neurones

• Resting potential


  The difference in potential between the inside and outside of a neuron when it is not transmitting an impulse
• Chapter 15 The Action Potential

  Thursday 2/11/2023
• Action potential


  • Electrical impulses transmitted by the nervous system
• Membrane potential


  Another word for a voltage or a difference in charge between two sides of a membrane
• Maintenance of resting potential


  1. Active transport of Na+ out and K+ into the axon by sodium-potassium-ATPase pump

  2. Facilitated diffusion of Na+ out and K+ into the axon through leaking channels
• At resting potential, the inside of the axon is 70 mV lower potential than the outside
• The resting potential is about -70 mV
• Generation of action potential

  1. Voltage-gated Na+ channels open

  2. Na+ rushes in, depolarizing the membrane

  3. Membrane potential reaches +40 mV

  4. Na+ channels close, K+ channels open

  5. K+ rushes out, repolarizing the membrane
• Action potentials are only generated if the potential difference reaches a value between -60 mV and -50 mV (threshold potential)
• The action potential is typically measured at about +40 mV
• The potential inside the axon goes from -70 to +40 mV in about 1 ms
• Membrane repolarization


  1. Na+ channels close, K+ channels open

  2. K+ diffuses out, restoring resting potential

  3. Potential briefly becomes more negative than resting potential (hyperpolarization)
• Refractory period


  Period after an action potential when another stimulus will not result in a second action potential
• The refractory period is about 2 ms
• Refractory period


  • Allows action potentials to be distinguished from one another
  • Ensures action potentials can only travel in one direction along the axon
• The intensity of a stimulus is indicated by the frequency of the action potentials, not their size
• All or nothing principle


  If the stimulus is below the threshold, there will be no action potential
• Myelinated neurones


  • Action potential jumps between nodes of Ranvier (saltatory conduction)
  • Greatly speeds up transmission of the impulse compared to non-myelinated neurones
• The longfin inshore squid, Doryteuthis pealeii, has the largest known axon, up to 1.5 mm in diameter
• Experimental observation of the action potential is done by placing electrodes inside and outside the axon and connecting them to an oscilloscope