CS 2016 Introduction to Practical 5 Nerve Conduction & Muscle Response Christian Stricker ANUMS/JCSMR - ANU THE AUSTRALIAN NATIONAL UNIVERSITY

CS 2016 Aims of the Practical At the end of this session, the student should be able to explain the following concepts: –electrical nerve stimulation incl. “effective” electrode; –nerve conduction velocity; –neuromuscular transmission and stimulus-evoked muscle contraction; and –summation of muscle responses.

CS 2016 Parts of This Practical 1.Electromyogram and stimulus strength 2.Stimulus condition 3.Nerve conduction velocity 4.Nerve stimulus and muscle contraction 5.Summation of the muscle response

CS 2016 Methods Recordings of ulnar nerve easier - closer to surface. –Funny bone (elbow). –Little finger side of wrist. Bipolar recording from muscle between two electrodes E 1 and E 2 against ground. Nerve stimulation via a bar electrode either at elbow or wrist. Recording via computer: you need to turn the box on… –Certain parameters can be changed. Top: stimulus properties. Right: recording conditions.

CS 2016 Electromyogram Normal polarisation of EMG: starts with a negative going potential. –Difference between E 1 and E 2 is recorded. –Initially, AP starts on E 1 causing negativity. –At the same time, no potential change at E 2. –At the end, E 1 with no change, but E 2 with negativity; hence a positive going deflection. If inversed, switch connections around on pre-amp.

CS 2016 EMG – Analysis How to measure M-wave: –First negativity [µV]. –Amplitude between most negative and positive peak [µV]. Plot amplitude of M-wave as a function of stimulus current (6 – 10 measurements). Determine stimulus threshold and Amplitude at supramaximal stimulation. (Values required for subsequent stimuli)

CS 2016 Stimulus: Anode vs. Cathode Anode: +, i.e. where to anions (-) flow. Cathode: -, i.e. where to cations (+) flow. At RMP, there is a voltage difference between the inside and outside; i.e. ~ -70 mV. If outside is made more -, transmembrane voltage drops and channels experience a depolarisation → AP. If outside is made more +, transmembrane voltage increases and channels experience a hyperpolarisation → no AP.

CS 2016 Stimulus Condition Swap stimulus bar around at same location (same “grooves”). Only one of the electrodes is effective at stimulation. The effective electrode will generate an EMG with shortest latency and vice versa. Determine which electrode it is.

CS 2016 Axon Fibre Recruitment Which fibres are recruited first? –An axon of a larger diameter will have a larger surface area → more current can be activated: First large diameter axons and later small ones are recruited. –Normal recruitment order is the other way around: First small cells with small axons and later increasingly larger axons: linear increase in force production with recruitment.

CS 2016 Conduction Velocity First stimulus at elbow (funny bone). Second stimulus at wrist: increase stimulus current by ≥ 6 units as the nerve is deeper in tissue. Time difference between the same two peaks.

CS 2016 Determine Conduction Velocity Measure time difference (on paper). Measure using a tape the difference in distance between two recording points (marked by two pen marks on skin): keep the tape a bit loose as the nerve does not quite go in a straight line. Determine conduction velocity as Compare to reference range: 50 – 60 m/s.

CS 2016 Nerve Stimulus & Muscle Contraction In addition to EMG, we now measure the force (pressure) exerted by the muscles. Delay between stimulus and muscle contraction. Trials show variability. –Use average of 5 trials. –Use a very small force on transducer (no force). –Maximal contraction is at peak of contraction.

CS 2016 Analysis of Experiment Determine the latency between the muscle action potential and the maximal force production. Analyse the latency of maximal force production as a function of stimulus strength: –Use sub-maximal stimuli as well. –Plot latency vs. stimulus strength. –Explain why this may be the case.

CS 2016 Summation of Muscle Response Two pulses are evoked on the nerve. If interval is shortened, force builds on a not-fully relaxed first response → summation.

CS 2016 Analysis of Summation Determine the peak force for each stimulus interval. Note that the force is measured in relative terms; the force indicated in Newtons does not correspond to the actual value (it is hard to calibrate the pressure transducers accurately). Express the peak amplitude in respect to the value at 500 ms (C 2 / C 1 ). Plot the relative maximal force produced with the second pulse as a function of stimulus interval. –Think of calcium homeostasis in influencing this relationship.

CS 2016 Answer 5 Questions In addition to the “normal” prac report, answer 5 questions for your write-up Due 5 o’clock on Monday 23 May Other Aspects There is a LabChart reader that you can download allowing you to read the files directly: If you want to take the data with you, copy the data into a Word file (Prnt Scrn) and take it with you. You then can measure from the Word file off-site.

CS 2016 That’s it folks…