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Action Potential & Propagation

Action Potential & Propagation. DENT/OBHS 131 Neuroscience. 2009. Ionic basis of APs. action potential: faithfully transmit information along the membrane ( axon ) of excitable cells allow rapid communication between distant parts of a neuron. Learning Objectives.

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Action Potential & Propagation

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  1. Action Potential & Propagation DENT/OBHS 131Neuroscience 2009

  2. Ionic basis of APs • action potential: • faithfully transmit information along the membrane (axon) of excitable cells • allow rapid communication between distant parts of a neuron

  3. Learning Objectives • Describe the roles of both sodium and potassium ions / voltage-gated channels before, during and after the action potential • Understand how the resistive & capacitive properties of neurons influence electrical signaling • Compare and contrast local circuit and saltatory propagation of action potentials

  4. 5 How many distinct ion channels are necessary for the AP? • 0 • 1 • 2 • 3 • 4

  5. 3 phases of the action potential • Resting • i.e. RMP • Depolarization • reversal of membrane potential • Repolarization • return of membrane potential to RMP

  6. General rule ENa +67 membrane potential (mV) • relationship between: • membrane potential • ion equilibrium potentials • if the membrane becomes more permeable to one ion over other ions then the membrane potential will move towards the equilibrium potential for that ion (basis of AP) RMP ECl -90 EK -98

  7. Depolarization • rapid opening of Na-selective channels • entry of Na “down” its electrochemical gradient • 1. membrane more permeable to Na than K • 2. membrane potential moves (rapidly) towards ENa • 3. because ENa is positive, the AP overshoots zero • 4. At the peak of the AP Na is the primary ion determining the membrane potential

  8. Repolarization • closure (inactivation) of Na-selective channels • slower opening of K-selective channel • 1. membrane more permeable to K than Na • 2. K moves out of cell • 3. membrane potential moves towards EK

  9. 2 independent channels • selective agents block the 2 components

  10. Voltage-gated ion channels • the opening and closing of AP Na and K channels are controlled by changes in the membrane potential

  11. What triggers an AP? • all-or-none • AP are not graded potentials • threshold • in order for an AP to occur the membrane must be depolarized beyond a threshold level • inward Na overcomes resting outward K movement • electrical stimulation • synaptic activation

  12. APs are regenerative • activation of Na channels is cyclical • initial depolarization • opening of Na channels • Na entry • etc..

  13. Learning Objective #1 • Describe the roles of both sodium and potassium ions / voltage-gated channels before, during and after the action potential

  14. AP review

  15. Learning Objective #2 • Understand how the resistive & capacitive properties of neurons influence electrical signaling

  16. How does an AP move? • Propagation • Aps are conducted along excitable cell membranes away from their point of origin • e.g. down the axon from cell soma to terminal

  17. Resistance ≈ how far it can get membrane resistance (rm) axon / dendrite diameter (d) axial, or internal, resistance (ri) rm ri ength constant  = “leaky pipe”

  18. Fat axons are fastest!

  19. Capacitance ≈speed • “bulk” solutions IN and OUT are neutral • the transmembrane potential difference exists within a narrow band just across the membrane • a capacitor • separates / stores charge • to change membrane potential • must add or remove charge • this takes time

  20. Summary • Capacitance - speed (time constant) • Resistance - distance (length constant) • How does neuron deal with these properties in order to have efficient AP propagation?

  21. Unmyelinated axons • local circuit propagation • slow • of the membrane during the AP is not restricted to a single spot • the inward current carried by Na ions during the AP depolarizes adjacent portions of the membrane beyond threshold and the regenerative AP travels along the membrane

  22. Refractory period • following a single AP a second AP cannot be generated at the same site for some time (absolute versus relative) • Na channels need to recover from inactivation • open K channels oppose inward Na movement

  23. Myelination • local circuit propagation is slow (< 2 m/s) • In motor neurons propagation is fast 100 m/s • Schwann cell / oligodendrocyte • envelop axons / layer of insulation • increase membrane resistance • less leaky • eliminate capacitance • less discharge • Nodes of Ranvier • discontinuity in myelin sheath (every few 200+ m)

  24. Saltatory conduction • APs are only generated at Nodes of Ranvier • high density of Na / K channels • current flows rapidly between nodes • little current leakage between nodes • AP “jumps” down fiber as successive nodal membrane capacitances are discharged

  25. Learning Objective #3 • Compare and contrast local circuit and saltatory propagation of action potentials

  26. propagation review Press button

  27. How can AP rise so fast (< 1 ms)? m= rmcm

  28. Membrane time constant • changing the membrane potential takes time • charging a capacitor is not instantaneous inject current V record voltage I m = rmcm ≈ 50 ms axon/dendrite

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