Content Summary:
There are three types of muscle tissue: skeletal, cardiac and smooth. Skeletal and smooth muscle have striations across the width of the muscle cell which allows for contraction. Skeletal muscle is considered to have voluntary control through nervous stimulation, while cardiac and smooth muscle have involuntary control. Smooth muscle lacks striations, has a single centrally located nuclei, and uses the circular and longitudinal arrangement of muscle fibers to make wavelike contractions. Skeletal and cardiac muscle tell you exactly where they are located, while smooth muscle is found in the digestive tract, blood vessels, bronchioles, urinary system and reproductive system. All three types are thought to contract by the sliding of thin filaments over thick filaments.


mu.jpg



Summary #1
Skeletal muscles are usually attached to bones by tendons. Each muscle has an insertion (the more movable attachment) and and origin ( the less movable attachment). There are 8 different types of muscle actions and each skeletal muscle can perform one or more. The movements are: extend-increase the angle at a joint, flex-decrease the angle at a joint, abduct-move a limb away from the body's midline, adduct-move a limb toward the body's midline, levate- move the insertion upward, depress-move the insertion downward, rotate-rotate a bone along its axis, and sphincter-constrict an opening. The agonist is the prime mover of any skeletal movement. Antagonistic muscles are flexors and extensors that act on the same joint while producing opposite actions.


musclefigure_front.jpg




Summary #2
Skeletal muscles can have different responses to muscle stimulation.
  • Twitch-contraction and relaxation of a muscle from a single electric shock
    • graded-increasing or decreasing the stimulus voltage
  • Summation-an immediate second electric shock produces a second piggy back twitch
  • Tetanus-a smooth and sustained muscle contraction. Also known as complete tetanus
    • incomplete-when stimulation delivers an increasing frequency of automatic shocks, twitches will continue to shorten as the contraction strength increases in amplitude
Contractions may be graded, which indicates the number of muscle fibers stimulated. Stimulation may be done by an electric stimulator which is called in vitro, or it can be done by motor axons which is called in vivo. Recruitment is when muscle contractions are made stronger by "calling in" more and larger motor units.

Types of contractions:
  • Isotonic-muscle contraction strength is constant and causes the muscle to shorten
  • Isometric-muscle tension does not cause shortening


isometric-isotonic.jpg


Summary #3
Cardiac muscle is regulated by autonomic motor neurons. Myocardial (cardiac) cells are short, branched, interconnected, striated and contain actin and myosin filaments within sarcomeres. They are joined to each other by electrical synapses called gap junctions located at the end of each myocardial cell. Intercalated discs within each cell connect the cells together mechanically and electrically. Within the myocardium (a mass of myocardial cells), AP's that originate at any point will spread to all the cells that are joined by gap junctions and they will function as a single unit. Epinephrine increases the ability of the myocardial cells to contract.


Cardiac_muscle_WITH_LABEL_SMALL.jpg


Application:
Every system that we have studied this far is important and has a significant effect on potential patients as every system affects our health in one way or another. As an OTA, I will see patients that may suffer from disorders such as Parkinson's, spastic paralysis, hemiplegia, quadriplegia, paraplegia, cardiac muscle damage and tremors. These all affect the muscular system or have an affect on it. My patients will expect me to be knowledgeable about their conditions so that I can help them in treatment. My goal is to help them lead the lives that they want and to function in ADL to the best of their abilities.

On a personal note, it was good for me to learn about how muscles work, what is needed for them to work properly and what happens when we exercise. Although I do not understand the process fully, it has been a good starting point for me to want to learn more.

Essential Questions:
The sliding filament theory is a theory that describes how actin thin protein filaments) slide over and between myosin (thick protein filaments) to cause contractions. The sliding filament theory of contraction has these steps:
  1. The myofiber shortens by movement of the insertion toward the muscle origin
  2. Shortening is caused by reducing the distance between Z lines within a sarcomere
  3. Sliding of the myofilaments shortens the sarcomere
  4. Asynochronous power strokes of myosin cross bridges slide the myofilaments
  5. The A bands stay the same length but are pulled toward the muscle origin
  6. I bands pull adjacent A bands closer together
  7. H bands are shortened as thin filaments on the sides of sarcomeres slide toward the middle


muscle2.jpg




Cross bridges extend from the myosin toward the actin to produce areas where the sliding of filaments can occur. Myosin proteins have structures that look like arms which extend from the axis of the thick filament and attach to actin on each side of the sarcomere. Each myosin head has an ATP-binding site that associates with an actin-binding site, splits ATP into ADP and P1. The myosin pulls the actin to make "bridges". When the cross bridge is formed, the bound P1 is released, a conformational change in the myosin takes place and the cross bridge forms a power stroke that moves the actin filament.

The role of calcium in muscle contraction is this: Ca2 attaches to troponin and causes movement of the troponin-tropomyosin complex and exposes binding sites on actin. Myosin cross bridges are then able to attach to actin and form the power stroke. If AP continue to be produced, the calcium release channels in the sarcoplasmic reticulum will remain open and Ca2 will continue to diffuse out. Ca2 concentration will remain high, allowing Ca2 to remain attached to troponin, the cross bridge cycle to continue and contraction will be maintained. To allow for muscle relaxation, AP must cease and the calcium release channels would close. Ca2+-ATPase pumps will move Ca2 from the sarcoplams to the sarcoplasmic reticulum. ATP is needed to power these pumps. Muscle relaxation takes place.