Lecture 4 â Action Potential 1 Lecture 5 â Action Potential 2
Lecture 4 – Action Potential 1 Action Potentials • Electrical signal that exceeds threshold voltage that causes a series of depolarisations down an axon towards a muscle or nerve cell • Ions are going to move in whichever direction brings the RMP closer to its own equilibrium potential (aka reversal potential) • At rest, leak channels are present and open, limited permeability Graded Potentials • Small fluctuation relative to RMP– before the charge reaches threshold and the AP is triggered, dissipates further from initial active area (loses charge) in BOTH directions • Polarity: the distance from 0 • Depolarised: closer to 0 – more positive • Repolarised: back to RMP • Hyperpolarised: further from 0 – more negative Voltage-gated Sodium Channel • Depolarises cell • Passive, open and close, sodium allowed • Fast activation gate, slow inactivation gate • Na+ channels - positive feedback system down the axon causing each subsequent voltage - gated channel to open Voltage-gated Potassium Channel • Repolarise cell as fast as possible so another AP can be fired • One gate that is very slow ❖ Frequency can be increased to increase the urgency of the signal. Amplitude of the signals never changes
Lecture 5 – Action Potential 2 Permeability Changes and Ion Fluxes During an AP • Rising phase due to Na+ influx, falling phase due to K+ efflux • Na+-K+-ATPase establishes concentration gradient to establish RMP • It is only the ions along the membrane that trade place almost no effect on concentration gradients as there's still a lot on K+ inside and Na+ outside Conduction velocity • AP moves down the axon, opening channels in a cascade, depolarising the adjacent areas as it moves forward. This brings the next set of voltage-gated channels to threshold in a wave of current. Behind it, the potassium channels are repolarising ready to fire another AP • APs don’t dissipate when branched • Determined by axon diameter, larger = faster • Myelination, myelinated = faster than unmyelinated Absolute Refractory Period • Period of time another action potential can’t fire
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Inactivation gate closes, previous active area must return to resting potential below threshold before another AP can be fired and the gated sodium channel can be opened again. This period of time is called the absolute refractory period. • Action potentials cannot summate (stack on top of each other) Relative Refractory Period • Period of time in which an action potential can be immediately fired however there is farther to travel to threshold and its harder to fire. This is because of the after-AP hyperpolarisation period • Prevents neurons from firing uncontrollably (note: epilepsy), stops action potential from doubling back on itself, limits and controls maximum frequency. Saltatory (to leap) Conduction • Speed of AP, the wider/larger the axon, the faster the conduction velocity • Myelination: fatty layer that encases the axon. Between each Shwann cell are Nodes of Ranvier. The AP is able to jump from node to node, repolarising at the previous node. This system was evolved due to the number of axons in the human body. Myelination is fully developed at 21years of age.
Lecture 5 – Action Potential 2 Permeability Changes and Ion Fluxes During an AP • Rising phase due to Na+ influx, falling phase due to K+ efflux • Na+-K+-ATPase establishes concentration gradient to establish RMP • It is only the ions along the membrane that trade place almost no effect on concentration gradients as there's still a lot on K+ inside and Na+ outside Conduction velocity • AP moves down the axon, opening channels in a cascade, depolarising the adjacent areas as it moves forward. This brings the next set of voltage-gated channels to threshold in a wave of current. Behind it, the potassium channels are repolarising ready to fire another AP • APs don’t dissipate when branched • Determined by axon diameter, larger = faster • Myelination, myelinated = faster than unmyelinated Absolute Refractory Period • Period of time another action potential can’t fire
•
Inactivation gate closes, previous active area must return to resting potential below threshold before another AP can be fired and the gated sodium channel can be opened again. This period of time is called the absolute refractory period. • Action potentials cannot summate (stack on top of each other) Relative Refractory Period • Period of time in which an action potential can be immediately fired however there is farther to travel to threshold and its harder to fire. This is because of the after-AP hyperpolarisation period • Prevents neurons from firing uncontrollably (note: epilepsy), stops action potential from doubling back on itself, limits and controls maximum frequency. Saltatory (to leap) Conduction • Speed of AP, the wider/larger the axon, the faster the conduction velocity • Myelination: fatty layer that encases the axon. Between each Shwann cell are Nodes of Ranvier. The AP is able to jump from node to node, repolarising at the previous node. This system was evolved due to the number of axons in the human body. Myelination is fully developed at 21years of age.
