3. Nerve Action Potential Flashcards
Describe in detail how a nerve voltage-gated channel operates
Sodium VG Channels
- At -70 mV (RMP), voltage gated sodium channels are closed (inactivation gate is open), meaning that there is no sodium current into the cell. Once the membrane is depolarised beyond its threshold (-55 mV) the activation gates open for about 0.5 msec allowing sodium to flood in, depolarising the cell further to about +30 mV only stopping due the fact that the inactivation gate closes automatically after 0.5 msec even though the activation gate is still open. So now at +30 the potassium voltage gated channels open (this is a bit weird to understand but they were activated at the same time that the sodium voltage gated channels were but they have a relatively slow response time compared to the sodium VG channels) allowing the cell to repolarise due to the rapid efflux of potassium ions from the inside of the cell to the ECF. This repolarises the cell back down back down to resting membrane potential (-70 mV). During the repolarising phase the gates on the sodium VG channels are resseting.
What is the difference between an inactivation and activation gate on a gated channel?
Activation Gate:
- When neuron is at RMP, the activation gate of the sodium channel closes and no sodium can move through the channel = The inactivation gate is open.
- When the cell membrane near the channel depolarizes, the activation gate swings open → Sodium enters cell.
- As long as the cell remains depolarized, activation gates in sodium channels remain open (hence why we need inactivation gates because otherwise this shit wouldnt stop)
Inactivation Gate:
- Inactivation gates are the outside intervention that stops the escalating depolarization of the cell.
- Has a delay of 0.5msec, which during that delay sodium channels are open allowing for enough sodium influx to create the rising phase of the action potential.
- When the slower inactivation rate finally closes, sodium influx stops and action potential peaks.
What are the main differences between a “graded” potential and an action potential?
“Graded” potential:
- Small deviations from RMP (-70mV)
- Hyperpolarisation or depolarisation
- Vary in amplitude depending on strength of stimulus
- Localised
- Occur usually in dendrites and cell body of neuron
- Stimulated by; Mechanical or chemical stimulation of membranes, or ions flow through ion channels and change membrane potential locally
Action potential:
- Not local
- Large deviations from RMP (to 0)
- Depolarisation
- Occurs through axon
Explain the structural and functional benefits of myelination.
- Node-to-node transmission is fucking quick (like almost no delay)
- Super fast with small sized axons (e.g. optic nerve)
- Metabolic energy conservation due to confinement of excitation to small nodal regions of the nerve
Describe the conduction of an action potential in an unmyelinated nerve fibre.
- AP propagates over surface of the axon membrane
- As sodium flows into cell during depolarisation, MP of adjacent areas is effected and voltage-gated sodium channels open
- The continues across membrane of axon
Describe the conduction of an action potential in a myelinated nerve fibre.
- Saltatory conduction
- Depol. only occurs at nodes of ranvier where there is a high desity of VG ion channels.
- Current carried by ion flows through ECF from node to node (that shit basically jumps)
Why does size matter when it comes to conduction speeds for both unmyelinated and myelinated nerve fibres?
- Conduction velocity is still dependant on diameter
Myelinated:
- The greater the fibre diameter in myelinated axons, the greater the distance between the nodes
- a smaller number of nodes permits a faster overall propagation of the AP
- best part of myelinated fibres is their small diametre resulting in less nodes → fast bois (e.g. optic nerve)
Unmyelinated:
- Large unmeylinated fibres conduct impulses faster due to thier large diamter
What are the differences between the absolute and relative refractory periods in nerve transmission?
Absolute:
- Denotes the interval following an AP during which a second AP cannot be excited, regardless of the intensity of the applied stimulus.
Relative:
- A second AP may be elicited, provided that a greater-than-usual stimulus in applied
How does the absolute refractory period relate to the opening and closing of the voltage-gated channels?
The reason an absolute refractory period exists is because the Na+ channels haven’t reset yet.
- During the absolute refractory period, no stimulus can trigger another action potential as the inactivation gate is closed and the activation gate is open
- During the relative refractory period, only a larger than normal stimulus can initiate a new action potential as the inactivation gate will begin to open as the activation gate resets itself.
Why are refractory periods necessary for nerve action potential transmission?
- For unidirectional propagation of the action potential
- Sodium channels reset to their original position while potassium channels remain open.
