SLIDING-FILAMENT THEORY OF MUSCULAR
CONTRACTION
The sliding filament theory states that “the
ACTIN filaments at each end of the sarcomere
slide INWARD on MYOSIN filaments, pulling the
Z-lines toward the center of the sarcomere and
thus shortening the muscle fiber”
As ACTIN filaments slide over MYOSIN filaments
the H-zone and the I-band of the Sarcomere shrink
The flexion of MYOSIN crossbridges as they pull on
the actin filaments is responsible for the movement
of the ACTIN filaments
This action causes only a very small displacement
Very rapid repeated flexions must occur in many
crossbridges throughout the entire muscle for
measurable movement to occur
before crossbridges can flex they must first attach
to the actin filament
when the sarcoplasmic reticulum releases calcium
ions, they bind with troponin, a protein situated at
regular intervals along the actin filament
this causes a shift in another protein tropomyosin,
which runs along the length of the ACTIN filament
in the groove of the double helix
the MYOSIN crossbridge attaches much more
rapidly to the ACTIN filament allowing flexion to
occur
TYPES OF MUSCLE ACTION
Concentric actions
Occur when tension developed in crossbridges
is enough to overcome any resistance to
shortening
Isometric Contraction
Occur when the tension in the crossbridges
equals the resistance to shortening and muscle
length remains the same
Eccentric Contractions
Occur when the tension developed in the
crossbridges is less then the resistance and the
muscle lengthens despite contact between the
MYOSIN crossbridge heads and ACTIN
filaments
Understanding the Process- Stimulus to
Contraction
Using a visual representation DESCRIBE YOUR
UNDERSTANDING of the process we go
through in order to contract a muscle cell.
Use the following terms in your description and
illustrations:
Stimulus
Brain
Spinal Cord
Motor Neuron
Axon
Motor Endplate
Neuromuscular junction
Ach-Acetylcholine
Ca+
Myofibril
Sarcomere
Z-line
A-band
I-band
H-zone
M-line
Actin Filament
Myosin Filament
Troponin
Tropomyosin
Binding Sites