5. Control of the Cardiovascular system 1

Question 1

What does membrane potential depend on?

Answer 1

Flow of K+ out of cells

Question 2

What equation is used to predict what a potential will be across a semi-permeable membrane?

Answer 2

Nernst equation

Question 3

If the membrane is only permeable to K+ at rest (diastole)…

Answer 3

Then the potential across it will equal the K+ equilibrium potential, (EK)

Question 4

Which equation can account for relative permeabilities of several ions simultaneously? Why is this a better indicator of membrane potential?

Answer 4

Goldman-Hodgkin-Katz equation

Membrane is never uniquely selective to 1 ion

Question 5

Briefly describe an AP in a nerve

Answer 5

Increase in Na+ permeability
Na+ floods in
Depolarises cell
Moves towards Na+ equilibrium potential
(Doesn’t get there as Na+ channels close)
Gradual increase in K+ permeability taking +charge out of cell to repolarise it and restore membrane potential

Question 6

Time duration of an AP in a nerve

Answer 6

2 ms

Question 7

Time duration of a cardiac AP

Answer 7

200-300 ms

Very long compared with nerves

Question 8

What does the duration of a cardiac AP control?

Answer 8

Controls the strength and duration of contraction of the heart
Long, slow contraction is required to produce an effective pump

Question 9

Absolute refractory period (ARP)

Answer 9

time during which no AP can be initiated regardless of stimulus intensity

Question 10

Relative refractory period

Answer 10

period after ARP where an AP can be elicited but only with stimulus strength larger than normal

Question 11

Why is the relative refractory period useful in cardiac cells?

Answer 11

Useful as can’t re-stimulate too quickly

Allows heart to fill before next stimulus

Question 12

What are refractory periods caused by?

Answer 12

Na+ channel inactivation
As membrane becomes more and more negative, more and more Na+ channels become available (recover) to be activated if re-stimulated
Na+ channels recover from inactivation as the membrane repolarises

Question 13

Tetanic stimulation

Answer 13

sustained contraction following a series of stimuli

Question 14

Describe repolarisation in skeletal muscle

Answer 14

Repolarisation occurs very early in the contraction phase making re-stimulation and summation of contraction possible
Tetanic stimulation can be produced

Question 15

What is the consequence of a long refractory period in cardiac muscle?

Answer 15

Long refractory period means it is not possible to re-excite the muscle until the process of contraction is well underway
Hence cardiac muscle cannot be tetanised

Question 16

Describe the phases of an action potential in the ventricles

Answer 16

Phase 0: Upstroke: Caused by Na+ channels opening, influx of Na+ and depolarisation towards Na+ equilibrium potential
Phase 1: Early repolarisation (increase in K+ conductance of membrane)
Phase 2: Plateau (Ca2+ channels open)
Phase 3: Repolarisation (K+ channels opening)
Phase 4: Diastole, Resting membrane potential

Question 17

Action potential profiles in the heart

Answer 17

Different parts of the heart have different AP shapes

Caused by different ion currents flowing and different ion channel expression in cell membrane

Question 18

Describe the electrical properties of the heart

Answer 18

Intrinsic
Capable of independent spontaneous generation and coordinated propagation of electrical activity
Specialised conduction system
Can beat independently even if separated from its nerve supply

Question 19

Describe the extrinsic nerve supply to the heart

Answer 19

Comes from the autonomic nervous system

Serves to modify and control the intrinsic beating established by the heart

Question 20

What is the resting membrane potential in SAN?

Answer 20

No resting membrane potential in SAN

Always oscillating

Question 21

What channels exist in SAN? What is the consequence of this?

Answer 21

Most channels exist in SAN

Exception is IK1: there is no IK1 (So membrane potential is never very stable)

Question 22

What is the upstroke produced by in the SAN? What is the consequence of this?

Answer 22

Upstroke produced by Ca2+ influx

So upstroke is quite slow

Question 23

What types of Ca2+ channel are in the SAN?

Answer 23

L-type (cause main upstroke)

T-type (activate at more negative potentials)

Question 24

What leads to repolarisation in the SAN?

Answer 24

Inactivation of Ca2+ channels

Question 25

Na+ influx in SAN

Answer 25

Very little Na+ influx | In a different type of Na+ channel so only small depolarisation

Question 26

What effect does increased sympathetic stimulation to the SAN have? What neurotransmitter is involved?

Answer 26

Noradrenaline SAN depolarise much more quickly and reaches threshold potential more quickly Fire off AP more quickly Heart rate increases

Question 27

What effect does increased parasympathetic stimulation to the SAN have? What neurotransmitter is involved?

Answer 27

ACh | Slows diastolic depolarisation in SAN, slower to reach threshold potential, slows heart rate

Question 28

Chronotropy

Answer 28

Describes changes in heart rate

Question 29

Positive chronotropic effect

Answer 29

Faster heart rate

Question 30

Iontropy

Answer 30

Describes cardiac contractility

Question 31

Positive inotropic effect

Answer 31

Increase in contractility

Question 32

How do the cardioregulatory centre and vasomotor centres in the medulla modulate intrinsic heart rate?

Answer 32

PNS innervation inhibits heart rate (Vagus nerve) | SNS innervation increases heart rate and contractility (sympathetic nerves)

Question 33

Location of sinoatrial node

Answer 33

Lies just below the epicardial surface at the boundary between right atrium and superior vena cava

Question 34

What does spontaneous depolarisation in the SAN allow?

Answer 34

Allows the heart to generate its own rhythm (autorhythmicity)

Question 35

Internodal fibres

Answer 35

Rapid conduction tracts to stimulate atrial myocardium

Question 36

Atrioventricular node

Answer 36

Specialised cells to delay wave of excitation and insulate from superior ventricular myocardium

Question 37

Bundle of his

Answer 37

Rapid conduction cells to transport an insulated wave of excitation

Question 38

Ventricular fibres

Answer 38

Propagate the impulse across the ventricular myocardium

Question 39

Describe the cardiac conduction system

Answer 39

Start at SAN Excitation passes along internodal fibres: rapid conduction across atria Blood pushed into ventricles Conduction slowed at AVN: delay wave of excitation from moving down into ventricles to allow filling of ventricles from contraction of atria Excitation moves down bundle of His which possesses rapid conduction fibres (slightly insulated) to apex Wave of excitation spreads up base causing ventricular excitation

Question 40

What is the impulse propagation of the cardiac AP due to?

Answer 40

a combination of passive spread of current and the existence of a threshold which, once reached, causes the cell to generate its own AP.

Question 41

What do gap junctions allow in reference to a cardiac cell?

Answer 41

Greatly reduce membrane resistance allowing current to easily leak from one cell to a neighbouring cell.

Question 42

What do intercellular communication and impulse conduction from one cell to the next relies on?

Answer 42

Gap junctions

Question 43

Where do gap junctions form in the heart?

Answer 43

At intercalated discs

Question 44

What are there many of in gap junctions? What do these do?

Answer 44

Formed of connexons Connexons consist of connexins Form a tube allowing a low resistance conduction pathway from one cardiac cell to another