1. a1

2. Muscle Proteins
In the thin filaments actin proteins are strung together like a bead of pearls
In the thick filaments myosin proteins look like golf clubs bound together

3. Muscle Proteins
In this first graphic, the myosin binding sites on the actin proteins are readily visible.
The regulatory proteins troponin and tropomyosin have been added to the bottom graphic: The myosin binding sites have been
covered

4. Myosin binding site exposed
Muscle Proteins
In this graphic the troponin-tropomyosin complex has slid down into the “gutters” of the actin molecule unblocking the myosin binding site
The troponin-tropomyosin complex can slide back and forth depending on the presence of Ca2+
Myosin binding site exposed

5. Muscle Proteins
Ca2+ binds to troponin which changes the shape of the troponin-tropomyosin complex and uncovers the myosin binding sites on actin

6. Muscle Proteins
Besides contractile and regulatory proteins, muscle contains about a dozen structural proteins which contribute to the alignment, stability, elasticity, and extensibility of myofibrils
Titan is the third most plentiful protein in muscle, after actin and myosin - it extends from the Z disc and accounts for much of the elasticity of myofibrils
Dystrophin is discussed later as it relates to the disease of muscular dystrophy
Titin is the largest known protein, consisting of 34,350 amino acids. Titin, also known as connectin is a protein that is important in the contraction of striated muscle tissues. Dystrophin, not Titin, is the protein absent in muscular dystrophy.

7. The Sliding-Filament Mechanism
With exposure of the myosin binding sites on actin (the thin filaments)—in the presence of Ca2+ and ATP—the thick and thin filaments “slide” on one another and the sarcomere is shortened

8. The Sliding-Filament Mechanism
The “sliding” of actin on myosin (thick filaments on thin filaments) can be broken down into a 4 step process

9. Step 1: ATP hydrolysis
Step 2: Attachment

10. Step 3: Power Stroke
Step 4: Detachment

11. The Sliding-Filament Mechanism
None of this occurs without intracellular Ca2+ release from its storage sites in the sarcoplasmic reticulum.

12. Contraction and Movement Overview Interactions Animation
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13. Length-Tension Relationship
Sarcomere shortening produces tension within a muscle
Limited contact between actin and myosin
Compressed thick filaments

14. Excitation-Contraction Coupling
We will come back to the term excitation-contraction coupling in a little bit
Before we can describe the
entire process, from
thinking of moving a
muscle to actual contraction
of sarcomeres, we must
first explore the processes
that occur at the neuromuscular junction

15. Neuromuscular Junction
Excitation-Contraction coupling (EC coupling) involves events at the junction between a motor neuron and a skeletal muscle fiber
A single muscle cell from a biceps may contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into sarcomeres.

16. Neuromuscular Junction
An enlarged view of the neuromuscular junction
The presynaptic membrane is on the neuron while the postsynaptic membrane is the motor end plate on the muscle cell. The two membranes are
separated by a space,
or “cleft”