Muscular System

 Muscles

• “machines” of the body
• Muscle make up nearly half the body’s mass. The essential function of muscle is contraction or shortening. This unique characteristic sets muscle apart from other tissues in the body. All body movements depend on the muscles. Thus, muscles can be viewed as the “machines” of the body.

Functions of the muscles

1. Produces movement. All movements of the human body are result of muscular contraction.
2. Maintaining posture. The skeletal muscles in the body maintain posture.
3. Stabilizing joints. Presence of muscle tendons reinforces and stabilizes joints that have poorly fitting articulating surfaces.
4. Generating heat. Heat is a by-product of muscle activity. This heat is essential in maintaining normal body temperature.

Types of muscles

Skeletal muscles

• Also called: voluntary muscle, striated muscle
• This type of muscle attaches to the body’s skeleton. Because of their attachment to the bony part of the body smoother contours of the body are formed. Skeletal muscle fibers are cigar-shaped, multi-nucleate cells and are the largest of the muscle fiber types. This is the only muscle type that can be controlled consciously, thus it is a voluntary muscle. Since its fibers appear to be striped it is known as striated muscle.

Summary of the characteristic of Skeletal Muscles:

1. Subject to conscious control.
2. Multinucleated.
3. Cigar-shaped.
4. Attaches to the body’s skeleton
5. Soft and fragile.
6. Its tissue can rapidly contract and with great force.
7. Tires easily.
8. Contraction is abrupt and rapid.

What is the reason why skeletal muscles are not ripped apart as they exert effort?

• Skeletal muscles do not rip apart after exerting effort because thousands of skeletal muscle fibers are bundled together by connective tissues which are responsible for providing strength and support to the muscle as a whole.

Structure or Parts of Skeletal Muscle

1. Endomysium – a delicate connective tissue sheath that encloses each skeletal muscle fiber.
2. Perimysium – a coarse fibrous membrane that wraps the sheathed muscle fibers.
3. Fascicle – bundle of fibers formed from group of sheathed muscle fibers wrapped by perimysium.
4. Epimysium – a tough overcoat of connective tissue that bounds together fascicles. This is the connective tissue that covers the entire muscle.
5. Tendons – these are cordlike structures that are formed from epimysia. These are composed of mostly collagenic fibers that can cross rough bony projections. Aside from anchoring muscles, tendons are very important in providing durability.
6. Aponeuroses – these are sheet like structures that attaches muscles indirectly to bones, cartilages or connective tissue coverings of each other.

Smooth Muscles

• Also called: visceral muscles, non-striated muscles, involuntary muscles
• Smooth muscles, unlike skeletal muscles, have no striations. It is controlled involuntarily, meaning to say individuals cannot consciously regulate it. If skeletal muscles are found in the bones, smooth muscles are found on the walls of hollow visceral organs such as the stomach, urinary bladder and respiratory passages. The main function of smooth muscles is to propel substances along a definite tract or pathway within the body. These muscles have only one nucleus and are spindle-shaped.

Summary of the characteristics of smooth muscles

1. Involuntary control.
2. Found on the walls of the hollow visceral organs.
3. Have no striations.
4. Propels substances along a fixed tract inside the body.
5. Spindle-shaped.
6. Have a single nucleus.
7. Arranged in layers or sheets. Most often, it is arranged in two layers, one circular the other longitudinal.
8. Alternately contracts and relaxes to change the shape and size of an organ.
9. Contraction of these muscles is slow and sustained.

Cardiac Muscle

• Unlike the other two types of muscles mentioned above, cardiac muscle is only found in one place in the body – the heart. The function of the heart as the pump – propelling blood into the blood vessels and to all tissues of the body, is carried out because of the presence of cardiac muscle. This muscle has similarities to skeletal muscles as it is striated and smooth muscles as it is involuntary and cannot be controlled consciously. Cardiac muscle fibers are branching cells joined together by special junctions called intercalated discs.

Summary of the characteristics of cardiac muscle

1. Only found in the heart.
2. Striated.
3. Involuntary control.
4. Cushioned by small amounts of soft connective tissue.
5. Arranged in spiral or figure 8-shaped bundles.
6. Contracts at a steady rate set by the heart’s pacemaker.

Types of Muscles in the Body
Characteristic	Skeletal	Smooth	Cardiac
Location in the body	Attached to bones	Located in the walls of the hollow visceral organs with the exception of the heart	Walls of the heart
Muscle fiber shape and appearance	Single, cigar shaped, very long, cylindrical, striated, multinucleated cells	Singe, no striations, fusiform, cells have single nucleus	Branching cell chains, uninucleate, with striations, intercalated discs
Regulation of contraction	Voluntary	Involuntary	Involuntary
Speed of contraction	Slow to fast	Very slow	Slow
Rhythmic contraction	No	Yes, in some	Yes