Generating an Action Potential

• when a neurone’s (axon’s) voltage increases beyond resting potential (-70mV)

• generates an action potential

• increase in voltage = depolarisation - voltage becomes more positive

• axon membrane becomes more permeable to Na+ so more move in and increase positive charge

• once an action potential is generated, it is generated at the next node of Ranvier (Mexican wave)

Start of depolarisation

• stimulus - provides enough energy to cause voltage-gated Na+ channels in axon membrane to open

• Na+ diffuse into axon as K+ diffuse out

• slight increase in voltage

Depolarisation

• if above threshold of -55mV, provides more energy, enabling more voltage-gated Na+ channels to open (more Na+ diffuses out and same quantity of K+ diffuses out)

• causes sharp increase in voltage/depolarisation

• peaks at +40mV as voltage-gated Na+ channels will close when this is reached in axon

Repolarisation

• K+ channels open at +40mV

• two channels are open - twice the amount of K+ diffusing out

• causes decrease in voltage (more negative) as more positive ions outside compared to inside

Hyperpolarisation

• continues until overshoot beyond resting potential - refractory period (temporarily go beyond -70mV (resting potential) to approx. 80mV)

—→ every couple of milliseconds, one part of the axon will go through all of these voltage changes - so once +40mV is reached, the next part of the axon will start its depolarisation wave (happens at all nodes of Ranvier until it is passed on to the next neurone)

‘All or Nothing Principle’

• threshold is at -55mV

• if depolarisation does not exceed this then action potential/impulse is not created

  —→ if stimulus is not large enough, it will not provide enough energy to open enough Na+ channels to go above -55mV

• if depolarisation does exceed -55mV then the stimulus will always peak at the same maximum voltage

• bigger stimuli increase the frequency of action potentials

Refractory period

• temporarily go up to around -80mV

• membrane cannot be stimulated during this period as Na+ channels are recovering (closed)

  • ensures that discrete impulses are produced - each action potential is separate from another (no overlaps to process and identify stimulus in detail)

  • ensures action potential can only travel in one direction - prevent spreading of Na+ along axon so that threshold potential is reached and a response can occur

  • limits number of impulse transmission (action potentials that can happen in a set amount of time) - prevents overreaction to a stimulus

Speed of conduction

• myelination and saltatory conduction

  • axon is conductive (long fibre that carries nerve impulse)

  • Schwann cells wrap around axon to form myelin sheath (lipid - insulation)

  • gaps in myelin sheath are nodes of Ranvier

  • action potential has to jump between nodes of Ranvier (only place where ions can move in/out to generate action potentials)

  • speeds up conduction as only have to generate an action potential in limited spaces (short distances)

  • if unmyelinated - every part of axon would have to generate and action potential so it would take ages to reach end of axon

• axon diameter

  • wider the diameter - faster the speed of conduction

  • as less leakage of ions so action potentials can travel faster

• temperature

  • higher temperature (up to certain temp) - faster the speed of conduction

  • as ions move faster so facilitated diffusion occurs faster

  • enzymes involved in respiration work faster so more ATP for active transport of Na+/K+ pump