Muscle Contraction
Muscle contraction is the process by which skeletal muscle develops tension and produces force. In skeletal muscle, contraction begins with excitation of the muscle fiber and ultimately causes calcium ions to regulate the interaction between the contractile proteins actin and myosin.
The mechanism is commonly explained using the sliding filament theory. During contraction, actin filaments slide past myosin filaments, causing the sarcomere to shorten. The actin and myosin filaments themselves do not become shorter.
Before studying the contraction mechanism, it is useful to understand the basic structure of skeletal muscle, especially the arrangement of myofibrils, sarcomeres, actin, and myosin.
Steps of Muscle Contraction
The following sequence explains the major muscle contraction steps in skeletal muscle.
A motor neuron carries an action potential toward the neuromuscular junction. When the action potential reaches the axon terminal, voltage-gated calcium channels open and calcium enters the nerve terminal.
The entry of calcium into the nerve terminal causes synaptic vesicles containing acetylcholine (ACh) to release their contents into the synaptic cleft.
ACh binds to nicotinic acetylcholine receptors on the motor end plate. This produces depolarization of the muscle fiber membrane. If threshold is reached, a muscle action potential is generated.
The muscle action potential spreads across the sarcolemma and travels deep into the muscle fiber through the transverse (T) tubules.
The electrical signal in the T-tubules triggers calcium release from the sarcoplasmic reticulum. The calcium concentration around the myofibrils therefore rises rapidly.
Calcium binds to troponin C on the thin filament. This changes the position of the troponin-tropomyosin complex and exposes the myosin-binding sites on actin.
The energized myosin head attaches to an exposed binding site on actin. This forms an actin-myosin cross-bridge.
The myosin head pivots and pulls the actin filament toward the center of the sarcomere. This movement is called the power stroke.
A new ATP molecule binds to the myosin head. This causes myosin to detach from actin.
ATP is hydrolyzed to ADP and inorganic phosphate. The released energy returns the myosin head to its energized position, allowing another cycle to occur if calcium remains available.
Sliding Filament Theory
The sliding filament theory explains how skeletal muscle shortens during contraction.
According to this theory, thin actin filaments slide toward the center of the sarcomere between the thick myosin filaments. The actin and myosin filaments do not themselves become shorter. Instead, their degree of overlap increases.
As more sarcomeres shorten, the myofibrils shorten, and the muscle fiber can generate force and shorten.
Sarcomere Contraction Diagram
Figure: Sarcomere structure showing the relationship between relaxed and contracted states.
What Happens to the Sarcomere?
| Structure | Change During Contraction |
|---|---|
| Sarcomere | Shortens |
| Z discs | Move closer together |
| I band | Becomes shorter |
| H zone | Becomes shorter and may disappear |
| A band | Remains essentially unchanged |
| Actin filament | Does not shorten; slides toward the M line |
| Myosin filament | Does not shorten |
Role of Calcium in Muscle Contraction
Calcium is the key link between electrical excitation and mechanical contraction.
At rest, tropomyosin covers the myosin-binding sites on actin. When calcium is released from the sarcoplasmic reticulum, it binds to troponin C. This shifts the regulatory proteins and allows myosin to interact with actin.
When calcium is removed from the cytoplasm and returned to the sarcoplasmic reticulum, the binding sites become covered again and the muscle fiber relaxes.
Role of ATP in Muscle Contraction
ATP is essential throughout the cross-bridge cycle. It is required for several important processes involved in normal contraction and relaxation.
- ATP binding allows myosin to detach from actin.
- ATP hydrolysis provides energy for repositioning the myosin head.
- ATP provides energy for calcium reuptake into the sarcoplasmic reticulum through calcium pumps.
Without adequate ATP, normal cross-bridge detachment and calcium reuptake cannot continue.
Excitation-Contraction Coupling
Excitation-contraction coupling refers to the sequence connecting the electrical excitation of a muscle fiber with the development of mechanical force.
The sequence can be summarized as:
↓
Sarcolemma and T-tubules activated
↓
Calcium released from sarcoplasmic reticulum
↓
Calcium binds to troponin C
↓
Actin-myosin interaction
↓
Cross-bridge cycling
↓
Force development and sarcomere shortening
Muscle Contraction and Relaxation
Contraction and relaxation are closely linked processes. Contraction occurs when calcium is available to permit actin-myosin interaction. Relaxation begins when calcium is actively transported back into the sarcoplasmic reticulum.
As cytoplasmic calcium concentration falls, calcium dissociates from troponin. Tropomyosin again covers the binding sites on actin, reducing further cross-bridge formation.
Types of Skeletal Muscle Contraction
Isometric Contraction
During an isometric contraction, the muscle develops tension without significant change in its overall length.
Holding a heavy object stationary is a simple example.
Isotonic Contraction
In an isotonic contraction, the muscle changes length while producing force. Isotonic movement can be further described as concentric or eccentric.
Concentric Contraction
A concentric contraction occurs when a muscle produces force while shortening.
Eccentric Contraction
An eccentric contraction occurs when a muscle produces force while lengthening under an external load.
Changes During Muscle Contraction
Several measurable changes occur when skeletal muscle contracts. These changes include mechanical, electrical, and biochemical events.
- Muscle tension increases.
- Calcium concentration in the muscle fiber cytoplasm increases.
- Actin-myosin cross-bridge cycling occurs.
- Sarcomeres shorten during shortening contractions.
- The I band becomes shorter.
- The H zone becomes shorter or disappears.
- The A band remains essentially unchanged.
- ATP is consumed.
- Heat is produced as a consequence of metabolic activity.
Muscle Contraction Diagram: What to Identify
When studying a muscle contraction diagram, focus on the sequence rather than memorizing the picture alone. A typical diagram should help you identify the relationship between actin, myosin, calcium, troponin, tropomyosin, and the sarcomere.
For examination purposes, pay particular attention to the changes in the I band, H zone, A band, and Z discs during contraction.
Why Does a Muscle Contract?
At the molecular level, contraction occurs because myosin heads repeatedly interact with actin and pull the thin filaments toward the center of the sarcomere. The combined shortening of many sarcomeres produces shortening and force development at the muscle-fiber level.
The process is therefore not simply a matter of the muscle proteins “shrinking.” Instead, the filaments slide past one another as cross-bridge cycling continues.
High-Yield MBBS Points
- Muscle contraction begins with excitation of the muscle fiber.
- Acetylcholine is released at the neuromuscular junction.
- The action potential travels along the sarcolemma and T-tubules.
- Calcium is released from the sarcoplasmic reticulum.
- Calcium binds to troponin C.
- Tropomyosin moves away from the myosin-binding sites on actin.
- Myosin binds to actin and forms cross-bridges.
- The power stroke moves actin toward the center of the sarcomere.
- ATP is required for myosin detachment and continued cross-bridge cycling.
- The sarcomere shortens during a shortening contraction.
- The I band decreases during contraction.
- The H zone decreases or disappears.
- The A band remains essentially unchanged.
- Actin and myosin filaments do not themselves shorten.
- Calcium reuptake into the sarcoplasmic reticulum is important for relaxation.
Quick Revision
Nerve impulse → ACh release → muscle action potential → T-tubules → Ca2+ release → troponin C → actin-myosin interaction → cross-bridge cycling → power stroke → sarcomere shortening.
| Question | Answer |
|---|---|
| What is the main theory explaining muscle shortening? | Sliding filament theory |
| What ion initiates the interaction between actin and myosin? | Calcium (Ca2+) |
| Where is calcium stored? | Sarcoplasmic reticulum |
| Which protein binds calcium? | Troponin C |
| Which band remains unchanged? | A band |
| Which bands/zones become shorter? | I band and H zone |
| What provides energy for cross-bridge cycling? | ATP |
Frequently Asked Questions
What are the steps of muscle contraction?
The major steps are action potential generation, acetylcholine release, muscle fiber depolarization, action potential propagation through T-tubules, calcium release, calcium binding to troponin, cross-bridge formation, power stroke, ATP-dependent myosin detachment, and repeated cross-bridge cycling.
What is the sliding filament theory?
The sliding filament theory states that muscle contraction occurs because actin filaments slide past myosin filaments toward the center of the sarcomere. The filaments themselves do not become shorter.
What happens to the A band during muscle contraction?
The A band remains essentially unchanged in length because it corresponds to the length of the thick myosin filaments.
What happens to the I band during contraction?
The I band becomes shorter as the actin filaments move toward the center of the sarcomere.
What is the role of calcium in muscle contraction?
Calcium binds to troponin C and causes a regulatory change that exposes the myosin-binding sites on actin, allowing cross-bridge formation.
Why is ATP needed for muscle contraction?
ATP is needed for myosin detachment from actin, re-energizing the myosin head, and supporting calcium reuptake during relaxation.
Related Muscle Physiology Topics
- 3 Types of Muscle Tissue: Skeletal, Cardiac, and Smooth Muscle
- Structure of Skeletal Muscle: Muscle Fibers, Myofibrils, Sarcomeres, Actin and Myosin
- Properties of Skeletal Muscle
After understanding muscle contraction, the next step is to learn how the motor neuron communicates with the skeletal muscle fiber at the neuromuscular junction.
Image Credits & Licenses
Sarcomere contraction diagram: “Sarcomere relaxed contracted.PNG” by Chippolito, Wikimedia Commons, licensed under CC BY-SA 3.0.
The image is used to illustrate the structural changes between relaxed and contracted sarcomeres.
Medical Disclaimer: The information provided in this article is strictly for educational, study, and exam-preparation purposes. It does not constitute professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for clinical decisions.