Neurons - Generation and Transmission of Action Potentials Flashcards
Hyperpolarization
When the resting membrane potential becomes more negative (moves closer to Ek than ENa)
Depolarisation
When the resting membrane potential becomes more positive (moves closer to ENa than Ek)
What is an action potential?
A brief fluctuation in membrane potential caused by a transit opening of voltage-gated ion channels, which spreads, like a wave, along axon.
What are the interchangeable terms for action potentials?
Spike, nerve impulse, discharge
Stick to action potential
When do action potentials occur?
After the membrane potential reaches a certain voltage called the ‘threshold’
What is the significance of the action potential?
Information is coded in the frequency of action potentials
A.P’s can be regarded as a form of language by which neurons communicate
What other key element are action potentials involved in?
Signal transmission along (often very long) axons that can be up to 0.5/1m long
Does the membrane depolarise or hyperpolarise to reach the threshold for a membrane potential?
It depolarises
The membrane potential voltage threshold to begin an action potential
~-55mV
What is the slow depolarisation evoked by?
A stimulus
What happens in the first stage of the action potential?
After the membrane potential reaches threshold it undergoes a fast (ie in milliseconds) depolarisation to ~ +30mV (overshoot)
Voltage of the “overshoot” in an action potential
+30mV
Stage 2 of an action potential
Repolarisation where the membrane potential regains its original polarity
Is coming back to a negative charge and hence becoming less positive
Stage 3 of an action potential
After-hyperpolarization
What stages of an action potential are the ‘Absolute Refractory Period’?
stages 1 and 2
What occurs in the ‘Absolute Refractory Period’?
The action potential cannot respond to another stimulus at this point
This means that the nerve cell is when a nerve cell is NOT excitable
What stages of an action potential are the ‘Relative Refractory period’?
stage 3
What is the Relative refractory period?
the part of an action potential where a strong stimulus could invoke an action potential but may not reach as high a point as it has to depolarise a lot more
What happens to the channels in the cell membrane once the threshold has been reached (stage 1 of an action potential)
- There is a sudden activation(opening) of the voltage gated Na+ channels causing a fast depolarization as the cell becomes more permeable to Na+
- At this point the ratio of K+:Na+ goes from 40:1 to 1:20 as the membrane potential shifts towards that of Ena (+60mV) but this is SHORT lasting
What can the stimulus be?
Physical = electrical current, mechanical stretch Chemical = drug or synaptic excitation
What happens to the channels in the cell membrane in stage 2 and 3 of an action potential?
The voltage gated sodium channel quickly inactivates
This is followed by the opening of the voltage gated K+ channel, leading to re-polarisation and after-hyperpolarization.
The membrane potential shifts towards EK+ and the ration becomes 100:1 (K+ and Na+ respectively) before returning to the original 40:1
Two mechanisms for stopping the excessive influx of Na
Voltage gated Na+ channels:
Activation and Inactivation gate
What is the key role of the voltage gated Na+ channel?
When the voltage threshold is reached, sodium channels open and Na+ ions move into the cell along BOTH the concentration gradient and electrical gradient
The influx of Na+ slows down and then stops when:
- The inside potential becomes positive and moves towards ENa, and thus attracts less Na+ ions
- Na+ channels inactivate
What are the key components of the voltage sensor?
Has polar charges
Is extremely sensitive
Undergoes a conformational change
Is closed so the solute cannot pass through
Why does Na+ go inside the cell?
- Due to the presence of the electrical gradient
2. Due to the concentration gradient, ie from an area of high to low concentration
What is Voltage gated Na+ channels activiation gate
Gate is normally closed.
When the threshold is met, the voltage sensor detects a change in membrane potential causing the gate to open, thus allowing Na+ ions to flow into the cell and causing rapid depolarisation.
It inactivates once the membrane potential within the cell becomes positive and hence no Na+ ions are no longer travelling down the conc gradient
What is Voltage gated Na+ channels inactiviation gate
Closed when the inner membrane potential is negative.
Once a depolarisation of the membrane potential to the threshold is detected, the gate is retracted.
When the membrane potential becomes positive, the gate closes the channel so the membrane potential can re-polarise
The voltage of resting membrane potential
-70mV
Voltage of action potential “peak”
+30mV
Total amplitude of action potential
100 mV
What does “graded” mean
The small sub-thresholds of depolarisation/hyperpolarisations that by themselves do not cause an action potential to occur
What event is each action potential?
An all or none event
