Cell membrane potential

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  • 1. Neuron Membranes & the Action Potential Chapter 9: Nervous System Unit 3: Integration and Coordination
  • 2. Cell Membrane Potential
    • The surface of the cell membrane is usually polarized (charged), with respect to the inside.
      • This polarization arises from an unequal distribution of positive and negative ions between sides of the membrane.
        • This polarization is particularly important in the conduction of muscle and nerve impulses.
  • 3. Distribution of Ions
    • The distribution of ions inside and outside cell membranes is determined in part by pores or channels in those membranes.
      • Some channels are always open, and others can be opened and closed.
      • Most channels are selective and only allow one type of ion/molecule through.
  • 4. Resting Membrane Potential
    • Active transport creates a concentration gradient across the cell membrane of sodium and potassium ions.
      • Na+/K+ pumps work to move Na+ out of the neuron and K+ into the neuron.
      • Uses ATP as energy source
        • ATPase breaks down ATP into ADP + P
        • Similar effects as those we saw in the ATPase of myosin heads in muscle fibers
  • 5.  
  • 6. Resting Membrane Potential
    • The Na+/K+ pump creates:
      • High concentration of sodium outside
      • High concentration of potassium inside
    • Negative amino acids are found in abundance inside the cell
      • Would make the intracellular fluid more negative, but…
    • Chlorine ions (Cl-) are found in abundance outside the cell
      • Counteract the negative of the amino acids
  • 7.  
  • 8. Resting Membrane Potential
    • If these were the only factors involved, the neuron would be neutral
    • However, K+ leaks out of the cell through leakage channels that are always open
    • Positives leaving the intracellular space means the overall charge is negative inside
      • (-70mV)
  • 9. Potential Changes
    • When neurons are excited (stimulated)
      • Affect the resting potential in a particular region of a nerve cell membrane.
      • If the 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 Depolarizing .
  • 10. Potential Changes
    • Changes in potential 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.
  • 11.  
  • 12. Action Potential
    • An action potential can be thought of as the “firing” of the neuron.
    • Action potentials will propagate down the length of a neuron’s axon
    • Action potentials are the electrical signals that move down a neuron
  • 13. Action Potential
    • Many subthreshold potential changes must combine to reach threshold, and once threshold is achieved, an event called Action Potential occurs.
      • At the threshold potential, permeability to ions changes suddenly at the region of the cell membrane being stimulated.
      • This is due to the presence of voltage-gated ion channels
        • Channels that respond to changes in membrane potential (voltage)
        • There are VGICs that are permeable to only K+ and others that are permeable to only Na+
  • 14.  
  • 15.  
  • 16. Action Potential
    • When threshold is reached:
      • 1. VGICs that are permeable to Na+ open
        • Na+ diffuses into neuron
        • Neuron’s membrane potential rises from -70mV to +40mV ( depolarizes )
      • 2. Na+ channels close as K+ VGICs open
        • K+ diffuses out of neuron
        • Neuron’s membrane potential repolarizes , going from +40mV to nearly -85mV
  • 17. Action Potential
    • When threshold is reached:
      • 3. The cell is now in what is called the refractory period (it’s too negative)
        • Neuron will not respond to further stimulation at this time
        • This is due to K+ channels being open a bit too long
      • 4. All VGICs return to normal and the neuron is ready to “fire” again
  • 18.  
  • 19. Nerve Impulse
    • When an action potential occurs in one region of a Neuron membrane, it causes 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 .
          • Animation
  • 20. Events Leading to the Conduction 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.
  • 21. 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.
  • 22. 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, where the exposed axon membrane contains sodium and potassium channels.
          • Nerve impulses jump from node to node, and are many times faster than conduction on an unmyelinated axon.
  • 23. 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.
  • 24. 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.