The document describes the process of muscle contraction. It begins with the muscle fiber being stimulated, which causes calcium ions to be released from the sarcoplasmic reticulum. The calcium ions then attach to troponin and tropomyosin, exposing active sites on the thin filaments. Myosin cross-bridges attach to these active sites and pivot, pulling the thin filaments toward the center of the sarcomere and causing the muscle fiber to contract. This cycle of cross-bridge formation and pulling of the thin filaments continues as long as calcium ions are present.

1. Fig. 10-3.
Myofibrils are
surrounded by
calcium-
containing
sarcoplasmic
reticulum.

2. Fig. 10-7. Review of thin and
thick filament structure

3. Fig. 10-9. Overview of
the process

4. Fig. 10-9. Overview of
the process
The muscle fiber is
stimulated.

5. Fig. 10-9. Overview of
the process
The muscle fiber is
stimulated.
Ca2+ ions are released.

6. Fig. 10-11. “End-
on” view of thick
& thin filaments,
showing the
effect of calcium
ions after release
from the S.R.

7. Fig. 10-9. Overview of
the process
The muscle fiber is
stimulated.
Ca2+ ions are released.

8. Fig. 10-9. Overview of
the process
The muscle fiber is
stimulated.
Ca2+ ions are released.
Thin filaments move to
middle of sarcomere.

9. Fig. 10-12
Calcium attaches to
troponin/ tropomyosin; they
roll away, exposing the
active site on actin.

10. Fig. 10-12
Myosin cross-bridges
attach to active site on
actin.
After attachment, the
cross-bridges pivot,
pulling the thin
filaments.

11. Fig. 10-12
A fresh ATP replaces the
ADP+Pi, allowing myosin
and actin to detach.
Energy from the
splitting of the fresh
ATP allows
repositioning of the
myosin head.

12. Fig. 10-12
This leads back to Step
1, which continues the
cycle as long as
calcium ions are
attached to
troponin/tropomyosin.

13. Fig. 10-9. Overview of
the process
The muscle fiber is
stimulated.
Ca2+ ions are released.
Thin filaments move to
middle of sarcomere.

14. Fig. 10-9. Overview of
the process
The muscle fiber is
stimulated.
Ca2+ ions are released.
Thin filaments move to
middle of sarcomere.
Muscle fiber contracts.

15. Fig. 10-9. Overview of
the process
The muscle fiber is
stimulated.
Ca2+ ions are released.
Thin filaments move to
middle of sarcomere.
Muscle fiber contracts.
Muscle tension
increases.