nerve impulses Flashcards
Describe the structure of a myelinated motor neurone
dendrite
cell body
axon
myelin sheath
node of ranvier
axon terminal
Describe resting potential
Inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside).
Explain how a resting potential is established across the axon membrane in
a neurone
● Na+/K+ pump actively transports:
○ (3) Na+ out of axon AND (2) K+
into 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; membrane permeability to Na+ increases
● Na+ diffuse into axon down electrochemical gradient (causing depolarisation) - Depolarisation
● If threshold potential reached, an action potential is generated
● As more voltage-gated Na+ channels open (positive feedback effect)
● So more Na+ diffuse in rapidly - Repolarisation
● Voltage-gated Na+ channels close
● Voltage-gated K+ channels open; K+ diffuse out of axon - Hyperpolarisation
● K+ channels slow to close so there’s a slight overshoot – too many K+ diffuse out - Resting potential
● Restored by Na+/K+ pump
Draw / label a graph showing an action potential
- stimulus
- depolarisation
- repolarisation
- hyperpolarisation
- resting potential
see flash card
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
Explain how the passage of an action potential along non-myelinated and
myelinated axons results in nerve impulses
Non-myelinated axon
● Action potential passes as a wave of
depolarisation
● Influx of Na+
in one region increases
permeability of adjoining region to Na+ by
causing voltage-gated Na+ channels to open
so adjoining region depolarises
Myelinated axon
● Myelination provides electrical insulation
● Depolarisation of axon at nodes of Ranvier only
● Resulting 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; depolarisation occurs along whole length of axon
○ So nerve impulses take longer to reach neuromuscular junction; 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
● Time taken to restore 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
Describe the factors that affect speed of conductance
Myelination
● Depolarisation at Nodes of Ranvier only → saltatory conduction
● Impulse doesn’t travel / depolarise whole length of axon
Axon diameter
● Bigger diameter means less resistance to flow of ions in cytoplasm
Temperature
● Increases rate of diffusion of Na+ and K+ as more kinetic energy
● But proteins / enzymes could denature at a certain temperature
