Muscles & Movement

In a relaxed skeletal muscle sarcomere, the H zone is best described as the region that contains:

• Aonly thin (actin) filaments
• Bonly thick (myosin) filaments✓
• Coverlapping thick and thin filaments
• Dthe Z line and its attached actin filaments

Answer: The H zone lies in the centre of the A band and contains only thick (myosin) filaments, with no overlap from thin filaments. During contraction it narrows as thin filaments slide inward.

During the sliding-filament mechanism of skeletal muscle contraction, which of the following remains constant in length?

• Athe I band
• Bthe H zone
• Cthe sarcomere
• Dthe A band✓

Answer: The A band corresponds to the length of the thick (myosin) filaments, which do not shorten. The sarcomere, I band and H zone all shorten as thin filaments slide over the thick filaments toward the centre.

When a skeletal muscle is stimulated, Ca²⁺ ions trigger contraction primarily by:

• Abinding directly to the myosin head and activating its ATPase
• Bphosphorylating myosin light chains via a kinase
• Cbinding to troponin, causing tropomyosin to shift and expose myosin-binding sites on actin✓
• Dopening voltage-gated Na⁺ channels in the sarcolemma

Answer: Ca²⁺ released from the sarcoplasmic reticulum binds to troponin, changing its shape so that the attached tropomyosin moves away from the actin-binding sites. This exposes the sites, allowing myosin heads to form cross-bridges.

In the cross-bridge cycle of skeletal muscle, the binding of a new ATP molecule to the myosin head causes:

• Athe power stroke that pulls the actin filament
• BCa²⁺ to be released from the sarcoplasmic reticulum
• Cthe myosin head to bind to the actin filament
• Dthe myosin head to detach from the actin filament✓

Answer: ATP binding to the myosin head reduces its affinity for actin, causing detachment of the cross-bridge. Subsequent hydrolysis of ATP to ADP + Pi then re-cocks the head ready to bind again.

Creatine phosphate is important in actively contracting muscle because it:

• Aregenerates ATP rapidly from ADP without requiring oxygen✓
• Bacts as the immediate substrate for the myosin ATPase
• Cis the main store of glucose released during anaerobic respiration
• Dbinds Ca²⁺ to terminate contraction

Answer: Creatine phosphate donates its phosphate group to ADP to regenerate ATP anaerobically and almost instantly, buffering ATP concentration during the first few seconds of intense activity before aerobic respiration increases.

Compared with fast-twitch (Type II) fibres, slow-twitch (Type I) skeletal muscle fibres typically have:

• Afewer mitochondria and less myoglobin
• Blarger stores of glycogen and phosphocreatine for anaerobic respiration
• Cmore mitochondria, more myoglobin and a richer capillary supply✓
• Da faster rate of ATP hydrolysis by their myosin ATPase

Answer: Slow-twitch fibres are adapted for sustained aerobic activity: they contain many mitochondria, abundant myoglobin (giving a red colour) and a dense capillary network to supply oxygen, allowing fatigue-resistant contraction.

Which sequence correctly describes events at the neuromuscular junction after an action potential reaches the motor neurone terminal?

• ACa²⁺ influx → acetylcholine release → binding to receptors on the sarcolemma → depolarisation of the muscle fibre✓
• Bacetylcholine release → Ca²⁺ influx → Na⁺ efflux → hyperpolarisation of the muscle fibre
• CK⁺ influx → noradrenaline release → binding to receptors → depolarisation of the muscle fibre
• Dacetylcholinesterase release → Ca²⁺ influx → muscle relaxation

Answer: Depolarisation of the presynaptic terminal opens voltage-gated Ca²⁺ channels; Ca²⁺ influx triggers exocytosis of acetylcholine, which diffuses across the cleft and binds to nicotinic receptors on the motor end plate, depolarising the sarcolemma.

The enzyme acetylcholinesterase, present in the synaptic cleft of the neuromuscular junction, is essential because it:

• Asynthesises acetylcholine from acetyl CoA and choline in the presynaptic terminal
• Bhydrolyses acetylcholine so that the muscle fibre is not continuously stimulated✓
• Cactively transports Ca²⁺ back into the sarcoplasmic reticulum
• Dbinds acetylcholine receptors to amplify the depolarisation

Answer: Acetylcholinesterase breaks down acetylcholine into choline and ethanoic acid, removing it from the receptors. This prevents continuous depolarisation and allows the muscle to relax; the choline is recycled into the presynaptic neurone.

Which statement correctly distinguishes cardiac muscle from skeletal muscle?

• ACardiac muscle is myogenic and its cells are joined by intercalated discs, whereas skeletal muscle requires nervous stimulation✓
• BCardiac muscle is unstriated, whereas skeletal muscle is striated
• CCardiac muscle fatigues rapidly, whereas skeletal muscle never fatigues
• DCardiac muscle is under voluntary control, whereas skeletal muscle is involuntary

Answer: Cardiac muscle is myogenic, generating its own rhythm of contraction, and its branching cells are connected by intercalated discs (with gap junctions) that allow rapid spread of excitation. It is striated like skeletal muscle but is involuntary and fatigue-resistant.

A researcher measures the length of the A band and the I band in a sarcomere before and after contraction. Which result would be expected?

• ABoth the A band and the I band shorten
• BThe A band shortens while the I band stays the same length
• CThe A band stays the same length while the I band shortens✓
• DBoth the A band and the I band stay the same length

Answer: The A band length is fixed by the length of the myosin filaments and does not change during contraction. The I band (region of thin filaments not overlapping myosin) shortens as the thin filaments slide further into the A band.

In smooth (involuntary) muscle, contraction differs from that of skeletal muscle because smooth muscle:

• Acontains no actin or myosin filaments
• Blacks the regular sarcomere arrangement, so it appears non-striated under the microscope✓
• Cis stimulated only by acetylcholine and never by hormones
• Dcannot sustain prolonged contraction

Answer: Smooth muscle does contain actin and myosin, but they are not arranged into regular sarcomeres, so no striations are visible. Smooth muscle contracts slowly, can sustain tension for long periods, and responds to both nerves and hormones.

A sprinter performing a 100 m race relies heavily on which combination of energy supply within the leg muscles?

• APredominantly slow-twitch fibres using aerobic respiration
• BPredominantly cardiac fibres using fatty acid oxidation
• CPredominantly slow-twitch fibres using creatine phosphate only
• DPredominantly fast-twitch fibres using ATP, creatine phosphate and anaerobic glycolysis✓

Answer: Explosive, short-duration activity recruits fast-twitch (Type II) fibres, which generate ATP rapidly from stored ATP, the creatine phosphate system and anaerobic glycolysis. These pathways supply energy quickly but fatigue rapidly through lactate accumulation.