10.2b Microscopic Anatomy of Skeletal Muscle

Primary cells forming skeletal muscle

Skeletal muscle fibers

Skeletal muscle fibers contain

Cytoplasm

Sarcoplasm

Cytoplasm of a muscle cell

Muscle cell length

10 to 500 micrometers in diameter

10 micrometers to 30 centimeters in length

myoblasts (blastos= germ)

muscle cell with potential of being mucle fiber

Myoblasts fuse and form

• skeletal muscle fibers during development

• muscle cells are multinucleated

Satellite Cells

• the myoblasts that don’t fuse with muscle fibers during development

• remain in adult skeletal muscle tissue

• also called stem cells

If skeletal muscle is injured

• Satellite cells can be stimulated to fuse with damaged skeletal muscle fibers

• assists to a limited extent in repair & regeneration

Sarcolemma (lemma= husk)

plasma membrane of a skeletal muscle fiber

Transverse tubules (T-tubules)

(trans= across)

(versus= to turn)

• tunnels in the sarcolemma

• links outer membrane to the inside

• sends signals for muscle movement

Myofibrils

• bundles of myofilaments within skeletal & cardiac muscle cells

• 80% of skeletal muscle fibers

• long, cylindrical structure

• contain myofilaments & help muscles contract effectively

Sarcoplasmic Reticulum (SR)

(rete= a net)

• endoplasmic reticulum found in muscle cells

• stores calcium ions required for contraction

• net around muscle fibers

• helps muscles contract by releasing calcium when triggered by electrical signals

Calmodulin

senses calcium for muscle contraction

Calsequestrin

• stores calcium in muscles

• helps control calcium levels for muscle movement

Myofiliaments

(filum= thread)

• tiny threads in muscles

  • myosin: thick filament

  • actin, tropomyosin, troponin: thin filament

• help muscles move

Thick Filaments

• 200-500 myosin proteins

• has 2 strands; each has a head and a tail

• head helps muscles move by using energy from ATP & contains a binding site for actin

• tails are intertwined & point towards the center

Thin Filaments

• half the diameter of thick filaments

• composed of actin, tropomyosin, and troponin

Actin

• F-actin: 2 twisted necklaces made of G-actin heads

• beads have spots for myosin heads to attach during muscle contraction (myosin binding site)

  • helps muscles move

Tropomyosin

• thin string-like protein that blocks myosin binding sites on actin in resting muscles

• prevents contraction until calcium binds to troponin

Troponin

• protein attatched to tropomyosin

• contains the binding site for calcium

Sarcomere

(meros= part)

• small parts in muscle fibers with thick & thin filaments

  • help muscle move

• their number depends on muscle fiber length

• composed of overlapping thick and thin filaments

What holds thin filaments in place

Z discs

• looks zigzagged from the side

I band

• contains only thin filaments

• bisected by the Z disc

• looks light under microscope

• when muscles fully shorten, thin filaments slide along thick filaments - making I bands disapear

A band

• central region of sarcomere

• contains entire thick filament

• thin filaments partially overlap the thick filament on each end of an A band

• appears dark when viewed under microscope

• does not change in length under muscle contraction

H zone (H band)

• most central portion of the A band

• does not have thin filament overlap

• only thick filaments are present

• during maximum muscle shortening, this zone disappears when thin filaments are pulled past thick filaments

M line

• thin transverse protein structure in the center of the H zone

• keeps thick filaments in place during muscle movements

Striations

• stripes seen under a microscope in skeletal muscle

• caused by arrangement of thick and thin filaments

Anistropic

area appars dark when viewed under polarized light

Isotropic

area appears light when viewed under polarized light

Connectin

• protein in muscle fibers

• keeps muscle parts in place & aids in muscle elasticity by creating tension during contraction

Dystrophin

• helps muscles work by linking parts inside muscle cells to the outer structure

• issues with dystrophin can cause muscular dystrophy- affecting muscle strength

Nebulin

• protein in muscles that helps thin filaments be the right length

• aids in muscle contraction by binding to actin

Glycogen

immediate fuel molecule

Myoglobin

• binds oxygen when muscle is at rest

• releasees it for use during muscle contraction

Creatine Phosphate

provides muscle fibers with fast ATP