Skeletal Muscles as Effectors: How We Move
Hello Biologists! This chapter is all about how your body translates a signal from your nervous system (like "lift that heavy book!") into actual movement. We are studying skeletal muscles—the engines of voluntary movement—and how they work at a microscopic level.
Understanding muscle contraction is crucial because it links the nervous system (control) to the response (action), showing exactly how organisms interact with and respond to their environment. Don't worry if the details of the proteins seem complex; we will break them down step-by-step!
1. Structure of Skeletal Muscle (3.4.4.1)
1.1 Muscles in Antagonistic Pairs (3.4.4.2)
You might know that muscles always pull; they never push. This fact means that muscles must work in pairs, called antagonistic pairs, to move a bone back and forth.
- When the biceps contracts, the arm flexes (pulls up).
- When the triceps contracts, the arm extends (pulls down).
These muscles pull against an incompressible skeleton—meaning the bones themselves cannot be compressed—which allows the force generated by the muscle fibres to be effectively transmitted for movement.
1.2 Gross and Microscopic Structure
A skeletal muscle is highly organised. Let’s zoom in:
- The whole muscle is made up of bundles of muscle fibres (muscle cells).
- Each fibre is encased in a membrane called the sarcolemma. The cytoplasm is called the sarcoplasm.
- The sarcoplasm contains many long, cylindrical structures called myofibrils.
- Myofibrils are the fundamental units of contraction and are made up of repeating units called sarcomeres.
The Ultrastructure of a Myofibril
Myofibrils have a characteristic striped or striated appearance due to the arrangement of two types of protein filaments:
- Actin: The thin filaments.
- Myosin: The thick filaments, which have tiny projections called myosin heads.
The structure of a single repeating unit, the sarcomere, is defined by bands:
- Z-lines: These mark the boundaries/ends of the sarcomere.
- I-band (Isotropic/Light band): Contains only actin filaments. It shortens during contraction.
- A-band (Anisotropic/Dark band): Contains the entire length of the myosin filaments, which also overlap with actin filaments. This band’s length does not change during contraction.
Key Takeaway: The sarcomere is the basic unit of muscle contraction, consisting of overlapping thick (myosin) and thin (actin) filaments.
2. The Sliding Filament Theory of Muscle Contraction (3.4.4.1)
The Sliding Filament Theory (SFT) explains how muscles contract. It states that the muscle contracts when the actin and myosin filaments slide past one another, causing the sarcomere to shorten. The filaments themselves do not shorten.
2.1 The Roles of Key Proteins
The ability for actin and myosin to interact is regulated by two other essential proteins located on the actin filament: Tropomyosin and Troponin (Troponin is the structure to which Calcium ions bind, which then causes Tropomyosin to move).
- Tropomyosin: In a relaxed muscle, this protein wraps around the actin filament, physically blocking the sites where myosin heads want to attach.
- Calcium Ions (\(\text{Ca}^{2+}\)): These are the "switch." When released, they bind to the regulatory protein associated with tropomyosin, causing the tropomyosin to move away from the binding sites.
2.2 The Contraction Cycle: Actomyosin Bridge Formation
Contraction requires a nervous signal (action potential) and is a cycle powered by ATP.
Step 1: Excitation and Calcium Release
The nerve impulse reaches the muscle fibre. This signal causes the sarcoplasmic reticulum (a specialised endoplasmic reticulum) to release stored calcium ions (\(\text{Ca}^{2+}\)) into the sarcoplasm.
Step 2: Exposure of Binding Sites
The \(\text{Ca}^{2+}\) ions bind to the protein complex associated with tropomyosin. This binding causes the tropomyosin molecule to change shape and move, uncovering the myosin-binding sites on the actin filament.
Step 3: Actomyosin Bridge Formation
The energized myosin head (which is holding ADP and Pi) now binds to the exposed site on the actin filament, forming a cross-bridge known as an actomyosin bridge.
Step 4: The Power Stroke
The binding triggers the release of ADP and Pi. The myosin head changes its angle (pivots), pulling the actin filament along the myosin filament. This is the power stroke, and it causes the sarcomere to shorten.
Step 5: ATP Binding and Detachment
A new molecule of ATP must bind to the myosin head. The binding of fresh ATP causes the myosin head to instantly detach from the actin filament (breaking the actomyosin bridge).
Step 6: Resetting the Myosin Head
The ATP is quickly hydrolysed (\(\text{ATP} \rightarrow \text{ADP} + \text{Pi}\)) by ATPase in the myosin head. This hydrolysis provides the energy to 're-cock' the myosin head, returning it to its high-energy, resting position, ready to bind again further down the actin filament.
The cycle repeats as long as \(\text{Ca}^{2+}\) ions (the signal) and ATP (the fuel) are present.
Analogy Tip: Think of the myosin head like a person using a rope.
(1) The head grabs the rope (actin).
(2) It pulls (power stroke).
(3) It lets go (requires ATP).
(4) It moves to a new position further along the rope (ATP hydrolysis).
Key Takeaway: Contraction involves a cycle of binding, pivoting, detaching, and resetting the myosin heads, driven by ATP and regulated by calcium ions and tropomyosin.
3. Energy Supply for Muscle Contraction (3.4.4.2)
Muscle contraction is one of the most energetically expensive processes in the body. ATP is needed for three main reasons:
- To detach the myosin head from the actin filament (allowing the cycle to continue).
- To reset (re-cock) the myosin head via hydrolysis.
- To actively pump \(\text{Ca}^{2+}\) ions back into the sarcoplasmic reticulum during relaxation.
3.1 The Role of Phosphocreatine
While respiration (both aerobic and anaerobic) is the ultimate source of ATP, muscle cells need a way to generate ATP instantaneously for sudden, high-intensity movements (like a quick jump or a heavy lift). This is where phosphocreatine comes in.
Phosphocreatine is a rapidly accessible reserve of phosphate that can regenerate ATP from ADP very quickly.
The reaction is catalysed by the enzyme creatine kinase: \[ \text{ADP} + \text{Phosphocreatine} \longrightarrow \text{ATP} + \text{Creatine} \]
This supply is very fast but very limited. It provides ATP for the first few seconds of maximal exertion, buying time until respiration can catch up.
Common Mistake Alert!
Don't confuse ATP's role! ATP causes the myosin head to detach. A lack of ATP (as occurs after death, leading to rigor mortis) means the myosin heads stay attached, locking the muscle in a contracted state.
4. Slow and Fast Skeletal Muscle Fibres (3.4.4.2)
Not all skeletal muscles are the same. Muscles contain a mix of two main fibre types, each adapted for a different job.
4.1 Slow (Slow-Twitch) Muscle Fibres (Type I)
These fibres are built for endurance and sustained activity. They contract slowly but can keep going for long periods without fatiguing.
- Location: Found in large proportions in postural muscles (like the back and legs) that must maintain contraction against gravity.
- Energy Supply: Primarily aerobic respiration.
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Key Properties:
- High concentration of myoglobin (an oxygen-storing protein, making them look red).
- Many mitochondria (to produce large amounts of ATP aerobically).
- Rich capillary network (high blood supply for oxygen delivery).
4.2 Fast (Fast-Twitch) Muscle Fibres (Type II)
These fibres are built for rapid, powerful contractions over a short duration. They fatigue quickly because they rely on less efficient energy pathways.
- Location: Found in large proportions in muscles used for rapid movement (like the biceps or eye muscles).
- Energy Supply: Primarily anaerobic respiration and phosphocreatine.
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Key Properties:
- Low myoglobin and low capillary density (making them appear white or pale).
- Fewer mitochondria.
- Large stores of glycogen and high concentration of enzymes for glycolysis (anaerobic respiration).
- High concentration of creatine kinase (to rapidly use phosphocreatine).
Quick Review: The Three Essentials for Contraction
To contract, a muscle fibre needs:
1. Signal: \(\text{Ca}^{2+}\) ions (to move tropomyosin).
2. Protein Interaction: Actin and Myosin (to slide past each other).
3. Energy: ATP (to detach and reset myosin heads).
Did you know? The ratio of slow-twitch to fast-twitch fibres is largely determined by genetics, which is why some people are naturally better at long-distance running (endurance) while others excel at sprinting (power).
Key Takeaway: Slow-twitch fibres are aerobic, red, and fatigue resistant; fast-twitch fibres are anaerobic, white, and powerful but fatigue quickly.