Microscopic Anatomy and Organization of Skeletal Muscle

Muscle refers to….

A tissue, an organ, a fiber

Muscle Tissue

skeletal, cardiac, or smooth

Muscle organ

bicep brachii

A muscle fiber 

a single cell of muscle tissue 

Epimysium

Dense CT covering the entire muscle 

Perimysium

CT surrounding each muscle fascicle

Endomysium 

Surrounds each muscle fiber, sub diving the fascicles

Functions of these connective tissue 

provide strength, insulation and connection to tendons. Transmits contraction force to bone. 

Connective tissue layer are

continuous with tendons

What composes muscle fiber 

many myofibrils 

Myofibrils

• Cylindrical bundles inside each muscle fiber.

• Contain sarcomeres arranged end-to-end → responsible for striations and contraction.

Sarcolemma

• The muscle cell membrane; conducts electrical impulses from the neuron into the muscle cell.

• Important for initiating contraction through depolarization.

Sarcomere

• Smallest component capable of contraction

• Shortens during contraction

sarcomere

What proteins make up a sarcomere

• actin, tropomyosin and troponin (also known as microfilaments)

• Myosin; motor protein

Z disc 

• Define sarcomere boundaries and anchor actin filaments 

H zone 

Only myosin; disappears when fully contracted

I band

Thin actin filaments; shortens during contraction

A - band

thick myosin filament; stays same length

M line

Actin (thin filament)

Contains binding sites for myosin; moves during contraction.

Myosin (thick filament)

Motor protein with heads that attach to actin and perform the power stroke.

Hierarchy (small → large) of muscle 

Myofibril — muscle fiber — fascicle — whole muscle

Tropomyosin

when resting blocks the myosin binding site on the actin protein

Troponin

reacts to Calcium and moves tropomyosin

Activation of tropomyosin example

1. Nerve impulse → ACh released → binds to receptors on motor end plate.

1. Action potential spreads through sarcolemma & T-tubules.

2. Ca²⁺ released from SR via RyR channels.

3. Ca²⁺ binds to troponin, moving tropomyosin off actin’s binding sites.

4. Cross-bridge cycle:

   • Myosin binds actin → power stroke (actin pulled toward M-line).

   • ADP + Pi released → new ATP binds → myosin detaches and resets.

Cycle repeats as long as Ca²⁺ and ATP are available

Relaxation of tropomyosin

Ca²⁺ pumped back into SR; tropomyosin re-blocks actin sites.

T tubules 

• Invaginations of sarcolemma that carry the action potential deep into the fiber.

• Work closely with sarcoplasmic reticulum (SR).

Sarcoplasmic Reticulum (SR):

• Modified smooth ER that stores and releases Ca²⁺ for contraction.

• Terminal cisterns form part of a triad with a T-tubule.