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Experiment: Muscle APs

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In this experiment we will look at EMG activity more closely. The muscle activity we saw in our previous experiment was the result of many muscle fibers contracting at once. When you add up the electrical activity from each fiber, you get a large distributed EMG signal that we can pick up using surface electrodes. But what about the signal to each fiber? How are these contractions controlled?

Background

There are two things that determine strength of contraction: 1) the rate of action potentials that occur in the nerve cell and its accompanying muscle fibers, and 2) the “number” of motor units recruited. The rate of action potentials is code very similar to what you have seen in the cockroach leg. The more action potentials, the more contractions occur. Recruitment, however, is a new concept for us here.

In a relatively weak muscle contraction you recruit motor units with the smallest number of muscle fibers first. These small motor units produce small twitch forces when they contract. If you increase the strength of the contraction you gradually recruit larger and larger motor units with greater numbers of muscle fibers. They produce larger and larger twitch forces. At the same time that you are gradually recruiting more and more motor units, you are also firing them faster and faster (more twitches per second equals more force!). All of these twitches are added together in the muscle. This is why motor units are so cool.... they convert the code of the nervous system into a perfectly adjusted muscle force that pulls on a tendon connected to a bone - and allow you to chew your food, kick a soccer ball, press a keyboard button, or hug a loved one.

We can use our EMG SpikerBox to record the individual muscle action potentials that occur as you contract muscles! One of the simplest EMG experiments takes advantage of the fact that most of the muscles that control your hand are in your forearm. Move your fingers up and down. These are controlled by the flexor digitorum superficialis and extensor digitorum communis muscles in your forearm. Now move your fingers from side to side. These are controlled by the dorsal interossei muscles which are actually some of the few muscles inside of your hand.. Since our EMG SpikerBox is only detecting local electric activity of muscles, and we put electrodes on the top of your hand, would you expect a difference if you moved your fingers up and down vs left or right? Let's see!


Video explanation of experiment.

Materials Required

To record Muscle APs, you will need to the following things.

  • EMG SpikerBox
  • Popsicle Sticks (x2)
  • Brass Fastners (x3, these typically are 1" and come in a small box of 100 from an Office Store)
  • Hand Drill
  • Glue or Electrical tape

Once everything is gathered, we can prep for our experiment.

Equipment Setup

Before we get started with recording Muscle APs, we are going to need to build our recording electrodes. First we will need to gather two Brass Fasteners that we will use as electrodes.

Brassfastener.png

Place the Brass Fastener through one end of a popsicle stick. It helps if you can make a small whole first so that the wood does not split.

PopsicleStick.png

Do this to 2 popsicle sticks, then attach them together with a third fastener.

EMGScissors.png

Experiment

  1. Attach the Red and Black clips to one lead from each of the Brass fasteners.
    AttachEMGSmallElectrode.png
  2. Locate the indent (valley) between the two center knuckles on your hand
    HandPlacement.png
  3. Place the electrodes over the back of your hand between the knuckles as shown below.
    HandwithElectrodes.png
  4. Hook up the EMG SpikerBox to the cables connected to the Brass Fasteners. You will also want to connect the white cable to a part of your body as this is our reference electrode.
  5. Turn on the EMG SpikerBox.
  6. Move your hand as if you are doing the opposite of what Dr. Spock does.
    Spock.png
  7. Plug in your SmartPhone as see if you can detect any Motor APs. You may find you have to "think about moving" so that you only generate a single twitch and a single AP!
Spikes.png

Educational Standards

Core Concepts Covered in this Lesson Plan
1.b. Each neuron communicates with many other neurons to form circuits and share information.
1.c. Proper nervous system function involves coordinated action of neurons in many brain regions.
1.d. The nervous system influences and is influenced by all other body systems (e.g., cardiovascular, endocrine, gastrointestinal and immune systems).
2.a. Sensory stimuli are converted to electrical signals.
2.b. Action potentials are electrical signals carried along neurons.
2.c. Synapses are chemical or electrical junctions that allow electrical signals to pass from neurons to other cells.
2.d. Electrical signals in muscles cause contraction and movement.
2.f. Communication between neurons is strengthened or weakened by an individual's activities, such as exercise, stress, and drug use.
2.g. All perceptions, thoughts, and behaviors result from combinations of signals among neurons.
3.b. Sensory circuits (sight, touch, hearing, smell, taste) bring information to the nervous system, whereas motor circuits send information to muscles and glands.
3.d. Complex responses occur when the brain integrates information from many brain circuits to generate a response.