Cell Biology: Action Potential

Action Potential

  • All cells have RMP (resting membrane potential)
  • Some cells can get excited
    • Neurons
    • Muscle cells
  • Measuring Action Potential
    • Using electrode
  • Steps of Action Potential
    • Sodium (Na^+) and Potassium (K^+) play key roles in making action potentials
    • When cell is at rest
    • High concentration of Na^+ outside of the cell and lower inside
    • Low concentration of K^+ outside of the cell and higher inside
    • . Voltage-Gated ion channels
    • Triggered by rise in membrane potential beyond threshold
    • Sodium ion channels open
      • Sodium flows into the cell
      • Driven by concentration gradient
      • Driven by electrical gradient (more - inside attracting +)
      • .Influx of sodium depolarizes the membrane and triggers the opening of potassium channels
    •  Potassium channels open
      • Potassium ions flow out of the cell
      • Following concentration gradient and electrical gradient
      • Repolarizing
        • Making the inside more negative again
    • Refractory Period
      • After an action potential 
      • Sodium channels are more resistant to opening -- so the potential can’t go both ways 
      • Cannot move back to a location where it has already occurred -- must go forward
    • Graded Potentials 
      • Graded potentials: small voltage fluctuations across the cell membrane 
      • Due to change in ion concentration
      • Hyperpolarization: charge of the membrane increases
      • When a negative voltage is applied
      • inhibitory
      • Hyperpolarizing phase = potassium efflux
      • Depolarization: membrane charge decreases
      • excitatory
      • When a positive voltage is applied 
      • Depolarizing phase = sodium influx
    • Steps

     1. Stimulus: sodium flows into the cell        Na channels open        If stimulus strong enough, depolarization passes threshold 2. Depolarization (continued): more Na channels open        Membrane potential increasing 3. Peak: Na+ channels inactive, K channels opened        Membrane potential decreases 4. Repolarizing: potassium leaves the cell making it more negative again 5. Hyperpolarization: K+ channels open a little too long, but then inactivate 6. Back to resting: Na and K pump bring it back to beginning

  • Some scenarios
    • If Na voltage gated channels are inactive, K channels not open
    • Leak channels open so it goes slow
    • Na wants to reach 66+ without K interfering
    • Na channels active, K+ channel opened
    • Fighting each other
  • Types of Na channels
    • Closed: can open with a strong stimulus
    • Inactivated: no stimulus will let Na through
  • AP are all or nothing, if the stimulus not strong enough to pass threshold, nothing will occur
    • No external energy required from AP, using the gradient
    • All ions that cause AP follow passive flow
    • Na and K ATPase creates gradient

Propagation of Action Potentials 

  • AP can only move forward, like a domino effect
  • Large depolarizations affect neighboring regions, causing them to depolarize
  • Signal regenerated without loss of intensity over distance
  • Refractory Period: time after AP when it is difficult or impossible to generate another AP on the same axon
    • Absolute refractory period: sodium channels are inactivated and no AP 
    • Relative refractory period: sodium channels are closed but a large stimulus could open them, unlikely
    • Makes sure that the AP won’t occur backwards 

 \n Regulating Speed of Axonal Transmission

  • Determined by the rate of AP propagation along an axon

  • Rate limiting step: spread of depolarization since it is passive

  • Resistance of Cytoplasm

    • Resistance to ion flow
    • Determines how easily/ how far charge can move under plasma membrane
    • Axons with a larger diameter have less resistance
    • Speed increase as diameter increases (sq root of diameter)

  • Increasing Speed of Propagation

    • Myelin: wrapped around axon to insulate
    • Made by schwann cells in p nervous system, and oligodendrocytes in c nervous system 
    • Saltatory conduction
    • AP jumping from one Node of Ranvier to other
    • Increases speed of propagation
    • Na can only flow in on the nodes
    • 20 times faster than continuous propagation along the axon

Neurotransmission

  • Propagation of electrical signal from one neuron to another
    • Passes signal through synapse
  • Electrical synapse: passing current
    • AP in presynaptic cell causes AP in postsynaptic cell
    • Current goes through gap junctions
    • Can only be excitatory
  •  Chemical synapse: passing neurotransmitters

  

  1. Nerve impluse
  2. Voltage gated calcium channels open
  3. Calcium channels trigger synaptic vessels holding neurotransmitters to fuse with the presynaptic membrane and release neurotransmitters into synapse
  4. Neurotransmitters binds to postsynaptic receptors, causing the receptor to become a channel and open letting Na or Cl flow in
  5. If Na flows in, Na gates open and AP is excitatory (keeps going)
  6. If Cl flow in, Cl gates open and AP is inhibitory (stops AP)

  • Post synaptic potentials

    • Excitatory (EPSP)
    • Result in depolarization of postsynaptic membrane
      • Approaches threshold
    • Inhibitory (IPSP)
    • Result in hyperpolarization of postsynaptic membrane
      • Does not get to threshold)

  • Terminating Transmission

    • Limiting the function of the neurotransmitter
    • Enzymatic Degradation: enzyme breaks up the neurotransmitter so it is no longer useful
    • Reuptake: neurotransmitter is brought back into the presynaptic used to be recycled

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