Physiology and Pharmacology
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| Basic mechanism of action potential generation | show 🗑
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| What is threshold | show 🗑
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| Current flow via passive diffusion | show 🗑
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| show | Length constant = square root(rm/ri)
This means the length of the axon that the signal will decline by half.
Rm = membrane resistance per unit axon
Ri = internal resistance per unit axon
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| show | T = Rm.Cm
Means the time taken for the membrane potential to be changed
Rm = membrane resistance per unit area of membrane
Cm = membrane capacitance per area of membrane
This depends on membrane resistance
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| show | If by passive diffusion - along the axon due to current leaking out (length constant) signal declines. Response rises slower as current decreases (time constant)
This means it cannot be by passive diffusiom
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| show | NA influx triggers depolarisation. Current flows to adjacent region depolarising it. This opens Na channel in these areas, generating a new action potential. The action potential is regenerated along the axon. This can be mediated by any ion.
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| Refractory period | show 🗑
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| Local circuits in impulse propagation | show 🗑
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| show | Result in changes to surface potential. This allows electrical activity of neurons to be measure using extracellular electrodes. We can measure changes in surface potential and record the events caused by movement of ions e.g. ECG
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| show | Depolarisation caused at A, with a strong action potential seen (10 mV). A block is present between A and B. B records a weaker AP (1-2mV). Moving along the axon local currents get weaker, resulting in weaker AP due to local current flow
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| show | Rate of charge entry (current) through active patch of membrane
Spread of local currents (depends on membrane length constant). Further movement = faster AP
Speed of membrane depolarisation by local current flow (depends on membrane time constant)
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| show | Governs the length of an axon ahead of the impulse that can be depolarised to threshold. Long length constant means more distant areas of membrane ahead can be depolarised to threshold, so increases conduction velocity
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| show | Internal resistance is inversely dependant upon axon cross sectional area.
Membrane resistance is inversely dependant on axon circumference.
Length constant is proportional to square root (a/2) so bigger axons conduct faster
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| show | No effect - decrease in membrane potential is cancelled out by a corresponding increase in membrane capacitance
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| show | Axon is wrapped in many layers of Schwann cells.
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| Effect of myelination on length constant | show 🗑
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| Effect of myelination on time constant | show 🗑
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| show | An action potential can only be generated at a node of Ranvier. Local currents travel further to depolarise the membrane, generating APs at each node. This makes the AP seem to jump along the neuron, known as Saltatory conduction
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| show | In myelinated neurons, voltage gated Na channels are packed at high density at the nodes of Ranvier. High packing density gives a large inward current, with gives a high conduction velocity.
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| show | Time constant
Length constant
Magnitude of the Na influx
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| show | Neuron with 2 nodes separated by a block placed in a bath. When electrically stimulated a small depolarisation is produced at beginning and end with nothing between. When a third node is placed halfway through the block, the AP is detected in the block
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| Comparing effect of axon diameter in myelinated and non-myelinated neurons | show 🗑
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| show | Alpha - proprioception, somatic motor
Beta - touch, pressure
Gamma - motor to muscle spindles
Delta - pain, cold, touch
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| Role of myelinated neurons less than 3 um diameter | show 🗑
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| show | Dorsal root - pain, temperature, mechanoreceptor
Sympathetic - Postganglionic sympathetic
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| Diseases involving decreased conduction velocity | show 🗑
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