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The Muscular System

Muscles

Muscles make up almost half the weight of the body

there are 650 different muscles in the human body

the size of your muscles is influenced by how much you use them. this is why speed skaters have large leg muscles

Individual elongated muscle cells can be up to 12 inches, or 30 centimetres, in length

at about the age of 40, the number and diameter of muscle fibers begin to decrease, and by age 80, up to 50 percent of the muscle mass may be lost. exercise and good nutrition help to minimize this loss.

muscle is a general term for all contractile tissue.

Functions of Muscular Tissue

Like nervous tissue, muscles are excitable or “irritable”

they can respond to a stimulus

Unlike nerves, however, muscles are also:

Contractible (they can shorten in length)

Extensible (they can extend or stretch)

Elastic (they can return to their original shape)

Types of Muscular Tissue

Muscle	Location	Function	Appearance	Control
Skeletal	skeleton	movement, heat, posture	striated, multi-nucleated (eccentric), fibers parallel	voluntary
Cardiac	heart	pump blood continuously	striated, one central nucleus	involuntary
Visceral (smooth muscle)	G.I. tract, uterus, eye, blood vessels	Peristalsis, blood pressure, pupil size, erects hairs	no striations, one central nucleus	involuntary

The Muscular System

Skeletal muscle is the only organ of the muscular system

Skeletal muscle is composed of skeletal muscle tissue and also contains nervous tissue, blood vessels and connective tissue

Half of the body’s weight is muscle tissue

Skeletal muscle = 40% in males, 32% in females

Cardiac muscle = 10%

Skeletal muscle fibers (cells) are arranged into bundles called fascicles

Fascicles are bound by connective tissue

Connective Tissue Coverings

Deep fascia – Surrounds entire skeletal muscle and extends beyond its length

Surrounds an individual skeletal muscle, separating it from other muscles

Fascia may extend beyond the ends of the muscle to become a tendon

Fascia may connect muscle to muscle and is called an aponeurosis

Epimysium – Closely surrounds skeletal muscle, binds fascicles together

Perimysium – Surrounds each fascicle

Endomysium – Surrounds each muscle fiber (cell)

Characteristics of Skeletal Muscle Tissue

Long, thin contractile fibers that are Striated

Striations due to arrangement of thick and thin filaments

Under voluntary control

Attached to the bones of the skeleton by tendons

Allow for movement, facial expressions, breathing, swallowing, writing, talking and singing, posture, heat production, joint stability

Skeletal Muscle Arrangement

A single muscle cell is a muscle fiber

Fibers are made up of myofibrils

Myofibrils are made up of thick and thin filaments

Sarcolemma – muscle cell membrane

Sarcoplasm – muscle cell cytoplasm

Skeletal Muscle Cells

Myofibrils are striated

Striations due to arrangement of thick and thin filaments

Seen as alternating areas of light and dark bands

The length of each myofibril is divided into repeating units called sarcomeres

A sarcomere is the functional unit of skeletal muscle

Sarcomere Arrangement

Sarcomere Structure

Sarcomere exists from Z-line to Z-line

A-Band is dark middle band- Overlapping think and thin filaments

I-Band – ends of A-Band, thin filaments only

Z-line is in the middle if the I-Band

Myosin filaments are held to the Z-line by titin proteins

Microscopic anatomy of a skeletal muscle fiber

Thick Filament Structure

Composed of many myosin molecules

Each myosin molecule has a tail region and 2 globular heads (crossbridges)

Thin Filament Structure

Composed of actin protein

2 strands of globular actin molecules twisted into a helix

Actin filaments have binding sites for myosin cross bridges

Tropomyosin protein spirals around actin helix

Troponin protein (3 subunits) is attached to actin and holds tropomyosin in place

Call this the troponin-tropomyosin complex

Specialized Organelles of Skeletal Muscle

Sarcoplasmic Reticulum (SR) – a type of ER

Surrounds each myofibril, running parallel to it

Stores calcium, when stimulated, calcium diffuses into sarcoplasm

Transverse Tubules (TT)

Extends into sarcoplasm as invaginations continuous with sarcolemma

T tubules run between cisternae of SR

Filled with extracellular fluid

Cisternae of SR and TT form a triad near where thick and thin filaments overlap

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