Peter

Skeletal Muscle
Peter Mundy

Introduction
Skeletal muscles are striate voluntary muscles which are attached to the bones of the skeleton throughout the body by tendons. They are used to facilitate movement in vertebrate animals by applying force to bones and joints via contraction caused by electrical stimulation from nerve impulses.

Contents
1. Skeletal Muscle Anatomy.
2. Structure of a cell.
3. Fiber Types.
4. General Characteristics of Type I and Type II Muscle Fibers

Skeletal muscle anatomy
The skeletal muscles are found in defined functional groups held together by layers of connective tissue. The deep fascia, an irregular connective tissue, fills the space between the muscle groups and aids in functioning skeletal muscular contraction. The deep fascia provides passageways for blood vessels and nerves. Each muscle is surrounded by the epimysium, an outer layer of connective tissue, which defines and protects the muscle. Muscle fibers are organized into groups named fascicules. The fascicules contain 10-100 muscle fibers covered in a layer of connective tissue named the perimysium. All muscles fibers are surrounded by a third layer of connective tissue named the endomysium. Every layer of connective tissue is contiguous with tendons, which are connected to bones or other connective muscular tissue.

Structure of a cell
Each muscle fiber covers the entire length of the muscle. The sarcolemma, a plasma membrane, covers each muscle fiber. The muscle fibers are multinucleated, each nuclei is located just under the surface of the sarcolemma. Neuromuscular junctions, also known the synapses, are where muscle fibers are stimulated by motor neurons. (1) Acetylcholine, a neurotransmitter chemical, is released by neurons into the synaptic region to cause fiber contraction.

Skeletal muscle cells have longitudinal bundles called myofibrils (formed up of myofilaments, a contractile filament) in the cytoplasm. (2) These myofilaments do not extend the entire length of the muscle fiber, but are arranged in compartments called sarcomeres. (1) Sarcomeres are contractile units which are joined throughout the full muscle fiber. The thicker band, found in the sarcomere center, is known as the A band which is accompanied by thin filaments on either side. One end of the thin filaments is anchored to the Z line in the I band which separates adjacent sarcomeres and the other end partially overlap the thick filaments. (2)

There are two main types of myofilament, thin filaments and thick filaments. Within the central region of the sarcomere there is no overlap between thin and thick filaments. This is known as the H zone. Within the center of the H zone there is a band named the M line which link the center regions of thick filaments. (2) The M line and thick filaments are linked to the titan protein fibers of the Z line. The space between overlapping thick and thin filaments is bridged by projections called cross bridges from the thick filaments. (2) Within a cross section each thick filament is surrounded by six thin filaments in the projection of a hexogen while each thin filament is surrounded by three thick filaments in a triangular projection. Thick filaments contain myosin, a contractile protein. Thin filaments contain the contractile protein, actin and two other proteins, troponin and tropomyosin. (2)

Fiber Types
Skeletal muscles contain two types of fibers, Type I and Type II, used to produce ATP; the amount of each varies from muscle to muscle, and from person to person. (4)

I. Type I (Slow Oxidative)
A. Fiber characteristics
1. Fatigue resistant
2. High capacity for aerobic energy supply
3. Limited potential for rapid force development
(athletes most likely to have high percentage – endurance athletes)

II. Type IC
A. Fiber characteristics
1. Very few Type IC fibers
2. Are a less oxidative form than Type I

III. Type IIA (Fast Oxidative Glycolytic – FOG)
A. Fiber characteristics
1. Low aerobic power but greater than Type IIB
2. Greater resistance to fatigue than Type IIB
3. Possess good aerobic and anaerobic characteristics
4. Rapid force development
5. High anaerobic power
(athletes most likely to have a high percentage – 400, 800 meter sprinters, middle distance swimmers, hockey)

IV. Type IIB (Fast Glycolytic)
A. Fiber characteristics
1. Possess good anaerobic characteristics
2. Poor aerobic characteristics
3. MAY be a pool of unused fibers, upon recruitment MAY transform to IIA
4. Reduction in Type IIB fibers occur with heavy resistance training
(athletes most likely to have a high percentage – sprinters, weight lifters, field event athletes)

V. Type IIC
A. Fiber characteristics
1. Very few in number in humans (0-5% of the fibers)
2. More oxidative than IIA and IIB

VI. Type IIAB
A. Fiber characteristics
1. A transitional or intermediate fiber type

(6) NSCA, Plyometrics and Speed Development, 2002))
(Ingjer 1969; Staron, Hikida, and Hagerman 1983; Staron et al. 1991; Staron et al. 1994)

General Characteristics of Type I and Type II Muscle Fibers

CHARACTERISTICS---------------TYPE I-------------------------------Type II
Force Production-------------------Low----------------------------------High
Intramusculer ATP Stores-------- Low----------------------------------High
Intramuscular PC Stores----------Low----------------------------------High
Contraction Speed-----------------Slow---------------------------------Fast
Endurance -------------------------High----------------------------------Low
Glycogen Stores-------------------No difference------------------------No difference
Aerobic Enzyme Activity----------High----------------------------------Low
Capillary Density------------------High----------------------------------Low

(5) Fleck, Steven J., Kraemer, William J. Designing Resistance Training Programs, 2ed))

References
(1) Bob Sorensen, Clark Gedney, Mary Gray, Properties of Skeletal Muscle.
(2) Various Biology-online.com sections
(3) University of Colorado, MUSCLE PHYSIOLOGY
(4) Wikipedia, Skeletal muscle
(5) Fleck, Steven J., Kraemer, William J. Designing Resistance Training Programs, 2ed
(6) NSCA, Plyometrics and Speed Development, 2002))
(Ingjer 1969; Staron, Hikida, and Hagerman 1983; Staron et al. 1991; Staron et al. 1994)

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"Experience is a hard teacher because she gives the test first, the lesson afterwards." - Vernon Sanders Law