1. Nerve & Muscle Physiology
Jeff Ericksen, MD
VCU Health Systems PM&R

2. Topics * Relevant anatomy Cell functions for signal transmission
Transport, resting potential, action potential generation & propagation
Neuromuscular transmission
Muscle transduction
Volume Conductor theory
Relevant anatomy for nerve conduction and muscle contraction will be covered. We will look at how the cell membrane functions as a signal generator and conductor. Conversion or transduction of the peripheral nerve signal into a mechanical event will also be discussed as well as how these events are recorded in the electrodiagnosis laboratory.

3. Acknowledgements Electrodiagnostic Medicine by Daniel Dumitru, MD
Chapter 1: Nerve and Muscle Anatomy and Physiology
Superb text covering all aspects of EMG/NCS

5. Cell membrane Necessary for life as we know it Border role for cell
Separates intracellular from extracellular milleau
Allows ion and protein concentration gradients to exist
Creates electric charge gradients

6. Cell membrane Provides structure for cell
Modulates cell interaction with environment
Mechanical, hormone-receptor
Controls material flow into/out of cell
Nutrition/waste management

7. 3 Key Membrane Components
Lipids 45-49%
phospholipids, cholesterol & glycolipids = amphipathic molecules
Polar = hydrophilic vs. nonpolar = hydrophobic
Proteins 45-49%
Carbohydrates 2-10%

9. Lipid characteristics
Membrane phospholipids have polar head group with 2 nonpolar tails
In water - nonpolar tail groups form an inside excluding water
2 arrangements possible
Micelle = tails inside, heads face out
Bilayer

11. Lipid bilayer or fluid mosaic model
Phospholipid sheet with tails aligned in center, heads facing out for a head-tail-head sandwich
No H2O at center, 75 Angstroms
Model as 2-D liquid with 2 degrees of freedom of motion for lipid
Long axis rotation
Lateral diffusion
Center of the bilayer is free of water
Width is about 75 angstroms
3rd degree of mobility is for lipid to flip flop from one side to another in bilayer
Cholesterol molecules are woven into bilayer, possibly for stability and reduces permeability to small molecules.
Glycolipids exist on extacellular outer surface only, bound to carbohydrates, function unknown.

15. Proteins in membrane provide cell functions
2 membrane protein types
Transmembrane = integral - across whole layer, amphipathic
Hydrophobic midportion acts with lipid layer tails
Hydrophilic section faces intra/extra environment
Peripheral proteins - inside or outside of bilayer

16. Proteins

18. Membrane transport
Lipid soluble molecules cross readily but large water soluble molecules need transport across bilayer
Transport proteins - specific for ion or molecule to cross
Channel proteins - span bilayer, large center, allow ion/molecule passage based on size
Carrier proteins - binding with specific material, conformational change then crossing membrane

19. Membrane transport Diffusion Active transport
Driven by kinetic energy of random motion
Thru lipids or proteins
Follows concentration gradient
Active transport
Needs energy source
Fights concentration or energy gradient

20. Simple vs. Facilitated diffusion
Crosses membrane bilayer or channel without binding
Increases with kinetic energy + lipid solubility + concentration gradient
Protein channels specific for ions, often gated by cell functions
Facilitated
Transmemb proteins
Needs protein binding, conformational change
Speed of transport limited by conformational change
Simple diffusion speed is dependent on kinetic energy and strength of concentration or charge gradient.
Protein channel control with ligand or voltage gating.
Ligand gating = binding molecule induces conformational change to allow transport.
Voltage gating = specific voltage change induces conformational change.

21. Membrane transport Carrier proteins Energy Diffusion Active transport
Channel protein
Energy
Simple diffusion
Facilitated
diffusion
Diffusion
Active transport

22. Active transport
Acting on semi-permeable membrane allows the cell to maintain a high intracellular concentration vs. extracellular fluid
Requires active process as diffusion would eventually equilibrate concentrations across membrane

23. Active transport
Transmembrane carrier protein uses ATP energy to pump ions against concentration gradient to develop transmembrane resting potential

24. Resting membrane potential
Excitable cells can generate and conduct action potentials over distances
Intracellular space carries potential difference of mV, inside with negative charge excess relative to outside

25. Resting membrane potential created by semi-permeable membrane and ions
Extracellular
440 20
560
Intracellular
Na 50
K 400
Cl 52

27. Nernst used thermodynamics in 1888 to determine work done by membrane
Work to move ion against concentration gradient is opposite to work to move against electrochemical gradient
Can calculate contributions from different ions
K = -75 mV, Na = +55 mV

28. Nomenclature
Polarized membrane: Intracellular potential is negative relative to extracellular space
Depolarization = less polarization of the membrane -80mV -> +20mV
Hyperpolarization = more polarization of membrane -80mV -> -100mV

29. Na influx with K efflux
Na driven by negative charge excess inside + concentration gradient
K driven by concentration gradient
If continued, would lose resting potential

30. Na - K ATP dependent pump
Plasma membrane structure uses active transport
2 K in for 3 Na out actively
Thus 3 Na must diffuse in for 2 K out

32. Membrane potential from Goldman-Hodgkin-Katz equation
Resting potential mostly from K contributions
If sudden Na permeability change, potential approaches Nernst Na potential rapidly
Action potential!

33. Voltage dependent ion channels
Ion flow across through membrane channels is initiated by membrane potential changes
If potential exceeds a threshold, rapid increase in Na permeability followed by later K permeability increase

34. Voltage dependent ion channels
Extracellular Na activation gate with intracellular inactivation gate and slow K activation gait
Conformational changes due to membrane potential changes influence ion permeability

35. Voltage gated channels

36. Channels and voltage influence
If resting potential depolarized by mV, then activation gate opened with 5000x increase in Na permeability followed by inactivation gate closure 1 msec later
Slow K activation gate opens when Na inactivation gate closes to restore charge distribution, slight hyperpolarization

39. Refractory periods
Absolute = state when activation gait cannot be reopened with a strong depolarization current, the membrane potential is relatively more positive
Relative = state when activation gait can be reopened by strong depolarizing current as membrane potential returns to equilibrium state

40. Action potential timing

43. Action potential propagation
Na + charge influx spreads longtiduinally down path of least resistance to induce depolarization in adjacent membrane, some transmembrane spread
As + charge builds up, attracts intracellular - charges and they are neutralized by new ICF + charges

44. AP propagation
Less electrochemical hold of ECF + charges which migrate and allow depolarization of membrane further
Process is repeated down axon until end is reached
AP is identical to AP from upstream nerve area, all or none event

47. Nerve membrane modeling
Capacitor = charge storage device, separate poles separated by a nonconducting material or dielectric
Hydrophobic center to lipid bilayer is good dielectric, allows membrane to function well as a capacitor

48. Nerve membrane modeling
Resistor = direct path to current flow but with some impedance
Nerve axon has both transmembrane resistance as well as longitudinal resistance

49. Current spread
Membrane capacitor model suggests transmembrane resistance is high, hence current flows more longitudinally vs. transmembrane capacitance flow or ionic channel resistance flow

50. Slow process
Longitudinal AP spread requires sequential depol. to threshold, membrane capacitor discharge and then alteration of proteins to turn on Na activation channels. This process can be slow.
Hence unmyelinated nerve conducts slowly = m/sec.

51. Need velocity to interact with environment!
longitudinal resistance will speed
diameter will resistance
Eliminate need to fire all surrounding tissue will velocity of conduction
Insulate nerve to prevent leakage, spread out the gated Na channels
Myelin & Nodes of Ranvier

52. Myelin
All peripheral nerve axons surrounded by plasma membrane of a Schwann cell
Single layer of membrane = unmyelinated nerve, multiple layers = myelinated nerve
Gap between Schwann cell covers = node of Ranvier

53. Myelinated axons
Outer myelin sheath + axon plasma membrane = axolemma covering axoplasm
Schwann cell membrane has lipid sphingomyelin, highly insulating
No Na channels under myelin, only at nodes. K channels under myelin in perinodal area

56. Current conduction with myelin insulation
AP at node, Na charge influx and current spreads longitudinally down axon
Minimal leak between nodes, reduced by 5000 vs. unmyelinated nerve
Charge separation, reduced protein leak channels & increased membrane resistance account for this

57. Current conduction
Circuit is closed by efflux of ionic current at node
Na ions accumulate beneath node, reduces electrochemical pull on ECF Na above node, they migrate back to upstream node to close loop
Above tends to increase + charge inside membrane or depolarize to give AP

58. AP generation at node
Nodes contain high # Na channels which open with depolarization
Na influx starts process again

60. Myelin effects Conduction velocity increases
Current and action potential jumps from node to node = saltatory conduction
Optimal internodal length is 100x axon diameter
Optimal myelin/axon ratio is 60/40

62. Neuromuscular junction, transducing the electrical signal to mechanical force

63. Multiple branches from large motor axons

64. What happens if varying myelin and diameter in branches?

65. NMJ anatomy Presynaptic Postsynaptic Terminal axon sprout
Mitochodria
Synaptic vesicles = ACH
Presynaptic membrane
Postsynaptic
Motor endplate
Single muscle fiber
Mitochondria
Ribosomes
Pinocytotic vesicles
Postsynaptic membrane
ACH receptors

68. NMJ Electrochemical conduction
Considerable slowing in smaller diam less myelinated branches
AP depolarizes terminal axon, Na conductance increases
Calcium conductance also dramatically increased
Influx Ca++ in terminal axon
Possibly facilitates fusion of ACH vesicles with presynaptic membrane

69. Electrochemical conduction….
Vesicular fusion with presynaptic membrane
Open to synaptic cleft, release quantum of ACH
100 vesicles per AP in mammals, 10k ACH per vesicle
Ca++ stays in terminal axon 200 ms, keeps axon readily excitable for repeat stimulation

70. ACH release
Rapid diffusion across cleft in .5 msec timing, bind receptors
Large transmembrane proteins with ACH site and ion channel
Ligand activated vs. voltage activated
ACH binding induces conformational change in ion channel
1 ms opening of cation specific channel = Na, K, Ca, repels anions with charge

71. Postsynaptic ion channel opening with ACH binding
Predominant influx is Na, K blocked by electrochem gradient, Ca concentration gradient not that large
Na influx locally depolarizes muscle membrane= endplate potential reversal which is not propagated = EPP
Single packet of ACH from vesicle gives MEPP

72. Muscle action potential
Generated if sufficient ACH released to cause postsynaptic membrane to reach threshold, muscle membrane depolarized and propagated impulse follows
Muscle AP travels along muscle membrane = sarcolemma
Similar to nerve, increased Na permeability in + feedback loop

73. T-tubules
Small volume favors K accumulation during repolarization after AP, tends to make membrane easy to depolarize again
Penetrate into muscle to spread AP into fiber
High surface area of T-tubules increases capacitance qualities and slows conduction in muscle

74. Excitation-Contraction
AP in T-tubule induces Ca++ release in SR terminal cisternae, exposure for 1/30 sec, then reuptake via pump
Ca++ bind to troponin C, induces conformational change of troponin complex and influences tropomyosin to actin relationship - mechanical force

80. The End!