There are three different kinds of muscles throughout the body, the skeletal, smooth and cardiac. The skeletal muscles are attached to the bones by tendons. The more moveable attachment, the insertion, is pulled toward the less moveable attachment the origin. An agonist muscle if a prime mover of any skeletal movement. The antagonistic muscles are flexors and extensors that act on the same joint to produce opposite actions. The smooth muscles are in the blood vessels and the cardiac muscle of heart. This unit talks about the structure of the different muscles, including the types of contractions, the mechanisms of the contractions and the regulation of contractions. We also learn what type of energy is required to keep are muscles moving. What happens to the muscles when we are exercising, when they become fatigued and about muscle damage and repair.

Types of Skeletal Muscles:
Extensor—increases the angle at a joint
Flexor—decreased the angle at a joint
Abductor—moves the limb away from the body
Adductor—moves the limb toward the body
Levator—moves insertion upward
Depressor—moves insertion downward
Rotator—rotates a bone along its axis
Sphincter—constricts an opening

Structure of Skeletal Muscles:
Within the connective tissue there are tendons which form a sheath around and into the skeletal muscle. This first layer is called epimysium. Then the connective tissue divides the muscle into bundles called fascicles. Perimysium is the connective tissue around the fascicles. There is a plasma membrane called the sarcolemma which surrounds these first layers. The inner most layer of muscle fibers is the endomysium. Skeletal muscles have the same organelles that are present in other cells but they also have multiple nuclei, called multinucleated. These muscles are striated which the other muscle groups do not have.

This film is on skeletal muscle structure. It shows where everything is located as described above. I thought it was helpful. Please take a look at it.
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The Mechanisms of Contractions:
With the skeletal muscles being striated there is a look of dark and light bands going through the muscles. The dark bands are called A bands, and the light bands are called I bands. Within these two bands, A and I, there is a thin dark line which is called the Z line. When we look at the total structure of a muscle fiber we see that each cell is packed with myofibrils, they extend the length of the fiber. The myofibrils contain even smaller structures called myofilaments, which are filled with proteins. Myofilaments can either be thick or thin. The A band is dark with thick filaments containing myosin. The I band is light with thin filaments containing actin. There is also an area where actin and myosin do not overlap; this is called the H band. Then at the center of the I band there is the Z line/disc where the actins attach forming a repeating pattern that serves as the basic subunit of striated muscle contraction. This unit is the sarcomeres. These sarcomeres are units of skeletal muscle consisting of the components between the two Z discs at each end, they are contractible. Muscle contraction takes place because the myofibrils get shorter, how this happens is by the thin filaments sliding over and between the thick filaments. Of course there has to be something anchoring these bands, which would be the M lines, these are proteins which anchor the myosin when it is contracting. There is also an elastic protein called Titin which attaches the myosin to Z discs this then helps with the recoil of a muscle.

This film is on the striations of skeletal muscles, showing where the A band, I band, Z discs and H zones are. It is a simple explaination of how a contraction works. It kind of looks like the picture in our book but has the animation to show how it all works, only 30 secs. I thought it was helpful. Was easier to look at this short video than try to explain it in words. Please take a look at it.
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Contractions of Skeletal Muscles:

Isotonic Contractions: Muscle changes length, it shortens and moves the load, force remains constant. The thin actin filaments are sliding across the myosin.
Isometric Contractions: Tension of the muscle increases but it does not shorten or lengthen. The cross bridges are forming and pulling but the actin filament is not moving.
Muscle Twitch: The response of a muscle to a single brief threshold stimulus. The strength will depend on the number of motor units that are activated.
The phases of a muscle contraction include: The Latent Period—where the muscle tension is starting; Period of Contraction—this is where the muscle fibers shorten; Period of Relaxation—Ca++ re-enters the sarcoplasmic reticulum.
Summation: Is when a second electric shock has been delivered right after the first, producing a second twitch that partially rides piggyback on the first twitch. This second twitch will be stronger than the first. This will only happen if repolarization has not been completed.
Incomplete Tetanus: Is when the increasing frequency of the shocks are happening automatically, the relaxation time between the twitches gets shorter and shorter but the strength of the contraction increases in amplitude. Calcium is increasing in the cytoplasm.
Complete Tetanus: Will happen when there are no visible relaxations between all the twitches. The contractions are smooth and sustained.

Cardiac and Smooth Muscles:

Although it is striated, cardiac muscle differs from skeletal muscle in that it is highly branched with cells connected by overlapping projections of the sarcolemma called intercalated discs. These discs contain desmosomes and gap junctions. In addition, cardiac muscle is auto rhythmic, generating its own action potential, which spreads rapidly throughout muscle tissue by electrical synapses across the gap junctions.

Smooth muscle does not have the striated appearance of skeletal muscle. This is due to its irregular arrangement of actin and myosin filaments. Also, the sarcolemma does not form a system of transverse tubules. As a result, contraction is controlled and relatively slow. These qualities are appropriate for smooth muscle function.

In addition to the thick myosin and thin actin filaments, smooth muscles also possess non-contracting intermediate filaments. The intermediate fibers attach to dense bodies that are scattered through the sarcoplasm and attached to the sarcolemma. During contraction, the movement of myosin and actin is transferred to intermediate fibers, which pull on the dense bodies, which, in turn, pull the muscle cell together. In this way, the dense bodies function similarly to the Z discs in striated muscles.

smooth muscle contracted,
smooth muscle contracted,


This disease is probably more about the nervous system but I feel it also has to do with the muscle system because of the affects it has on the muscles of its victims. Friedreich’s ataxia is a rare, inherited disease that causes nervous system damage which leads to loss of control of muscle movements. Ataxia is defined as trouble coordinating movements. This disease may be considered a neurological disorder but it has a great affect on the muscle system also. The disease is called an autosomal recessive genetic disorder, meaning the child has to get a copy of the defective gene from both parents. It is the most common form of hereditary ataxia affecting about 1 in every 50,000 people in the United States and it does not care about gender. It starts out damaging the nervous system, spinal cord and the nerves that control muscle movements. Patients may have trouble walking, muscle weakness, speech problems, involuntary eye movements, heart palpitations and develop scoliosis.

We have a family here in Watertown that has this. The parents are fine and so is their oldest daughter. The second daughter was diagnosed when she was very young. She passed away in her mid 20s, if I remember right, from heart failure. Their only son was also diagnosed with this disease. He graduated with my daughter and at that time he was still walking. I knew of the daughter because of working in the clinic, so when things started to change for their son I was more curious as to what was happening with him. Over the last few years his condition has deteriorated, he is now in the wheel chair fulltime. When talking with him on the phone it can be hard to understand him. The oldest daughter is married and they have adopted children as to not pass this gene on to others in their family.

We see other muscle disorders in the clinic anything from pulled muscles to restless leg syndrome. My father has essential tremors and we can see how things are changing with him. He doesn’t write checks anymore because he shakes so much. I wonder if I will have tremors also as I continue to get older because I seem to be getting other genetic things from my father, like the high cholesterol but yet I am thankful after knowing this family and their disease that was passed to their children.


Describe the sliding filament theory of muscle contraction and why it is called the sliding filament theory. Describe the action of the cross bridges that cause a power stroke. What is the role of calcium and ATP in muscle contraction and relaxation?

A muscle will shorten because of the movement of the insertion (moveable attachment) toward the origin (less moveable attachment) of the muscle. The distance between the Z lines is reduced because of the shortening of the myofibrils and the sarcomeres. The sliding of the myofilaments will shorten the sarcomeres but the length of each filament stays the same during a contraction. The thin filaments (actin) are pulled over the thick filaments (myosin) by asynchronous power strokes of the myosin cross bridges. The A bands are pulled toward the origin of the muscle but their length remains the same during a contraction. The I bands become shorter as the adjacent A bands are pulled together. The contraction will shorten the H bands as the thin filaments are pulled toward the middle of the sides of the sarcomeres.
Myosin the thick filaments remain central and constant in length while areas of the sarcomere become shorter. The I bands also changed in length, these are rich in actin, the thin filament. The theory states that the sliding of actin past myosin generates muscle tension.

The Cross-Bridge Cycle:
Cross bridges are formed by the heads of myosin molecules that extend toward and interact with actin. ATP is broken down to ADP and Phosphate (P), releasing energy which is stored in the myosin head, from a previous cycle of movement. The sliding filaments are produced by actions of the cross bridges. The myosin head has an ATP binding site and cannot bind with actin until it is cocked. The power stroke happens by exerting force on the actin. As calcium levels rise, the cycles of cross bridges and contractions will occur but will stop as the calcium level deceases. The reason the calcium levels decrease is because of being pumped back into the sarcoplasmic reticulum on a continuous basis.
Walk along” theory of contraction:

Mechanism of Skeletal Muscle Contraction,
Mechanism of Skeletal Muscle Contraction,

Mechanism of Skeletal Muscle Contraction,

All information for this page was found at the following sites: Our textbook, Human Physiology, 12th Edition, Stuart Ira Fox;; National Institute of Neurological Disorders; Miller Keane, Encyclopedia & Dictoinary of Medicine, Nursing and Allied Health, 5th Edition; Anatomy & Physiology for Dummies, Donna Rae Siegried, Wiley Publishing. all pictures and videos are noted along with them in the area of the page.