Electrical Properties of Nerve Cells

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Electrical Properties of Nerve Cells

• 10.5.12

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The resting membrane potential
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Action potential mechanism
The rapid opening of voltage-gated Na channels
allows rapid entry of Na, moving membrane
potential closer to the sodium equilibrium
potential (40 mv)
A cell is polarized because its interior is
more negative than its exterior.
Repolarization is movement back toward the
resting potential.
The slower opening of voltage-gated K channels
allows K exit, moving membrane potential
closer to the potassium equilibrium potential
(-90 mv)
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Important Potentials

• Resting membrane potential is -70mV
• Depolarization peak is at 40mV
• Hyperpolarization peak is at -90mV
• Threshold potential is about -55mV

• 40mV is the Na equilibrium potential
• -90mV is the K equilibrium potential

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Na equilibrium potential
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K equilibrium potential
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Graded Potential

• A weak stimulus can depolarize or
  hyperpolarize the membrane generating a
  membrane potential which is not enough to
  generate an action potential. This is known as
  graded potential
• Graded potential causes potential change in
  limited areas
• The graded potential spreads along the membrane
  by changing the charge on the membrane
  capacitance and by flowing through opened
  channels

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Graded Potential

• As the current flows along the membrane, some of
  the current leaks through open channels in the
  neighboring areas. As a result the membrane
  potential progressively decreases with increasing
  distance from the source point
• This spatial pattern is exponential and the
  distance where the voltage changes to 37 of its
  original value is the length constant

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The size of a graded potential (here, graded
depolarizations) is proportionate to the
intensity of the stimulus.
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Graded potentials can be EXCITATORY or INHIBITORY
(action potential (action potential
is more likely) is less likely)
The size of a graded potential is proportional to
the size of the stimulus.
Graded potentials decay as they move over
distance.
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• Remember
• Membrane potential changes due to change in
  stored charge on membrane capacitor
• Membrane conductance changes due to flow of ions
  through gated channels during graded and action
  potentials

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Excitable cells

• As most neurons and muscle cells are much longer
  than their length constants, the graded impulses
  disappear when flowing along the cell, thus the
  responses cannot deliver signals from one end to
  the other in the cell
• Excitable cells are distinguished by their
  ability to generate active potentials that can
  propagate without losing their amplitude

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Excitable nerve cells

• A typical neuron has a dendritic region and an
  axonal region.
• The dendritic region is specialized to receive
  information whereas the axonal region is
  specialized to deliver information.
• Nerve cells have a low threshold for excitation.
  The stimulus may be electrical, chemical or
  mechanical

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Dendrites receive information and undergo graded
potentials.
Neuron
Axons undergo action potentials to deliver
information, typically neurotransmitters, from
the axon terminals.
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• Two types of physicochemical disturbances
• Local, non-propagated potential (Graded
  potentials)
• Propagated potentials, Action potentials or nerve
  impulse

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All-or-None Principle

• The all or none feature of action potential
  implies that stimulus less than certain threshold
  level of depolarization results in a graded
  response which would not be transferred. However
  a stimulus big enough to move the membrane
  potential beyond the threshold will generate
  action potential that can propagate to distant
  regions of the cells

• Threshold potential of-55mV corresponds to the
  potential to which an exccitable membrane must be
  depolarized in order to initiate an action
  potential

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• Throughout depolarisation, the Na continues to
  rush inside until the action potential reaches
  its peak and the sodium gates close.
• If the depolarisation is not great enough to
  reach threshold, then an action potential and
  hence an impulse are not produced.
• This is called the All-or-None Principle.

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Spatial or temporal summation

• Graded responses can interact with each other and
  can be spatially or temporally summed
• If two graded potentials occur at the same time
  in close enough /same places, their effects add
  up. This is called spatial summation
• If two graded potentials occur at the same place
  in succession, their effects add up. This is
  called temporal summation
• As an analogy, spatial summation is like using
  many shovels to fill up a hole all at once.
  Temporal summation is like using a single shovel
  to fill up a hole over time. Both methods work to
  fill up the hole

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Graded and action potential in neurons

• In neurons, the axon hillock (initial point of
  axon) has the lower threshold with relatively
  high densities of Na channels and is thought to
  be the principal trigger zone
• The graded responses produced throughout the
  dendrites or cell body is summed spatially and
  temporally, and if the summed response is large
  enough to pass the threshold, an action potential
  will be generated at axon hillock.

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Oneway propagation of the AP
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Oneway propagation of the AP
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Oneway propagation of the AP