Lab 8 - Electromyography - EMG Flashcards
What was done in this lab
Explore the electrical activity of skeletal muscle by recording an electromyogram (EMG) from a volunteer. You will examine the EMG of both voluntary and evoked muscle action, and use this technique to measure nerve conduction velocity.
What are muscle cells known as and why
As muscle fibres as they are usually elongated in one direction.
What is a muscle made up of
1000s of such elongated fibres
How are groups of fibres arranged within a muscle
Bundled into fasicles which are surrounded by a connective tissue sheath known as the perimysium
What do several fasicles group together to make
Muscle
What surrounds the muscle
Epimysium
What are individual fibres seperated by
Endomysium
How are skeletal muscles attached to bone
By tendons
What is each individual fibre innervated by
A branch of a motor axon
What happens under normal circumstances
A neuronal action potential activates all of the muscle fibers innervated by the motor neuron and its axonal branches. The motor neuron, together with all of the individual muscle fibres that it innervates, is termed a motor unit
What happens when muscle fibers are activated
Initiation of an action potential (either voluntarily, or as a result of electrical stimulation of a peripheral nerve), conduction of the action potential along the nerve fibre, release of neurotransmitter at the neuromuscular junction and depolarisation of the muscle membrane with resultant contraction of the muscle fibers.
What is electromyography
a technique that measures the electrical activity of the muscles and the nerves controlling the muscles. The data recorded is an Electromyogram
What are the 2 methods of recording EMG
- Needle electrodes inserted through the skin into the muscle (we will not do this)
- Electrodes placed on the skin surface- DID THIS
What does size and shape of waveform measured provide information about
The ability of the muscle to respond when the nerves are stimulated
When is EMG in clinical setting most often used
When people have symptoms of weakness, and examination shows impaired muscle strength.
What can EMG help do
Differentiate muscle weakness caused by neurological disorders from other conditions.
What does EMG provide you with
A depiction of the timing and pattern of muscle activity during complex movements.
What happens in a raw surface EMG signal
Reflects the electrical activity of the muscle fibers active at that time. Motor units fire asynchronously and it is sometimes possible, with exceedingly weak contractions, to detect the contributions of individual motor units to the EMG signal. As the strength of the muscular contraction increases, however, the density of action potentials increases and the raw signal at any time may represent the electrical activity of perhaps thousands of individual fibers.
What does raw EMG signal during voluntary contractions indicate
Intensity of EMG activity
What is coactivation
- Contraction of a muscle leads to more minor activity in the antagonist muscle. Muscle is activated cooridinately with another muscle. Agonist and antagonist.
- Coactivation is a small increase in activity in a non active muscle that is in opposition to a muscle that is contracting forcefully. The coactivation of opposing muscles most likely aids in joint stabilization.
What is consequence of agonist - antagonist muscle coactivation
For movement mechanics (such as facilitating faster movements, and effects on action stability) and implication for movement optimisation (fine movement) and joint stability.
How are evoked EMG signals produced
By electrical stimulation of a motor nerve supplying a muscle.
What is the abductor pollicis brevis muscle
A member of the thenar muscle group on the palmar surface of the hand = MEDIAN NERVE
Where is the median nerve easy to stimulate
At the wrist and elbow
What is the latent period
Time it takes for the muscle to contract in response to the electrical pulse is recorded
- Short delay from the time where the action potential reaches the muscle until tension can be observed in the muscle.
How is median nerve stimulated
Brief electrical pulses are administered through the skin to the nerve
What is the speed of the response dependent on
The conduction velocity
What is the range of normal conduction velocities
50 - 60 m/s
However it varies from person to person and one nerve to another
What causes muscles to react in abnormal ways
Nerve and muscle disorders
What can help detect the presence, location and extent of diseases that damage muscle tissue (such as muscular dystrophy) or nerves (such as amyotrophic lateral sclerosis,
Measuring the electrical activity in muscles and nerves
What happens when an external nerve stimulation is applied
The volunteer will feel a brief ‘pinch’, a tingling sensation and a twitching of the muscle.
How long is the electrical pulse in these exercises
Very brief - less than a ms
What happens when investigating voluntary change in contractile force
Record EMG during voluntary muscle contractions, and investigate how contractile force changes with increasing demand.
What happens when investigating coactivation
Examine the activity of antagonist muscles and the phenomenon of coactivation.
What happens when investigating evoked EMG
Record EMG responses evoked by stimulating the median nerve at the wrist.
What happens when investigating nerve conduction velocity
Measure nerve conduction velocity from the difference in latencies between responses evoked by nerve stimulation at the wrist and the elbow.
Why is integrated activity used in assessment of muscle function
Easier to quantitate
What does baseline ( 0 books ) mean
Any EMG result when no book is applied, but the arm is still
What units is the amplitude measured in?
mV
What happened to the amplitude with increased weight?
Was there a pattern in this change?
As weight increases, amplitude increases.
As number of books increases, amplitude increases.
What type of muscle contraction have you been doing for exercise 1
Voluntary contraction
Unlike the discrete waveform from an electrocardiogram, the electromyogram waveform is irregular. Why do you suppose this is?
Unlike, the heart, skeletal muscle does not contract in a fully synchronized fashion. Instead, an EMG is recording the unsynchronized electrical activity of many muscle fibers.
How the EMG trace changed when you added weights to your arm
The heavier the load, the greater the recorded activity. Greater motor unit activity is required to generate more force.
Explanations of coactivation in exercise 2
For Bicep contraction, bicep amplitude is much higher than tricep contraction. The phenomenon of agonist-antagonist muscle coactivation has consequences for movement mechanics (such as facilitating faster movements, and effects on action stability) and implication for movement optimisation (fine movement) and joint stability.
How many muscle fibres are there per motor unit in biceps or triceps?
Larger muscles eg those involved in movement 150-500 muscle fibres.
What is occurring in triceps during bicep contraction
Triceps relax while biceps contract
What units is latency measured in?
In units of time – seconds, hours or minutes
How did the latent period change with an increase in pulse current? why?
Does not change action potentials travels constant speed. But wavelength and amplitude changes.
List the physiological events that occur during the latent period.
Initiation of action potential in nerve fibers; conduction of the impulse along the nerve; transmission at the neuromuscular junction; initiation of action potential in muscle fibers.
Which of the contributions to the latent period (that you listed in exercise 3) depends on the position of the stimulating electrode?
Conduction of the impulse along the nerve.
What are the units of nerve conduction velocity?
m/s
How fast does an electrical impulse travel through your median nerve (or ulnar nerve)?
50 m/s
Based on your results and calculations for nerve conduction velocity, how long would it take for a nerve impulse to travel from the spinal cord to the big toe? Assume that the distance traveled is one metre.
Conduction velocity varies among nerves, but can be at least 50 meters per second in large motor fibers. At that rate, it would take 20 milliseconds (0.02 s) for a nerve impulse to travel one meter.
What disease could slow down nerve conduction velocity and why?
- Measuring the electrical activity in muscles and nerves can help detect the presence, location and extent of diseases that damage muscle tissue (such as muscular dystrophy) or nerves (such as amyotrophic lateral sclerosis, aka Lou Gehrig’s disease, ALS).
- Carpal tunnel syndrome due to slower conduction velocities may also be caused by injury or damage to a nerve (such as carpal tunnel syndrome)
= Multiple Sclerosis – affects cns, demyelinating disease (attacks myelin sheath).
Was there variability in the nerve conduction velocity amongst members of your group? What are some explanations for this?
Depending upon the quality of their data the students may or may not see a difference in conduction velocity between volunteers. There is considerable variation among individuals for many physiological processes
