Transmission of nerve impulses

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Transmission of nerve impulses
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axon
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During a nerve impulse
Na
axon
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During a nerve impulse
Na
K
axon
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Resting state
More positive outside
axon
Less positive - inside
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During an action potential
Na
axon
Less positive - inside
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During an action potential
Na
axon
Inside becomes DEPOLARISED
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During an action potential
Na
axon
Inside becomes DEPOLARISED
Action potential is generated
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During an action potential
K
axon
Inside becomes negative again
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Resting state
More positive outside
axon
Less positive - inside
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Transmission of nerve impulses

• Neurones transmit impulses as electrical signals
• These signals pass along the cell surface
  membrane of the axon as a nerve impulse

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Transmission of nerve impulses

• It is NOT the same as an electric current passing
  down a wire (which is much faster)
• The mechanisms is the same throughout the animal
  kingdom

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Transmission of nerve impulses

• Experiments have been carried out using squid
  axons which are big enough to have electrodes
  inserted in them.
• One electrode can be placed inside the axon and
  one on its surface.

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Transmission of nerve impulses

• When at rest, the inside of the membrane has a
  negative electrical potential compared to the
  outside.
• This difference in potential is called the
  resting potential and is typically about between
  -70 mV to -80 mV
• In this resting state the axon is said to be
  polarised.

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Transmission of nerve impulses

• This is maintained because the neurone has an
  internal composition which is different to the
  outside.
• Sodium ions and potassium ions are transported
  across the membrane against their concentration
  gradients by active transport

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Transmission of nerve impulses

• Carrier proteins pick up Na ions and transport
  them to the outside.
• At the same time K ions are transported into the
  axon.
• This is known as the sodium-potassium pump and
  relies on ATP from respiration

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Transmission of nerve impulses

• Inside the axon there are large numbers of
  negatively charged organic ions which can not
  move out of the axon.

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Transmission of nerve impulses

• The Na ions are passed out faster than the K
  ions are bought in.
• Approx. three Na ions leave for every two K
  ions that enter.
• K ions can also diffuse back out quicker than
  Na ions can diffuse back in.
• Net result is that the outside of the membrane is
  positive compared to the inside.

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Axon membrane
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Axon membrane
Inside axon
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The action potential

• A nerve impulse can be initiated by mechanical,
  chemical, thermal or electrical stimulation
• When the axon is stimulated the resting potential
  changes.
• It changes from 70 mV inside the membrane to 40
  mV
• For a very brief period the inside of the axon
  becomes positive and the outside negative

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The action potential

• This change in potential is called the action
  potential and lasts about 3 milliseconds
• When an action potential occurs, the axon is said
  to be depolarised.
• When the resting potential is re-established the
  axon membrane is said to be repolarised

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depolarisation
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depolarisation
reploarisation
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depolarisation
reploarisation
overshoot
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direction of impulse
depolarisation
reploarisation
overshoot
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Depolarisation

• When the membrane depolarises changes occur in
  the membrane to the permeability of both Na ions
  and K ions

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Depolarisation

• When the axon is stimulated, channels open on its
  cell surface which allow Na ions to pass
  through.
• Na ions flood in by diffusion
• The Na ions create a positive charge of 40 mV
  inside the membrane, reversing the resting
  potential and causing the action potential

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Repolarisation

• Potassium channels open in the membrane and K
  ions diffuse out along their concentration
  gradient.
• This starts of repolarisation
• At the same time, sodium channels in the membrane
  close preventing any further influx of Na ions.

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Repolarisation

• The resting potential is re-established as the
  outside of the membrane becomes positive again
  compared to the inside.
• So many K ions leave that the charge inside
  becomes more negative that it was originally.
• This shows up as an overshoot.

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resting potential (no net ion movement)
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Na start to move in
resting potential (no net ion movement)
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Na ions diffuse in rapidly
Na start to move in
resting potential (no net ion movement)
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K ions diffuse out rapidly
Na ions diffuse in rapidly
Na start to move in
resting potential (no net ion movement)
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K ions diffuse out rapidly
Na ions diffuse in rapidly
Sodium ions pumped out potassium ions pumped in
Na start to move in
resting potential (no net ion movement)
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Repolarisation

• The potassium channels close and the
  sodium-potassium pump starts again.
• Normal concentrations of sodium and potassium
  ions is re-established.
• The membrane is once again at its resting
  potential

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direction of impulse

- - - - - - - -
- - - - - - - -

a) In the resting axon, there is a high conc. of
Na ions outside and a high conc. of K ions
inside. But the net effect is that the outside is
positive compared to the inside giving the
resting potential
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Leading edge of impulse
-
- - - - - - -
- - - - - - -
-
b) The axon is stimulated producing an action
potential, setting up local circuits on the axon
membrane
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direction of impulse
Na
- -
- - - - - -
- - - - - -
- -
Na
c) Sodium ions rush into the axon along a
diffusion gradient depolarising the membrane
causing an action potential
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direction of impulse
K
- -
- - - - - -
- - - - - -
- -
K
d) As the action potential passes along the axon
potassium ions diffuse out along a concentration
gradient, starting off the process of
repolarisation
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direction of impulse
K
K
Na
- -
- - - - - -
- - - - - -
- -
K
K
Na
e) The sodium-potassium pump is re-established,
fully repolarising the membrane