Action potentials Flashcards
What is the resting membrane potentials
20-90mV
What are the conc of ions present in the ICF and ECF
Na - 145 ECF, 15 ICF
K - 4 ECF, 150 ICF
Cl - 110 ECF, 10 ICF
What ions can permeate membrane at rest
K+ (impermeable to Na+)
How is the potential gradient/ resting membrane potential created
The K+ moves out of the cell leaving more positive ions outside and more negative within the cell. Na/K pumps also help by pumping out 3 Na+ ions and only taking 2 K+ in (the pump is electrogenic)
What is the threshold for the action potential
-55mV
What causes the rising phase
Influx of sodium via voltage gated Na channels
What causes the downstroke (falling phase)
K+ moving out of the axon via voltage gated K channels
What are the types of gated opening systems
Ligand (receptor)
Voltage (RMP changes/meets threshold)
What are ion channels
Membrane proteins
Aqueous channel through membrane
What are the names of gates on voltage gates channels
M gate
H gate
What position are the Sodium channel gates in during closed, open and refractory states
Closed - M closed H open
Open - M open H open
Refractory - M open H closed
What are the steps of generation of an action potential to cause overshoot (spike)
Stimulus - depolarisation
RMP moves towards threshold
Gated ion channels close
Threshold reached
Na+ channels open Na+ influx
Greater depolarisation (rising phase)
All Na+ channels open
MP overshoots 0mv
What happens once overshoot occurs
When MP reaches 35mV
Na+ channels shut (refractory phase)
K+ channels open
K+ moves out of cell (downstroke)
MP returns to resting
Ion channels return to resting state
Excitability restored
What is happening once the threshold is reached
voltage-gated Na+ channels open
Na+ diffuse in; - further depolarisation
Positive feedback involved here
What occurs at the peak
Na+ channels close; voltage-gated K+ channels open;
K+ diffuse out; - repolarisation
When does the H gate open and M gate close again
At -70mV
What is the purpose of the refractory period
Limits maximum firing frequency of action potentials in axons
Ensures unidirectional propagation of action potentials
Prevents summation of action potentials
Prevents summation of contractions in cardiac muscle – the cardiac AP lasts as long as the ventricular contraction
How is an action potential propogated
An AP in one section of axon sets up longitudinal current flow
This depolarises adjacent ‘resting’ parts of the axon
The AP is regenerated further along the axon
More current flows, and the next region of axon is activated
Action potentials travel along the axon as waves of depolarisation
What determines speed of action potential propogation
The speed of AP propagation increases with the axon diameter
Large axons conduct impulses more rapidly than small ones
Rapid conduction is achieved only with very large axons…
…. But evolution of myelin greatly increases speed of AP propagation
What axon type would propogate an AP fastest
Large diameter and myelinated
How does a myelin sheath increase AP velocity
Forms insulating layer reducing the leakage of current from the axon
What is saltatory conduction
In myelinated nerve, the passive currents spread further along the axon
There are fewer regeneration steps per unit length of axon
Thus, the AP propagates more rapidly than in unmyelinated axons
What are the function of peripheral nerves
Some sensory and some motor
What are the myelinated axons in cutaneous nerves
Abeta
Adelta
What axon type is unmyelinated
C fibres
What function to all axon types have in common
Mechanoreceptors
What are the function of C fibres
Mechanoreceptors
Thermoreceptors (hot and hot)
Nociceptors (pain)
Which axon types are responsible for taste
A delta
