Pearson Edexcel International A Level Biology: Skeletal Muscle, Cardiac Control and Adrenaline (YBI11) – Revision Notes

These notes condense statements 7.9 to 7.15 of the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2, including Core Practical 17. Topic 7 is International A2 (IA2); Topics 1 to 4 are International AS (IAS). There are no tiers. For full explanations see the study guide, then test yourself with the practice questions.

Also useful: the course hub, the printable checklist, the free diagnostics and the exam preparation page.

7.9 Movement at a joint

Structure	Joins	Key property
Tendon	Muscle to bone	Inelastic collagen, transmits the pull
Ligament	Bone to bone	Slightly elastic collagen, stabilises the joint
Skeleton	n/a	Rigid levers; joint is the pivot
Flexor	n/a	Contracts to bend the joint (angle decreases)
Extensor	n/a	Contracts to straighten the joint (angle increases)

Antagonistic pair: when one muscle contracts the other relaxes. Muscles only pull. Elbow: biceps (flexor) and triceps (extensor).

7.10 Fibre structure and types

Skeletal muscle fibre: one long multinucleate cell with sarcolemma, T tubules, sarcoplasm, sarcoplasmic reticulum (stores Ca2+), many mitochondria and myofibrils made of sarcomeres.

Band	Contains	During contraction
A band	Thick filaments (full length)	Stays the same length
I band	Thin filaments only	Shorter
H zone	Thick filaments only	Shorter, may disappear
Sarcomere (Z to Z)	One A band and half an I band at each end	Shorter

	Slow twitch	Fast twitch
Speed and force	Slow, sustained	Fast, powerful
Fatigue	Slow to fatigue	Fatigue quickly
Respiration	Aerobic	Anaerobic glycolysis, phosphocreatine
Mitochondria and myoglobin	Many and high	Fewer and low

7.11 Sliding filament theory in steps

1. Action potential travels along the sarcolemma and T tubules.
2. Sarcoplasmic reticulum releases Ca2+.
3. Ca2+ binds troponin, which changes shape.
4. Tropomyosin moves, exposing myosin-binding sites on actin.
5. Myosin head (with ADP and phosphate) binds actin: cross-bridge.
6. Power stroke: head tilts, actin pulled inwards, ADP and phosphate released.
7. ATP binds the head, so it detaches from actin.
8. ATPase on the myosin head hydrolyses ATP; the head returns to its original position and can bind again.
9. Relaxation: Ca2+ pumped back by active transport and tropomyosin blocks the sites.

Distinction: ATP is needed to detach the head (step 7) and to re-set it (step 8).

7.12 The heartbeat

Myogenic means the cardiac muscle starts its own contraction. Sequence:

1. SAN (right atrium) generates the wave of excitation: the pacemaker.
2. Atria contract together.
3. Non-conducting tissue stops the wave crossing straight to the ventricles.
4. AVN delays the wave, so the atria finish emptying.
5. Bundle of His carries it down the septum.
6. Purkyne fibres spread it up the ventricle walls from the apex.
7. Ventricles contract from the bottom upwards.

ECG feature	Meaning
P wave	Atrial depolarisation
QRS complex	Ventricular depolarisation
T wave	Ventricular repolarisation
Close R waves, rate above 100	Tachycardia
Spread-out R waves, rate below 60	Bradycardia
Irregular spacing, no clear P waves	Atrial fibrillation
Early extra beat then a pause	Ectopic beat

Heart rate (beats per minute) = 60 ÷ R-to-R time in seconds.

7.13 Cardiac output and its control

Quantity	Formula
Stroke volume	End-diastolic volume − end-systolic volume
Cardiac output	Stroke volume × heart rate
Heart rate	Cardiac output ÷ stroke volume

Method in steps: write the formula, put values in with units, convert cm³ to dm³ by dividing by 1000, and give the answer with units.

Worked reminder: stroke volume 62 cm³ and heart rate 78 beats per minute give 62 × 78 = 4836 cm³ per minute, or 4.84 dm³ per minute.

Control by the medulla oblongata:

• Cardiovascular control centre, with an accelerator pathway (sympathetic neurones to the SAN, faster rate) and a decelerator pathway (vagus nerve to the SAN, slower rate).
• Chemoreceptors (carotid arteries, aorta) detect a fall in pH from more carbon dioxide; baroreceptors detect blood pressure.
• Ventilation centre: receives chemoreceptor input and sends impulses to the diaphragm and intercostal muscles to increase rate and depth.

7.14 Adrenaline

• Source: adrenal glands. Transport: blood. A hormone, so slower but longer-lasting than a nerve impulse.
• Heart rate and stroke volume rise; ventilation rises; bronchioles dilate.
• Liver glycogen is broken down to glucose, which enters the blood.
• Blood is diverted to the muscles: arterioles to muscles dilate, arterioles to gut and skin constrict.

7.15 Core Practical 17: spirometer traces

Measure	How to get it from the trace
Tidal volume	Peak to trough of one breath
Breathing rate	Breaths ÷ time in minutes
Respiratory minute ventilation	Tidal volume × breathing rate
Oxygen consumption	Fall in baseline volume ÷ time

The baseline falls because the carbon dioxide is absorbed and oxygen is used. Compare rest with exercise, giving values with units, then comment on the change (for example, “rate and tidal volume both rose, so minute ventilation rose from X to Y”).

Must-know distinctions

• Tendon (muscle to bone, inelastic) and ligament (bone to bone, slightly elastic).
• Flexor (decreases the joint angle) and extensor (increases it).
• Myogenic (the heart muscle starts its own beat) and neurogenic (needs a nerve impulse each time). Nerves only alter the rate of the heart.
• Accelerator pathway (sympathetic, to the SAN) and decelerator pathway (vagus nerve, to the SAN).
• Tidal volume (one breath) and minute ventilation (tidal volume × breathing rate).
• Cardiac output (per minute) and stroke volume (per beat).

Worked reminders

Sarcomere from a micrograph. Magnification ×25 000, image length 50 mm. Actual length = 50 ÷ 25 000 = 0.002 mm = 2.0 µm. Convert mm to µm by multiplying by 1000.

ECG. Paper speed 25 mm per second, R-to-R distance 30 mm. Time = 30 ÷ 25 = 1.2 s. Heart rate = 60 ÷ 1.2 = 50 beats per minute, which is bradycardia (below 60).

Spirometer. Tidal volume 0.55 dm³, 20 breaths per minute. Minute ventilation = 0.55 × 20 = 11 dm³ per minute. If the baseline falls 2.4 dm³ in 8 min, oxygen consumption = 2.4 ÷ 8 = 0.30 dm³ per minute.

Percentage change. (new − old) ÷ old × 100. Stroke volume 48 to 62 cm³: 14 ÷ 48 × 100 = 29.2%.

Method in steps for a spirometer trace question:

1. Read the time scale and count the breaths in a stated time to get the breathing rate.
2. Measure peak to trough for tidal volume, using the volume scale.
3. Multiply tidal volume by breathing rate for minute ventilation.
4. Measure the fall in the baseline over a stated time for oxygen consumption.
5. Compare rest and exercise using values and units, and say what the change means.

Quick self-test

1. State the function of a tendon and a ligament.
2. Which band of a sarcomere stays the same length during contraction?
3. Name the two proteins that sit on the thin filament and control access to myosin-binding sites.
4. State two roles of ATP in contraction.
5. Why does the AVN delay the impulse?
6. Heart rate 80 beats per minute and stroke volume 60 cm³: calculate cardiac output in dm³ per minute.
7. Tidal volume 0.60 dm³ and breathing rate 14 per minute: calculate minute ventilation.
8. The R-to-R time on an ECG is 0.50 s. Calculate the heart rate and name the condition.
9. Stroke volume rises from 48 cm³ to 62 cm³. Calculate the percentage increase.
10. Give two differences between fast and slow twitch fibres.
11. Name the nerve that slows the heart.
12. State two effects of adrenaline that increase the supply of glucose or oxygen to muscle.

Answers

1. A tendon joins muscle to bone and transmits the pull; a ligament joins bone to bone and holds the joint.
2. The A band.
3. Troponin and tropomyosin.
4. It detaches myosin from actin, and its hydrolysis by ATPase re-sets the myosin head (also used to pump Ca2+ back).
5. So the atria finish contracting and emptying before the ventricles contract.
6. 80 × 60 = 4800 cm³ per minute = 4.8 dm³ per minute.
7. 0.60 × 14 = 8.4 dm³ per minute.
8. 60 ÷ 0.50 = 120 beats per minute: tachycardia.
9. (62 − 48) ÷ 48 × 100 = 29.2%.
10. Slow twitch has more mitochondria and myoglobin and fatigues slowly; fast twitch contracts faster and more powerfully but fatigues quickly.
11. The vagus nerve.
12. Any two of: increased heart rate, glycogen converted to glucose in the liver, dilated bronchioles, blood diverted to muscle.

Where marks are usually lost

• Saying the muscle “pushes” or naming the wrong joint partner structure.
• Stating that the A band or filaments shorten.
• Omitting ATP when explaining detachment of the myosin head.
• Saying calcium “attaches to actin” instead of troponin.
• Writing that the AVN speeds up or “starts” the heartbeat.
• Stating that nerves start the heartbeat rather than the SAN.
• Forgetting the unit or mixing cm³ and dm³.
• Describing the ECG as “faster” without a value or a named feature.
• Describing adrenaline as a nerve signal.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification, Issue 2, February 2021, Pearson Education Limited. Statements 7.9 to 7.15 of Topic 7, including Core Practical 17.