Cardiac and skeletal muscle

Question 1

What is meant by antagonistic muscle pairs?

Answer 1

Muscles can only pull, so they work in pairs to move bones around joints. Pairs of flexors and extensors pull in opposite directions, agonist contracts while the antagonist relaxes.

Question 2

What are extensors?

Answer 2

Muscles that increase the angle at a joint and act to extend limbs.

Question 3

What are flexors?

Answer 3

Muscles which decrease the angle at a joint and act to bend limbs.

Question 4

What do tendons and ligaments do?

Answer 4

Tendons (inelastic tissue) connect muscles to incompressible skeleton.
Ligaments (elastic tissue) join bones at joints.

Question 5

Describe the gross structure of skeletal muscle

Answer 5

Muscle cells are fused together to form bundles of parallel muscle fibres (myofibrils).
Arrangement ensures there is no point of weakness between cells.
each bundle is surrounded by endomycium: loose connective tissue with many capillaries.

Question 6

Where are slow and fast-twitch muscle fibres found in the body?

Answer 6

Slow-twitch: sites of sustained contraction like the calf muscle
Fast-twitch: sites of short-term, rapid, powerful contraction like biceps.

Question 7

Explain the structure and properites of slow-twitch muscle fibres

Answer 7

Glycogen store: many terminal ends can be hydrolysed to release glucose for respiration.
Contain myoglobin: higher affinity for oxygen than haemoglobin.
Many mitochondria for more ATP.
Surrounded by blood vessels for high supply of oxygen and glucose.

Question 8

Explain the structure and properties of fast-twitch muscle fibres

Answer 8

Large store of phosphocreatine
More myosin filaments
Thicker myosin filaments
Higher concentration of enzymes involved in anaerobic respiration
Extensive sarcoplasmic reticulum for rapid uptake and release of Ca2+

Question 9

Describe the microscopic structure of skeletal muscle

Answer 9

Myofibrils: site of contraction
Sarcoplasm: shared nuclei and cytoplasm with lots of mitochondria and sarcoplasmic reticulum
Sarcolemma: folds inwards towards sarcoplasm to form transverse tubules

Question 10

How is muscle contraction stimulated?

Answer 10

1. Neuromuscular junction: action potential = voltage-gated Ca2+ channels open
2. Vesicles move towards & fuse with presynaptic membrane
3. Exocytosis of ACh which diffuses across synaptic cleft
4. ACh binds to receptors on Na+ channel proteins on skeletal muscle cell membrane
   Influx of Na+ = depolarisation

Question 11

What does depolarisation of the sarcolemma cause?

Answer 11

It triggers the sarcoplasmic reticulum to release calcium ions. This stimulates muscle contraction.

Question 12

What is the sarcoplasmic reticulum?

Answer 12

A type of endoplasmic reticulum found in muscle cells which is specialised to store and release calcium ions.

Question 13

What does Ca2+ binding to troponin cause?

Answer 13

It pulls on tropomyosin which moves and frees up the actin binding sites which allows myosin to bind.

Question 14

What is the difference between troponin and tropomyosin?

Answer 14

Troponin is the small protein bound to tropomyosin which binds to Ca2+ and pulls on tropomyosin to move it away from the binding sites on actin.
Tropomyosin is the large protein composed of two alpha helixes which blocks the myosin binding sites on actin.

Question 15

What is an ATPase?

Answer 15

An enzyme which catalyses the hydrolysis of ATP to ADP + Pi

Question 16

Define the term myogenic

Answer 16

A type of muscle which can initiate its own contraction without outside stimulation from nervous impulses.

Question 17

What type of muscle is myogenic?

Answer 17

Cardiac muscle

Question 18

What is the sinoatrial node (SAN)?

Answer 18

A group of cells in the wall of the right atrium that generate electrical activity, causing the atria to contract. The SAN is the heart’s pacemaker.

Question 19

What is the atrioventricular node (AVN)?

Answer 19

A group of cells located between the atria that slow down the wave of excitation and pass it between the ventricles, along the bundle of His.

Question 20

What is an electrocardiogram (ECG)?

Answer 20

A graph showing the electrical activity in the heart during the cardiac cycle.

Question 21

Explain the characteristic patterns displayed on a typical ECG

Answer 21

P wave - depolarisation of atria during atrial systole
QRS wave - depolarisation of ventricles during ventricular systole
T wave - repolarisation of ventricles during ventricular diastole

Question 22

Name some heart defects that ECGs can help diagnose

Answer 22

Symptoms of cardiovascular disease. including arrhythmia and tachycardia

Question 23

How can ECGs be analysed to determine heart defects?

Answer 23

-The shape and size of the peaks on the graph provide information about the functioning of areas on the heart and the electrical conduction pathway.
-The frequency of the peaks provides info about the frequency of stimulation which can be used to diagnose tachycardia and bradycardia.

Question 24

State the formula for cardiac output.

Answer 24

stroke volume (V) x heart rate (R)

Question 25

What controls heart and ventilation rate?

Answer 25

The autonomic nervous system

Question 26

Why do heart and ventilation rates increase during exercise?

Answer 26

To increase oxygen supply for respiring tissues and rapidly remove CO2.

Question 27

Name the receptors involved in changing heart rate and state their location.

Answer 27

Baroreceptors (detect changes in blood pressure) - carotid body
Chemoreceptors (detect changes in pH e.g. due to increase in CO2 concentration) - carotid body and aortic body.

Question 28

Name the receptors involved in changing ventilation rate.

Answer 28

Chemoreceptors and stretch-mediated receptors in muscles and tendons.

Question 29

How does the body respond to an increase in blood pressure?

Answer 29

1. Baroreceptors send more impulses to the cardioinhibitory centre in the medulla.
2. More impulses to SAN down the vagus nerve via the parasympathetic nervous system
3. Stimulates release of acetylcholine, which decreases heart rate

Question 30

How does the body respond to a decrease in blood pressure?

Answer 30

1. Baroreceptors send more impulses to cardioacceleratory centre in the medulla.
2. More impulses to SAN via sympathetic nervous system.
3. Stimulates release of noradrenaline, which increases heart rate and strength of contraction.

Question 31

How does the body respond to an increase in CO2 concentration?

Answer 31

1. Chemoreceptors detrect pH decrease and send more impulses to cardioacceleratory centre & ventilation centre of medulla.
2. More impulses to SAn via sympathetic nervous system.
3. Heart rate increases, so rate of blood flow to lungs increases = rate of gas exchange and ventilation rate increases.