Electrical Properties of the Heart Muscle

Question 1

Describe the general structure of cardiac muscle fibres

Answer 1

Similar to skeletal muscles, composed of long thin myofibrils and contract as sarcomere shortens
Cardiac muscle fibres are connected via gap junctions

Question 2

What are intercalated discs

Answer 2

Sites of thickening of sarcolemma where cells join together

i think these are like gap junctions that allow transfer of things like ions between fibres, specific to myocardial cells

Question 3

What does it mean when myocytes form electrical/functional syncytium

Answer 3

Where cells contract in a synchronous fashion - this is important for the pumping action of the heart.

Question 4

Which ion has the greatest influence on magnitude of the resting potential and why

Answer 4

K+

There is a substantial K+ gradient and the membrane is relatively permeable to K+

Question 5

Are the equilibrium potentials of K+ and Na+ relatively more or less negative than the resting potential

Answer 5

K+ is more negative

Na+ is a lot more positive

Question 6

What is an equilibrium potential of an ion

Answer 6

the equilibrium potential is the membrane potential where the net flow through any open channels is 0.

Question 7

Describe the phases of a cardiac muscle’s action potential

Answer 7

• Na+ floods in, down gradient, membrane potential rises to 20-30mV and Na+ channels are inactivated
• K+ permeability increases and K+ leaves at a higher rate down gradient
• Ca2+ channels simultaneously open and Ca2+ flows in causing a plateau of the membrane potential
• K+ efflux exceeds Ca2+. Ca2+ channels are inactivated.
• Membrane potential falls to K+ equilibrium

Question 8

What is the absolute refractory period

Answer 8

When sodium channels close at the peak of an action potential and remain closed during the plateau phase. Muscle stimulation cannot happen in this period.

Question 9

What is the Relative Refractory Period (RRP)

Answer 9

Between -50mV and complete repolarisation a further action potential can be generated but this requires a greater than normal stimulation

Question 10

What is the importance of the long plateau phase in the cardiac action potential

Answer 10

The plateau outlasts the mechanical activity, so individual contractions cannot fuse into a maintained titanic contraction as in skeletal muscle.
This is important as the heart has to beat rhythmically.

Question 11

What is pacemaker tissue in the heart

Answer 11

Denervated cardiac muscle continues to contract rhythmically

Question 12

What does it mean that pacemaker tissues have automaticity

Answer 12

They have the ability to initiate their own beat. Cells of this tissue can spontaneously depolarise

Question 13

Give examples of pacemaker tissues in the heart

Answer 13

sinoatrial (fastest)
atrioventricular nodes
Bundle of His
Purkinje fibres

Question 14

Describe the shape of pacemaker tissue action potentials

Answer 14

Upstroke is more gradual, no depolarisation plateau

Question 15

Describe the process of an action potential of in pacemaker tissue

Answer 15

• Depolarisation happens at threshold potential. Causing Na+ channels to open and followed by Ca2+ influx
• Depolarisation due to K+ efflux
• Cells have an unstable resting potential and gradually depolarise from -60 to -40mV due to slow continuous influx of Na+ and decreased efflux of K+

Question 16

What is the role of the Sinoatrial Node (SAN)

Answer 16

Serves as the primary pacemaker of the heart and determines the rate at which the whole heart beats

Question 17

What is the role of the Sinoatrial Node (SAN)

Answer 17

Serves as the primary pacemaker of the heart and determines the rate at which the whole heart beats
(has the fastest rate of drift of the resting potential??)

Question 18

Where is the SAN

Answer 18

Right atrium

Question 19

Where does the wave of electrical activity from the SAN spread

Answer 19

• Across the right and then the left atrium (via the bachmanns bundle)
• Enters the Atrioventricular (AV) node

Question 20

What is the role of the AV node

Answer 20

Acts as a conductor of impulses from the atria to the ventricles
Impulse conducts very slowly through the AV node producing the delay that allows the ventricles to fill with blood after the atria have contracted.

Question 21

After the AV node where does the electrical impulse go

Answer 21

From the AV node through the Bundle of His (right and left branches).
These then subdivide into a complex network of conducting fibres, Purkinje system
Allows almost simultaneous contraction of the right and left ventricles

Question 22

What is an ECG

Answer 22

An electrocardiogram is a graphic made by an electrocardiograph that records the electrical voltage in the heart in the form of a continuous strip graph

Question 23

What is the P wave formed by on an ECG

Answer 23

The conduction of the electrical impulse through the atria, this is the first bump on the ECG

Question 24

What forms the PR interval on the ECG

Answer 24

The delay in the AV node forms much of the PR interval along with part of atrial repolarisation

Question 25

What forms the QRS complex on the ECG

Answer 25

The spread of electrical activity through the ventricular myocardium via the bundle of His and purkinje fibres.
Seen as the main spike in the ECG

Question 26

What forms the T wave on the ECG

Answer 26

The repolarisation of the ventricles

Question 27

What is the normal range for the PR interval

Answer 27

120-200ms

Question 28

What is the normal range for the QRS complex

Answer 28

80-120ms

Question 29

What does a long QRS complex indicate

Answer 29

Conduction abnormalities

Question 30

What does a shorter and longer PR interval indicate

Answer 30

Shorter - leaky annulus fibrosis (Wolf-parkinson-white syndrome)
Longer - AV conduction block

Question 31

What is the consequence of an AV block

Answer 31

A pacemaker site distal to the block will become the new pacemaker for the heart
These secondary pacemaker sites generally have a slower intrinsic rate.
Therefore an AV conduction block = Ventricular bradycardia