• Neurones?transmit?electrical impulses, which travel extremely quickly along the neurone?cell surface membrane?from one end of the neurone to the other
• Unlike a normal electric current, these impulses are?not?a flow of electrons
• These impulses, known as?action potentials, occur via very?brief changes in the distribution of electrical charge?across the cell surface membrane
  • Action potentials are caused by the?rapid movement of sodium ions and potassium ions?across the membrane of the?axon

Resting potential

• In a?resting axon?(one that is?not transmitting impulses), the?inside?of the axon always has a slightly?negative electrical potential?compared to outside the axon
• This?potential difference?is usually about?-70mV?(ie. the inside of the axon has an electrical potential about 70mV?lower?than the outside)
• This is called the?resting potential
• Several factors contribute to?maintaining?the resting potential:

How the resting potential is maintained table

The resting potential of an axon and how it is maintained

Action potentials

• There are?channel proteins?in the?axon membrane?that allow?sodium ions?or?potassium ions?to pass through
• These open and close?depending on the electrical potential?(or voltage)?across the axon membrane?and are known as?voltage-gated channel proteins?(they are?closed?when the axon membrane is at its?resting potential)
• When an?action potential?is stimulated (eg. by a receptor cell) in a neurone, the following steps occur:
  • Sodium channel proteins?in the axon membrane?open
  • Sodium ions?pass?into the axon?down the?electrochemical gradient?(there is a greater concentration of sodium ions outside the axon than inside. The inside of the axon is negatively charged, attracting the positively charged sodium ions)
  • This?reduces the potential difference?across the axon membrane as the?inside?of the axon becomes?less negative?– a process known as?depolarisation
  • This triggers?voltage-gated?sodium channels?to open, allowing?more sodium ions?to enter and causing?more depolarisation
  • This is an example of?positive feedback?(a small initial depolarisation leads to greater and greater levels of depolarisation)
  • If the potential difference reaches around?-50mV?(known as the?threshold value),?many more channels open?and?many more sodium ions enter?causing the inside of the axon to reach a potential of around?+30mV
  • An action potential is generated
  • The depolarisation of the membrane at the site of the first action potential causes?current?to flow to the?next section?of the?axon membrane,?depolarising?it?and causing sodium ion voltage-gated channel proteins to open
    • The 'flow' of current is caused by the diffusion of sodium ions along the axon from an area of high concentration to an area of low concentration
  • This triggers the?production of another action potential?in this section of the axon membrane and the process continues
  • In the body, this allows action potentials to begin at one end of an axon and then pass along the entire length of the axon membrane

How an impulse is transmitted in one direction along the axon of a neurone

Repolarisation and the refractory period

• Very shortly (about 1 ms) after an action potential in a section of axon membrane is generated,?all the sodium ion voltage-gated channel proteins in this section close, stopping any further sodium ions diffusing into the axon
• Potassium ion voltage-gated channel proteins?in this section of axon membrane now?open, allowing the diffusion of potassium ions?out of the axon, down their concentration gradient
• This?returns the potential difference to normal?(about -70mV) – a process known as?repolarisation
  • There is actually a short period of?hyperpolarisation. This is when the potential difference across this section of axon membrane briefly becomes?more negative than the normal resting potential
• The potassium ion voltage-gated channel proteins then?close?and the sodium ion channel proteins in this section of membrane?become responsive to depolarisation again
  • Until this occurs, this section of the axon membrane is in a?period of recovery?and is?unresponsive
  • This is known as the?refractory period

How the membrane potential changes during an action potential