Contraction of Skeletal Muscle Cells
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Module: Support and Movement
Activity: Animations
Title: Contraction of Skeletal Muscle Cells
Introduction
1. Describe the series of events started by a nerve impulse arriving at a neuromuscular junction.
The nerve impulse causes release of neurotransmitter which in turn depolarize the muscle membrane,
leading to the release of calcium ions into the sarcoplasm. The calcium ions bind to the regulatory proteins
and trigger contraction.
Muscle Cell Structures
2. Define these muscle structures:
Myofibrils
Rod-like structures that extend the length of the cell and
contain repeating units called sarcomeres.
Sarcomeres
Contractile units of a skeletal muscle cell that
contain actin (thin) and myosin (thick) filaments.
3. Describe actin and myosin filament function. Attachment of myosin to actin
and subsequent sliding of myosin against actin results in muscle contraction.
3. Explain the importance of the myosin binding sites on the actin
filament.
` Provide the location for the head on the myosin cross bridge to attach to the
actin filament which initiates the movement of the filaments against each
other.
4. What is the function of the tropomyosin-troponin complex?
Troponin contains binding sites for calcium ions and is attached to actin and tropomyosin and blocks the
myosin binding sites when the muscle is at rest.
5. What affect does calcium ions have on these filaments?
In the presence of calcium ions troponin binds with calcium and pulls the tropomyosin away from the
myosin binding sites and uncovers them.
6. Identify these parts of the myosin filament and the
function of each.
Myosin tail
tail that allows for the power stroke.
Myosin crossbridge
Head of the myosin molecule that contains ATP binding sites and attaches to actin during muscle
contraction
7. Explain the function of ATP with regard to the “power stroke.”
ATP provides the energy to detach the myosin head from actin and reposition the head to its ready state so
that it can perform another power stroke upon reattachment to actin.
8. Identify the titin filament and describe its function.
The elastic titin filament that allows the sarcomere to return to its resting length
after contracting or stretching
Contraction of a Sarcomere
9. Ca+2 ions entering the sarcoplasm triggers a contraction cycle. Describe each of the following
steps of a contraction cycle.
a. ATP Hydrolysis
ATP provides the energy to move myosin molecules back into the energized
configuration necessary to perform the power stroke.
b. Crossbridge Attachment
Myosin temporarily binds with actin.
c. Power Stroke
Upon attachment ot actin, the myosin head snaps back to a low energy position and moves the actin
filament toward the center of the sarcomere. Phosphate and ADP are released from the myosin head
during this process.
d. Crossbridge Detachment
Binding of ATP to myosin crossbridge detaches myosin from actin.
10. How long will this process continue?
Contraction continues if electrical impulses continue to promote release of more calcium ions and if ATP
molecules are generated in adequate concentrations.
Contraction and Movement
11. Describe how each of the following successively contributes to gross body
movement.
Myosin
Pulls actin filaments, sliding them toward the center of the sarcomere.
Actin
When the actin filaments slide towards the center of the sarcomere, causes the
sarcolemma to move.
Dystrophin proteins and the sarcolemma
The actin filaments are connected to the sarcolemma through dystrophin proteins.
Muscle cell
When the sarcolemma shortens the muscle cell contracts.
Entire Muscle
As muscle cells contract, the entire muscle shortens and exerts a force on the attachments of the
muscle. .
Tendon & Bone
The contraction force of the muscle is applied to the bone by way of the tendon which connects the
muscle to the bone.
12. Contrast muscle origins and insertions.
Contraction force brings the insertion site of the muscle towards its origin point of attachment..