The document contains 14 questions about skeletal muscle structure and function from a magazine editor. Responses are provided that explain: 1) How skeletal muscles provide movement, heat and posture through muscle contraction triggered by acetylcholine. 2) Excitability is shared by muscles and nerves, and contractility relates to the agonist/antagonist concept. 3) Structures unique to skeletal muscle fibers and those involved in contractility vs excitability.

1. Letters to the Magazine Editor of Anatomy Geographic
1. Explain how skeletal muscles provide movement, heat, and posture. Are all
of these functions unique to muscles? Explain your answer.
Your skeletal muscle makes bones movable by receiving acetylcholine, which is
found directly in the central and peripheral nervous system. It is received from a
motor nerve that triggers an action potential across the muscle fibers. Actin and
myosin are found deep within the muscle fibers, and are responsible in the
muscle contraction process. The chemical reactions within your muscles that
lead to muscle contractions are the reason heat is generated. Remember that
energy conversion will always produce heat.
2. The characteristic of excitability is shared by what other system? Relate
contractility and Extensibility to the concept of agonist and antagonist
discussed in chapter 10.
Excitability is the ability to receive and respond to stimulus, stimulus is usually a
chemical neurotransmitter, hormone, or pH change. Contractility is the ability to
shorten and thicken forcibly, especially unique to the muscular system. So in this
case, excitability is the agonist when comparing it to contractility because it has
more responsive characteristics which include receiving and responding to
stimulus. The characteristic of excitability is also shared with the nervous system.
3. What structures are unique to skeletal muscle fibers? Which of the
structures involved primarily in contractility and which are in involved in
excitability?
There are more than 600 skeletal muscles in the body. Collectively, they
constitute 40%-50% of our body weight. The structures of our cardiac system are
very unique to the skeletal muscle fibers. The skeletal muscle fibers are fibers
bound together by connective tissue that blood vessels and nerves run through.
4. Explain how the structure of the myofilaments is related to their function.
Microfilaments are solid rods made of a protein known as actin. When it is first
produced by the cell, actin appears in a globular form. In microfilaments,
however, which are also often referred to as actin filaments, long polymerized
chains of the molecules are intertwined in a helix, creating a filamentous form of
the protein (F-actin). All of the subunits that compose a microfilament are
connected in such a way that they have the same orientation. Due to this fact,
each microfilament exhibits polarity, the two ends of the filament being distinctly
different. This polarity affects the growth rate of microfilaments, one end typically

2. assembling and disassembling faster than the other. Microfilaments are typically
nucleated at the plasma membrane. Therefore, the edge of a cell generally
contains the highest concentration of microfilaments. When found directly
beneath the plasma membrane, microfilaments are considered part of the cell
cortex, which regulates the shape and movement of the cell's surface.
Consequently, microfilaments play a key role in development of various cell
surface projections. ©3
5. Explain how the sliding filament theory allows for the shortening of a
muscle fiber.
Sliding filament theory is a model used to explain the mechanism by which
muscles contract. The contraction of skeletal muscle, which is what makes
movement possible, occurs in three ways. Concentric muscle contraction
involves the shortening of muscle fibers, as in the lifting phase of a bicep curl,
while eccentric muscle contraction is made possible by the lengthening of muscle
fibers, as in the lowering phase of a bicep curl. Isometric contraction is another
possibility, during which the muscle does not change in length while sustaining a
contraction, as in stopping the weight midway through a bicep curl and holding
the elbow at 90 degrees. Sliding filament theory describes the process that
makes these changes in muscle length, and therefore muscle contraction,
possible. ©4
6. Compare and contrast the role of Ca++ in excitation, contraction, and
relaxation of a muscle cell.
The role of Ca++ in excitation and contraction is the release of it into the
sarcoplasm from the SR where it will bind to troponin molecules in the thin
myofilaments. In relaxation, the SR begins pumping Ca++ back into the sac, and
as Ca++ is removed from troponin molecules, it returns to its original position. ©1
7. People who exercise seriously are sometimes told to work a muscle until
they "feel the burn". In terms of how muscle is able to release energy,
explain what is going on in the muscle early in the exercise and when the
muscle is "burning."
When the muscle cells are deprived of oxygen, the cells produce ATP through
the anaerobic fermentation. Glycolysis occurs in the cytoplasm and does not
need oxygen. In the glycolysis pathway, two ATPs are produced as net products.
Also, anaerobic fermentation occurs. The pyuvate is converted into lactic acid
and regnerate NAD using this process.© 5

3. 8. Describe the anatomical arrangement of a motor unit. Contrast fine and
gross motor units
This neuron is one of several nerve cells that enter a muscle in a
bundle called a motor nerve. One motor neuron and muscle fibers
make a functional unit called a motor unit. Impulse conduction by one
motor unit may stimulate 12 motor neurons to contract at one time,
while another may stimulate 100. ©1 A fine motor unit is one which
helps with small movements such as grabbing something whereas a
gross motor unit helps with big movements such as jumping.
9. Using fiber types, design a muscle for a marathon runner and a different
muscle for a 100-yard-dash Sprinter. Explain your choice.
A muscle for a 100 yard dash sprinter would need to have quicker twitch
contractions because oxygen doesn’t need to reach every part in his body
for the short distance he runs. However, a marathon runner would need
more oxygen in order to reach his whole body for the entire race. ©1
10. Explain the meaning of a "unit of combined cells" as it relates to cardiac
Muscle. How does the structure arrangement affect its function?
The phrase “unit of combined cells,” is referring to a syncytium in the cardiac muscle
which is an electrically coupled mass. This is formed because the cardiac muscle,
unlike the skeletal muscle, forms strong electrically coupled junctions with other
fibers. This function and the branching of other fibers allow the cardiac fibers to form
syncytium which is where the phrase comes from. The structure arrangement of a
band around the heart chamber allows for the muscle to conduct a single impulse
along a sarcolemma that is continuous, which makes the cardiac muscle self-
exciting, easier for impulses. ©1
11. Describe Rigor Mortis.
Rigor Mortis is the stiffening of something after it dies. It lasts approximately 72
hours depending on the temperature in the place the body is located. The muscles
cannot relax, causing the skeletal muscles to contract. The membranes of the
muscles become more permeable to calcium ions, the calcium ions make the actin
and myosin work together. A cool thing about Rigor Mortis is that it can be used to
tell the time of death depending on the occurring factors. ©1

4. 12. Describe in detail the 4 factors that influence the strength of muscle
contractions.
Number of fibers active, increased muscle size, frequency of stimulation, and the
length. ©1
13. What are the phases of a twitch contraction? What molecular events occur
during each of these phases?
The phases of a twitch contraction are the latent period, the contraction phase, and
the relaxation phase, which make a twitch lasting no longer than 1/10th of a second.
During the latent period, stimulated impulses travel through the sarcolemma and T
tubules to the SR, where calcium ions are triggered into the sarcoplasm. Contraction
starts when the calcium binds to troponin and the myofilaments begin to slide.
Finally, relaxation begins when the myofilaments stop sliding. ©1
14. How does the treppe effect relate to the warm-up exercises of athletes?
The treppe effect relates to the warm-ups of athletes because calcium is released
from muscle leading muscles to have stronger muscle contractions to improve
muscle perfomance. ©1