lecture 4 : cardio vascular control 1

Question 1

how is the resting membrane potential maintained?

Answer 1

• the membrane is only permeable to K+ at rest
• the potential across it will equal the K+ equilibrium potential
• the concentration of K+ can be further maintained using sodium potassium ATPases
• sodium gets taken out and K is put in

Question 2

what would happen without the ATPases

Answer 2

the concentration gradients would collapse

Question 3

how does the membrane potential change?

Answer 3

the membrane potential will change with changes to the permeability of the membrane to various ions

Question 4

what can we use to calculate the membrane potential?

Answer 4

either with the nernst equation (less accurate)

with goldman bodkin katz equation

this takes into account the actual permeabilities

Question 5

what controls the strength of the heart beat

Answer 5

• the duration of action potential controls the duration of the contraction of the heart
• long slow contraction is needed to provide an effective pump

Question 6

what is absolute refractory period?

Answer 6

this is the time during which no action potential can be initiated regardless of the intensity of the stimulus

Question 7

what is a relative refractory period?

Answer 7

this is the period after the absolute period where an action potential can happen but only with a stimulus larger than normal

Question 8

what are the refractory periods caused by?

Answer 8

they are caused by Na+ channel inactivation

Question 9

why is the relative refractory period important?

Answer 9

• you cannot get a thing called tetanus which is present in skeletal muscles
• this means the the heart can refill up with blood during diastole

Question 10

what are the phases of the cardiac action potential?

Answer 10

phase 0: upstroke 
phase 1: early depolarisation 
phase 2: plateau 
phase 3: repolarisation 
phase 4: resting membrane potential

Question 11

what is upstroke caused by?

Answer 11

the upstroke is caused by an increase in the permeability of the membrane to sodium

Question 12

small repolarisation

Answer 12

increase in channels that give rise to a current called a transient outward current

• this is carried by potassium ions
• very brief

Question 13

what brings about contraction

Answer 13

• the increase in the permeability of the cell to calcium ions
• the influx of calcium promotes the release of more Ca2+ channels from the stores in the sarcoplasmic reticulum

Question 14

how does the depolarisation take place

Answer 14

• slow increased in potassium happening

- that opens up which stops the contraction and brings it back to the resting membrane potential

Question 15

why is the plateau phase prolonged?

Answer 15

• balance of inward movement of calcium and outward movement of potassium

Question 16

how does depolarisation happen?

Answer 16

specialised potassium current

Question 17

why are there different shapes of action potential profiles?

Answer 17

• different parts of the heart have different action potential shapes due to different ionic currents flowing

Question 18

how are the electrical properties of the heart intrinsic?

Answer 18

• capable of independent spontaneous generation and coordinated propagation of the electrical activity
• there is a specialised conduction system
• the heart can beat independently even after being separated from its nervous supply

Question 19

what modulates the pacemaker activity?

Answer 19

the autonomic nervous system

Question 20

show the action potential of the ventricular cell:

Answer 20

insert pic

Question 21

show the action potential of the SA node cell?

Answer 21

insert pic

Question 22

why does the the action potential in the SA node look different?

Answer 22

• this is because not all ion channels exist in the SA node
• there is no K1 ion channel
• this means there is never a stable resting potential
• the upstroke is not caused by sodium
• the upstroke is caused by calcium entry

Question 23

what two types of channels are there

Answer 23

T type - activate at more negative potentials than 
L type 
(also present in smooth muscle) 

L type-

Question 24

what modulates the heart rate?

Answer 24

the sympathetic and parasympathetic nervous system

these are attached to the SA nodal cell

Question 25

what modulates the heart rate?

Answer 25

the sympathetic and parasympathetic nervous system

these are synapsed to the SA nodal cell

Question 26

neurotransmitter released?
parasympathetic side?

sympathetic side ?

Answer 26

acetylcholine

noradrenaline

Question 27

what happens if you increase the sympathetic side?

Answer 27

there would be a lot more noradrenaline at the nerve endings

• the pacemaker cells depolarise much more quickly so they fire action potentials more quickly than normal so the
• heart rate increases
• contractility increases

affects SA node

Question 28

what happens if you increase the parasympathetic side?

Answer 28

it slows down the depolarisation
therefore slowing down the heart rate

affects SA node
vie the vagus nerve

Question 29

where does the SA node sit?

Answer 29

• it lies just below the epicardial surface at the boundary between the right atrium and the superior vena cava

INSERT PIC

Question 30

what is the cardiac conduction pathway?

Answer 30

• starts at sinoatrial node
• moves across internal fibres
• conduction is slowed down at the atrioventricular node
• moves to he bundle of His
  which moves the conduction to the apex of the heart
• then the conduction moves to the ventricular fibres which then spread the wave of excitation across the ventricle

Question 31

what is the propagation of the cardiac action potential caused by?

Answer 31

• a combination of passive spread of a current

- the existence of threshold which once reached can generate its own action potential

Question 32

what is the function of gap junctions?

Answer 32

greatly reduces the membrane resistance which allows the current to leak between one cell and the next cell

Question 33

what is the structure of a gap junction?

Answer 33

INSERT PIC

Question 34

what kind of membrane does the SA node have?

Answer 34

an unstable membrane potential

Question 35

what is the order of excitation through the heart?

Answer 35

• SA node - atrial bundles - AV node - bundle of His - L and R bundle branches - ventricular purkinje system