Student Handout
Muscle
Introduction
In these experiments, you will explore how muscles work and examine some of
the properties of muscle fatigue. You will electrically stimulate the nerves of the
forearm using the Isolated Stimulator built into your PowerLab. You and your
group will be able to examine recruitment, summation and tetanus. In addition,
you will use a hand dynamometer to examine grip force and the ability to sustain
it under different conditions
Background
The skeleton provides support and articulation for the body. Bones act as support
structures and joints function as pivot points. Skeletal, or striated, muscles are
connected to the bones either directly or by tendons, strong bundles of collagen
fibers.
Two or more muscles usually work antagonistically. In this arrangement, a
contraction of one muscle stretches, or elongates, the other (Figure 1).
Figure 1. Biceps/triceps - an example of two muscles working antagonistically.
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©2009 ADInstruments
Student Handout
Muscle
Skeletal muscle is composed of long, multinucleate cells called fibers grouped
into fascicles (Figure 2).
Figure 2. Skeletal muscle structure.
A single motor neuron, and all the muscle fibers that it innervates, is known as a
motor unit (Figure 3).
Figure 3. A motor unit.
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©2009 ADInstruments
Student Handout
Muscle
An action potential in a motor neuron induces an action potential in the muscle
fibers it innervates by releasing the neurotransmitter acetylcholine into the
neuromuscular junction. This muscle action potential causes a brief increase in
the intracellular concentration of calcium ions [Ca2+], and activates the contractile
molecular machinery inside the fiber. This requires the use of intracellular
supplies of adenosine triphosphate (ATP) as the energy source. The result is a
brief contraction called a ‘twitch’.
A whole muscle is controlled by the firing of up to hundreds of motor axons.
These motor nerves control movement in a variety of ways. One way in which the
nervous system controls a muscle is by adjusting the number of motor axons
firing, thus controlling the number of twitching muscle fibers. This process is
called ‘recruitment’.
A second way the nervous system controls a muscle contraction is to vary the
frequency of action potentials in the motor axons. At stimulation intervals greater
than 200 ms, intracellular [Ca2+] is restored to baseline levels between action
potentials and the contraction consists of separate twitches.
At stimulation intervals between 200 and 75 ms, [Ca2+] in the muscle is still above
baseline levels when the next action potential arrives. The muscle fiber therefore
has not completely relaxed and the next contraction is stronger than normal. This
additive effect is called summation.
At even higher stimulation frequencies, the muscle has no time to relax between
successive stimuli. The result is a smooth contraction many times stronger than a
single twitch: a ‘tetanic’ contraction. The muscle is now in a state of ‘tetanus’.
When external nerve stimulation is applied, the volunteer will feel a brief ‘pinch’, a
tingling sensation, with a twitching of the muscle. It may feel similar to the static
discharge you feel when you rub your feet on the carpet and then touch a metal
object. In our exercises, each electrical pulse is very brief (less than a
millisecond). The voltage of these electrical pulses is not high enough to cause
injury or permanent damage. There are no risks associated with these small
currents. Nothing is inserted into the skin, so there is no risk of infection.
In Exercise 1, you will observe muscle responses without recording them. In
Exercises 2 to 4, you will use a transducer to measure forces generated by the
adductor pollicis muscle.
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©2009 ADInstruments
Student Handout
Muscle
In the last exercise, the grip force exerted by the hand is recorded with a grip
force transducer as you investigate the phenomenon of muscle fatigue.
Figure 4. Some muscles of the forearm and hand.
What you will do in the laboratory
You will complete five exercises:
1. Nerve stimulation. You will observe the effects of electrical stimuli on a
student volunteer using the nerves of the forearm.
2. Twitch response and recruitment. You will record and measure the
muscular twitch response to nerve stimulation, and demonstrate recruitment in
the twitch response as the stimulus strength increases.
3. Summation. You will measure the effects of changing the interval between
paired stimulus pulses.
4. Tetanus. You will induce and observe a short tetanic contraction in a student
volunteer.
5. Grip force and fatigue. You will calibrate a hand dynamometer and measure
the decline in maximal force during a sustained contraction.
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©2009 ADInstruments