myelination of nerve fibers allows for salutatory conduction

Non Myelinated Myelinated

Non Myelinated No myelin layers Impulse is 1m/s Schwann Cells make myelin that wraps round axon Impulse jumps between Nodes of Ranvier 100m/s

Resting Potential (–70mV) Overall there is a charge imbalance Negative charge on the inside Positive charge on the outside Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential

Resting Potential (–70mV) No transmission = resting potential There IS a potential difference across the membrane Due to an imbalance of positive and negative charges across the membrane Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential

Resting Potential (–70mV) Sodium potassium pump pumps 3 sodium ions out for every 2 potassium ions in Membrane is also 50 times more permeable to potassium ions Potassium ions leak back across the membrane faster than sodium ions Build up of positive ions on outside of membrane creates difference in charge Sodium concentration gradient very steep Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential

Action potential Rapid change in membrane potential Depolarisation – negative to positive Repolarisation – positive to negative (inside membrane of axon) Understanding: An action potential consists of depolarization and repolarization of the neurons

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 Understanding: An action potential consists of depolarization and repolarization of the neurons

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 Understanding: An action potential consists of depolarization and repolarization of the neurons

Resting Potential (–70mV) Returns to resting potential before another impulse can be sent Sodium potassium pump moving sodium ions out, and potassium ions in Understanding: An action potential consists of depolarization and repolarization of the neurons

Label the diagrams Summarise what happens during each stage Include the following on the diagrams: Label pumps/proteins What are pumps/proteins doing at each point K+ and Na+ movement Overall charges inside and outside The potential inside the neuron (+30/-70mV) Understanding: An action potential consists of depolarization and repolarization of the neurons

Action Potential Depolarisation Repolarisation Sodium channels Sodium ions Potassium channels Potassium ions Charge change inside neuron Overall charge in neuron after

Charge change inside neuron Overall charge in neuron after Action Potential Depolarisation Repolarisation Sodium channels Open Close Sodium ions Diffuse into neuron Stay inside neuron Potassium channels Closed Potassium ions Diffuse out of neuron Charge change inside neuron Negative to positive Positive to negative Overall charge in neuron after Positive (+30mV) Negative (-70mV)

Nerve impulses Start at one end of a neuron Propagated along axon to other end of neuron Ion movements that depolarise one part of the neuron trigger depolarisation of the neighboring part Only move in one direction as they are only initiated at one end of the neuron Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential

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 Some 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 Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential

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 Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach 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 Skills: Analysis of oscilloscope traces showing resting potentials and action potentials.

Synapses Junctions between cells in the nervous system Chemicals (neurotransmitters) send signals across synapses Understanding: Synapses are junctions between neurons and between neurons and receptor on effector cells

Synaptic Transmission Nerve impulse propagated along pre- synaptic neuron until it reaches the end of the neuron and the pre-synaptic membrane Understanding: Synapses are junctions between neurons and between neurons and receptor on effector cells

Synaptic Transmission 2. Depolarisation of pre- synaptic membrane causes calcium ions (Ca2+) to diffuse through channels in the membrane into the neuron Understanding: Synapses are junctions between neurons and between neurons and receptor on effector cells

Synaptic Transmission 3. Influx of Ca2+ causes vesicles containing neurotransmitter to move to the pre-synaptic membrane and fuse with it Understanding: Synapses are junctions between neurons and between neurons and receptor on effector cells

Synaptic Transmission 4. Neurotransmitter is released into the synaptic cleft by exocytosis Understanding: When pre-synaptic neurons are depolarized they release a neurotransmitter into the synapse

Synaptic Transmission 5. Neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post synaptic membrane Understanding: When pre-synaptic neurons are depolarized they release a neurotransmitter into the synapse

Synaptic Transmission 6. The binding of the neurotransmitter to the receptors causes adjacent sodium channels to open Understanding: When pre-synaptic neurons are depolarized they release a neurotransmitter into the synapse

Synaptic Transmission 7. Sodium ions diffuse down their concentration gradient into the post-synaptic neuron, causing it to reach its threshold potential Understanding: A nerve impulse is only initiated if the threshold potential is reached

Synaptic Transmission 8. An action potential is triggered in the post- synaptic membrane and is propagated along the neuron Understanding: Synapses are junctions between neurons and between neurons and receptor on effector cells

Synaptic Transmission 9. The neurotransmitter is rapidly broken down and removed from the synaptic cleft Understanding: Synapses are junctions between neurons and between neurons and receptor on effector cells

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 Applications: Secretion and reabsorption of acetylcholine by neurons at synapses

Acetylcholinesterase Breaks acetylcholine into chlorine and acetate Reabsorbed back into pre-synaptic neuron where it is converted back into a neurotransmitter Applications: Secretion and reabsorption of acetylcholine by neurons at synapses

Neonicotinoids What are they? What do they do? Advantages? Arguments against? Applications: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors