By: Brittney Bennett

Overview of the Skeletal Muscle

Skeletal Muscle BasicsThe skeletal muscle is a type of striated muscle and is usually attached to the skeleton. The function of skeletal muscles is to control all of the body locomotion. All muscles maintain posture, stabalize joints and generate heat for the human body. Each skeletal muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers and connective tissue. There are three connective tissue sheaths:
endomysium- a fine sheath composed of reticular fibers surrounding each muscle fiber
perimysium- fibrous connective that surrounds groups of muscle fibers called fascicles
epimysium- overcoat of dense regular connective tissue that surrounds the entire muscle

This diagram shows what the skeletal muscle is composed of.

external image Skeletal_Muscle_Fibers.gifPicture courtesy of http://www.medicalook.com/systems_images/Skeletal_Muscle_Fibers.gif


Each skeletal muscle has bundles of different layers that it is made of. The first is the muscle itself also known as the organ. Then there is the portion of the muscle called the fasicle. The next layer is the muscle fiber or the cell. Following the cell is the organelle of bundles of myofilaments called the myofibril. Next is the sarcomere which is a segment of a myofibril. The last bundle of a skeletal muscle is the myofilament which is an extended macromolecular strucutre. In detail, each myofibril contains actin and myosin filaments. Each myofilament conatinsthick and thin filaments. The thick extend the entire length of an A band while the thin extend across the length of the I band and half way into the A band. TheZ-disc is a sheet of proteins that anchors the thin filaments and connects the myofibrils to one another.

This diagram shows the organization of a typical muscle from a gross anatomical view.
external image sketchmuscle1.gifPicture courtesy of http://www.uoguelph.ca/zoology/devobio/miller/sketchmuscle1.gif

Skeletal Muscle Tissue

  • packaged in skeletal muscles that attach to and cover the bony skeleton
  • has obvious stripes called striations
  • controlled voluntarily (by conscious control)
  • contracts rapidly but tires easily
  • is responsible for overall body motility
  • extremely adaptable
  • can exert forces ranging from a fraction of an ounce to over 70 pounds
This is a picture of human skeletal muscle tissue.
external image 800px-Human_skeletal_muscle_tissue_3_-_TEM.jpgPicture courtesy of http://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Human_skeletal_muscle_tissue_3_-_TEM.jpg/800px-Human_skeletal_muscle_tissue_3_-_TEM.jpg
Muscle Cells
Muscle cells are also called fibers. They have a cyndrical and elognated shape and are multinucleated. Each muscle has a nuclei that is located in the peripheral aspect of the cell which is located under the plasma membrane. This vacates the central part of the muscle fiber for myofibrils.

Skeletal Muscle Nerve and Blood Supply

Each skeltal muscle is served by one nerve, an artery and one or more veins. Each fiber is supplied with a nerve ending that controls contraction. Skeletal muscles contract voluntary by somatic nerve stimulation. However, these type of muscles have the ability to contract involuntarily through reflexes. Contracting fibers require continuous delivery of oxygen and nutrients by the arteries. Wastes must be removed by the veins.

Skeletal Muscle Attachments

Most skeletal muscles span joints and are attached to the bone in at least two places. When muscles contract the movable bone the muscle's insertion moves toward the immovable bone which is the muscle's origin. The origin of skeletal muscles is an attachement to a bone closer to the centre of the body's axis such as the scapula. The insertion of the muscle is attached across the joint to a bone that is farther fromt he body's axis such as the humerous. Muscles attach:
directly- epimysium of muscle is fused to the periosteum of a bone
indirectly-connective tissue wrappings extend beyond muscle as a tendon or aponeurosis

Types of Skeletal Muscle Fibers

The type of protein contained in myosin is one way to categorize a type of skeletal muscle fiber. Myosin is one of the most important proteins that takes responsiblitly for the ability of a muscle to contract. Using this type of classification we can distinguish the two major types of fibers for skeletal muscles which are Type I and Type II.Type I (slow twitch) skeletal muscle fibers are reddish, good for endurance such as long distance running and are slow to tire due to their use of oxidative metabolism. Type II fibers ( fast twitch) appear to be whittish and are used for short bursts of power and speed. This type of skeletal muscle fiber uses both anaerobic metabolism and oxidative metabolism. Type II fibers are quicker to fatigue and therefore is used for sports such as gymnastics, sprinters and weightlifters. In Russia today many parents test their children for their type of muscle fiber in order to determine what sports they would be naturally good at!

This graph shows that Kenyans dominate in marathons. The reason for this is because Kenyans have primarily Type I muscle fibers.
external image runninggraph.jpgPicture courtesy of http://images.google.com/imgres?imgurl=http://www.bio.davidson.edu/courses/genomics/2004/Wilson/picrender.jpg&imgrefurl=http://www.bio.davidson.edu/courses/genomics/2004/Wilson/endurancegene.html&h=460&w=446&sz=50&hl=en&start=1&tbnid=CFij-G8UoSNklM:&tbnh=128&tbnw=124&prev=/images%3Fq%3DType%2BI%2Bmuscle%2Bfiber%26gbv%3D2%26hl%3Den%26safe%3Dactive


Microscopic Anatomy of a Skeletal Muscle Fiber

  • each fiber is a long, cundrical cell with multiple nuclei just beneath the sarcolemma
  • fibers are 10 to 1,000 micrometers in diameter and up to hundreds of centimeters long
  • each cell os a syncytium produced by fusion of embryonic cells
  • sarcoplasm has numerous glycosomes and a unique oxygen-building protein called myoglobin
  • fibers contain usual organelles, myofibrils, a sarcoplasmic reticulum and T-tubules

Skeletal Muscle Contraction

In order to contract a skeletal muscle must:
  • be stimulated by a nerve ending
  • propagate an electrical current, or action potential, along its sarcolemma
  • have a rise in intracellular CA ion levels which is the final trigger for a contraction
Linking the electrical signal to the contraction is called Excitation Contraction Coupling.
These three diagrams show the process of a skeletal muscle contraction.
external image pretox-moa1.jpg

external image muscle2.JPG

external image me4a.gifPicture courtesy of http://images.google.com/images?gbv=2&hl=en&safe=active&q=Cross+Bridge+Cycling+of+the+skeletal+muscle

Nerve Stimulus of a Skeletal Muscle

Skeletal muscles are stimulated by motor neuronsof the somatic nervous system. Axons of these neurons travel in nerves to the muscle cells. The axons of motor neurons branch profusely as they enter the muscles. Each axonal branch forms a neuromuscular junction with a single muscle fiber.

Contraction of Skeletal Muscle Fibers

Contraction refers to the activation of a myosin's cross bridges. Shortening occurs when tensin generated by the cross bridge exceeds the forces opposing the shortening. Contraction ends when the cross bridges become inactive and the tension generated declines and relaxation is induced.

Contraction of the Skeletal Muscle on the Organ Level

The contraction of muscle fibers is similar. The two types of muscle contractions are:
isometric contraction- increasing the muscle tension (muscle does not shorten or lengthen)
isotonic contraction- decreasing the muscle length (muscle shortens)

Graded Muscle Responses

Graded muscle responses are the variations in degrees of a muscle contraction and are required for proper control of skeletal movement. Muscle responses are determined by the changing in frequency of stimulation and the changing strength of the stimulus. There are different types of muscle responses according to varying stimuli. A single stimulus results in a single contractile called a muscle twitch. Since a muscle does not have time to completely relax, the frequently delivered stimuli increases the contractile force which is referred to as a wave summation. When stimuli are more rapidly delievered it results in incomplete tetanus. Then, if stimuli are given quickly enough the result is complete tetanus. Another part of muscle response is the stimulation strength. One type of strength is the threshold stimuluswhich is stimulus strength when the first observable muscle contraction occurs. Beyond a threshold stimulus the muscle contracts more vigorously as the stimulus strength increases. The force of a contraction is controlled precisely by the multiple motor unit summation of the motor unit. This is called recruitment which brings more and more muscle fibers into play causing more muscle tension.