CTB2 - Mechanical and Electrical Properties of the Heart

Question 1

What are the three types of muscle cell within the heart? State the purpose of each.

Answer 1

Atrial cardiomyocytes - works with ventricular cardiomyocytes for synchronised contractions.
Ventricular cardiomyocytes - works with atrial cardiomyocytes for synchronised contractions.
Specialised excitatory and conductive fibres - generation and conduction of electrical impulses (also are weak contractorsj)

Question 2

What key feature distinguishes cardiomyocytes from other cells?

Answer 2

Segmentation

Question 3

Give the purpose of gap junctions between cardiomyocytes.

Answer 3

Rapid exchange of ions - ensures depolarisation and rest of action potential, allowing cardiomyocyte contraction.

Question 4

What is the sarcolemma?

Answer 4

Plasma membrane of cardiomyocytes - consists of glycocalyx.

Question 5

What are myofibrils?

Answer 5

Protein fibres consisting of actin and myosin protein filaments, which slide past one another during contraction as part of the sliding filament theory.

Question 6

What are the contractile units of cardiomyocytes? How are these separated.

Answer 6

Sarcomere. Separated by Z lines/discs.

Question 7

What are T tubules and what are their purpose?

Answer 7

Invagination of the cell surface. Attached to sarcoplasmic reticulum so reach deep into the cell. Ensure that depolarisation spreads all over the cell.

Question 8

What is the sarcoplasmic reticulum and what does it do?

Answer 8

Membrane bound structure. Contains intracellular store of calcium ions where are released upon stimulation by extracellular calcium ions. Vital for muscular contraction.

Question 9

What is excitation - contraction coupling? Briefly describe.

Answer 9

Process that allows for rhythmic contractions of cardiac muscle fibres following excitation which is spread across the cardiac muscle fibre by the presence of the T tubules.

Question 10

What are the four stages of the action potential?

Answer 10

Depolarisation. Repolarisation. Hyperpolarisation. Resting potential.

Question 11

Briefly describe the movements of ions during a cardiac action potential.

Answer 11

At resting potential, potassium ion channels open and sodium and calcium are closed.

Initial membrane depolarisation (phase 0) causes fast sodium channels to open, causing an influx of sodium ions. Calcium channels open too, allowing for rapid increase.

Phase 1 - sodium ion channels close.

Phase 2 - calcium ions out and potassium in but are balanced hence the plateau which allows for longer contraction.

Phase 3 - rapid repolarisation. Calcium channels close and potassium slowly leaks into cell.

Phase 4 - small excess of potassium ions but resting potential reset.

Question 12

What are the two systems controlling the heart rate?

Answer 12

Intrinsic and extrinsic.

Question 13

Give brief overview of the hearts intrinsic control of heart rate.

Answer 13

Referred to as the cardiac cycle. SA node acts as pacemaker producing an electrical signal. Spreads across atria causing their contraction - blood ejected into ventricles. Impulse reaches AV node following short delay (allows atria to completely empty/ventricles to completely fill). AV node sends impulse down the bundle of his and Purkinje fibres. Wave spreads from apex across the heart, causing ventricular contraction.

Question 14

What are the four relay stations within the intrinsic heart control mechanism? Describe each briefly in its role and location.

Answer 14

SA node - specialised cardio myocytes in upper top corner right atrium. Pacemaker cells - initiate the electrical impulse.

AV node - specialised cardio myocytes in lower bottom corner of right atrium. Conduct impulse to the bundle of his.

Bundle of his - conductive fibres (two sets for left and right). Located in interventricular septum. Send electrical impulse to heart apex.

Purkinje fibres - conductive fibres at heart apex allowing for wave of depolarisation to spread across the heart aiding ventricular contraction.

Question 15

What is ventricular tachycardia?

Answer 15

Abnormal heart rhythm where the ventricles contract irregularly. Means that they do not fill properly, so less efficient oxygen delivery to tissues (and also return to lungs).

Question 16

Can the heart beat if there is no blood supply?

Answer 16

Does not need a blood supply, just needs oxygen.

Heart is independent of the central nervous system - its intrinsic control means that it can beat itself.

Question 17

What nervous system is involved in the extrinsic control of the heart? Give its two branches and the effect of each branch.

Answer 17

Controlled by the autonomic nervous system. Branches are sympathetic nervous system (increases heart rate as per fight or flight response) or parasympathetic nervous system (decreases heart rate as per rest and digest response).

Question 18

What nerves are involved in the sympathetic and parasympathetic extrinsic control of heart rate? Give their outcome.

Answer 18

Sympathetic cardiac nerves - increase heart rate and contraction force.
Vagus nerve - decreases heart rate (as parasympathetic effect).

Question 19

What hormones are released during the extrinsic control of heart rate, for the sympathetic and parasympathetic systems respectively?

Answer 19

Sympathetic - epinephrine and norepinephrine (catecholamine).
Parasympathetic- acetylcholine.

Question 20

In addition to increasing heart rate, give some other outcomes of the sympathetic nervous system during a fight or flight response?

Answer 20

Vasoconstriction of blood vessels. Increasing breathing rate and frequency. Blood diverted away from non essential organs. Pupils dilate.

Question 21

What are the two periods of a Cardiac cycle?

Answer 21

Systole - ventricular contract ejecting blood into the aorta and pulmonary artery.
Diastole - ventricles relax as the atria continue to fill them with blood.

Question 22

Does the cardiac cycle refer to the electrical or mechanical properties of the heart?

Answer 22

Includes both the electrical activity and the mechanical processes as a result of them.

Question 23

Give an approximation for resting heart rate.

Answer 23

Approx 60-80bpm.

Question 24

Which cells in the body contain the most mitochondria?

Answer 24

Cardiac muscle cells contain most mitochondria (approx 40%). Skeletal muscle is often mistakenly thought to contain the most.

Question 25

What is the structural difference between male and female hearts?

Answer 25

Generally, female hearts contain less cardiac muscle.

Question 26

If you are feeling hot, what are some potential responses that would occur in the body?

Answer 26

Sweating - increased loss of water means increased heat loss.
Vasodilation of blood vessels near skin - attempts to lose more heat, but also causes flushing.
Blood flow to internal organs reduced - blood flow to skin is greater therefore cardiac output has been redistributed.
Increased heart rate/cardiac output - to ensure enough blood is delivered both to skin and internal organs,

Question 27

What is fainting? What are the two types of fainting?

Answer 27

Also referred to as syncope. Temporary reduced blood flow to brain.

Neurally-mediated syncope - glitch in ANS.
vasovagal syncope - heart beat stops temporarily meaning reduced blood flow to brain.

Question 28

How is fainting reversed?

Answer 28

Sympathetic nervous system activity - increases heart rate and vasoconstriction to stabilise blood pressure.