Physiology: cardiac muscle

Question 1

CARDIAC MUSCLE CELLS
Describe the shape
Are they striated or not?
What structure connects adjacent cardiomyocytes? What 2 sub-structures does it contain?
How many nuclei per cell?
Size of mitochondria, and implications of this?

Answer 1

Short, branching.
Yes
Intercalated discs - desmosomes and gap junctions
1-2
Large - fatigue resistant.

Question 2

PACEMAKER CELLS
What locations are they found in?
Pacemaker cells create action potentials.
Are APs created spontaneously by the heart, and can they be moderated by the ANS and other factors?
What are the 3 components of AP’s made by pacemaker cells?

Answer 2

SA node, AV node, bundle of His/AV bundle
Yes, yes
Pacemaker potential, depolarisation, repolarisation

Question 3

PACEMAKER CELLS
What is happening with ion influx/efflux at:
- Pacemaker potential
- Depolarisation
- Repolarisation

Answer 3

Na+ channels open (influx), K+ channels closing
Ca2+ channels opening (influx)
Ca2+ channels inactivate, K+ channels opening (efflux)

Question 4

PACEMAKER-CONDUCTION SYSTEM
Describe the conduction pathway of the heart

Answer 4

SA node –> AV node –> AV bundle/bundle of His –> Purkinje fibres (subendocardial branches)

Question 5

PACEMAKER-CONDUCTION SYSTEM
Describe how impulses go from the SA node to the AV node; and the SA node to the left atrium

Answer 5

Excite cells in the right atrium; pass through 3 internodal tracts to the AV node
Travel through Bachmann’s bundle to excite the left atrium

Question 6

PACEMAKER-CONDUCTION SYSTEM
How does the AV node delay the AP?
What is the importance of this?

Answer 6

Smaller diameter of cells, fewer gap junctions
Allows atria to complete contraction and for ventricles to fill, before ventricles contract

Question 7

PACEMAKER-CONDUCTION SYSTEM
Does the fastest pacemaker determine the HR?
When the SA node sets the HR, it’s called the ______ rhythm
What are the speeds of the
- SA node
- AV node
- AV bundle + Purkinje fibres

Answer 7

Yes
Sinus rhythm
60-75 beats/min, 50 beats/min, 30-40 beats/min

Question 8

MODULATION OF HR
What is the effect of the SNS on HR?

Answer 8

Causes adrenal medulla to secrete adrenaline
Symp nerves T1-5 arise from cardioacceleratory centre, and synapse with SA and AV node, ventricles, coronary arteries
In the AV node, increases conduction
- Increases Na+ and Ca+ permeability –> increased rate of pacemaker potential and decreased delay –> increased HR

Question 9

MODULATION OF HR
What is the effect of the PNS on HR?

Answer 9

Nerve synapses from cardioinhibitory centre to dorsal motor nucleus of vagus
Here, vagus nerve projects to SA and AV node.
In AV node, decreases conduction
- Decreases Na+ and Ca+ permeability and increases k+ permeability –> decreased rate of pacemaker potential and decreased delay –> increased HR

Question 10

MODULATION OF HR
Effect of temperature?

Answer 10

Increased temperature (eg. fever) increases HR, cooling slows the heart (eg. open heart surgery)

Question 11

CARDIOMYOCYTE EXCITATION
What are the channels involved?

Answer 11

Na+ channels
L type calcium channels
Transient K+ channels
Delayed rectifier K+ channels

Question 12

CARDIOMYOCYTE EXCITATION
Label the 4 phases of the cardiomyocyte AP

Answer 12

Question 13

CARDIOMYOCYTE CONTRACTION
Describe how this process occurs

Answer 13

AP enters from adjacent cell
AP runs down T tubules of contractile cardiomyocyte
AP causes voltage gated/L type Ca2+ channels to open. This leads to Ca2+ influx into the cell. Ca2+ binds to ryanodine receptors of SR, which releases further Ca2+
Increased cytosolic Ca2+ initiates cross bridge cycling

Question 14

CARDIOMYOCYTE CONTRACTION
- Is contraction more or less dependent on T tubules (compared to SR) than skeletal muscle, why?
- Why do cardiomyocyte APs/contractions last longer than skeletal muscle?
- Why is this important?

Answer 14

More - cardiomyocyte SR has less Ca2+
Long plateau phase, longer absolute refractory period of Na+ channels
Enables sustained contraction - enables ejection of blood from the heart

Question 15

CARDIOMYOCYTE CONTRACTION
- How does SNS/catecholamines impact force of myocyte contraction?

Answer 15

Stimulate adenylate cyclase –> increased PKA –> increased phosphorylation of L type Ca2+ channels, increasing their open state probability and duration

Question 16

CARDIOMYOCYTE RELAXATION
What 2 processes enable this to occur?

Answer 16

Ca2+ ATPase on SR pumps Ca2+ back into SR
3Na+/2Ca2+ exchanger pumps Ca2+ out of the cell.