15.1.12
Sliding Filament Theory Contraction 2
Role of Calcium Ions
Role of Calcium Ions
When muscle cells are stimulated, there is an influx of calcium ions. The ions play an important role in initiating muscle contraction. The steps involved are:


Depolarisation
Depolarisation
- Muscle contraction is initiated when an action potential arrives at a neuromuscular junction from a motor neurone.
- The action potential causes depolarisation of the sarcolemma.
- Depolarisation spreads along the T tubules and into the sarcoplasm.


Influx of calcium ions
Influx of calcium ions
- Depolarisation of the T tubules stimulates the sarcoplasmic reticulum (SR).
- The SR releases Ca2+ ions into the sarcoplasm.


Tropomyosin
Tropomyosin
- Ca2+ ions bind to a protein attached to tropomyosin.
- Tropomyosin is a protein that blocks the actin-myosin binding site.
- Binding of Ca2+ ions causes the protein to change shape.
- Altering the protein causes tropomyosin to be moved. The actin-myosin binding site is no longer blocked by tropomyosin.


Actin-myosin cross bridge
Actin-myosin cross bridge
- The myosin head can now bind to the actin filament.
- The bond between actin and myosin is called the actin-myosin cross bridge.


ATP hydrolase
ATP hydrolase
- Ca2+ ions also activate ATP hydrolase.
- ATP hydrolase is an enzyme that hydrolyses ATP to ADP and inorganic phosphate. This process releases energy that can power muscle contraction.
Actin-Myosin Cross Bridges
Actin-Myosin Cross Bridges
The influx of Ca2+ ions to the sarcoplasm allows myosin and actin filaments to bind, creating an actin-myosin cross bridge. The roles of the cross bridges in muscle contraction are:


Bending of myosin heads
Bending of myosin heads
- When Ca2+ ions activate ATP hydrolase, ATP is hydrolysed and energy is released.
- The energy released from this reaction causes the myosin head to bend.
- The movement of the myosin head causes the actin filament to slide past the myosin filament.
- The actin filament is pulled by the myosin head because of the actin-myosin cross bridge.


Breaking the cross bridge
Breaking the cross bridge
- After the actin filament has slid past the myosin filament, the actin-myosin cross bridge is broken. This is driven by energy from ATP.
- The myosin head is no longer attached to the actin filament.


Forming a new cross bridge
Forming a new cross bridge
- The myosin head bends back to its original position after it is released from the actin binding site.
- The myosin forms a new cross bridge with a binding site further along the actin filament.


Contraction
Contraction
- The cycle of forming and breaking actin-myosin cross bridges occurs quickly and continuously.
- As actin filaments are pulled past the myosin filaments, the overall result is the shortening of the sarcomere.
- Shortening of the sarcomere causes muscle contraction.
Halting Contraction
Halting Contraction
Muscle contraction is stopped when the muscle cells are no longer stimulated. The steps involved are:


Removal of calcium ions
Removal of calcium ions
- If action potentials are no longer stimulating the muscle cells, the release of Ca2+ ions by the sarcoplasmic reticulum (SR) will stop.
- The Ca2+ ions are transported back into the SR by active transport.


Movement of tropomyosin
Movement of tropomyosin
- Removal of Ca2+ ions means that the protein attached to tropomyosin undergoes a conformational change.
- The protein changes shape. This causes tropomyosin to shift so that it is blocking the actin-myosin binding sites.
- Myosin heads can no longer bind to actin filaments.


Sarcomere lengthens
Sarcomere lengthens
- Myosin heads can no longer bind to actin filaments.
- The actin filaments return to their resting position.
- The sarcomere lengthens again. The muscle is no longer contracting.
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
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