The Nerve Impulse.

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The Nerve Impulse.
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The Neuron at Rest

• The plasma membrane of neurons contains many
  active Na-K-ATPase pumps.
• These pumps shuttle Na out of the neuron and K
  into the neuron when ATP is hydrolyzed.
• Three Na are pumped out of the neuron at a time
  and two K ions are pumped in

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• This creates a concentration gradient for Na.
  As Na accumulates on the outside of the neuron,
  it tends to leak back in.
• Na must pass through proteins channels to leak
  back through the hydrophobic plasma membrane.
  These channels restrict the amount of Na that
  can leak back in.
• This maintains a strong positive charge on the
  outside of the neuron

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• The K inside the neuron also tends to follow its
  concentration gradient and leak out of the cell.
• The protein channels allow K to leak out of the
  cell more easily.
• As a result of this movement in Na and K ions,
  a net positive charge builds up outside the
  neuron and a net negative charge builds up
  inside.

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• This difference in charge between the outside and
  the inside of the neuron is called the Resting
  Potential.
• The resting potential in most neurons is
• 70 mV.
• When the neuron is at rest, it is polarized

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Initiation of the Action Potential

• A change in the environment ( pressure,
  heat,sound, light) is detected by the receptor
  and changes the shape of the channel proteins in
  part of the neuron usually the dendrites.
• The Na channels open completely and Na ions
  flood into the neuron. The K channel close
  completely at the same time and K ions can no
  longer leak out of the neuron in that particular
  area.

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• The interior of the neuron in that area becomes
  positive relative to the outside of the neuron.
• This depolarization causes the electrical
  potential to change from 70 mV to 40 mV
• The Na channels remain open for about 0.5
  milliseconds then they close as the proteins
  enter an inactive state.
• The total change between the resting state (-70
  mV) and the peak positive voltage ( 40mV) is the
  action potential ( about 110 mV)

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• The spike in voltage causes the K pumps to open
  completely and K ions rush out of the neuron.
  The inside becomes negative again. This is
  repolarization.
• So many K ions get out that the charge goes
  below the resting potential. While the neuron is
  in this state it cannot react to additional
  stimuli.
• The Refractory period lasts from 0.5 to 2
  milliseconds.
• During this time, the Na-K-ATPase pump
  reestablishes the resting potential.

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Transmission of the impulse

• The stimulus induces depolarization in a very
  small part of the neuron, at the dendrites.
• The sequence of depolarization and repolarization
  generates a small electrical current in this
  localized area.
• The current affects the nearby protein channels
  for Na and causes them to open.

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• When the adjacent channels open, Naions flood
  into that area of the neuron and an action
  potential occurs. This in turn will affect the
  areas next to it and the impulse passes along the
  entire neuron.
• The electric current passes outward over the
  membrane in all directions BUT the area to one
  side is still in the refractory period and is not
  sensitive to the current. Therefore the impulse
  moves from the dendrites toward the axon.

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Threshold stimulus

• Action potentials occur only when the membrane in
  stimulated (depolarized) enough so that sodium
  channels open completely.
• The minimum stimulus needed to achieve an action
  potential is called the threshold stimulus.
• If the membrane potential reaches the threshold
  potential (generally 5 - 15 mV less negative than
  the resting potential), the voltage-regulated
  sodium channels all open. Sodium ions rapidly
  diffuse inward, depolarization occurs.

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All-or-None Law

• Action Potentials occur maximally or not at all.
• In other words, there's no such thing as a
  partial or weak action potential. Either the
  threshold potential is reached and an action
  potential occurs, or it isn't reached and no
  action potential occurs.
• However, different neurons have different
  densities of Na channels and therefore have
  different APs

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• The AP remains constant as it travels down the
  neuron. Its amplitude is always the same because
  it corresponds to wide open Na channels.
• The frequency of the AP can change.

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Conduction Velocity

• impulses typically travel along neurons at a
  speed of anywhere from 1 to 120 meters per second
  
• the speed of conduction can be influenced by
• The diameter of a fiber. Velocity increases as
  diameter increases.
• Temperature. As temperature increases, the
  velocity increases. Axons of birds and mammals
  can be very small because of the high body
  temperature.
• the presence or absence of myelin.

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• Neurons with myelin (or myelinated neurons)
  conduct impulses much faster than those without
  myelin.
• Because fat (myelin) acts as an insulator,
  membrane coated with myelin will not conduct an
  impulse.
• So, in a myelinated neuron, action potentials
  only occur along the nodes and, therefore,
  impulses 'jump' over the areas of myelin - going
  from node to node in a process called saltatory
  conduction (the word saltatory means 'jumping')

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Summary

• The Action Potential, or nerve impulse is an
  electrochemical event involving the rapid
  depolarization and repolarization of the nerve
  cell membrane.
• The axon terminals of one neuron do not touch the
  dendrites of other neurons. What happens when the
  impulse reaches the axon terminal?