5.3.4(Nerve impulses: transmission)

Describe the steps for transmission of a nerve impulse and the formation of a local current

• An action potential occurs, sodium ion channels open at that point in the membrane

• The open sodium channels allow sodium ions to diffuse across the membrane from the region of higher concentration outside the neurone into the neurone. The concentration of sodium ions inside the neurone rises at the point where the sodium ion channels are open

• Sodium ions continue to diffuse along the neurone, away from region of high concentration.

• The local current causes depolarisation further along the neurone causing the voltage-gated sodium channels there to open. The open channels allow rapid influx of sodium ions causing an action potential. The action potential has therefore moved along the neurone

Define what is meant by a local current

• The movement of sodium ions down a concentration gradient in the nerve.

Describe and explain what occurs to the area behind the AP after the AP has moved

• As the area of the depolarisation moves upstream, channels in the region behind the AP close

• This causes repolarisation of the membrane

• Channels in the regoin are unresponsive

• This ensures that the nerve impulse can only travel in one direction

Describe what is meant by the term saltatory conduct

• In myelinated neurones the AP only occurs at nodes of Ranvier

• The AP jumps from one node to the next

• longer local currents

Explain why APs can only occur at the nodes of RAnvier

• it is where voltage gated sodium and potassium channels are located

• No diffusion of Na+ or K ions is able to occur in myelinated nerve membrane

• No depolarisation can occur so threshold is never met

Describe and explain the advantages of myelination

• Myelinated fibres conduct more quickly than unmyelinated:

  • Myelin sheath acts as an insulator

  • Lack of sodium and potassium gates in myelinated region

  • Depolarisation only occurs at notes of Ranvier only

  • So longer local currents

  • AP jumps from one node to another - saltatory conduct

Describe and explain why neurone transmission is described an “all or nothing” response

• Any stimulus always produces the same size action potential, given that it has caused the v to reach the threshold

• The frequency of the AP varies based on the strength of the stimulus

• The stronger the stimulus, the more frequent the generation of APs

• This is because, the higher the intensity of the stimulus, the more sodium channels opened, which causes more generator potentials

• Therefore more action potentials are generated in the same time span