Muscles and Movement

Introduction to Muscles and Movement

Welcome to one of the most dynamic chapters in your Biology A-Level! Have you ever wondered how your body manages to pull off a sprint, or how you can stand perfectly still for minutes? It all comes down to the incredible coordination between your skeleton, muscles, and connective tissues. In this guide, we will break down the "machinery" of movement, from the big structures you can see to the microscopic proteins that slide past each other every time you blink.

1. The Support Team: Skeleton, Tendons, and Ligaments

Movement isn't just about muscles; it’s a team effort. To move a limb, your muscles need something to pull against, and they need to be attached securely.

The Skeleton: Acts as a sturdy frame. It provides a system of levers that muscles pull on to create movement.

Tendons: These are tough, non-elastic cords of tissue that connect muscle to bone. Because they don't stretch much, the force of the muscle contraction is passed directly to the bone.

Ligaments: These connect bone to bone. They are slightly elastic to allow for joint movement but strong enough to keep your joints stable and prevent them from popping out of place (dislocating).

Analogy: Think of your bones as the wooden parts of a puppet, your muscles as the person pulling the strings, and the tendons as the strings themselves!

2. The Structure of Muscle Fibres

Muscle isn't just one solid "blob" of tissue. If you looked at a skeletal muscle under a microscope, you would see it is made of thousands of long, cylindrical cells called muscle fibres.

These fibres are special because:

1. They are multinucleate (they have many nuclei) because they are so long.
2. They contain smaller units called myofibrils.
3. These myofibrils are made of even smaller protein filaments called myofilaments: Actin (the thin filament) and Myosin (the thick filament).

3. Fast Twitch vs. Slow Twitch Muscle Fibres

Not all muscles are created equal! Depending on whether you are running a marathon or lifting a heavy box, your body uses different types of fibres.

Slow Twitch Fibres

These are designed for endurance and posture. Features:
- They contract slowly but can keep going for a long time without getting tired.
- They rely on aerobic respiration, so they have many mitochondria.
- They are rich in myoglobin (a protein that stores oxygen), which gives them a dark red color.
- They have a very good blood supply (lots of capillaries) to keep oxygen flowing.

Fast Twitch Fibres

These are designed for short bursts of speed and power. Features:
- They contract very quickly and powerfully but tire out (fatigue) fast.
- They rely more on anaerobic respiration.
- They have fewer mitochondria and less myoglobin, so they look paler.
- They store lots of creatine phosphate and glycogen for quick energy.

Key Takeaway: Marathon runners have a high percentage of slow-twitch fibres, while Olympic sprinters have more fast-twitch fibres!

4. The Sliding Filament Theory

This is the "how-to" of muscle contraction. It explains how muscles get shorter (contract) without the individual protein filaments actually shrinking. Instead, they slide past each other.

Don't worry if this seems complex at first! Let's look at the "players" involved:

- Actin: Thin filaments with binding sites for myosin.
- Myosin: Thick filaments with "heads" that look like tiny golf clubs.
- Tropomyosin: A long protein strand that blocks the binding sites on actin when the muscle is relaxed.
- Troponin: A smaller protein attached to tropomyosin that acts like a "lock."

The Step-by-Step Process:

1. Activation: An action potential reaches the muscle, causing calcium ions (\(Ca^{2+}\)) to be released from the sarcoplasmic reticulum into the muscle fibre.

2. Unblocking: The \(Ca^{2+}\) binds to troponin. This causes the troponin to change shape, which pulls the tropomyosin away from the binding sites on the actin.

3. Binding: Now that the sites are open, the myosin heads bind to the actin, forming a cross-bridge.

4. The Power Stroke: The myosin head bends, pulling the actin filament toward the center of the sarcomere. This releases ADP and inorganic phosphate (\(P_{i}\)).

5. Detachment: A new molecule of ATP binds to the myosin head. This causes the head to release the actin.

6. Resetting: The enzyme ATPase (located on the myosin head) breaks down the ATP into ADP and \(P_{i}\). The energy released "cocks" the myosin head back to its original high-energy position, ready to bind again.

Quick Review: Muscle contraction requires both Calcium (to clear the path) and ATP (to provide the energy for movement and detachment).

5. Energy for Contraction

Muscles need a constant supply of ATP. Since ATP is used up very quickly during exercise, the body has several ways to make more:

1. Aerobic Respiration: Used during low-intensity exercise (requires oxygen).
2. Anaerobic Respiration: Used during high-intensity exercise; produces lactate (see the "Respiration" chapter for more details on the fate of lactate).
3. Creatine Phosphate: A molecule stored in muscles that can quickly donate a phosphate group to ADP to make ATP instantly. This lasts for only about 5–10 seconds of maximal effort.

6. Summary and Exam Tips

Common Mistakes to Avoid:

- Mixing up Tendons and Ligaments: Remember: Tendons = To bone. Ligaments = Like to Like (bone to bone).
- Forgetting ATP's role in relaxation: ATP is needed for the myosin head to detach. This is why "rigor mortis" happens—without ATP after death, muscles stay locked in a contracted state!
- Confusing Troponin and Tropomyosin: Just remember that Troponin is the "Trigger" that binds to Calcium.

Key Takeaway Table:

Element: Calcium Ions (\(Ca^{2+}\))
Role: Binds to troponin to expose binding sites.

Element: ATP
Role: Provides energy for the power stroke and allows myosin to detach from actin.

Element: ATPase
Role: Hydrolyses ATP to reset the myosin head.

Note: For how the heart muscle differs and how it is controlled by the medulla oblongata, please refer to the chapter on "Heart Control, Homeostasis and Thermoregulation".