3.6.2.1 - Nerve Impulses Flashcards
Describe resting potential
- The Inside of an axon has a negative charge relative to the outside of the axon
-70mv
Why does the Inside of an axon have a negative charge relative to the outside of the axon?
There are more positive ions outside compared to inside
Explain how a resting potential is established across the axon membrane in
a neuron
● Na+/K+ pump actively transports:
> (3) Na+ out of the axon AND (2) K+ into the axon
● Creating an electrochemical gradient:
> Higher K+ conc. inside AND higher Na+ conc. outside
● Differential membrane permeability:
> More permeable to K+ → move out by facilitated diffusion
> Less permeable to Na+
(closed channels)
Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential
- Stimulus
● Na+ channels open so the membrane permeability to Na+ increases
● Na+ ions diffuse into the axon down an electrochemical gradient (causing depolarisation) - Depolarisation
● If threshold potential is reached, an action potential is generated
● As more voltage-gated Na+ ion channels open (positive feedback effect)
● So more Na+ diffuse in rapidly - Repolarisation
● Voltage-gated Na+ ion channels close
● Voltage-gated K+ ions channels open so K+ ions diffuse out of axon - Hyperpolarisation
● K+ ion channels are slow to close so there’s a slight overshoot – too many K+ diffuse out - Resting potential
● Is restored by Na+/K+ pump
Describe the all-or-nothing principle
● For an action potential to be produced, depolarisation must exceed threshold potential
● Action potentials produced are always same magnitude / size / peak at same potential
> Bigger stimuli instead increase frequency of action potentials
What does a bigger stimuli do to action potentials?
> Bigger stimuli increase frequency of action potentials
Explain how the passage of an action potential along non-myelinated axons results in nerve impulses
- Action potential passes as a wave of
depolarisation - The Influx of Na+
in one region increases
permeability of its adjoining region to Na+ by
causing voltage-gated Na+ channels to open
so adjoining region depolarises
Explain how the passage of an action potential along myelinated axons results in nerve impulses
- Myelination provides electrical insulation
- The depolarisation of axon is at the nodes of Ranvier only
- This Results in saltatory conduction (local
currents circuits) - So there is no need for depolarisation along
whole length of axon
Suggest how damage to the myelin sheath can lead to slow responses and /or jerky movement
- Less / no saltatory conduction so depolarisation occurs along the whole length of axon
> So nerve impulses take longer to reach neuromuscular junction so there is a delay in muscle contraction - Ions / depolarisation may pass / leak to other neurones
> Causing wrong muscle fibres to contract
Describe the nature of the refractory period
- The time taken to restore the axon to resting potential when no further action potential can be generated
- As Na+ channels are closed / inactive / will not open
Explain the importance of the refractory period
- Ensures discrete impulses are produced (action potentials don’t overlap)
- Limits frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus)
> Higher intensity stimulus causes higher frequency of action potentials
> But only up to certain intensity - Also ensures action potentials travel in one direction – can’t be propagated in a refractory region
In the second half of the (1) period an action potential can be produced but requires (2) stimulation
to reach (3)
(1) refractory
(2) greater
(3) threshold
What are the factors that affect speed of conductance
1) Myelination
2) Axon diameter
3) Temperature
Explain how myelination affects the speed of conductance
● Depolarisation at Nodes of Ranvier only → saltatory conduction
● Impulse doesn’t travel / depolarise whole length of axon
Explain how axon diameter affects the speed of conductance
● Bigger diameter means less resistance to flow of ions in cytoplasm
Explain how temperature affects the speed of conductance
● Increases rate of diffusion of Na+ and K+ ions as they have more kinetic energy
● But proteins / enzymes could denature at a certain temperature
Describe the structure of a myelinated motor neuron
> Dendrite
Cell body
(Soma)
Axon
Myelin sheath (made of schwann cells)
Nodes of ranvier
Axon terminal
Which direction does a nerve impulse travel in a motor neuron?
cell body to axon terminal
