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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| show | Cells contain electrolytes that can carry ions through the axon, so current can flow like any charged ion. However this is not a possible mechanism as current can leak out the axon via resting membrane conductance, so the current gets weaker along
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| Length constant | show 🗑
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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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| Action potential propagation in unmyelinated axons | show 🗑
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| show | The time following one action potential during which another cannot be generated (absolute) or when the threshold for another AP is higher (relative). The area of an axon behind an action potential enters a refractory period due to inactivation of channel
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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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| Evidence for local circuits - Hodgkin | show 🗑
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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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| show | Membrane resistance is increased by myelination, as the membrane is less leaky
Increasing Rm increases the membrane length constant
Increasing length constant increases conduction velocity
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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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| show | In myelinated neurons there is a linear relationship. In non myelinated neurons conduction velocity is proportional to the square root of the radius. Myelinated neurons out perform non-myelinated except at very small diameters
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| Role of myelinated neurons 2-20 um diameter | show 🗑
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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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