Micro Muscle: Muscle Responses II
I. Muscle Response
Previously we have looked at the molecular process that occurs when the muscles receive
a signal to contract, but what happens at the tissue level? Aside from the filaments of the
sarcomeres sliding past each other what other effects does muscle contraction have on the body?
Much like the nervous system, the muscular system operates off of signals that have electrical
properties. For an entire muscle fiber to contract it must receive adequate amount stimuli before
the muscle fiber actually contracts. Once the stimuli reach a set strength, an action potential
immediately sweeps over the entire muscle fiber via the sarcolemma causing the release of
calcium ions into the myofibrils which results in contraction. The minimum stimuli strength to
cause this reaction is called the threshold stimulus. Having a threshold stimulus control the
contraction of the muscle fibers ensures that only specific events cause contraction and not
ambient, random signals from other metabolic processes. This muscle stimulus is generated by
the motor neuron which releases enough acetylcholine to bring the fibers in its motor unit to
the threshold, generating a muscle impulse in each muscle fiber.
1. Recall from the previous lesson: What is a motor unit?
What does acetylcholine do?
2. What is a threshold stimulus?
3. What structure of the muscle fiber carries the muscle impulse throughout the muscle fiber?
Remember from the beginning of the unit that skeletal muscle contractions are referred to as a
twitch. A twitch contains two phases: a contraction phase and a relaxation phase. During the
contraction period, the muscle fiber pulls actively at its attachments. After, comes a period of
relaxation where the pulling force declines. A twitch has a brief delay between the time of
stimulation and the beginning of contraction. This lag time is called the latent period, which in
humans can be less than 2 milliseconds.
Another factor that affects muscle contraction is the resting length of the fiber. In fibers
that are stretched beyond their normal resting length, the force of contraction is decreased due to
the sarcomeres being stretched to the point where myosin in the thick filaments cannot reach the
actin binding sites on the thin filaments. Conversely, if the fibers are at short lengths, the
sarcomeres become compressed and continued shortening cannot occur. Obviously, muscles
work best when they are in a normal operating position <i.e.> not in an awkward position when
trying to perform activities.
4. Give a real life example of when a muscle is stretched too far to perform normal contraction.
5. What are the two phases of a muscle twitch?
II. Muscle Tissue Response.
Muscle fibers that are exposed to a series of stimuli of increasing frequency will
ultimately reach a point where it is unable to fully relax before the next stimulus occurs that
signals another contraction. During this time other muscle fiber twitches combine to sustain the
whole muscle contraction. This sustained contraction of individual twitches combining their
forces to maintain muscle contraction is called summation. When the resulting forceful,
sustained contraction lacks partial relaxation it is called tetanic contraction. An extreme
example of this is lockjaw.
As stated before, muscle contractions are a result of signals from motor neurons to
muscle fibers. The number of muscle fibers that a single motor neuron controls varies greatly.
What can be said about motor units though is that there is a pattern between the numbers of
muscle fibers per motor neuron. The fewer the muscle fibers per motor neuron, the more precise
movements occur. These motor units create movements with high degrees of control. Opposite
of that, the higher the numbers of muscle fibers per motor neuron, the less control of movement
and precision will result. These are larger muscles that need a lot of force to accomplish their
normal functions and do not need high degrees of precision to operate.
1. Why is lockjaw considered an example of tetanic contraction?
2. Name a body part that is controlled by a motor unit with a low number of muscle fibers.
3. Name a body part that is controlled by a motor unit with a high number of muscle fibers.
Since muscle units control a relatively small amount of an entire muscle’s fibers, other
levels of organization are needed to control the whole muscle. Motor units must work together
to achieve whole muscle contraction. Muscles though want to be as efficient with energy use as
possible as to not expend ATP and other energy sources needlessly. That is why not all motor
units within a muscle have the same threshold stimulus. Some motor units can operate on a
relatively low threshold stimulus while other motor units activate at progressively higher and
higher levels of stimuli. If there is a low force of resistance acting against muscle contraction,
then lower numbers of muscle fibers are needed to carry out muscle contraction. Conversely, if
there is a large amount of resistance to muscle contraction, more muscle fibers are needed to
contact the muscle. This addition of multiple muscle units to perform a single contraction is
called recruitment. As the intensity of muscle stimuli increases the recruitment of motor units
continues until all possible motor units are activated in that muscle.
4. What would be an example of a situation where there would be low recruitment when
contracting a muscle?
5. What would be an example of a situation where there would be high recruitment of motor
units when contracting a muscle?
Even though you may not be aware of it, a lot of muscles in your body maintain a certain
number of fibers that undergo sustained contraction. This is called muscle tone. Muscle tone is
the response to stimuli generated by the spinal cord and travels to a few muscle fibers. This is
particularly important in maintaining posture. Muscles in the neck, trunk, and lower limbs create
the upright posture that allows the body to maintain their head upright, stand, or sit. When tone is
not maintained the body collapses, such as when a person loses conciseness.
6. Why is muscle tone important?
III. Fast and Slow Twitch Muscle Fibers
Muscle fibers are not created equal. They can operate at various speeds of contraction.
Broadly speed of contraction can be divided into two groups: Fast twitch and slow twitch. Fast
twitch fibers, also known as white fibers, have less myoglobin (recall from earlier lessons that
myoglobin is an oxygen binding protein found in the sarcoplasm), poorer blood supply. They
can be found in the eye and hand. These fibers also have a more extensive SR which releases
and stores greater amounts of calcium ions and they produce ATP faster than slow twitch
muscles. Because of these factors fast twitch muscle operate quicker than the slow twitch
Slow twitch fibers on the other hand operate as their name suggests, slower than that of
fast twitch muscles. Another name given to slow twitch muscles is red fiber muscles due to their
red appearance because it contains high levels of myoglobin. Unlike fast twitch muscles, these
tissues are more adequately supplied with blood and contain many mitochondria to supply the
fibers with ATP they need to operate. Because they have high numbers of ATP, they can
contract for long periods of time before they become fatigued. These types of muscles are found
in the back and legs.
1. What advantages to fast twitch muscle give to the body?
2. What advantages to slow twitch muscles bestow on the body?
IV. Fatigue
After muscle has been used for a persistent, prolong period of time, it starts to lose its
ability to contract. This is called muscle fatigue. This is caused by various conditions such as
decreased blood flow, chemical imbalance (ion imbalances), and psychological loss of
motivation to continue muscle activity. Most likely though, it is the result of lactic acid
accumulation in the muscles. After oxygen has been depleted in the muscle tissues, ATP begins
to be created in a way that bypasses oxygen altogether (anaerobic respiration). This allows the
muscle to keep being used, but the side effect is that lactic acid is created in the muscle that
prevents muscles from completely responding to stimulus due to the acid lowering the pH of the
muscle tissues. Lactic acid can only be removed through the blood stream where other enzymes
break it down.
Another common occurrence that affects muscles is cramps. A cramp is a sustained,
powerful, involuntary contraction. Cramps usually occur when changes in the extracellular fluid
between the motor neuron and the muscle fiber occur that result in imbalances of electrolyte
concentration. For the most part this imbalance happens when there is a decreased amount of
electrolytes in the area of the cramp. This then in turn triggers an uncontrolled stimulation of the
muscle fiber.
1. What are some of the causes of muscle fatigue?
2. What causes a cramp?
3. What is a popular source of electrolytes for athletes? (Think sports drinks)
Review: Complete questions 14. 15, 16, and 18 on page 299
Within Chapter 9 of the new book you will find descriptions for the following proteins.
Protein Name
Protein Identification Chart