3.6.3 Skeletal muscles

What is the primary function of antagonistic muscle pairs?

• To compress the skeleton

• To work against each other to move bones

• To produce hormones

• To store calcium ions

Answer explanation

Antagonistic muscle pairs work against each other to create movement by contracting and relaxing, allowing bones to move at joints. This is essential for coordinated motion, making the correct answer 'To work against each other to move bones'.

Which of the following best describes the ultrastructure of a myofibril?

• A single muscle fibre

• A bundle of actin and myosin filaments

• A collection of muscle cells

• A network of blood vessels

Answer explanation

A myofibril is best described as a bundle of actin and myosin filaments, which are the key proteins involved in muscle contraction. This structure is essential for the function of muscle fibers.

What role does calcium ions play in myofibril contraction?

• They provide energy for contraction

• They bind to tropomyosin to expose binding sites

• They bind to actin to initiate contraction

• They bind to myosin to initiate contraction

Answer explanation

Calcium ions bind to tropomyosin, causing a conformational change that exposes the binding sites on actin for myosin. This is crucial for initiating muscle contraction.

Which molecule is directly responsible for the power stroke in muscle contraction?

• Actin

• Myosin

• ATP

• Calcium ions

Answer explanation

ATP is crucial for muscle contraction as it provides the energy needed for the myosin heads to perform the power stroke, pulling actin filaments and causing muscle shortening.

What is the role of phosphocreatine in muscle contraction?

• It stores calcium ions

• It provides a rapid source of ATP

• It breaks down actin

• It forms the actinomyosin bridge

Answer explanation

Phosphocreatine serves as a rapid source of ATP during muscle contraction, allowing for quick energy replenishment. This is crucial for sustaining high-intensity activities, making it essential for muscle function.

Which of the following is a characteristic of slow skeletal muscle fibres?

• They fatigue quickly

• They have a high density of mitochondria

• They are used for short bursts of power

• They contain less myoglobin

Answer explanation

Slow skeletal muscle fibres are characterized by a high density of mitochondria, which supports endurance activities by providing sustained energy. In contrast, they fatigue slowly and are not primarily used for short bursts of power.

Where are fast skeletal muscle fibres typically located?

• In muscles used for posture

• In muscles used for endurance activities

• In muscles used for quick, powerful movements

• In the heart

Answer explanation

Fast skeletal muscle fibers are primarily found in muscles responsible for quick, powerful movements, such as sprinting or weightlifting, due to their ability to contract rapidly and generate high force.

What is the primary role of myosin in muscle contraction?

• To bind calcium ions

• To hydrolyse ATP

• To form cross-bridges with actin

• To store energy

Answer explanation

The primary role of myosin in muscle contraction is to form cross-bridges with actin. This interaction is crucial for muscle contraction, as it allows myosin to pull on actin filaments, leading to muscle shortening.

How does ATP contribute to muscle relaxation?

• By binding to actin

• By detaching myosin heads from actin

• By increasing calcium ion concentration

• By forming cross-bridges

Answer explanation

ATP contributes to muscle relaxation by detaching myosin heads from actin. When ATP binds to myosin, it causes the myosin heads to release from the actin filaments, allowing the muscle to relax.

What is the role of tropomyosin in muscle contraction?

• It binds to myosin

• It covers binding sites on actin

• It hydrolyses ATP

• It stores calcium ions

Answer explanation

Tropomyosin plays a crucial role in muscle contraction by covering the binding sites on actin filaments. When calcium ions bind to troponin, it causes tropomyosin to shift, exposing these sites for myosin to attach and initiate contraction.