SKELETAL MUSCLE TISSUE

• Skeletal Muscle - Skeletal muscles are all the muscles of the body that are attached to the bones of the body or “the skeleton”. Contraction of these muscles permit movements of the extremities, the eyeballs, the tongue, the chest cavity, and much more. These contractions are consciously controlled or in other

words, the contraction of skeletal muscle is voluntary. Forces exerted by skeletal muscle are tremendous. For example, uncontrolled contractions (spasming) of skeletal muscle during seizure activity can snap the diaphysis of a long bones, and the human calf muscle can, on its own, withstand forces of over a ton.

• Terminology - A specialized terminology is used to describe muscles or muscle tissue. The cell membrane is called the sarcolemma, the cytoplasm is called the sarcoplasm, the endoplasmic reticulum is

sarcolemma

• called the sarcoplasmic reticulum, and the muscle cell itself is called a muscle fiber. This terminology is specific to muscles and is derived from the Greek root “sarkos” which means “flesh”.

Muscle fiber

Sarcoplasmic reticulum

• Structure - Skeletal muscles can be described as “bundles within bundles within bundles”. Each individual skeletal muscle fiber consists of bundles of myofibrils and is encased in a connective tissue called the endomysium.

• Groups of skeletal muscle fibers are in turn gathered into bundles which are wrapped in more connective tissue called the perimysium. These bundles are in turn grouped together and enclosed in a layer of dense connective tissue called the epimysium which forms individual muscles.

• Each individual muscle is wrapped in a tough layer of fibrous connective tissue called the fascia. These are continuous with tendons and the periosteum of bones.

• Muscle Fibers - (myocytes or muscle cells) - Muscle fibers are the basic unit of the muscle itself. It is the smallest functional unit or cell that a muscle can be divided into. Muscle fibers are made up of smaller sub-units called myofibrils.

• These are in turn composed of threadlike myofilaments which are composed of the proteins actin and myosin. Actin filaments are thin and appear as light bands under the microscope, while myosin filaments are thick and appear dark.

• These have an overlapping structure and are held together by chemical cross bridges. The overlapping structure gives skeletal muscle a banded or striped look under the microscope.

***(notice also the multiple peripheral nuclei seen in this picture)

• These alternating light and dark bands are called striations and give skeletal muscle its alternate name, and more accurate name, voluntary, striated muscle.

• A closer look at the striations reveals that the myofibrils are composed of repeating units of actin and myosin called sarcomeres. A sarcomere represents the smallest individual contractile unit of a muscle fiber. It is measured from the point where actin myofilaments overlap to the next actin overlap. The actin overlap is called the “Z” line, so a complete sarcomere extends from Z line to Z line.

Part of a myofibril

• The alternating dark bands seen in a typical light microscope picture are due to the thicker areas of myosin overlap within the sarcomere.

General Characteristics

1. Location – usually attached to

bone. ***(Note that the periosteum is continuous with

the tendon which is continuous with the fascia)

2. Cell Type or description – long, cylindrical, parallel fibers arranged as bundles within bundles.

(1-40mm long)**(Note the construction – bundles of actin &

myosin form myofilaments; bundles of

myofilaments form myofibrils; bundles of

myofibrils form muscle fibers; bundles of

muscle fiber bundles form a muscle.)

3. Myofibril

arrangement-- Densely fill cell from

end to end

- Orderly; with dark

striations visible.

4. Location of nucleus

or nuclei –

Multiple, peripherally located nuclei

**(Note the strong presence of striations)

5. Vascular Supply

and relative rank

- Good blood supply,variable with activity

- Ranks 2nd among muscle tissues

6. Description of

contraction and control factor –

- Vigorous, of relatively

short duration, fibers

contract independently

- Voluntary contraction

7. Alternative

names –

Voluntary, striated

Microscopic Views of Skeletal Muscle (Schematic)

Longitudinal View (long axis)

Note the parallel fibers and dark striations.

Note the parallel fibers, dark striations, and the multiple, peripheral nuclei.

Another Longitudinal View

Cross-sectional View (short axis)

Note the peripherally located nuclei and the “bundled” look.

Perimysium

endomysium

Another Cross-sectional view

Note the peripherally located nuclei and the cylindrical (circular ends) of the bundles.

Some slides include both views in one!!

Area of cross-section

Area of longitudinal section

Important Questions:

1. How can skeletal muscle be identified by longitudinal view?

2. How can skeletal muscle be identified by cross-sectional view?

Answer #1 - In the longitudinal

view there will be parallel fibers and multiple peripheral nuclei.

Answer #2 –In the cross-sectional

view there will be

peripheral nuclei and

the appearance of

“cylindrical bundles”.

Additional Pictures of Skeletal Muscle