Nervous System: Action Potential Generation and Transmission part 1 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)
Action Potentials
A brief fluctuation in membrane potential caused by the transient opening of voltage-gated ion channels which spread along an axon
The membrane potential voltage threshold to begin an action potential
~-55mV
What are action potentials the key element of in neurons?
The process of signal transmission along axons
Does the membrane depolarise or hyperpolarize to reach the threshold for a membrane potential?
It depolarises
Stage 1 of an action potential
After the membrane potential reaches threshold it undergoes a fast 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
Stage 3 of an action potential
After-hyperpolarization
What stages of an action potential are the ‘Absolute Refractory Period’?
stages 1 and 2
What stages of an action potential are the ‘Relative Refractory period’?
stage 3
What is the Absolute Refractory Period?
when a nerve cell is NOT excitable i.e. a stimulus would not cause another action potential to occur
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 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 happens to the channels in the cell membrane in stage 2 and 3of an action potential
the voltage gated sodium channels inactivate which is followed by the opening of voltage gated K+ channels leading to a repolarisation and after-hyperpolarization and the membrane potential shifts towards EK+ and ratio becomes 100:1 before returning to original 40:1
Two mechanisms for stopping the excessive influx of Na+
Voltage gated Na+ channels have an activiation and inactivation gate
What is Voltage gated Na+ channels activiation gate
This is a gate that is normally closed. When threshold is met, the voltage sensor detects the change in membrane potential causing the gate to open, allowing Na+ ions to flow into the cell causing a rapid depolarisation. It inactivates once membrane potential within the cell becomes positive so Na+ ions are no longer travelling down conc gradient
What is Voltage gated Na+ channels inactiviation gate
Is closed when the inner membrane poential is negative . Once a depolarization of the membrane potential to threshold is detected, the gate is retracted. When the membrane potential becomes positive the gate closes the channel so the membrane potential can repolarise
Voltage of resting membrane potential
-70mV
Voltage of action potential “peak”
+30mV
Total amplitude of action potential
100mV
What does “graded” mean
These are small subthreshold depolariseation/hyperpolaeisaitons that by themselves do not cause an action potential to occur
When is an action potential “automatic”
Once the membrane potential has reached the threshold of -55mV
What path does current flow through
the once of LEAST resistance
Direction of current with charge outside membrane
Current flows from positive to negative
Rule #2
Wehn the current generated from an outside source flows through the cell membrane from
outside > inside = hyperpolarisation (MP is more -ve)
inside > outside = depolarisaiton (MP is more +ve)
Where in the neuron are action potentials first generated
The action initial segment (axon hillock)
Why are action pottentials generated in the action initial segment (axon hillock) first?
It has the lowest threshold (is a trigger zone) because of the high density of voltage gated Na channels
What are excitatory postsynaptic potentials (EPSPs)?
is the change in membrane potential of the postsynaptic neuron cell that causes the depolarisation of the axon initial segment to threshold. They are caused by synaptic transmission of stimuli from prestynaptic axons to dendrites and cell bodies
When do action potentials go once generated in the axon initial segment?
Down to cell body/along axon
