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									Cell Membrane Potential



 Chapter 9: Nervous System
 Unit 3: Integration and Coordination
Neuron Communication
 Neurons communicate with one another
  through lots of nerve action potentials
  (nerve impulses)
 Generation of action potentials depends
  on two features of the cell membrane
  1. A resting membrane potential
  2. Existence of ion channels
Cell Membrane Potential
 A Membrane Potential is when body cells
  exhibit a difference in the amount of electrical
  charge on the inside and outside of the
  membrane.
 Cells with a membrane potential = polarized
       Like voltage stored in a battery!

    Polarization occurs due to an unequal distribution of
     positive and negative ions between sides of the
     membrane.

    Resting membrane potential = neurons at “rest”,
     not conducting action potentials
Flow of Ions
 Connecting the terminals of a battery with a
  piece of metal to a radio an electrical current
  flows from the battery allowing you to listen to
  music
 In living tissues, a flow of ions generates this
  electrical current
 Ions can flow across the membrane through
  ion channels.
    Some channels are always open, and others can be
     opened and closed.
       Channels can also be selective.
What are Ion Channels?
 When open, ion channels allow specific
  ions to diffuse across the membrane (high
  to low)
 Also, + charged ions will move towards a -
  charged area and vice versa
 There are 2 Types of ion channels
  1. Leakage Channels
  2. Gated Channels
Types of Ion Channels
 Leakage channels - allow a slow but steady
  stream of ions to leak across the
  membrane
   Example - Sodium (Na +), Potassium (K+), Cl-
 Gated channels - open and close on
  command
   Example - Voltage-gated channels open in
    response to a change in membrane potential
Resting Potential
 Two main ions are involved in potentials,
  K+ and Na+.
 Resting potential arises from the
  unequal distributions of various ions in
  cytosol and interstitial fluid.
 In a resting neuron, the outside of the
  membrane has a + charge and inside
  the membrane has a - charge.
   This creates potential energy, -70mV
Potential Changes
 Nerve cells exhibit electrical
  excitability.
   Changes (stimuli) usually affect the resting
    potential in a particular region of a nerve
    cell membrane.
      When a membrane’s resting potential
       decreases (as the inside of the membrane
       becomes less negative when compared to the
       outside), the membrane is said to be
       Depolarized.
Changes in Membrane Potential
 Changes in the resting potential of a
  membrane have two main phases:
   1. Depolarizing Phase - reversal of charge inside to
      outside
   2. Repolarizing Phase - membrane polarization is
      restored to resting state
      Changes are directly proportional to the intensity of
       the stimulation.
        If additional stimulation arrives before the effect of
         previous stimulation subsides, summation takes
         place.
           As a result of summated potentials, a level called
            Threshold Potential may be reached.
Action Potentials
 An action potential (AP) is a sequence of
  rapidly occurring events that decrease and
  reverse the membrane potential before
  eventually restoring it to the resting state
Generation of Action Potential
 Many subthreshold potential changes must
  combine to reach threshold and create an
  Action Potential.
   At the threshold potential, permeability suddenly
    changes at the region of the cell membrane being
    stimulated.

      Channels highly selective for sodium ions open
       and allow sodium to diffuse freely inward.

         As sodium ions diffuse inward, the membrane loses its
          negative electrical charge and becomes Depolarized.
Action Potential Continued
 As sodium ions diffuse inward, the membrane
  loses its negative electrical charge and becomes
  depolarized.
    At almost the same time, potassium ions diffuse
     outward, and the inside of the membrane becomes
     negatively charged once more.

       The membrane become Repolarized, and it remains
        in this state until stimulated again.
Major Events of Action Potential
 When the membrane reaches threshold,
  sodium channels open, some sodium
  diffuses in, and the membrane is
  depolarized.
    Soon afterward, potassium channels open.
      Potassium ions diffuse out, and the membrane
       is repolarized.
Nerve Impulse
 Axons are capable of AP’s, dendrites and the
  cell body are not.
 AP’s in one region of a nerve cell membrane
  cause a bioelectric current to flow to adjacent
  portions of the membrane.
    This Local Current stimulates the adjacent
     membrane to its threshold level and triggers
     another action potential.
       A wave of action potentials move down the axon to
        the end.
          This propagation of action potentials along a nerve
           axon constitutes a Nerve Impulse.
Events of a Nerve Impulse
1. Neuron membrane maintains resting
   potential.

2. Threshold stimulus is received.

3. Sodium channels in a local region of the
   membrane open.

4. Sodium ions diffuse inward,
   depolarizing the membrane.
Events Continued
5. Potassium channels in the membrane
   open.
6. Potassium ions diffuse outward,
   repolarizing the membrane.
7. The resulting action potential causes a
   local bioelectric current that stimulates
   adjacent portions of the membrane.
8. Wave of action potentials travels the
   length of the axon as a nerve impulse.
Impulse Conduction
 A myelinated axon functions as an
  insulator and prevents almost all ion flow
  through the membrane it encloses.
   Nodes of Ranvier between adjacent
    Schwann cells interrupt the sheath.
      Action potentials occur at these nodes, and
       jump from node to node.

        This is called saltatory conduction.
Speed of Nerve Impulses
 The speed of nerve impulse conduction is
  proportional to the diameter of the axon.

   The greater the diameter, the faster the
    impulse.

      An impulse on a thick, myelinated motor neuron of
       skeletal muscle and travel 120 meters/second!
      A thin, unmyelinated skin neuron might be 0.5
       meters/second.
All-or-None Response
 Nerve impulse conduction is an all-or-
  none response.
   If a neuron responds at all, it responds
    completely.
      A nerve impulse is conducted whenever a
       stimulus of threshold intensity or above is
       applied to an axon, and all impulses carried on
       that axon are of the same strength.
         A greater intensity of stimulation does not produce
          a stronger impulse, but more impulses per
          second.

								
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