Transmission of Nerve Impulses (CIE A Level Biology)

• 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 the outside of the axon
• This potential difference is usually about -70mV (i.e. 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

Factor	How the factor contributes to maintaining the resting potential
Sodium-potassium pumps in the axon membrane	These pumps move sodium ions (Na+) out of the axon and potassium ions (K+) into the axon The pump proteins use the energy from the hydrolysis of ATP to continue moving these ions against their concentration gradients
Many large, negatively charged molecules (anions) inside the axon	This attracts potassium ions, reducing the chance of them diffusing out of the axon
Impermeability of the axon membrane to ions	Sodium ions cannot diffuse through the axon membrane when the neurone is at rest
Closure of voltage-gated channels (required for action potentials) in the axon membrane	Stops sodium and potassium ions diffusing through the axon membrane

Maintaining the Resting Potential Diagram

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

A Nerve Impulse Only Travels in One Direction - Diagram

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 the 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 the 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

Membrane Potential Changes During an Impulse - Diagram

How the membrane potential changes during an action potential