Can a muscle fiber (cell) contract “halfway” or incompletely?
No, muscle cells can’t contract halfway or do a lazy contraction
(unlike people, they don’t have the choice to be lazy!)
B.What is the name of this concept?
This is known as the ALL OR NONE PRINCIPLE.
c.What does this mean about the way sarcomeres act during a
This means that when actin slides in the sarcomere, it slides as
far as it can. The sarcomeres always shorten to their fullest
possible extent, which is what creates the strongest possible
contraction of the cell.
Muscles are made up of many motor units. What is a motor unit?
A motor unit is a group of muscle cells (fibers) that are under
the control of one nerve. (The nerve controls the contraction of
the muscle cells). For example, in the leg a typical motor unit
is made of 1,000 muscle cells all controlled by one nerve. The
motor unit is similar to a unit/group of soldiers that are led by
one commander. Instead of a group of soldiers, the motor unit is
a group of cells that contract together. The “leader” of the
unit is the nerve.
If the electrical voltage (stimulus/shock) applied to a muscle is
gradually increased, what happens to the number of motor units which
contract? What does this mean happens to the muscle contraction?
The number of motor units (groups of cells) contracting increases
as the voltage (electrical current) is increased. This causes
the muscle contraction to get stronger.
a. If a subthreshold stimulus (shock) is used on a muscle,
how many motor units in the muscle will contract?
b. If a threshold stimulus is used on a muscle, how many motor units
in the muscle will contract?
c. If a maximal stimulus is used, what happens?
Motor Unit Summation
a. A subthreshold stimulus will not cause any motor units to
contract because it is too weak to cause cells to contract.
b. Threshold stimulus: Only one motor unit (group of cells) will
contract out of the many motor units in the muscle.
c. Maximal stimulus: Causes ALL motor units in the muscle to
contract, producing the strongest contraction possible.
. A maximal stimulus is one that is strong enough to cause ________
motor units in a muscle to contract.
A maximal stimulus is one that is strong enough to cause ALL
motor units in a muscle to contract.
b. Motor Unit Summation means that as the stimulus (electrical voltage)
is _______________, the ______________ of a muscle
b. Motor Unit Summation means that as the stimulus (electrical
voltage) is increased, the strength of a muscle contraction
c. Motor Unit Summation explains how one muscle can have
contractions of varying or different _________________--like a
pat, slap, or punch done by the same muscle.
Motor Unit Summation explains how one muscle can have
contractions of varying or different strengths.
Motor Unit Summation is also called ______________.
Recruitment [Because as you increase the electrical current, you
are recruiting (getting) more and more cells to contract.]
Sometimes, after a period of rest, when a muscle first contracts, not
all of its calcium is released from the sarcoplasmic reticuli.
The Treppe Effect/ Staircase Phenomenon
In this graph the stimulus (voltage) is NOT being increased
but remains constant.
page 17 graph
. Since only some of the calcium is released, will the muscle contract
fully the first few times it contracts?
No, the contraction will not be as strong as it should be if all
of the calcium is not released by the SR.
What is this concept known as?
The Treppe Effect.
What do these contractions look like? Because of this, what is this
concept also called?
The Treppe Effect is a series of contractions that gradually get
stronger. The contractions look like steps going up because they
increase in size, so this concept is also called the Staircase
. What kind of stimuli (shocks) are used to produce this and how many
are given per second?
The shocks should be maximal (normally they would cause a full
muscle contraction) and be given at low frequency, or 10-20
shocks per second.
Wave Summation means that several individual muscle contractions
can be ___________ together so that they create a contraction that
is _________________ than a single contraction
Wave Summation, Incomplete and Complete Tetanus
Wave Summation means that several individual muscle contractions
can be added (summated) together so that they create a
contraction that is stronger than a single contraction.
How is Wave Summation created in a muscle?
Wave Summation is created when electrical shocks are applied to
to the muscle while it is already contracting
(mid-contraction), using 20-40 stimuli (shocks) per second.
When many contractions are summated, this leads to a type of Wave
Summation called tetanus.
In Complete Tetanus many contractions fuse together, producing one
long, powerful contraction
How is Complete Tetanus created in a muscle?
a. The muscle is not allowed to relax between stimuli (shocks) by
applying the stimuli at a high frequency—using 40-50 stimuli
What specifically happens in muscle cells that causes Complete
Calcium cannot be returned to the sarcoplasmic reticulum between
shocks because they occur too rapidly. Calcium has to be returned
to the SR to end any contraction. When calcium remains in the
sarcomeres this creates a single, long, strong contraction—
Why is Complete Tetanus NOT a regular event in the body?
Complete tetanus occurs when the muscle is stimulated at 40-50
shocks/sec., but the typical firing of nerves in the body is
around 25 stim/second. *(However, incomplete tetanus, where the
contractions do not all fuse together, does happen regularly in
the body. In incomplete tetanus the muscle does relax slightly
between contractions—which looks like a long, quivering
contraction). See in above diagram.
How fast does a nerve have to fire action potentials onto a muscle to
cause the Treppe Effect?
. (Treppe Effect)
a. At 10-20 shocks per second—---low frequency.
Overall, what causes the Treppe Effect to occur? (What is the main
thing that causes the contractions to get stronger?)
The main cause is a gradual release of calcium by the SR. It
takes several contractions for all of the calcium to be released.
WHY does the amount of calcium released by the sarcoplasmic reticuli
affect the strength of muscle contractions?
The amount of calcium determines the number of active sites that
will be uncovered and thus, the number of myosin heads that can
attach to actin. The more myosin heads that attach, the stronger
Muscle Tone can be described as a ________________ contraction
because only a _____________ number of motor units are
contracting at any one time.
Muscle Tone can be described as a partial contraction
because only a limited number of motor units are contracting at
any one time.
What term describes the type of contraction that Muscle Tone is?
Can the Sliding Filament Theory be used to explain Muscle Tone?
No, because actin doesn’t slide in a Muscle Tone contraction.
What is happening in Muscle Tone?
There is an increase in tension (the ‘tone’) in the muscle
because myosin heads attach to actin. (Myosin heads do NOT swing
and pull on actin however—so actin doesn’t slide).
e. Can a Muscle Tone contraction be used to move your arms and legs?
e. No, Muscle Tone contractions can’t move arms or legs.
f. What two things are Muscle Tone contractions used for?
f. Muscle Tone contractions: 1) Keep postural muscles taunt/tight
to keep head upright, torso and calf muscles tight
2) Are use to hold limbs in position (stabilize them) after they
have already been moved
a. There is an energy-production system used in skeletal muscle known as
the Phosphagen System. What is creatine phosphate?
a. Creatine phosphate (CP) is an energy source that is stored in
skeletal muscle to prevent muscle from totally running out of
ATP. Each CP is a “backup energy supply” that is rapidly turned
into 1 ATP when ATP levels are low.
b. At what point during physical exercise is creatine phosphate used?
b. At the very beginning of intense, short-term exercise. It
provides energy for the first 15 seconds of exercise.
c. Why is creatine phosphate used at this point in physical exercise?
c. It quickly provides energy for contraction while the slower
processes that create energy from food (aerobic and anaerobic
metabolism) are not yet producing ATP.
a. What is Fatigue?
a. The deterioration (decline) of muscle contraction, leading to the
total loss of the muscle’s ability to contract.
b. What are the main causes of Fatigue?
b. Insufficient energy (ATP) production, often due to lack of oxygen
and lack of sugar in the muscle.
c. What specifically happens in the muscle when Fatigue occurs?
c. Without ATP, the sarcoplasmic reticulum is unable to store
calcium. Calcium leaks out of the SR, causing myosin to attach
to actin, producing tetanus. Eventually, collapse of the
contraction occurs. The tetanus that occurs with fatigue is seen
in rigor mortis, when muscles stiffen a few hours after death.
d. How are cramps or contractures related to Fatigue?
d. Contractures are a type of cramping in muscle that is the start
of fatigue, a temporary state of tetanus that often occurs in the
hands (as with Writer’s Cramp).
a. How much ATP is created from 1 glucose by this method?
Aerobic metabolism creates 36 ATP from each molecule of glucose.
*Other foods besides sugar, like protein and fat, are also turned
into ATP using the aerobic process.
60. There are two methods the body uses to make energy (ATP) from
food sources. The method called aerobic metabolism creates the
most energy from food of the two methods.
b. What makes this process so good at producing energy?
b. The use of oxygen allows the greatest amount of energy to be
made from foods. The word “aerobic” means that oxygen is used.
c. What type of skeletal muscle cells are best at producing energy, and
thus, use this method?
c. Slow twitch or slow oxidative cells
d. What does this mean about how long these cells can contract before
fatiguing (tiring out)?
d. Able to contract for long periods of time (fatigue-resistant).
e. What special molecule do these cells contain and how does it help in
the energy-making process?
e. Slow twitch cells contain myoglobin, a molecule that allows them
to store oxygen. It is also is responsible for the dark color of
the cells (which we call “dark meat” in chicken or turkey).
a. What is the alternate method of producing energy?
a. Anaerobic metabolism (or fermentation).
b. Why is it less efficient at making energy?
b. It is less efficient at making energy from food because it
does not use oxygen.
c. What are the major drawbacks to a muscle cell using this method?
c. Only a limited amount of energy is produced, 2 ATP per glucose.
This only allows the cell to contract for brief periods of time
before tiring out. Anaerobic metabolism also produces lactic
acid which contributes to the poor ability to make ATP and
creates a burning, painful sensation in muscle.
d. What one advantage does it have over aerobic metabolism?
d. The process of producing ATP by anaerobic respiration is 2 ½
times faster than producing it by aerobic respiration. Thus, it
is a process that provides energy for muscle contraction during
the first minute of any type of exercise. It also is the process
used during on and off bursts of exercise like tennis, soccer, or
a short swim.
e. What type of skeletal muscle cells use this method?
e. Fast twitch or fast glycolytic.
f. Even though these cells are not designed for endurance, what are
they designed for?
f. Quick, powerful contractions that give speed in a short activity
like sprinting and weight lifting.
g. In some people endurance training can cause fast twitch to turn into
intermediate fibers. What does this mean happens to these fibers?
g. When fast twitch turn into intermediate fibers, they have the
endurance of slow twitch fibers. This means the intermediate
fiber is a type of fast twitch that is not only good for speed
but also for endurance (doesn’t tire out quickly).