Sliding Filament Theory 

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The sliding filament theory explains muscle contraction based on how muscle fibers (actin and myosin) slide against each other to generate tension in the overall muscle.

Step 1: A muscle contraction starts in the brain, where a signal is sent to the motor neuron (a).  The combination of the motor neuron and the skeletal muscle fibers make up a motor unit.

Color the motor neuron (a) yellow.
Vesicles that contain the neurotransmitter, acetylcholine. Color vesicles (b) gray .
Color the triangles that represent the acetylcholine (c) orange  .
Acetylcholine reaches the receptors (d) Color the receptors brown .
The gap between the neuron and muscle fiber is the synapse (e). Color this area light green .
The impulse travels down membrane, or sarcolemma. Color the sarcolemma (f) dark green  .

Step 2: The impulse travels moves into the transverse tubules where it causes calcium to be released from the sarcoplasmic reticulum.

Color the transverse tubule (g) light blue and the circles that represent calcium(h) dark blue .
The sarcoplasmic reticulum (i) is only partially pictured, color this structure pink .

Step 3: Calcium binds to the actin and causes it to change shape so that it can interact with myosin.

Color the actin myofilament (j) red . 

Step 4: The change in shape allows myosin heads to form cross-bridges between the actin and myosin.

Color the myosin filament (m) yellow .
Color the cross bridges and their heads (k) purple .

Step 5: Energy from ATP is used to create a "power stroke" between the two filaments. The actin filament  slides inward and shortens, or contracts, the whole muscle.

Color the ATP orange .


1.  The motor unit consists of muscle fibers and what else?

2.  What neurotransmitter is needed to initiate a muscle contraction?

3.  What substance causes the actin to change shape?

4.  How does myosin and actin interact with each other?

5.  What substance provides the energy for muscle contraction? 

7.   Where is calcium released from?

8.   What is the gap between the neuron and muscle fiber called?

9.  What are the two filaments found in muscles?

10.   Place the steps in order:

___  Cross bridges form                              
___ Signal travels down the motor neuron
___ Calcium released from sarcoplasm     
___ ATP creates a power stroke, shortening actin filament
___  Actin changes shape

sliding filament coloring

Other Resources on Muscles

Muscular System Unit - Resources for Anatomy and Physiology Class

Resources for Learning the Sliding Filament Model of Muscle Contraction

Sarcomere Coloring

Muscle Anatomy Crossword Puzzle

Magic School Bus Works Out - Season 3/ Episode 9 (See Netflix)