Skeletal Muscle Structure

What are the three types of muscular tissue?

Skeletal, cardiac, and smooth.

What common properties do muscular tissues share?

Excitability, contractility, extensibility, and elasticity.

Describe the structure of skeletal muscle fibers.

Skeletal muscle fibers are huge, cigar-shaped, multinucleate cells that are striated due to the arrangement of proteins.

What is the type of control over skeletal muscle?

Skeletal muscles are voluntary muscles, meaning they are subject to conscious control.

Excitability

Ability to send electric action potentials

Contractility

Shorten with force

Extensibility

Stretch or extend

Elasticity

Return to original length

Skeletal muscle

• Huge, cigar-shaped, multinucleate cells (multiple nuclei per cell)

• Largest of the muscle fiber cells “striated muscle” due to obvious stripes (arrangement of proteins)

• Voluntary muscles - as only type subject to conscious control (in addition to reflex action)

• Only respond to nervous system stimuli

• “Skeletal, striated, voluntary”

Endomysium (skeletal muscle fiber structure)

Connective tissue sheath around a muscle fiber

Fascicle (skeletal muscle fiber structure)

A bundle of muscle fibers

Perimysium (skeletal muscle fiber structure)

Coarser fibrous membrane sheath around a bundle of fibers

Epimysium (skeletal muscle fiber structure)

An even ‘tougher‘ overcoat of connective tissue which covers the entire muscle

Tendon (skeletal muscle fiber structure)

Strong cords that bind the muscle via fusing with periosteum

Abduction

Is a movement away from the midline – just as abducting someone is to take them away

Adduction

Movement towards the midline

Extension

Refers to a movement that increases the angle between two body parts

Flexion

Refers to a movement that decreases the angle between two body parts

Lateral rotation

Rotating movement away from the midline

Medial rotation

Rotational movement towards the midline

Muscle functions

• Producing movement

• Maintaining posture

• Generating heat

• Stabilizing joints

Multinucleate

A cell that contains more than one nucleus

Sarcolemma

Plasma membrane that surrounds the muscle cells

Myofibrils

Long ribbon-like organelles that fill the cytoplasm

Sarcomeres

Chains of tiny contractile units lines up end to end making up the myofibril

• a portion of organelle, not a cell

• creates the banding pattern

  • alternating light and dark proteins/bands give a striated appearance

  • I Bands - “light” (no myosin)

  • A Bands - “dark” (both myosin/actin)

  • Z Disc - dark mid-line interruption of the light I Band (think end of sarcomere segment)

  • H Zone - lighter central area on the dark A Band (no actin)

  • M Line - tiny protein rods in the H zone that hold thick filaments together (middle of sarcomere)

Sarcomere structure

• Myosin Filament - “thick filament” made of myosin protein. Contain ATPase which split ATP in order to generate energy for contraction

• Extend length of Dark A Band. Ends studded with projections called myosin heads or cross bridges which link to the thin filament during contraction. Actin Filament - “thin filament” made of actin protein. Anchored to z-disc (disc-like membrane). Light I bands made up of only the thin filaments from two adjacent sarcomeres. Overlap thick filaments but do not connect when muscle is relaxed allowing for a “bare zone”/H-zone which disappears during contraction

Sarcoplasmic reticulum

A specialized smooth endoplasmic reticulum that surrounds each myofibril

• major role is to store and release calcium during contraction stimulation