Module 3: action potentials Flashcards
what sends actions potentials in a neuron?
the Axon
what other words are used to describe an action potential?
spike
impulse
what is depolarisation in a neuron?
when the ICF becomes more positive than the resting membrane potential (-70mV+)
why does hyperpolarisation happen normally?
Na+ entering the cell
what is repolarisation in a neuron?
when the cell has gone from resting to depolarise and it goes back to resting again
why does repolarisation happen normally?
more K+ leaving the cell
what is hyperpolarisation?
when the ICF becomes more negative than the resting membrane potential (-70mV-)
what triggers an Action potential in neurons?
a local or graded potential that is large enough to trigger the postivie feedback cycle
describe the features of an action potential
- large change in voltage
- very brief (~1-2ms)
- revirsible
are local/graded potentials the same size?
No
what is the general threshold to trigger an action potential?
~50mV
where are action potentials initiaiated?
in the axon hillock
are action potentials the same size?
yes ~+30mV
what are the 3 states of the voltage gates sodium channel?
- open
- closed
- inactivated
describe how the sodium channel is positioned in the open position
gate is open and ball is at rest, sodium ions can pass down heir gradient as they wish
describe how the sodium channel is positioned in the closed position
the gate is closed, the ball is at rest, no ions can pass through but it is ready to open again at the next action potential
describe how the sodium channel is positioned in the inactivated position
the gate is open, the ball is blocking the channel so sodium ions cannot pass through. It isn’t able to open again until it moves to the closed position at the end of repolarisation
when do sodium channels open during an action potential?
at the threshold of ~-50mV
when do sodium channels become inactive in an action potential?
at the peak of the action potential ~+30mV
when do sodium channels become closed in an action potential?
either before the thereshold or during repolarisation, when it reaches -70mV resting potential
what are the 2 states of the potassium channel?
- open
- closed
when do potassium channels become open in an action potential?
at the peak of the action potenetial
when do potassium channels become closed in an action potential?
when the action potential becomes hyper polarised (lower than -70mV)
what is the positive feedback cycle in an action potential?
when a local or graded potential is big enough to cause a ripple effect within the Na+ channels, opening enough to trigger more opening, which floods the neuron with positive ions, making it depolarise more and more until an action potential is achieved
describe the ion flow of an action potential
- -70mV: both channels closed
- ripple of current causes a positive feeback cycle until the threshold is reached
- on reaching -50mV threshold all Na+ channels open and flood the neuron, rapidly depolarising it to +30mV
- once at +30mV the Na+ channels become inactive and the K+ channels open
- K+ channels cause repolarisation, dropping the voltage
- once voltage reaches -70mV the Na+ channels become closed
- K+channels still open, neuron becomes hyperpolar and K+ channel closes
- passive facilitated diffusion occurs due to the leak channels and the resting membrane potential is achieved again
how do local anaesthetics work?
they keep the Na+ channels in the inactive position so they cannot go back to closed, then open, which means an action potential cannot be triggered, causing the targeted sensory neurons to stop working, which in turn makes your face numb
what is action potential propagation?
the process of the action potential travelling from the axon hillock to the end of the axon
what is an absolute refactory period?
when all the Na+ channels are open or inactive so no more action potentials can be generated
at what point is an abosolute refactory period?
between the threshold and near back to resting potential
what is a relative refactory period?
when an action potential could be generated but it would need a large stimulus as
1. some Na+ channels are ready but some are still inactive, so cannot open or
2. all Na+ channels are ready but it’s hyperpolarised
what limits an action potential being triggered again?
the amount of Na+ channels open, as indicated by the refactory periods
what direction does an action potential travel?
from the axon hillock, away from the soma, to the terminal
what affects the speed of an action potential?
- diamter of the axon: thicker is faster
- temperature: not relevant humans as we are hod blooded, but faster the warmer
- degree of myelination: more myelination faster action potential AKA saltatory conduction
what is saltatory conduction?
when the myelination of an axon enables the action potential to jump along the axon to the next node of Ranvier, speeding up the pace
what is a node of Ranvier?
a space on an axon between the myelinated sheaths that exposes the Na+ and K+ gated voltage channels
describe multiple sclerosis
a degenerative disease where the body attacks the myelination of the neurons and breaks them down.
symptoms: pain, tingling, numbness, walking difficulty, fatigue, muscle spasms. etc
how does action potential nuance itself when it always triggers at +30mV?
frequency of action potentials, more potentials=stronger stimulus
