How Amplifiers Work (Week 6 Questions)

1.
"Where does the electrical conduction in your muscle of interest come from?"

The electrical conduction in the muscles of the neck come from the nerves that send signals through the muscles to tell it how to move. In order to understand how the signaling works, it is essential to understand the anatomy behind the muscles and the nerves. There are a myriad of muscles in the neck, and they are spread throughout four layers. The outer layer is mainly for movement of the upper limb and shoulder. The muscle in this layer is primarily called the trapezius muscle. The next two layers are made up of mostly splenius capitis and semi-spinalis capitis. Finally, the inner layer has small muscles and transverse-spinalis between the bone and the first two vertebrae. For the purposes of this test, the superficial muscles, such as the sternocleidomastoid, will be tested since surface electrodes are weak and cannot detect all four layers of the muscle.

The sternocleidomastoid muscle is responsible for tilting the head from side to side, rotating the face, flexing the neck, and raising the sternum. The sternocleidomastoid has an accessory nerve. This nerve essentially signals the muscle to move in the desired manner. The accessory nerve applies motor fibers. There are two components to the accessory nerve, and they are cranial and spinal. Since many of the cranial nerves serve the same function as other nerve fibers, it is more common to refer to these nerves as spinal accessory nerves. The nerve begins on the outside of the skull rather than inside and usually originates from neurons in the upper spinal cord. It enters the skull through the foramen magnum and exists through the jugular foramen. The nerve is then attached to the head. In the neck, the accessory nerve crosses the internal jugular vein. The nerve goes through the sternocleidomastoid muscle and sends the muscle motor branches, and the nerve continues to move through until it reaches the trapezius muscle and provides motor unnervation to part of this muscle. The figure shows how the accessory nerve attaches to both the sternocleidomastoid and trapezius muscle to send signals. 

2.

What does the electrical signal taken from their muscle of interest look like and why (i.e. shape when activated/deactivated, signal amplitude, etc)?

This is an example of an electrical signal taken from the trapezius muscle. The shape is similar to that of a combination of sinusoidal waves of different magnitudes. This also resembles other body signals such as the heart with EKG waves and those of the eyes with EOG waves.  The varying amplitudes of the waves on this graph show how the electrical impulses generated by the muscles are not just a single consistent wave but rather a series of waves of varying strength sent in a rapid sequence. This is what causes the filaments in the muscle to contract. Both frequency and magnitude increase when the muscle is being contracted harder or is doing more work and vice versa.

3.

Using 9V batteries, the two amplifiers in the INA2126 can produce signals in the range of -9V to +9V each. What should the gain used for the amplifiers be and why?

For the purposes of capturing EMG signals, the gain on the amplifiers should be set to about 3000. The reason for this is that when obtaining EMG signals from the skin, the electrical impulse is rather weak by the time it makes its way to the skin. Therefore in order to obtain a readable signal the gain must be relatively large. With the gain set to this level it enables us to view graph with enough detail to be able to set thresholds for using the data to activate motors using MATLAB. If the gain is set too high, the sensors will begin to read data from sources other than the desired muscles. For example, the sensors may begin to interpret signals from any other powered electrical device in the vicinity. That could lead to data that would be difficult to interpret due to the amount of “noise” present in the readings. In order to determine this number the equation A = VO / V_EMG must be used. In this equation, V_EMG is the voltage of the raw EMG signal collected at this skin by the sensor. Studies have shown that the normal V_EMG signal that is picked up by a sensor located on the surface of the skin will be .1 to .5 millivolts. Since the muscles used in this experiment are fairly large, it is safe to say that the reading will probably be closer to .5 mV. VO is the voltage output that the amplifier will then emit. The proportion of these signals gives the magnitude of the amplified signal. For the purposes of this project, the VO should be set to 1.5V, that way the DAQ will be able to pick up the signal and still provide a smooth enough graph to be interpreted. Since the average EMG signal given off by the muscles is approximately .0005V that would make the equation  look like A = VO/V_EMG = 1.5V/.0005V = 3000. Therefore the optimal amplification value is by a factor of 3000.

4.

Using the INA2126 datasheet, what should the value of the gain resistor R_G be, given the above information?

The gain was estimated above as approximately 2,000. Using this information and the INA2126 data sheet, the value for the resistor should be R_G = 80,000/ (2,000-1) = 40.0 ohms.  By using a resistance of 40.0 ohms, the electrons will be slowed down, voltage will decrease, and this will insure that the voltage of the power source will not receive signals that are higher than its capability. Additionally, it does not set the voltage too small so that the signals are weak. This enables the DAQ to work at an optimum voltage allowing for accuracy and at the same time, does not supply more voltage then can be handled. 

References

I. Ahmad, F. Ansari, U. Dey, A Review of EMG recording technique, 2nd ed., : International Journal of Engineering Science and Technology, 2012, p. .

II. Anderson, L. L., Ljaer, M., Sogaard, K., Sjogaard, G. (2007). Maximal muscle strength and EMG activity of the shoulder/neck muscles in females with work-related neck muscle pain. Journal of Biomechanics, 40(S72).

 

 

(2009). Sternocleidomastoid muscle [Online]. Available Telnet: wikipedia.org/wiki/sternocleidomastoidmuscle Directory: Wikipedia Works: Sternocleidomastoid muscle

(1848, 1993, 1995, 1996, 2001, 2004, 2007, 2010). Accessory nerve [Online]. Available Telnet: wikipedia.org/wiki/Accessory_nerve Directory: Wikipedia Works: Accessory nerve