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Cable Properties of Axons

Cable Properties of Axons. 24 February 2012. The Action Potential. Flows smoothly along length of axon Active process, but depends on passive spread of electrical potential. Ions and the action potential. How far does the voltage spread?. Potential is carried through the axon

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Cable Properties of Axons

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  1. Cable Properties of Axons 24 February 2012

  2. The Action Potential Flows smoothly along length of axon Active process, but depends on passive spread of electrical potential

  3. Ions and the action potential

  4. How far does the voltage spread? Potential is carried through the axon The length constant (l) is the distance over which the potential falls to 37% of its maximum (1/e).

  5. What does the length constant depend on? • Two resistances: • Membrane resistance (rm)—the resistance of the membrane to current flow (in Ohm-cm) • Internal resistance (ri)—the resistance of the axoplasm along the axon (in Ohms/cm) rm1/2 l = ri • One more resistance: • Input resistance--the average resistance to current flow through the axoplasm and membrane to the extracellular space

  6. Peak potential The peak voltage change in a neuron after current is injected, V0, is proportional to the current injected; the proportionality is the input resistance.

  7. Input resistance

  8. Specific resistance • The size of dendrites and axons vary from neuron to neuron • The composition of dendrites and axons vary from neuron to neuron • To compare neurons, we use specific resistances: • Specific membrane resistance (Rm) • Specific internal resistance (Ri) Rm = 2parmRi = ripa2 …where a is the radius of the fiber

  9. Effect of the diameter of the fiber Knowing the input, membrane and internal resistances, we can put them all together: RmRi1/2aRm1/2 rinput= 0.5 2p2a3 l = 2R As fiber radius increases, rinput decreases As fiber radius increases, l increases

  10. Membrane Capacitance • C=Q/V • Where C is capacitance, Q is the amount of charge (coulombs) and V is voltage. Units are coulombs/volt or Farads. • Almost all neurons have equivalent capacitance: 1mF

  11. Time Constant t The rise of the potential in a stimulated neuron can be described by the time it takes for the potential to reach 63% of its final value: t= (1-1/e) t = RC

  12. Cables vs. Neurons . t is not constant along the length of an axon. Why? Capacitance must be charged all along the length of the axon

  13. Three properties that describe electrical conduction in the neuron: • Input Resistance • The resistance of a neuron to stimulation; total resistance to current flow • Length Constant (l) • The distance across which a potential declines to 37% of its maximum value (V0) • Time Constant (t) • The time it takes for the membrane voltage (Vm) to reach 63% of its final value.

  14. Myelin and Neural Processing • Some neurons are covered with a “myelin sheath” • Myelin is produced by glial cells • Oligodendroglia (CNS) • Schwann cells (PNS)

  15. What does myelin do? Decreases Capacitance Increases membrane resistance Rm Increases length constant l Decreases time constant t How?

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