Biomedical Electronics & Bioinstrumentation

1. Biomedical Electronics & Bioinstrumentation Origin of Biopotentials (Part 2) ENT213/4 21/1/2009 Prepared by: MegatSyahirulAmin bin Megat Ali E-mail: megatsyahirul@unimap.edu.my

2. Contents • Introduction to Biopotential Recordings • Fundamentals of Electroneurogram (ENG) • Principles of Electromyogram (EMG)

3. Introduction • An electroneurogram is a method used to visualize directly recorded electrical activity of neurons in the central nervous system (brain, spinal cord) or the peripheral nervous system (nerves, ganglions). The acronym ENG is often used. • An electroneurogram is similar to an electromyogram (EMG), but the later is used to visualize muscular activity.

4. Introduction • An electroencephalogram (EEG) is a particular type of electroneurogram in which several electrodes are placed around the head and the general activity of the brain is recorded, without having very high resolution to distinguish between the activity of different groups of neurons.

5. Introduction • What can you tell from these pictures?

6. The Electroneurogram (ENG) • An electroneurogram is usually obtained by placing an electrode in the neural tissue. • The electrical activity generated by neurons is recorded by the electrode and transmitted to an acquisition system, which usually allows to visualize the activity of the neuron.

7. The Electroneurogram (ENG) • Conduction velocity in a peripheral nerve can be measured by stimulating a motor nerve at two points a known distance apart along its course.

8. The Electroneurogram (ENG) • These are done through subtraction of shorter from longer latency, which gives the conduction time.

9. The Electroneurogram (ENG) • Using the known distance, the conduction velocity can be obtained. D L1 - L2 Where, v : Velocity D : Distance between electrodes L1 : Time of longer latency L2 : Time of shorter latency v=

10. The Electroneurogram (ENG) • Recording of these signals can be done using concentric needle electrode or surface electrodes. • Nerve field potentials measurable by: • Stimulating motor and sensory nerve which results in field potentials for both active fibers. • Eliciting neural field potentials from purely sensory or motor nerves.

11. The Electroneurogram (ENG) • Application of ENG includes study on the following fields: • Sensory nerve field potentials. • Motor-nerve conduction velocity. • Reflexly evoked field potentials. • Important parameters to be observed: • Conduction velocity and latency. • Characteristics of field potentials evoked by stimulated nerve.

12. Sensory Nerve Field Potentials • Measured by applying stimulus on the sensory nerve.

13. Sensory Nerve Field Potentials • Ring-stimulating electrodes applied to the fingers. • Known distance allow computation of conduction velocity of sensory nerve. • Examples: • For ulnar nerve, potentials can be recorded as high as armpit. • For median nerve, potentials can be recorded at or above the elbow.

14. Sensory Nerve Field Potentials • Long stimulation pulse results in: • Muscle contractions. • Limb movements. • Undesired artifacts. • Overcoming the problems: • Comfortable limb positioning. • Relaxed posture. • Brief, intense stimulus.

15. Sensory Nerve Field Potentials • Desirable stimulus: • 100V amplitude. • Duration of 100-300µs. • Why do we need these characteristics? • Excites large, rapidly conducting sensory nerve fibers. • Does not elicit pain fibers and surrounding muscle. • Diagnose peripheral nerve disorders.

16. Motor-Nerve Conduction • Conduction velocity of motor nerves can measured as follows: • Example: • Diagnose motor neuron abnormality. Peroneal nerve of the left leg

17. Reflexly Evoked Potentials • Recording of a second potential after the initial response when the peripheral nerve is stimulated. • As the neural stimulus site is brought closer to the muscle, the latency of the 1st response decrease, while the latency of the 2nd response increase.

18. Reflexly Evoked Potentials • Indicates that the stimulus travels to the CNS proximally for some distance before travelling distally in the opposite direction. • This suggests that the signal travels along the sensory nerve to as far as the spinal cord to elicit a spinal reflex. • An electrical homolog of the simple “knee-jerk” response.

19. Reflexly Evoked Potentials • Example: Stimulate popliteal nerve and record at triceps sural.

20. Reflexly Evoked Potentials • Low intensity stimulus, only high-amplitude H-wave observable. • Medium intensity stimulus, both M- and H-wave of moderate amplitude can be seen. • High intensity stimulus, only high-amplitude M-wave observable.

21. Electromyogram (EMG) • Skeletal muscle functions via contraction of a motor unit. • The motor unit can be activated by volitional effort, where all constituent muscle fibers are activated synchronously. • In cross section, the motor unit are interspersed with fibers of other motor units.

22. Electromyogram (EMG)

23. Electromyogram (EMG) • The muscle fibers of a single motor unit (SMU) constitute a distributed, unit bioelectric source located in a volume conductor. • The volume conductor consists of all other active and inactive muscle fibers.

24. Electromyogram (EMG)

25. Electromyogram (EMG) • The following are the properties of an evoked extracellular potentials of SMU: • Triphasic waveform. • Duration of 3-15ms. • Amplitude of 20-2000µV. • Discharge frequency of 6-30 per second.

26. Electromyogram (EMG) • Disadvantage of using surface electrode: • Only used with superficial muscle. • Sensitive to electrical activity over too wide an area. • Types of electrode for deep tissue recording: • Monopolar • Bipolar • Multipolar insertion-type

27. Electromyogram (EMG)

28. Electromyogram (EMG)

29. Electromyogram (EMG) • (a) and (b) shows observable motor unit action potentials from normal dorsal interosseus muscle during powerful contraction. • (c) shows the interference pattern where individual units are not distinguishable. • (d) shows the interference pattern during very strong muscular contraction.

30. Electromyography (EMG) • Shape of SMU potentials considerably modified by diseases. • In peripheral neuropathy, partial denervation of muscles is followed by regeneration. • However, regenerating nerve fibers conduct more slowly than healthy axons.

31. Electromyography (EMG) • In addition, excitability of affected neurons are changed, resulting in slow conduction. • Neural impulse more difficult to initiate and longer time to reach the muscle. • Causes scatter and desynchronization of EMG signals.

32. Advancements in EMG • Mathematical modeling studies of single-fiber and SMU action potentials. • Signal processing methods. • Automatic techniques for: • Detection • Decomposition • Analysis

33. Further Reading… • Marieb E.N, Hoehn K. (2007). Human Anatomy & Physiology. 7th Ed., Benjamin Cummings. • Chapter 2 • Webster, J.G. (1997). Medical Instrumentation: Application and Design. 3rd Ed., Wiley. • Chapter 4

34. The End… “If we knew what we were doing, it would not be called research, would it?…"