Tuesday, September 22, 2009

The Skeletal Muscle

Objectives
At the end of the 60-minute period, at least 85% of the learners are expected to:
a. describe the structure and characteristics of skeletal muscles through small group discussion,
b. appreciate the role played and the characteristics of skeletal muscles
c. list down the different structures of the skeletal muscles.


Reference: SeeleyR.R., et al Essentials of Anatomy and Physiology 6th Edition, Mc-Graw Hill International.

Topic: Functions of the Muscles
Characteristics of Skeletal Muscles
Structure of Skeletal Muscles

I. Functions of the Skeletal Muscles
a. Body Movement. Contraction of the skeletal muscle is responsible for the overall movement of the body such as walking, running etc.
b. Maintenance of posture. Skeletal muscles constantly maintain tone, which keeps us sitting or standing erect.
c. Respiration. Muscles of the thorax are responsible for movement necessary for breathing.
d. Production of body heat. When skeletal muscles contract, heat is given off as a by-product.
e. Communication. Skeletal muscles are involved in all aspects of communication.
f. Constriction of organs and vessels.
g. Heart beat

II. Characteristics of Skeletal muscles
a. Contractility is the ability of skeletal muscles to shorten with force.
b. Excitability is the capacity of skeletal muscles to respond to stimuli.
c. Extensibility means that skeletal muscles can be stretched.
d. Elasticity is the ability of the skeletal muscles to recoil to their original resting length after they have been stretched.

III. Structure
Each skeletal muscle is surrounded by a connective tissue sheath called EPIMYSIUM or FASCIA. A muscle is composed of numerous visible bundles called MUSCLE FASCICULI which are surrounded by loose connective tissue called the PERIMYSIUM. A fasciculus is composed of several muscle cells or muscle fibers. Each muscle fiber is surrounded by loose connective tissue called ENDOMYSIUM.
The muscle contraction is much easier to understand when we understand the structure of a muscle cell. The cytoplasm of each of the muscle fiber is called the sarcoplasm, contains numerous myofibrils. Each myofibril is a thread like structure that extends from one end of the muscle fiber from one end of the muscle fiber to another. Myofibril contains two major kinds of protein fibers: myosin and actin.
The actin and myosin myofilaments are arranged into highly ordered repeating units along with the myofibril called sarcomere. Sarcomere is the basic structural units of the skeletal muscle. Each sarcomere extends from one Z disc to another Z disc.
Actin filaments or the thin filaments, resemble two-minute strands of pearl twisted together. Troponin molecules are attached at specific intervals along with actin myofilaments and provide calcium binding sites on the actin myofilaments. Tropomyosin filaments are located along the groove between the twisted strands action myofilaments subunits.
Myosin filaments of thick filaments, resembles bundles of minute golf clubs. The part of the myosin molecule that resembles the golf club heads can bind to the exposed attachment sites on the actin myofilaments.
The cell membrane of the muscle fiber is called the sarcolemma. The T-tubles are associated with a highly organized Smooth endoplasmic Reticulum called the Sarcoplasmic reticulum. It has a high concentration of calcium ion which is responsible for the muscle contraction.
The arrangement of the actin and myosin myofilaments in sarcomeres gives the myofibril a banded appearance. The light I band, which consists of only myosin myofilament, spans each Z disk and ends at the myosin myofilaments. A darker, central region in each sarcomere, called an A band extends the length of the myosin myofilaments. In the cenetrr of each filament is a second light zone called the H zone which consists of only myosin myofilament. The myosin myofilaments are anchored in the center of the sarcomere at a dark-staining band, called the M-line.


Skeletal Muscle

Sunday, September 20, 2009