FATIGUE
LEARNING OBJECTIVES
At the end of the lecture, the student should be able to:
1. Define Fatigue.
2. Types of Fatigue.
3. Causes of Fatigue.
4. Prepare Nerve and muscle for the experiment.
5. Record the graph of fatigue in power lab.
6. Analyse the graph of fatigue.
FATIGUE
LECTURE OUTLINE
DEFINITION :
An unpleasant feeling of weakness or tiredness caused by working,
stress or lack of sleep.
When a muscle is repeatedly stimulated, it becomes gradually less
excitable and ultimately ceases to respond. This phenomenon is
called fatigue.
Types Of fatigue:
Physical fatigue:
Inability to exert force with one's muscles to the degree that would
be expected given the individual's general physical fitness.
Mental fatigue:
Can manifest itself both as somnolence (decreased wakefulness) or
just as a general decrease of attention, not necessarily including
sleepiness.
CAUSES OF FATIGUE:
There are many possible physical and psychological causes of fatigue. Some of the more
common are:
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An allergy that leads to asthma.
Anemia (including iron deficiency anemia).
Depression or grief.
Persistent pain.
Sleep disorders such as ongoing insomnia.
Underactive or overactive thyroid gland.
Use of alcohol or drugs such as cocaine or narcotics, especially with regular use
Fatigue can also accompany the following illnesses:
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Anorexia or other eating disorders.
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Arthritis, including juvenile rheumatoid arthritis.
Cancer.
Congestive heart failure.
Diabetes.
Infection, especially one that takes a long time to recover from or treat such as bacterial
endocarditis (infection of the heart muscle or valves), parasitic infections, AIDS,
tuberculosis, and mononucleosis.
Kidney disease.
Liver disease.
Malnutrition.
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Certain medications may also cause drowsiness or fatigue, including antihistamines for
allergies, blood pressure medicines, sleeping pills, steroids, and diuretics.
CHRONIC FATIGUE SYNDROME (CFS):
Persistent fatigue unrelated to exertion, not substantially relieved
by rest and accompanied by the presence of other specific
symptoms for a minimum of six months. The disorder may also
be referred to as post-viral fatigue syndrome (PVFS, when the
condition arises following a flu-like illness).
METHODS OF ELICITING FATIGUE:
1. TRANSMISSION FATIGUE OR INDIRECT FATIGUE:
This type of fatigue is produced when the muscle is indirectly stimulated i.e by placing the
electrodes on the nerve.
When a repeated stimulus is applied to the muscle through the nerve, all the stored
acetylcholine at the neuromuscular junction is exhausted. Thus the nerve impulse cannot be
transmitted from the nerve to the muscle at neuromuscular junction. Hence the site of fatigue
is the neuromuscular junction and not the muscle itself.
2. CONTRACTION FATIGUE, MUSCLE FATIGUE OR DIRECT FATIGUE:
This type of fatigue is recorded by placing the electrodes directly on the muscle and stimulating
it repeatedly.
When the muscle contracts repeatedly (exercise) a metabolic end product Lactic acid
accumulates in the contractile tissue fibers causing failure of contraction. This is also due to
depletion of energy.
SOURCES OF ENERGY FOR THE MUSCLE:
ATP & ADP: Each high energy phosphate bond provides 7300 calories of energy.
ATP in a muscle can sustain muscle power for 3 sec. only.
Continuous supply is provided by:
1)Phosphocreatine-Creatine-system: Each high energy phosphate bond provides 10,300
calories.
2) Glycogen-Lactic Acid System: Each glucose mol. Splits into two pyruvic acid mol. And energy
is released to form four ATP mol.
3) Aerobic system: Oxidation of food stuffs in mitochondria to provide energy
In frogs, the isolated muscle cannot recover from fatigue but in human muscles the
formation of lactic acid is a reversible phenomenon and by giving rest to the muscle it can
recover and lactic acid in presence of Oxygen is converted back to pyruvic acid.
Pyruvic acid
Lactic acid
Oxygen
If the blood supply is arrested the muscle gets fatigued in lesser time due to non availability of
oxygen.
EXPERIMENT:
Here you will examine the muscle's response to a sustained series of stimuli at a high
frequency.
PROCEDURE:
1. Make sure the muscle is moist and is in contact with the stimulating electrodes.
2. In the Stimulator panel, insert your value for the supramaximal stimulus voltage that
you calculated in Exercise 1.
3. Set the Interval between pulses to 20 ms and the number of pulses to 1750.
Click Start. After a 50 ms delay LabTutor will stimulate the muscle for 35 seconds with pulses
at 20 ms intervals. Recording will continue for a further 10 seconds after the last stimulus.
ANALYSIS:
You will have a single record to analyze.
1. Place the Marker on the baseline immediately before the start of stimulation.
2. Position the Waveform Cursor at the time of maximal force generation. Click to place
the Δ Force value in the Value panel then drag it to the appropriate cell in the table.
3. Now measure the force when it has declined to a minimum, immediately prior to the
end of stimulation, and enter the value in the appropriate cell of the table.
The last column of the table shows the percentage decline in contraction force from maximum
to minimum.
Learning Resources:
Text and Images from :
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Practical Physiology for First year M.B.B.S
http://www.emedicinehealth.com/fatigue/article_em.htm.
http://www.nlm.nih.gov/medlineplus/ency/article/003088.htm