This article deals with basic techniques of neurophysiologic monitoring and stimulation. The described practices are used in medical diagnosis, surgery and treatment. Different devices with many functions and properties are used in stimulation. A system for electromyographic monitoring (EMG) and stimulation is proposed. The stimulating part of the prepared device described in this article is based on signal processing using an MSP430 microcontroller. This should certainly prove to be beneficial for future use, in a similar manner to the benefits bestowed by some current parts of modern, smaller and cheaper medical devices related to EMG.
Modern medical practice is based on high-tech
technology utilization. To this end, the most modern
technology is required to ensure continued progress in
the development of medical devices. Many forms of
bio-electric phenomena can be recorded with relative
ease. These include measurement, stimulation and
recording such as in the neurophysiologic intraoperative
monitoring (NIOM) used in medical
monitoring, treatment and surgery.
Techniques based on monitoring include:
electromyography (EMG), somato-sensory evoked
potentials (SEPs), motor evoked potentials (MEPs),
brainstem auditory evoked potentials (BAEPs),
electroencephalography (EEG), electrocardiography
(ECG) and electroneurography (ENG). Electrical
stimulation is required to record some of these signals,
with the stimulating and recording device parameters
dependent on the expected frequency range and signal
intensity - (Tab. 1,Tab. 2).
This paper proposes a system for stimulation and
EMG signal recording based on the MSP430
microcontroller, which is currently one of the most
advanced, fastest, ultra-low-power and most compact
The first two sections of this article discuss medical
measurement techniques for selected bioelectric
phenomena, and the design of the proposed system is
Intra-operative electromyography (EMG) provides
useful diagnostic and prognostic information in spinal
and peripheral nerve surgery . Basic techniques
include free-running EMG, stimulus-triggered EMG
and intra-operative nerve conduction studies. These
techniques can be used to monitor the following; (1)
nerve roots during spinal surgery; (2) the facial nerve
during cerebellopontine angle surgery and (3)
peripheral nerves during brachial plexus exploration
and repair. However, there are a number of technical
limitations which can cause false-positive or falsenegative
results, and these must be recognized and
avoided wherever possible .
EMG can be monitored in any muscle accessible to a
needle, wire, or surface electrode, so that mechanical
irritation of peripheral nerves or nerve roots results in
muscle activity in the corresponding musculature.
Recording and Stimulation
Trans-cranial stimulation activates spinal cord motor
fibers. It is important to localize motor tract deficits by
choosing appropriate muscles to record. The most
convenient recording is performed from at least two
muscles on either side below the surgical level, and
from one muscle above it which serves as a control
signal. The precise methodology involved in the choice
of stimulated muscles and instigated medical processes
is beyond the scope of this article .
Trans-cranial activation of subcortical motor tracts is
elicited most efficiently by anodal stimulation. Figure1
shows a typical electrical recording from a single nerve
fiber, including the dc offset potential (resting
potential) which occurs on membrane penetration. It
also shows the transient disturbance of membrane
potential (the action potential) when an adequate
stimulus is applied.
Conduction velocity in a peripheral nerve is
measured by stimulating a motor nerve at two points a
known distance apart along its course . Subtraction
of the shorter latency from the longer one gives the
conduction time along the segment of nerve between
the stimulating electrodes (Fig. 2). The conduction
velocity of the nerve can be determined when the
separation distance is known. This has great potential
clinical value, especially where conduction velocity in
a regenerating nerve fiber is slowed following nerve
Characterization and Interpretation
In order to understand the level of clinical
significance represented by a pattern of EMG activity,
the activity must be characterized beyond a simple burst
or train description . The most important feature
suggesting significance is its relationship to the surgical
events at that time. In addition, a number of electrical
features of EMG activity can suggest greater or lesser
degrees of irritation and therefore greater or lesser
The following EMG patterns are highlighted in
Figure 3: (A) a minor burst of activity occurring as a
lumbar root is manipulated; (B) a more intense burst
occurring on the background of an ongoing train of
activity; (C) intense ongoing trains of activity from
multiple motor units, denoting asynchronous activity.
(D) a residual train of activity as the effect of nerve root
irritation wanes and (E) an interference pattern in the
left gastrocnemius muscle after inadvertent trauma to
the corresponding nerve root.
The proposed EMG system comprises recording and
stimulating parts. The digital part of the system is
based on MSP430 microcontroller utilization. Here, a
pulse width modulation (PWM) signal generated by a
timer and D/A converter in the microcontroller is used
Theory of PWM signals
Pulse width modulation (PWM) is a powerful
technique for controlling analog circuits with processor
digital outputs. PWM is employed in a wide variety of
applications, ranging from measurement and
communication to power control and conversion.
Pulse-width modulation uses a square wave with a
modulated duty cycle which results in variation in the
average waveform value. When a square waveform f(t)
with a low value ymin, a high value ymax and a duty
cycle D is considered the resultant average value of the
waveform is given by:
As f(t) is a square wave, its value is ymax for 0 < t < D.T and ymin for D.T < t < T . Expression (1) then becomes:
The duty cycle “D” is the time an entity spends in an
active state as a fraction of the total time considered.
The microcontroller is used as a PWM signal
generator. This signal is fully programmable, so that
the frequency range, amplitude, and target signal
latency are easily set.
MSP430 Microcontrollers (MCUs) from Texas
Instruments (TI) are 16-bit, RISC-based, mixed-signal
processors designed specifically for ultra-low-power.
MSP430 MCUs have the right mix of intelligent
peripherals, ease-of-use, low cost and the lowest power
consumption for many applications .
Since the MSP430 MCU is designed specifically for
ultra-low-power applications, its flexible clocking
system, multiple low-power modes, instant wakeup and
intelligent autonomous peripherals enable true ultralow-
power optimization which dramatically extends
The MSP430 MCU clock system has the ability to
enable and disable various clocks and oscillators which
allow the device to enter various low-power modes
(LPMs). This flexible clocking system optimizes
overall current consumption by enabling the required
clocks only when appropriate .
The architecture, combined with five low power
modes is optimized to achieve extended battery life in
portable measurement applications. The device features
a powerful 16-bit RISC CPU, 16-bit registers, and
constant generators that contribute to maximum code
efficiency. The digitally controlled oscillator (DCO)
allows wake-up from low-power modes to active
mode, typically within 3 μs.
The MSP430F563x series are microcontroller
configurations with a high performance 12-bit analogto-
digital (A/D) converter, comparator, two universal
serial communication interfaces (USCI), USB 2.0,
hardware multiplier, DMA, four 16-bit timers, a realtime
clock module with alarm capabilities and up to 74
Typical applications for this device include analog
and digital sensor systems, digital motor control,
remote controls, thermostats, digital timers and handheld
meters. The MSP430F5638 is used as a generator
of PWM signals where the amplitude, latency and
frequency of the EMG stimulating signal can be set.
Timer_A is a 16-bit timer/counter with up to seven
capture/compare registers . This timer can support
multiple capture/compares, PWM outputs, and interval
timing, and it also has extensive interrupt capabilities.
Interruptions can be generated from the counter in
overflow conditions and from each capture/compare
Timer_A features include:
A synchronous 16-bit timer/counter with four operating modes
A selectable and configurable clock source
Up to seven configurable capture/compare registers Configurable outputs with pulse width modulation (PWM) capability
Asynchronous input and output latching
An interrupt vector register for fast decoding of all Timer_A interrupts
The DAC12_A module is a 12-bit, voltage output DAC, which can be configured in 8-bit or 12-bit mode and can be used in conjunction with the DMA controller. When multiple DAC12_A modules are present, they can be grouped together for synchronous update operation.
Features of the DAC12_A include:
12-bit monotonic output
8-bit or 12-bit voltage output resolution
Programmable settling time vs power consumption
Internal or external reference selection
Straight binary or 2's complement data format, right or left justified
Self-calibration option for offset correction
Synchronized update capability for multiple DAC12_A modules
Results of System Testing
Figure 5 depicts an example of a PWM signal where the amplitude can be set by a D/A converter. The frequency and latency of generated pulses are set in the time-interrupt routine. There is some noise problem when the device is powered by the USB cable. Although there is no apparent problem when the amplitude is 200 mV, the absolute noise destroys all useful signals at less amplitude.
The comparison of noises for different power supplies at low signal amplitudes is shown in Figure 6.
For further stimulations and measurements, it is necessary to use a battery powered device.
An EMG system for recording and stimulation is proposed in this article, and the stimulating device was developed and tested. For this stimulation, a modern MSP430 microcontroller was used to generate stimulation pulses with the configurable parameters of pulse width and amplitude, the number and frequency of pulses in the burst and the latency between bursts.
Our future task is to develop a wearable neurostimulator based on EMG technology, including an adaptive output control based on on-line signal DSP processing.
This material, and the MSP-TS430PZ100USB 100- Pin Socket Target Board and USB Programmer were financially supported by the: AV 4/0012/07 FEI.
The research described in the paper was financially supported by the Slovak Ministry of Education under VEGA Grant No. 1/0987/12.
Ing. Martin Nováček
Institute of Electronics and Photonics
Faculty of Electrical Engineering and Information Technology
Slovak University of Technology in Bratislava
Ilkovičova 3, SK-812 19, Bratislava
Phone: +421-2-602 91 213
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