Nerve Impulses (AQA A Level Biology)

Action Potentials

• 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

Action potentials

• There are channel proteins in the axon membrane that allow sodium ions or potassium ions to pass through

• When an action potential is stimulated (eg. by a receptor cell) in a neurone, the following steps occur:

• Sodium ion channels 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

• Depolarisation triggers more 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 potential), 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 sodium ions to diffuse to along the axon, depolarising the membrane in the next section of the axon and causing sodium ion voltage-gated channel proteins to open there

  • This is known as conduction and is sped up by the presence of Schwann cells

• 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

  • The refractory period is very important as it ensures that ‘new’ action potentials are generated ahead (ie. further along the axon), rather than behind the original action potential

  • This makes the action potentials discrete events and means the impulse can only travel in one direction. This is essential for the successful and efficient transmission of nerve impulses along neurones.

The five stages of an action potential: stimulus, depolarisation, repolarisation, hyperpolarisation and return to resting state

Nerve Impulses

• When receptors (such as chemoreceptors) are stimulated, they are depolarised

• If the stimulus is very weak or below a certain threshold, the receptor cells won’t be sufficiently depolarised and the sensory neurone will not be activated to send impulses

• If the stimulus is strong enough to increase the receptor potential above the threshold potential then the receptor will stimulate the sensory neurone to send impulses

• This is an example of the all-or-nothing principle

  • An impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential

• Rather than staying constant, threshold levels in receptors often increase with continued stimulation, so that a greater stimulus is required before impulses are sent along sensory neurones

The receptor potential increases as the strength of the stimulus increases. As the strength of stimulus increases beyond the threshold, the frequency (not amplitude) of impulses increases.