Describe the structure of a myosin molecule.

A myosin molecule has a long fibrous tail that lies along the shaft of the thick filament and a globular head that projects from it, carrying a binding site for actin and an ATPase active site.

Describe the structure of a thin filament.

A thin filament is a double helix of two chains of globular actin subunits, with a fibrous molecule of tropomyosin winding along the groove between them and a molecule of troponin attached at regular intervals.

What happens when calcium ions bind to troponin?

The troponin changes shape and drags the tropomyosin deeper into the groove of the thin filament, uncovering the myosin-binding sites on the actin.

Where do the calcium ions come from when a muscle is stimulated?

They are released from the sarcoplasmic reticulum, which opens its calcium channels when the action potential travels along the sarcolemma and down the transverse tubules.

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Human Musculoskeletal Anatomy: Skeletal Tissues

The sliding filament theory

3 things to know
Component 3

Overview

Structure of a myosin molecule, structure of a thin filament and power stroke of muscle contraction.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe what happens when a muscle relaxes.

Key term

Which protein makes up the thick filaments of a myofibril?

Key term

What keeps the myosin-binding sites on actin covered in a resting muscle?

Questions from this subtopic

Each one with the model answer underneath.

1
Which protein makes up the thick filaments of a myofibril?
Model answer
The thick filaments are made of myosin.
2
Describe the structure of a myosin molecule.
Model answer
A myosin molecule has a long fibrous tail that lies along the shaft of the thick filament and a globular head that projects from it, carrying a binding site for actin and an ATPase active site.
3
Describe the structure of a thin filament.
Model answer
A thin filament is a double helix of two chains of globular actin subunits, with a fibrous molecule of tropomyosin winding along the groove between them and a molecule of troponin attached at regular intervals.
4
What keeps the myosin-binding sites on actin covered in a resting muscle?
Model answer
The tropomyosin lies across the binding sites and physically blocks them, so no myosin head can attach.
5
What happens when calcium ions bind to troponin?
Model answer
The troponin changes shape and drags the tropomyosin deeper into the groove of the thin filament, uncovering the myosin-binding sites on the actin.
6
Where do the calcium ions come from when a muscle is stimulated?
Model answer
They are released from the sarcoplasmic reticulum, which opens its calcium channels when the action potential travels along the sarcolemma and down the transverse tubules.

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