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, so the head hydrolyses ATP and swings through the power stroke
• Bphosphorylating the myosin light chains via a calcium-dependent kinase, which is what allows cross-bridges to form in skeletal fibres
• Cbinding to troponin, causing tropomyosin to shift and expose myosin-binding sites on actin✓
• Dopening the voltage-gated Na⁺ channels of the sarcolemma (muscle membrane)

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⁺ release 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 into the terminal → Na⁺ efflux from the muscle fibre → hyperpolarisation of the sarcolemma
• CK⁺ influx into the terminal → noradrenaline release → binding to adrenergic receptors on the sarcolemma → depolarisation of the muscle fibre
• Dacetylcholinesterase release → Ca²⁺ influx → muscle relaxation (without depolarisation)

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 presynaptic terminals
• 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 (Type I) fibres using aerobic respiration, with oxygen delivered by a dense capillary network
• BPredominantly cardiac muscle fibres in the legs using fatty acid oxidation to release energy slowly
• CPredominantly slow-twitch fibres relying on the creatine phosphate store alone, which is exhausted within a few seconds
• 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.

What does the sliding filament theory propose?

• AActin and myosin both shorten, shortening the sarcomere
• BActin slides over myosin, shortening the sarcomere✓
• CMyosin slides over actin, lengthening the sarcomere
• DActin and myosin separate, lengthening the sarcomere

Answer: The filaments themselves keep their length; myosin heads pull actin past them so the sarcomere shortens. The I band and H zone narrow while the A band stays the same — which is how the theory was tested.

What is the role of calcium ions in muscle contraction?

• AThey move tropomyosin so actin can bind to tropomyosin
• BThey move tropomyosin so myosin can bind to actin✓
• CThey provide the energy needed for the power stroke
• DThey break the cross-bridge at the end of the stroke

Answer: Calcium released from the sarcoplasmic reticulum binds troponin, shifting tropomyosin off the binding sites so myosin heads can attach. ATP supplies the energy and detaches the head — calcium is the switch, not the fuel.

What is the role of ATP in the cross-bridge cycle?

• AAttaching the myosin head and holding it against the actin
• BDetaching the myosin head and recocking it for the next stroke✓
• CBinding to troponin so tropomyosin moves out of the way
• DPumping calcium ions out of the sarcoplasmic reticulum only

Answer: ATP binding causes the myosin head to release actin; its hydrolysis then re-cocks the head ready to bind again. Without ATP the heads stay attached, which is the basis of rigor mortis. ATP also powers the calcium pump, but that is a separate role.

How do slow twitch fibres differ from fast twitch fibres?

• AFewer mitochondria and myoglobin, suited to endurance
• BMore mitochondria and myoglobin, suited to short sprints
• CFewer mitochondria and myoglobin, suited to short sprints
• DMore mitochondria and myoglobin, suited to endurance✓

Answer: Slow twitch fibres respire aerobically, so they are rich in mitochondria, myoglobin and capillaries and resist fatigue. Fast twitch fibres rely on anaerobic glycolysis for rapid powerful contraction and tire quickly.

Why must muscles operate in antagonistic pairs?

• AMuscles can only push, so another is needed to reverse the movement
• BOne muscle contracts while the other also contracts equally
• COne muscle provides energy while the other provides movement
• DMuscles can only pull, so another is needed to reverse the movement✓

Answer: Contraction shortens a muscle and pulls a bone; nothing lets it push the bone back. So a second muscle on the opposite side contracts to reverse the movement while the first relaxes — biceps and triceps being the standard example.

What is a sarcomere?

• AThe region between two Z lines of a myofibril✓
• BThe region between two A bands of a myofibril
• CThe membrane surrounding a whole muscle fibre
• DThe store of calcium ions within a muscle fibre

Answer: The sarcomere is the repeating contractile unit running Z line to Z line. The membrane is the sarcolemma and the calcium store is the sarcoplasmic reticulum.

Why does phosphocreatine allow brief bursts of intense activity?

• AIt rapidly regenerates ADP from ATP without needing oxygen
• BIt is broken down directly to release energy for contraction
• CIt supplies glucose to the muscle for anaerobic respiration
• DIt rapidly regenerates ATP from ADP without needing oxygen✓

Answer: Phosphocreatine donates its phosphate to ADP, restoring ATP almost instantly and anaerobically. The store is small and lasts only seconds, but it bridges the gap before glycolysis and aerobic respiration catch up.

What happens to the H zone during contraction?

• AIt narrows as actin filaments slide towards the centre✓
• BIt widens as actin filaments slide away from the centre
• CIt stays the same width throughout the contraction
• DIt disappears because the myosin filaments shorten

Answer: The H zone contains myosin only, so it narrows as actin slides inwards. The A band, which is the full length of the myosin, does not change — evidence that the filaments slide rather than shorten.

What is the neuromuscular junction?

• AA synapse between a sensory neurone and a muscle fibre
• BA synapse between a motor neurone and a muscle fibre✓
• CA junction between two adjacent muscle fibres
• DA junction between a muscle fibre and a tendon

Answer: It works like a cholinergic synapse: acetylcholine crosses the cleft and depolarises the sarcolemma, spreading down the T-tubules to release calcium. It is always excitatory, unlike some synapses in the CNS.