1. The Muscular System
Skeletal muscle consists of numerous muscle cells called Muscle fibers.
Muscle fiber terminology and characteristics
Sarcolemma = plasma membrane of the muscle cell
Highly invaginated by transverse tubules (T tubules) that permeate the cell.
Sarcoplasma = cytoplasm of the muscle cell
Contains calcium-storing sarcoplasmic reticulum
Specialized endoplasmic reticulum of a muscle cell

2. The Muscular System Muscle fiber terminology and characteristics
Muscle cells are multinucleated
Nearly the entire volume of the cell is filled with numerous, long myofibrils.
Two types of filaments
Actin
Thin filaments consisting of two strands arranged in a double helix
Troponin and Tropomyosin = molecules that cover special binding sites on actin.

3. The Muscular System Two types of filaments
Myosin
Thick filaments
Each myosin filament forms a protruding head at one end.
Actin and myosin are parallel and arranged side by side.
Striated appearance produced from overlapping filaments
Each repeating unit of the pattern is called a sarcomere.
Each sacromere is separted by a Z-line, to which the actin filaments are attached
Myosin filaments are located between the actin, unattached to the Z-line.

4. The Muscular System Muscle contraction: sliding-filament model
ATP binds to a myosin head and forms ADP + Pi
When ATP binds to a myosin head, it is converted to ADP + Pi, which remains attached to the myosin head.
Ca++ exposes the binding sites on the actin filaments
Ca++ binds to the troponin molecule causing tropomyosin to expose positions on the actin filament for the attachment of myosin heads.

5. The Muscular System Muscle contraction: sliding-filament model
3. Cross bridges between myosin heads and actin filaments form.
1. When attachment sites on the actin are exposed, the myosin heads bind to actin to form cross bridges.
4. ADP and Pi are released and sliding motion of actin results.
1. The attachment of cross bridges between myosin and actin causes the release of ADP and Pi. This, in turn, causes a change in shape of the myosin head, which generates a sliding movement of the actin toward the center of the sarcomere.
2. This pulls the two Z-lines together, contracting the muscle fiber.

6. The Muscular System Muscle contraction: sliding-filament model
5. ATP causes the cross bridges to unbind.
1. When a new ATP molecule attaches to the myosin head, the cross bridge between the actin and myosin breaks, returning the myosin head to its unattached position
Without the addition of a new ATP molecule, the cross bridges remain attached to the actin filaments.
This is why corpses are stiff.

7. The Muscular System Neuromuscular junctions
Synapses between neurons and muscles
Muscle contraction is stimulated through the following steps:
Action potential generates release of acetylcholine.
Action potential generated on sacrolemma and throughout the T-tubules
Receptors on the sacrolemma initiate a depolarization event and action potential.
The action potential travels along the sacrolemma throughout the transverse system of tubules.

8. The Muscular System
Muscle contraction is stimulated through the following steps:
Sacroplasmic reticulum releases Ca++.
Myosin cross bridges form.
The Ca++ released by the sacroplasmic reticulum binds to troponin molecules on the actin helix, promoting tropomyosin molecules to expose binding sites for myosin cross-bridge formation.
If ATP is available, muscle contraction begins.

9. The Muscular System
Humans and other vertebrates have three kinds of muscles
Skeletal muscle
Attached to bones and causes movement of body
Smooth muscle
Lines the walls of blood vessels and digestive tract
No striation
No T-tubules, therefore, contraction is controlled and relatively slow.
Cardiac muscle
Striated
Highly branched with cells connected by gap junctions
Generates its own action potential, which spreads rapidly throughout muscle tissue by electrical synapses across gap junctions.