The Nerve Impulse Part 2
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The Nerve Impulse Part 2

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The Nerve Impulse Part 2 The Nerve Impulse Part 2 Presentation Transcript

  • The nerve impulse Part 2
  • Progress of an impulse
    • When an impulse reaches any point on the axon an action potential (AP) is generated
    • Small local currents occur at the leading edge of the AP
    • Sodium ions move across the membrane towards negatively charged regions.
    • This excites the next part of the axon so the AP progresses along its length
    • The local currents change the potential of the membrane, creating a new action potential ahead of the impulse.
  • stimulus The passage of an impulse
  • stimulus The passage of an impulse + + + + + - + + + + + - - - - - - + - - - - - +
  • stimulus The passage of an impulse + + + + + - + + + + + - - - - - - + - - - - - + Na + Na+
  • stimulus The passage of an impulse + + + + + - + + + + + - - - - - - + - - - - - + Na + Na+ local electrical circuit
  • The all or nothing law
    • An AP can only be generated if the stimulus reaches a certain threshold intensity
    • Below this threshold, no AP can be created
    • Once the threshold level is reached, the size of an impulse is independent of the stimulus
    • So, a greater stimulus does not give a greater action potential.
  • successive stimuli
  • successive stimuli increasing intensity of stimulation
  • successive stimuli increasing intensity of stimulation threshold intensity
  • successive stimuli increasing intensity of stimulation threshold intensity below threshold intensity: no action potentials
  • successive stimuli increasing intensity of stimulation below threshold intensity: no action potentials threshold intensity
  • successive stimuli increasing intensity of stimulation below threshold intensity: no action potentials threshold intensity action potentials generated
  • The all or nothing law
    • The difference between a weak and a strong stimuli is due to the frequency of the APs
    • A weak stimulus gives few APs
    • A strong stimulus gives more APs
    • … .(and is also likely to result in APs in more neurones)
  • The refractory period
    • Following the passage of an AP, there is a time delay before the next one can pass
    • This is called the refractory period
    • During this time sodium channels in the membrane are closed, preventing the inward movement of Na + ions
    • This is known as the absolute refractory period (about 1 ms)
  • neurone excitability 0 1 2 3 4 5 6 7 8 time / ms
  • neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability
  • neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus
  • neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus
  • neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus
  • neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus absolute refractory period
  • neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus absolute refractory period
  • neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus absolute refractory period normal resting excitability
  • neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus absolute refractory period relative refractory period normal resting excitability
  • neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus absolute refractory period relative refractory period normal resting excitability refractory period
  • The refractory period
    • The membrane starts to recover and the potassium channels open
    • Even before it is completely repolarised an AP can occur if the stimulus is more intense than the normal threshold level
    • This period is known as the relative refractory period and lasts about 5 ms.
  • The refractory period
    • The refractory period means that impulses can only travel one way down the axon as the region behind the impulse can not be depolarised.
  • The refractory period
    • It also limits the frequency at which successive impulses can pass along the axon
  • Speed of transmission
    • In myelinated neurones speed of transmission is up to 100 metres per millisecond.
    • In unmyelinated neurones it is much slower at about
    • 2 m ms -1.
  • Speed of transmission
    • Myelin speeds up the speed of the impulse by insulating the axon.
    • Myelin is fatty and does not allow Na + or K + to pass through it.
    • So depolarisation (and APs) can only occur at the nodes of Ranvier.
    • So the AP ‘jumps’ from one node to the next.
    • This is known as salatory conduction .
  • Salatory conduction
    • Advantages
    • Increase speed of transmission 100 fold.
    • Conserve energy as sodium-potassium pump only has to operate at the nodes and fewer ions have to be transported
    Nerve fibres growing through cylindrical Schwann cell formation .
  • axon myelin sheath
  • axon myelin sheath direction of impulse
  • axon myelin sheath direction of impulse + - + - + + - -
  • axon myelin sheath direction of impulse + - + - + + - - polarised depolarised
  • axon myelin sheath direction of impulse + - + - + + - - polarised depolarised local circuit
    • Any thing that affects the rate of respiration, such as temperature, will affect the transmission rate in a nerve.
    • This is because the restoration of the resting potential is an energy-requiring process relying upon ATP
  • Axon diameter
    • The thicker the axon, the faster the rate of transmission.
    • Probably due to the greater surface area of the membrane over which ion exchange can occur
  • Axon diameter
    • Giant axons found in some invertebrates (earthworms, marine annelids) are thought to be associated with rapid escape responses