75. The origin and conduction of the heart

Question 1

What is the heart

Answer 1

electrically controlled muscular pump

Question 2

what is autorhythmicity

Answer 2

the fact that the heart can beat rhythmically in the absence of external stimuli

Question 3

where in the heart does the excitation normally originate to initiate the heart beat

Answer 3

In Pacemaker cells in the Sino-Atrial Node

Question 4

where is the Sino-Atrial Node located

Answer 4

Upper Right Atrium (close to where the superior vena cava enters the right atrium)

Question 5

A heart CONTROLLED by the Sino-Atrial node is said to be in …. …..

Answer 5

sinus rhythum

Question 6

The cells in the SA NODE have ……. resting membrane potential

Answer 6

no stable

Question 7

The cells in the SA node generate

Answer 7

regular spontaneous pacemaker potentials

Question 8

The spontaneous pacemaker potentials takes the membrane to what

Answer 8

the threshold

Question 9

when the threshold is reached, what is generated

Answer 9

Action potential - This results in the generation of REGULAR SPONTANOUS ACTION POTENTIALS in the SA nodal cells

Question 10

what is the threshold for Intracellular Recording from SA Node Cell

Answer 10

-40mV (from an initial -60mV)

Question 11

what is the pacemaker potential

Answer 11

the slow depolarization of membrane potential to the threshold aka -40mV (from an initial -60mV)

Question 12

what causes the pacemaker potential

Answer 12

1. The funny current - slow Na+ influx through HCN channels which are open when the cell is hyperpolarized (more negative)
2. Decrease K+ efflux at the end of hyperpolarization period, less K+ leaves the cell which helps the inside of the cell to become less negative
3. Ca2+ Influx also start to enter the cell, contributing to the gradual depolarization. (less negative)

Question 13

once the threshold is reached it causes what

Answer 13

action potential (ie depolarization)

Question 14

The RISING PHASE OF ACTION POTENTIAL (i.e. DEPOLARISATION) is caused…. HINT - what channels and what influx

Answer 14

by opening of long lasting (L-TYPE) voltage-gated Ca++ CHANNELS
Resulting in Ca++ INFLUX

Question 15

what is the falling phase of action potentials called

Answer 15

repolarization

Question 16

what is repolarization caused by

Answer 16

• inactivation of L type Ca2+ channels
• activation of K+ channels resulting in K+ efflux

Question 17

cardiac excitement spreads from the Sino Atrial node to….?

Answer 17

Atrio-Ventricular Node

Question 18

how does excitement spread cell-to-cell

Answer 18

Via Gap Junctions (and maybe Desmosomes but i will check!)

Question 19

what is the AV node

Answer 19

a small bundle of specialized cardiac cells

Question 20

where is the AV node located

Answer 20

base of the right atrium

Question 21

what are the properties of the AV node cells

Answer 21

small in diameter, slow conduction velocity

Question 22

spread of excitation: From SA node to AV node: mainly cell-to-cell conduction via gap junctions; but there is also some

Answer 22

Internodal Pathways

Question 23

conduction is delayed where?

Answer 23

Av Node

Question 24

what does delay in conduction at the AV node allow

Answer 24

atrial systole (contraction) to proceed ventricle systole

Question 25

The Bundle of .....and its branches and the network of ......... allow rapid spread of action potential to the ventricles

Answer 25

His, Purkinje fibers

Question 26

Is the action potential in contractile cardiac muscle cells different to the action potential in pacemaker cells

Answer 26

YES

Question 27

the resting membrane potential remains at ................mV until the cell is excited (cardiac muscle cell)

Answer 27

-90mV

Question 28

The rising phase (depoalrisation) is caused by what in cardiac muscle cells

Answer 28

fast Na+ influx

Question 29

The rising phase (depoalrisation) in cardiac muscle cells reverses the membrane potential to ......mV

Answer 29

+20 mV

Question 30

what is phase 0 of Contractile Myocytes Action Potential

Answer 30

Fast Na+ influx

Question 31

what is phase 1 of Contractile Myocytes Action Potential

Answer 31

closure of Na+ channels and transient (brief) K+ efflux

Question 32

what is phase 2 of Contractile Myocytes Action Potential

Answer 32

mainly Ca2+ influx (through L type Ca2+ channels)

Question 33

what is phase 3 of Contractile Myocytes Action Potential

Answer 33

closure of Ca2+ channels and K+ efflux

Question 34

what is phase 4 of Contractile Myocytes Action Potential

Answer 34

resting membrane potential

Question 35

which phase of Contractile Myocytes Action Potential is the Plateau phase

Answer 35

2

Question 36

what is the plateau phase of Contractile Myocytes Action Potential

Answer 36

the membrane potential is maintained near the peak of action potential for a few hundred milliseconds

Question 37

Plateau phase is unique to Contractile Myocytes Action Potential or Pacemaker cells

Answer 37

Contractile Myocytes Action Potential

Question 38

the falling phase is which stage of the contractile myocyte potentials

Answer 38

3

Question 39

It is possible to record the spread of electrical activity through the heart from the skin surface. What is this test?

Answer 39

Electrocardiogram (ECG)

Question 40

how does an ECG work

Answer 40

the wave of depolarisation and repolarisation moves across the heart and sets up electrical currents which can be detected by surface electrodes

Question 41

what are the standard ECG limb leads

Answer 41

Lead 1 - Right arm and left arm Lead 2 - right arm and left leg Lead 3 - Left arm and Left leg

Question 42

what is the P wave

Answer 42

atrial depolarization

Question 43

what is the QRS complex

Answer 43

Ventricular depolarisation

Question 44

what is the T wave

Answer 44

ventricular repolarisation

Question 45

what is PR interval

Answer 45

largley AV node delay

Question 46

what is the ST segment

Answer 46

Ventricular systole occurs here

Question 47

What is TP interval

Answer 47

diastole occurs here

Question 48

whats the order of ECG waves and intervals

Answer 48

P, PR Interval, QRS complex, ST segments, TP interval

Question 49

which nerve is the parasympathetic supply to the heart

Answer 49

Vagus

Question 50

what does the vagus nerve do on normal resting conditions

Answer 50

slows intrinsic heart rate from 100bpm to a normal resting heart rate of 70bpm

Question 51

finish the sentence. Vagus nerve supplies the ........... and ........... And what does the Vagus Nerve do when stimulated

Answer 51

SA Node and AV Node. Slows rate of firing from SA Node and increases AV Node delay

Question 52

when the vagus is stimulated the slope of pacemaker potential ............ and takes less/more time to reach threshold

Answer 52

decreases. More.

Question 53

cardiac sympathetic nerves supply what

Answer 53

SA Node, AV Node and Myocardium

Question 54

sympathetic stimulation does what?

Answer 54

increases the rate of firing from SA Node and decrease AV Node delay and increases force of contraction

Question 55

what neurotransmitter is acting on which receptors for sympathetic stimulation

Answer 55

Noradrenaline acting on beta 1 adrenoreceptors

Question 56

pacemaker potential reaches threshold slower/quicker. Frequency of action potentials increase/decrease

Answer 56

quicker increase

Question 57

whats the main difference between cardiac myocytes and pacemaker cells

Answer 57

Cardiac Myocytes: Contraction and force Respond to electrical impulses Stable resting membrane potential with a plateau phase in their action potential. Pacemaker Cells: Heart rhythm Spontaneous depolarization Without a stable resting membrane potential.