6.2.1 Nerve impulses

Describe the structure of a myelinated motor neurone

• dendrite

• cell body

• myelin sheath: made of Schwan cells

• axon

• Node of ranvier

• axon terminal

Describe resting potential

inside of axon has a negative carge relative to outside (as more positive ions outside compared to inside)

Explain how a resting potential is established across the axon membrane in a neurone

• Na+ / K+ pump actively transports 3Na+ out of axon AND 2K+ into axon

• creating an electrochemical gradient:

  • higher K+ conc. inside and higher Na+ conc. outside

• differential membrane permeability:

  • more permeable to K+ → move out by facilitated diffusion

  • less permeable to Na+ (closed channels)

Explain how changes in membrane permeability lead to depolarisation and the f=generation of an action potential

1. stimulus

   • Na+ channels open; membrane permeability to Na+ increases

   • Na+ diffuses into axon down electrochemical gradient, causing depolarisation

2. Depolarisation

   • if threshold potential reached, an action potential is generated

   • as more voltage-gated Na+ channels open (positive feedback effect)

   • so more Na+ diffuse in rapidly

3. Repolarisation

   • voltage-gated Na+ channels close

   • voltage-gated K+ channels open, K+ diffuse out of axon

4. Hyper polarisation

   • K+ channels slow to close so there’s a sight overshoot - too many K+ diffuse out

5. Resting potential

   • restored by Na+ / K+ pump

Draw and label a graph showing a action potential

…

Describe the all-or-nothing principle

• for an action potential to be produced, depolarisation must exceed threshold potential

• action potentials produced are always same magnitude / size / peak at the same potential

  • bigger stimuli increase frequency of action potentials

Explain how the passage of an acrtion potential along a non-myelinated axon results in nerve impulses

• action potential passes as a waeof depolarisation

• influx of Na+ in one region increases permeability of adjoining region to Na+ by causing voltage-gated Na+ channels to open so adjoining region depolarises

Explain how the passage of an acrtion potential along a myelinated axon results in nerve impulses

• myelination provides electrical insulation

• depolarisation of axon at node of Ranvier only

• resulting in saltatory conduction (local currents circuits)

• so there is no need for depolarisation along whole lengt of axon

Suggest how damage to the myelin sheath can lead to slow responses and / or jery movement

• less / no saltatory conduction; depolarisation occurs along the whole length of the axon

  • so nerve impulses tak longer to reach neuromuscular junction; delay in muscle contraction

• ions / depolarisation may pass / leak into other neurones

  • causing wrong muscle fibres to contract

Describe the nature of the refractory period

• time taken to restore axon to resting potential when no further action potential can be generated

• as Na+ channels are closed / inactive / will not open

Explain the importance of the refractory period

• ensures discrete impulses are produced (action potentials don’t overlap)

• limits frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus)

  • higher intensity stimulus causes higher frequency of action potentials

  • but only up to certain intensity

• also ensures action potentials travel in one direction - can’t be propagated in a refractory region

Dewcribe the factors that affect speed of conducatnce

myelination:

• depolarisation at nodes of Ranvier → saltatory conduction

• impulse doesn’t travel / depolarise the whole length of the axon

axon diameter:

• bigger diameter means less resistance to flow of ions in cytoplasm

temperature:

• increases rate of diffusion of Na+ and K+ bas more kinetic energy

• but proteins / enzymes could denature at a certain temperature