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The Nerve Impulse Part 2
 

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