The Sliding Filament Theory
Microstructure of Skeletal
Muscle (recap)
Actin-Myosin Orientation:
• Myosin filament (thick).
• Have long rod-shaped tails with 2 globular
heads.
• The heads form cross bridges.
Microstructure of Skeletal Muscle (recap)
• Actin filament (thin).
• Tropomyosin is a rigid, rod-shaped protein which
lies in grooves on either side of actin.
• Troponin is complex of 3 globular proteins.
Structures involved
• Myofibril: A cylindrical organelle running the length of the
muscle fiber, containing Actin and Myosin filaments.
• Sarcomere: The functional unit of the Myofibril, divided
into: I, A & H bands.
• Actin: A thin, contractile protein filament, containing
'active' or 'binding' sites.
• Myosin: A thick, contractile protein filament, with
protrusions known as Myosin Heads.
• Tropomyosin: An actin-binding protein which regulates
muscle contraction.
• Troponin: A complex of three proteins, attached to
Tropomyosin.
Structure of Skeletal Muscle
Microstructure of Skeletal Muscle
Structure of Skeletal Muscle
The Sarcoplasmic Reticulum and
Transverse Tubules
Muscular Contraction
Excitation-Contraction Coupling
• Depolarization of motor end plate (excitation)
is coupled to muscular contraction
• Overview:
– Action potential travels down transverse tubules
and causes release of Ca+2 from SR
– Ca+2 binds to troponin and causes position change
in tropomyosin
=Exposing active sites on actin
– Strong binding state formed between actin and
myosin
– Contraction occurs
Muscle Contraction:
• The process of a muscle contracting can be divided into 5
sections:
• Excitation:
1. A nerve impulse (AP) arrives at the neuromuscular junction,
which causes a release of a chemical called Acetylcholine
(Ach) into the synaptic cleft.
- The presence of Acetylcholine causes Calcium (Ca+) to be
released from the sarcoplasmic reticulum.
2. In the presence of high concentrations of Ca+,
the Ca+ binds to Troponin  changing its shape.
- This moves Tropomyosin from the active site of
the Actin to expose uncovered sites.
- Therefore, Myosin filaments can now attach to
the Actin  forming a cross-bridge.
Tropomyosin, Ca+ & ATP (cont)
• Ca+ causes tropomyosin to be displaced.
• So it no longer blocks the myosin binding site
• So myosin and actin can bind together allowing cross
bridge cycling
3. The myosin head binds to the actin, tilts,
forcing the actin to move toward the myosin this requires ATP (energy)
4. When an ATP molecule binds to the Myosin
head, the Myosin detaches from the Actin and
the cross-bridge is broken
- When the ATP is then broken down the Myosin
head can again attach to an Actin binding site
further along the Actin filament and repeat the
'power stroke'.
• Energy released
produces crossbridge
movement (power
stroke).
• New ATP attaches to
myosin crossbridge to
dissociate from actin.
5. This process of muscular contraction can last
for as long as there is enough ATP and Ca+
stores.
- Once the impulse stops the Ca+ is pumped
back to the Sarcoplasmic Reticulum and the
Actin returns to its resting position causing the
muscle to lengthen and relax.
• During contraction,
neither myosin nor actin
filaments change in
length.
• They slide past each
other.
• A band remains same,
I band decreases.
• H zone decreases on
contraction.
Sarcomere Relaxed
Sarcomere Partially Contracted
Sarcomere Completely
Contracted
The Sliding Filament Theory…
• Therefore, when the muscle contracts 
sarcomeres become smaller
• However, the filaments do not change in
length.
• Instead they slide past each other (overlap)
• So actin filaments slide between myosin
filaments = the zone of overlap is larger
Muscular Contraction
The Sliding Filament Model
• Also called the swinging lever-arm model
• Muscle shortening occurs due to the
movement of the actin filament over the
myosin filament
• Formation of cross-bridges between actin and
myosin filaments
• Power stroke
• Reduction in the distance between Z lines of
the sarcomere
Therefore… sliding-filament theory:
Muscle fibers shorten or lengthen
because thick and thin myofilaments
slide past each other without the
filaments changing length.
• See http://thepoint.lww.com Animation:
Sliding Filament Theory.