Cell membrane potential

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Cell membrane potential

  1. 1. Neuron Membranes & the Action Potential Chapter 9: Nervous System Unit 3: Integration and Coordination
  2. 2. Cell Membrane Potential <ul><li>The surface of the cell membrane is usually polarized (charged), with respect to the inside. </li></ul><ul><ul><li>This polarization arises from an unequal distribution of positive and negative ions between sides of the membrane. </li></ul></ul><ul><ul><ul><li>This polarization is particularly important in the conduction of muscle and nerve impulses. </li></ul></ul></ul>
  3. 3. Distribution of Ions <ul><li>The distribution of ions inside and outside cell membranes is determined in part by pores or channels in those membranes. </li></ul><ul><ul><li>Some channels are always open, and others can be opened and closed. </li></ul></ul><ul><ul><li>Most channels are selective and only allow one type of ion/molecule through. </li></ul></ul>
  4. 4. Resting Membrane Potential <ul><li>Active transport creates a concentration gradient across the cell membrane of sodium and potassium ions. </li></ul><ul><ul><li>Na+/K+ pumps work to move Na+ out of the neuron and K+ into the neuron. </li></ul></ul><ul><ul><li>Uses ATP as energy source </li></ul></ul><ul><ul><ul><li>ATPase breaks down ATP into ADP + P </li></ul></ul></ul><ul><ul><ul><li>Similar effects as those we saw in the ATPase of myosin heads in muscle fibers </li></ul></ul></ul>
  5. 6. Resting Membrane Potential <ul><li>The Na+/K+ pump creates: </li></ul><ul><ul><li>High concentration of sodium outside </li></ul></ul><ul><ul><li>High concentration of potassium inside </li></ul></ul><ul><li>Negative amino acids are found in abundance inside the cell </li></ul><ul><ul><li>Would make the intracellular fluid more negative, but… </li></ul></ul><ul><li>Chlorine ions (Cl-) are found in abundance outside the cell </li></ul><ul><ul><li>Counteract the negative of the amino acids </li></ul></ul>
  6. 8. Resting Membrane Potential <ul><li>If these were the only factors involved, the neuron would be neutral </li></ul><ul><li>However, K+ leaks out of the cell through leakage channels that are always open </li></ul><ul><li>Positives leaving the intracellular space means the overall charge is negative inside </li></ul><ul><ul><li>(-70mV) </li></ul></ul>
  7. 9. Potential Changes <ul><li>When neurons are excited (stimulated) </li></ul><ul><ul><li>Affect the resting potential in a particular region of a nerve cell membrane. </li></ul></ul><ul><ul><li>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 . </li></ul></ul>
  8. 10. Potential Changes <ul><li>Changes in potential are directly proportional to the intensity of the stimulation. </li></ul><ul><ul><li>If additional stimulation arrives before the effect of previous stimulation subsides, summation takes place. </li></ul></ul><ul><ul><li>As a result of summated potentials, a level called Threshold Potential may be reached. </li></ul></ul>
  9. 12. Action Potential <ul><li>An action potential can be thought of as the “firing” of the neuron. </li></ul><ul><li>Action potentials will propagate down the length of a neuron’s axon </li></ul><ul><li>Action potentials are the electrical signals that move down a neuron </li></ul>
  10. 13. Action Potential <ul><li>Many subthreshold potential changes must combine to reach threshold, and once threshold is achieved, an event called Action Potential occurs. </li></ul><ul><ul><li>At the threshold potential, permeability to ions changes suddenly at the region of the cell membrane being stimulated. </li></ul></ul><ul><ul><li>This is due to the presence of voltage-gated ion channels </li></ul></ul><ul><ul><ul><li>Channels that respond to changes in membrane potential (voltage) </li></ul></ul></ul><ul><ul><ul><li>There are VGICs that are permeable to only K+ and others that are permeable to only Na+ </li></ul></ul></ul>
  11. 16. Action Potential <ul><li>When threshold is reached: </li></ul><ul><ul><li>1. VGICs that are permeable to Na+ open </li></ul></ul><ul><ul><ul><li>Na+ diffuses into neuron </li></ul></ul></ul><ul><ul><ul><li>Neuron’s membrane potential rises from -70mV to +40mV ( depolarizes ) </li></ul></ul></ul><ul><ul><li>2. Na+ channels close as K+ VGICs open </li></ul></ul><ul><ul><ul><li>K+ diffuses out of neuron </li></ul></ul></ul><ul><ul><ul><li>Neuron’s membrane potential repolarizes , going from +40mV to nearly -85mV </li></ul></ul></ul>
  12. 17. Action Potential <ul><li>When threshold is reached: </li></ul><ul><ul><li>3. The cell is now in what is called the refractory period (it’s too negative) </li></ul></ul><ul><ul><ul><li>Neuron will not respond to further stimulation at this time </li></ul></ul></ul><ul><ul><ul><li>This is due to K+ channels being open a bit too long </li></ul></ul></ul><ul><ul><li>4. All VGICs return to normal and the neuron is ready to “fire” again </li></ul></ul>
  13. 19. Nerve Impulse <ul><li>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. </li></ul><ul><ul><li>This Local Current stimulates the adjacent membrane to its threshold level and triggers another action potential. </li></ul></ul><ul><ul><ul><li>A wave of action potentials move down the axon to the end. </li></ul></ul></ul><ul><ul><ul><ul><li>This propagation of action potentials along a nerve axon constitutes a Nerve Impulse . </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Animation </li></ul></ul></ul></ul>
  14. 20. Events Leading to the Conduction of a Nerve Impulse <ul><li>1. Neuron membrane maintains resting potential. </li></ul><ul><li>2. Threshold stimulus is received. </li></ul><ul><li>3. Sodium channels in a local region of the membrane open. </li></ul><ul><li>4. Sodium ions diffuse inward, depolarizing the membrane. </li></ul>
  15. 21. Events Continued <ul><li>5. Potassium channels in the membrane open. </li></ul><ul><li>6. Potassium ions diffuse outward, repolarizing the membrane. </li></ul><ul><li>7. The resulting action potential causes a local bioelectric current that stimulates adjacent portions of the membrane. </li></ul><ul><li>8. Wave of action potentials travels the length of the axon as a nerve impulse. </li></ul>
  16. 22. Impulse Conduction <ul><li>A myelinated axon functions as an insulator and prevents almost all ion flow through the membrane it encloses. </li></ul><ul><ul><li>Nodes of Ranvier between adjacent Schwann cells interrupt the sheath. </li></ul></ul><ul><ul><ul><li>Action potentials occur at these nodes, where the exposed axon membrane contains sodium and potassium channels. </li></ul></ul></ul><ul><ul><ul><ul><li>Nerve impulses jump from node to node, and are many times faster than conduction on an unmyelinated axon. </li></ul></ul></ul></ul>
  17. 23. Speed of Nerve Impulses <ul><li>The speed of nerve impulse conduction is proportional to the diameter of the axon. </li></ul><ul><ul><li>The greater the diameter, the faster the impulse. </li></ul></ul>
  18. 24. All-or-None Response <ul><li>Nerve impulse conduction is an all-or-none response. </li></ul><ul><ul><li>If a neuron responds at all, it responds completely. </li></ul></ul><ul><ul><ul><li>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. </li></ul></ul></ul><ul><ul><ul><ul><li>A greater intensity of stimulation does not produce a stronger impulse, but more impulses per second. </li></ul></ul></ul></ul>

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