Electrical Station Lab 4.1
Electrical Systems in Health Fields

Action Potentials and Muscle Contraction

Station Lab 4.1: Electrical Systems in Health Fields
Action Potentials and Muscle Contraction 

 
http://en.wikipedia.org/wiki/Neuroeffector_junction

Lab Goal: The goal of this station is to use electromyography to see changes in electrical activity in the muscle, and to make observations about those changes over time.
Lab Materials Required: Vernier EKG/EMG Sensor, Hand Dynamometer, LabQuest® Mini Interface, Logger Pro® software, Human Physiology with Vernier Lab Manual, three disposable electrodes   
Objectives for Experiment:

• Identify general changes in electrical activity when flexing and relaxing the forearm muscles.
• Use an electromyogram to identify general trends in grip strength and amplitude with muscle fatigue
• Perform statistics on grip strength and EMG data to find mean strength and changes in amplitude over time.
• Correctly define the terms in bold from the introduction
• Correctly answer application questions relating to the lab experiment

Prerequisite knowledge: Minimum 6th grade reading level. All instructions provided.
Process Summary: At this station you will…

• Review the medical vocabulary and read instructions on how to perform an experiment with electromyography (e-leck-tro-my-AH-gra-fee), or EMG
• Experiment with contracting and relaxing your forearm muscles to see differences in electrical activity
• Experiment with muscle fatigue and record an electromyogram to see how electrical activity changes over time
• Use statistics to calculate changes in mean grip strength and amplitude of electrical impulses due to muscle.
• Answer questions related to the data you gathered

Applied Health Terminology:  Like many professions, the medical field has its own vocabulary. We have created flashcards to help you learn some of the vocabulary related to joints and movement that was used in this lesson.
You can find these flashcards on the computer at the site below.
Instructions to site:

• Click on the link below.

https://quizlet.com/85978026/electrical-station-41-muscle-fatigue-flash-cards/

• Then click on the “Flashcards” button in the upper left corner of the screen.
• For each card, say the word out loud and try to say what you think the definition is.
• Flip the card by clicking on it and say the definition listed out loud.

Evaluation: Your lab performance will be evaluated by the criteria (standards) you will find in this project’s rubric. A rubric is simply a table that states how you will be evaluated. Your coach will use this table to report your performance.

CLICK HERE TO GO TO THE RUBRIC

How does our brain tell our muscles to contract?

Interesting Facts to Know

The Nervous System

The brain is in charge of your body; however it is also connected to your spinal cord and many long electrical fibers called nerves. The spinal cord and nerves are what allow the brain to communicate with the body. Your brain, spinal cord and all of the nerves in your body make up the nervous(NUR-vus) system.

The brain has a lot of functions. Not only does it allow us to tell our bodies when to do something, like pick up a pencil or speak a sentence, it also keeps us going by causing our bodies to do things we have no control over.  The peripheral nervous system is further divided into two parts. The autonomic (aw-toe-NAWM-ic) nervous system is the part of the PNS that we do not have control over. This controls things like your heart beating, breathing, digestion, sweating and many more functions. The somatic (so-MAA-tick) nervous system is the part of the PNS that we do control. This controls all of the voluntary (or skeletal) muscles in your body.

Neurons

The entire nervous system is made up of individual cells called neurons (NUR-awns). What we know as nerves are actually long bundles of many neurons together. Neurons do not have the classic round shape that we think of most cells having. They actually have lots of different shapes because they can branch off in many directions. But a basic neuron has this structure:

Neurons transport messages between the brain and body using electrical signals called action potentials (ak-shun puh-TEN-chuls). The dendrites (DEN-drights) are little branches that receive the electrical signal from other neurons, and pass on the signal to the cell body or soma. The soma contains the neuron’s nucleus and organelles (or mini-organs of the cell). The electrical signal is then passed along from the soma to the axon, which is covered with the myelin sheath (MY-el-in SHEETH). The myelin sheath is a fatty layer that acts as an insulator, or a material that prevents the conduction of an electrical signal! Now why would we want to stop the electrical signal message being carried by the neuron? Read on my friend!

The axon has small spaces where it is not covered by the myelin sheath, and these are called the nodes of Ranvier (NODES of Ron-vee-AY), named for the French doctor who discovered them. The purpose of these nodes is to force the action potential to jump from one end of a myelin covering to another. This is the answer to our question above: myelin forces the electrical signal to be passed along the axon much faster by making it hop to each node! This is called saltatory conduction (SAL-tuh-tor-ee cun-DUCK-shun). Diseases like multiple sclerosis (MS), in which people have a slow decline is both sensation and the ability to move, is actually caused by a loss of the myelin sheath. It’s not that their neurons aren’t transporting the signal, it’s that the signal is just traveling much too slowly to be useful!

The last part of the neuron is the axon terminals, which is where the action potential is passed on. The place where an axon terminal meets another neuron or a target body part is called a synapse (SIN-aps). There are two types of synapses: electrical and chemical. In an electrical synapse, the action potential can be directly passed on because the two neurons are nearly touching. In a chemical synapse, the action potential is turned into a chemical signal using the release of neurotransmitters (nur-o-TRANS-mit-urs).

These are released into the space between the axon terminal and a dendrite of the next neuron, called the synaptic cleft. The dendrite of the next neuron has small cup-like particles called receptors (ruh-SEP-turs) that can bind the neurotransmitters. Once they bind, the next neuron begins creating a new action potential to continue passing on to the body.

Types of Neurons
Neurons are often described as being in one of three categories based on their function. Afferent (AFF-ur-ent) neurons are the neurons that allow us to sense things, such as pressure, temperature, pain, light, smell, taste and sound; that is why they are also often called sensory neurons. Afferent neurons always transport signals from the body to the central nervous system (the spinal cord and brain).
Efferent (EFF-ur-ent) neurons are the neurons that allow our brain to tell parts of our body to move (whether we have control over it or not). For example, the neuron that passes the message telling your intestines to contract, and the neuron that passes the message telling your muscles to bend your arm are both efferent neurons; but one is done automatically and the other is done because you decided to do it. Because these neurons tell our body when to move, they are also often called motor neurons. Efferent neurons always transport signals from the central nervous system to the body.

It is unfortunate that “afferent” and “efferent” are so similar, because this can be confusing! But it can be very helpful to think of this mnemonic (memory tool):

afferent neurons approach the central nervous system (aa) and efferent neurons exit (ee)

Delivering the message
As we said before, the neurons that transport the signal from the brain to our muscles to tell them to move are motor neurons. But what actually occurs to make this happen?

At the end of a nerve pathway from the brain to a muscle fiber there is a special kind of chemical synapse called the neuromuscular junction (NUR-o-musk-you-lur JUNK-shun). When the action potential reaches the axon terminal (1 in the picture), neurotransmitters (3) are released and they bind to receptors (4) on the muscle fiber (2). Once these bind to the outside of the muscle fiber, a complex chain of reactions causes the muscle fiber to fire its own action potential, which finally leads to contraction! Small organelles within the muscle fiber called mitochondria (my-toe-KAHN-dree-uh) (5 in the picture) are responsible for using nutrients to produce energy used for contraction.

Modified from http://commons.wikimediha.org/wiki/File:Synapse_diag4.png

Keep in mind that a motor neuron has many axon terminals, so it can stimulate multiple muscle fibers at once. A single motor neuron plus all of the muscle fibers it communicates with is called a motor unit.

Also, keep in mind that a whole muscle is made up of thousands of muscle fibers, so it will receive thousands of messages from many motor neurons at once before it contracts!

Strength of Contraction

First, when a single motor neuron fires, there is a range of strengths with which the muscle fibers in that motor unit can contract. But, the strength of the action potential delivered by the neuron never changes!

How does the neuron tell a muscle fiber to contract more or less? The answer is firing rate, or how many times the neuron fires on the muscle fiber in 1 second. A faster firing rate results in stronger contraction of the fiber, and a slower firing rate causes a weaker contraction of the fiber.

Second, the strength of the entire muscle depends on muscle recruitment, or the number of motor units that are stimulated to contract. A muscle contraction usually begins by only activating smaller motor units (in which a motor neuron only communicates with a few muscle fibers). As more strength is needed, larger motor units (in which a motor neuron communicates with many muscle fibers) are activated as well.

Believe it or not, it is possible for us to see this in action! We can use a process called an electromyography (e-leck-tro-my-AH-gruh-fee) to actually see and record the electrical activity in a muscle.
Electromyography is often abbreviated as EMG (much easier to say and remember!)  
EMG produces a record of the electrical activity in the muscle, called an electromyogram. An electromyogram is a long line with peaks and dips in it. If you have ever seen the wave on a heart rate monitor, it is very similar to that. An electromyogram has two main parts: frequency (FREE-kwen-see) and amplitude (AMP-lih-tude).

Frequency is the number of peaks in one second, and is related to how often action potentials are fired in 1 second. So, frequency shows us the firing rate of the motor units. The unit of frequency on an EMG is peaks per second.

Amplitude is the height of the peaks, and is related to the total amount of electricity being fired at once. Since the size of each action potential is always the same, the total amount of electrical energy depends on the number and size of motor units firing at the same time. So, amplitude shows us muscle recruitment (number of motor units active at one time). The unit of amplitude on an EMG is millivolts (mV).

So an electromyogram actually lets us see both of the factors that affect the strength of a muscle contraction!

Fatigue
In order to complete a contraction, your body needs oxygen and energy. All cells in the body have a type of organelle inside them (remember the mitochondria covered earlier?) that use the nutrients you eat and oxygen to produce a high-energy molecule called ATP.  ATP is then broken down to release energy that can be used to make a muscle fiber contract.

ATP

But when we start doing intense exercise such as doing many repetitions with a heavy dumbbell, or even just flexing a muscle for a long period of time, the amount of oxygen we are using is not enough to meet the demand of the muscles. When this happens, the body must use other ways of making ATP that do not require oxygen in order to make the muscles keep contracting. This is called anaerobic contraction (AN-air-o-bick con-TRACK-shun).

The problem is that these other processes are less effective (they produce less ATP) and they produce acid as a waste product. This acid then starts to build-up inside the muscle cell. It is believed that this acid build-up makes it harder for the muscle cell to create an effective contraction, known as muscle fatigue (fuh-TEEG), and it is also what causes the “burn”!

We can actually use EMG to observe how a muscle’s electrical activity changes with fatigue. Muscle fatigue is still a topic of intense research, and scientists have not been able to completely identify exactly how it works yet. But we do know that two things happen to the EMG reading as fatigue progresses:

1. A decrease in frequency is seen on the EMG.  It is thought that due to the acid build-up, it takes longer for muscle fibers to fire and transport their own action potentials in response to the motor neuron. This means the muscle fibers then take longer to contract.

AND

2. An increase in amplitude is seen on the EMG. It is thought that the loss of power in many muscle fibers leads to the recruitment of more motor units. These then coordinate with each other to fire at the same time, resulting in more electrical energy being released at once (which we see as higher amplitude). This allows the contraction to be held despite muscle fatigue.

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Links to Station 4.1Modules
Lab Intro | Lab Presentation and Practice | Communications Intro| Communications Presentation and Practice| Math

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