Transmission of Nerve Impulses

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Transmission of Nerve Impulses

• WALT
• Neurones transmit impulses as a series of
  electrical signals
• A neurone has a resting potential of 70 mV
• Depolarisation causes an action potential to be
  transmitted along the axon

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Resting Potential

• Experiments have been carried out using Giant
  Squid axons
• These are large enough to have microelectodes
  inserted into then to measure changes in
  electrical charge.
• One electrode is inserted into the axon and one
  is placed on the outside of the cell membrane

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Resting Potential

• The difference between the two potential charges
  is called the resting potential
• The membrane of a neuron is negatively charged
  internally with respect to outside
• This generates a potential difference of around
  - 50 - 90 mV (resting potential)

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Resting Potential
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Maintaining the Resting Potential

• Cation pumps (Na pumps) maintain active
  transport of K ions in and Na out of the
  neurone
• 3 Na ions are pumped out at the same time 2 K
  ions are pumped in
• This is done by the Sodium Potassium ATPase pump

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Sodium Potassium Pump
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Diffusion back

• Also within the membrane are channel proteins
  that allow both Na and K ions to diffuse back
  down their concentration gradient
• However there are many more K channels so K
  ions diffuse back much faster than the Na ions
• The net result is that the outside of the axon is
  positively charged compared to inside

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An Action Potential

• Action Potential
• An action potential is produced when membrane of
  neuron stimulated, the charge is reversed
• The inside of the axon was -70 mV and this
  changes to 40 mV and membrane is said to be
  depolarized

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An Action Potential

• A nerve impulse can be initiated by mechanical,
  chemical, thermal or electrical stimulation
• Experiment show that when a small electrical
  current is applied to the axon the resting
  potential changes from 70 mV to 40 mV
• This change in potential is called the action
  potential

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An Action Potential

• An Action Potential is produced due to a sudden
  increase in the permeability of the membrane to
  Na
• Na ions rush into neuron through the Na
  channels to depolarize the membrane, and then
  further increases its permeability to Na
• This leads to greater influx further
  depolarization --- positive feedback

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The Action Potential

• The Na ions move into the axon causing the
  charge to change to 40mV
• This reversal of charge causes the action
  potential

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The Action Potential

• When inside becomes sufficiently positively
  charged, permeability to Na ions start to
  decrease.
• At the same time as Na begins to move inward,
  K begins to move in the opposite direction along
  a diffusion gradient slowly until the membrane is
  repolarized.

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An Action Potential

• Within about 2 milliseconds, the same portion of
  the membrane returns to resting potential of -70
  mV inside this is called repolarisation
• Provided the stimulus exceeds a certain value
  (the threshold value), an action potential
  results.

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All or none response

• Above the threshold value, the size of the Action
  Potential ( A P ) remains constant, regardless of
  the size of the stimulus
• The size of the A P does not decrease as it is
  transmitted along the neuron but always remains
  the same

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Progression of The impulse

• When a nerve impulse reaches any point on the
  axon an action potential is generated.
• Small local circuits exist at the leading edge of
  the action potential.
• Sodium ions move towards the negatively charged
  regions.
• This excites the next part of the axon and so the
  action potential progresses

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The Refractory Period

• Absolute refractory period
• This lasts for about 1 msec during which no
  impulses can be propagated however intense the
  stimulus
• Relative refractory period
• This lasts for about 5 msec during which new
  impulses can only be generated if the stimulus is
  more intense than the normal threshold

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The refractory Period

• The refractory period ensures that
• Impulses can flow in only one direction as the
  region behind the impulse cannot be depolarised
• It limits the frequency at which successive
  impulses can pass along an axon.