1. The structure and function of myelinated nerve Mark Baker Neuroscience

2. Learning Objectives (all knowledge based): Provide an explanation of how impulses are propagated Describe how myelin works

3. Resting potentials and action potentials

4. 10 mM Na + 140 mM Na + 120 mM K + 2 mM K + Inside Outside K + channels Na + channel

5. 10 mM Na + 140 mM Na + 120 mM K + 2 mM K + +ve Inside Outside  -ve Na + channel K + channels

6. 10 mM Na + 140 mM Na + 120 mM K + 2 mM K + Inside Outside -80 mV Na + channel K + channels

7. 10 mM Na + 140 mM Na + 120 mM K + 2 mM K + Inside Outside -80 mV Na + channel K + channels

8. Reversal potential (E) for an ion is given by the Nernst equation: At 20 ºC, 58.2 log [out]/[in] At 37 ºC, 61.5 log [out]/[in] Thus a normal value of E K is negative and E Na is positive

9. Na + current generates up- swing of action potential Na + channel inactivation and K + channel activation underlie repolarization K + channel activation generates afterhyperpolarization

10. Function of myelinated nerve

11. nerve conducts action potentials by local circuit currents Function of myelinated nerve node of Ranvier internode outside inside direction of impulse optic nerve axons (Brady et al 1999) compact myelin Shiverer mouse (myelin basic protein null) local circuit current potentials recorded extracellularly axon longitudinal currents transmembrane current

12. Tasaki 1959 air gaps RR node single fibre teased out current across the myelin 0 1 2 ms -2 0 2 current (nA) transmembrane current at a node 0 1 2 ms -2 0 current (nA) 2 Function of myelinated nerve internode

13. Tasaki 1959 air gaps RR node single fibre teased out current across the myelin 0 1 2 ms -2 0 2 current (nA) transmembrane current at a node 0 1 2 ms -2 0 current (nA) 2 Function of myelinated nerve inward current outward current internode

14. Function of myelinated nerve Conduction velocity in a large myelinated axon is around 50 ms -1. An action potential at a single point lasts close to 0.5 milliseconds at body temperature. Therefore the action potential is around 25 mm (an inch) long. If there are nodes at 1 mm intervals, over 20 will be involved simultaneously in propagating a single impulse.

15. -80 +40 membrane potential (mV) 5 ms stimulator A B depolarizing afterpotential (DAP) Function of myelinated nerve action potential In the mammal repolarization occurs without K + channel involvement (shown by Chu et al 1979) H1H1 (cartoons only) (real thing!!) from Barrett EF and Barrett JN (1982) DAP

16. S.Y. Chiu, J.M. Ritchie, R.B. Rogart and D. Stagg, 1979 Function of myelinated nerve

17. Why worry about the DAP – just a detail isn’t it?? DAP is functionally important because a myelinated axon is easier to stimulate during the residual depolarization (following the refractory period). The DAP contributes to repetitive firing and sensory coding What is the process that allows repolarization of a node following an action potential? This has been a puzzle because kinetically fast delayed rectifier K + channels are not found at adult mammalian nodes of Ranvier. Problem solved by Barrett and Barrett 1982 – DAP is a part of the answer. Turns out if you know how the DAP is generated –you understand how the axon works.

18. Explain what electrical capacity is!! + - R C

19. + - C R + - time V  = RC

20. Explain what electrical capacity is!! + - C R + - time V  = RC time I initially behaves like a closed circuit, current flow determined by value of the resistor finally behaves like an open circuit, there is no current flow in the circuit at all What is the potential drop across the capacitor when it has fully charged, and the current in the circuit has dropped to zero?

21. Biological membranes provide large capacities (microscopic lipid bilayer with conducting solution on either side) usually taken to be 1  Fcm -2. Internodal membrane provides a capacity about 1000 times more than a node of Ranvier, so this will short-circuit the action currents close to the nodes, unless steps are taken to prevent it.

22. 100 pF capacitor (p pico, 10 -12 ; SI nomenclature)

23. In parallel, what is the total capacty? In series, what is the total capacty?

24. Axolemma Periaxonal space Myelin low capacitance- 1/C total = 1/C 1 +1/C 2 +…. 1/C n Axolemma large capacitance Barrett and Barrett resistance, makes myelin a poor insulator because there are current pathways across it. Without it, the node couldn’t repolarize, and the axon would not have a resting potential

25. Myelin provides a low capacity sheath – membrane stack has a much reduced capacity (like separating the two plates of a capacitor) The single internodal axon membrane (under the myelin) has a high capacity, so that action currents go straight through it – as though invisible, generating only a small change in potential across the membrane, the DAP If the myelin were a good insulator the axon simply wouldn’t work, because it would not have a resting potential, and the action potential at the node couldn’t repolarize!!!

26. Numerical simulation run at room temperature, Baker 2000 getting the model right Function of myelinated nerve Barrett and Barrett (1982) got the model right

27. Summary of function Myelinated nerve is a remarkable structure designed to efficiently propagate impulses at high speed. Ion channels and other proteins are targeted to discreet regions of axonal membrane in the formation of nodes and internodes. Only nodes are electrically excitable. The myelin sheath provides low internodal capacitance to allow energy efficient transmission, but is a relatively poor insulator, allowing internodal K + channels to set the axonal membrane potential.

28. Structure of myelinated nerve

29. Learning Objectives (all knowledge based): Be able to describe the basic structure of myelinated nerve. Be aware of the distribution of ion channels and cell-adhesion molecules that helps define distinct domains along an axon, and stabilizes the sheath

30. In the PNS Schwann cells engulph axons, and where the axon is greater than 1-2  m in diameter, the Schwann cell forms a myelin sheath around it. Axons are ensheathed sequentially by single Schwann cells. Schwann cells produce basement membrane that includes laminin a matrix protein that is essential for normal nerve development, function and regeneration.

31. From Poliak and Peles 2003

32. Oligodendrocyte Astrocyte foot process In the central nervous system, oligodendrocytes ensheathe several axons. Axons as small as 0.2  m in diameter are myelinated. Oligo’s can produce Myelin that spirals in oposite directions Astrocytic foot processes contact nodes.

33. Construction of myelinated nerve Myelinating cells cause clustering of Na + and K + channels and induce large axonal diameters

34. Na + channels (green) Caspr 1 (red) Fast K + channels (blue) rat optic nerve 10  M Matthew Rasband and Peter Shrager 2000 Rudolf Martini Peter Shrager Formation of nodes of Ranvier by Schwann cells

35.  -subunits interact with both intracellular and extracellular proteins, controlling Na + channel localization and contributing to the control of channel density  1 is crucial tenascin-R ankyrin-G Nf186contactin 11 22 Axon membrane Oligodendrocyte spectrin, actin cytoskeleton Ig-CAMs (lectin-like domains)  -subunit McEwen j. Biol Chem 279: 16044-16049 (2004)

36. D.P. Schafer and M. N. Rasband (2006) Glial CAMs recruit axonal CAMs at points of contact Axonal CAMs are attachment sites for cyto- skeletal proteins

37. Copyright ©2002 Society for Neuroscience Arroyo, E. J. et al. J. Neurosci. 2002;22:1726-1737 Figure 10.

38. Myelin structure

39. Myelin protein P0 is a cell adhesion molecule in Schwann cells extracellular membranes

40. Scherer and Arroyo 2002 EM of compact myelininterpretation

41. Summary of structure Myelinating cells in CNS and PNS differ Axon-satellite cell interaction is crucial for the formation of nodes of Ranvier e.g interaction of gliomedin in Schwann cells and NF186 is an important factor in Na + channel clustering Myelinated axon membrane incorporates domains typically expressing certain ion channels and cell adhesion molecules (CAMs) Sheath contains characteristic CAMs eg P0, and these stabilize myelin

42. References Structure of myelinated nerve eg: L. Shapiro et al (1996) Neuron 17: 435-449 (crystal structure of P0) M. Rasband and P. Shrager (2000) Ion channel sequestration in central nervous system axons. J Physiol. 525:63-73. E.J. Arroyo et al. (2002) Genetic dismyelination alters nodal structure J. Neurosci. 22:1726-1737 Y. Eshed et al. (2005) Neuron 47: 215-229 Giomedin mediates Schwann cell-axon interaction and the assembly of nodes of Ranvier Scherer and Arroyo (2002) Recent progress on the molecular organization of myelinated axons J Peripher Nerv Syst. 7:1-12 (Review) Poliak and Peles (2003) The local differentiation of myelinated axons at nodes of Ranvier. Nat Rev Neurosci. 4:968-80 (Review) D.P. Schafer and M.N. Rasband (2006) Current opinion in Neurobiology 16: 508-514. (Review) READ THIS

43. References Function of myelinated nerve eg: Try: M.D. Baker (2000) Trends in Neuroscience 23: 514-519 Chiu SY, Ritchie JM, Rogart RB and Stagg D (1979) A quantitative description of membrane currents in rabbit myelinated nerve. J Physiol. Jul;292:149-66 Barrett EF and Barrett JN (1982) Intracellular recording from vertebrate myelinated axons: mechanism of the depolarizing afterpotential. J Physiol. 1982 Feb;323:117-44.