Control Of Heart Function

Question 1

2 nodes of the heart

Answer 1

Sinoatrial node:pacemaker 60-100bpm. Found in junction if crista terminallims

Atrioventriculat node:Has pacemaker activity as it slows calcium mediated action potential. Found at triangle of koch

Question 2

What tract connects SAN and AVN

Answer 2

Internodal tracts which are specialized myocytes

Question 3

Which fibres have the fastest conduction velocity

Answer 3

Purkinje fibres

Question 4

3 phases of sinoatrial node excitation

Answer 4

Phase 4 pre potential: Na+ influx through funny channels. Nodal cells have no resting membrane potentials. Increase in mp

Phase 0 upstroke: calcium influx increases mp

Phase 3 repolarisation occurs due to k+ efflux

Question 5

Why do different parts of the heart have different action potential shapes

Answer 5

As different ion currents flow and different ion channel expressions in cell membrane

Question 6

Difference in length of AP between cardiac myocytes and nerves

Answer 6

Cardiac ap is longer 200-300ms vs 2-3ms
Longer as duration of AP controls duration of contraction and long slow contraction is needed for effective pump

Question 7

Ventricular cell action potential

Answer 7

5 phases

Phase 0 - Upstroke (-90 mV up to ~20-30 mV) → Na+ influx

Phase 1 - Early re-polarisation → Decrease in Na+ permeability & K+ efflux

Phase 2 - Plateau (maintains cell at a level of depolarisation at value of 0mV) → Due to Ca2+ influx

Phase 3 - Re-polarisation → Happens at ~270ms after stark of upstroke → K+ efflux

Phase 4 - RMP - Na+/K+ ATPase pump

Question 8

What two parts of CNS control heart

Answer 8

Autonomic NS- the cardioregulatory centre and vasomotor centres in medulla
Parasympathetic communicates through vagus nerve causing a decrease in heart rate by decreasing phase 4 slope of SA nodal cell thus decreases pressure

Symlathiect cimmunjcates via sympathetic nerves causing increase in heart rate (chronograph) thus increase in slop of phase 4. Increases force of contraction (inotropy) as increases ca2+ thus increase bp

Question 9

What do parasympathetic nerves consist of, and which part of the spinal cord do they arise from?

Answer 9

• Arise from cranial and sacral part of spinal cord
• They have pre-ganglionic fibres that release ACh as NT
• Post-ganglionic fibres also release ACh
• Important for controlling heart rate

Question 10

What type of receptor on the SA nodal cell receives the post-ganglionic fibre? For pns

Answer 10

M2 muscarinic receptor → g-coupled receptor

Coupled with Gi protein which inhibits adenylyl cyclase which prevents conversion of ATP to protein kinase A

Question 11

What do sympathetic nerves consist of, and which part of the spinal cord do they arise from?

Answer 11

• Arise from thoracolumbar part
• Pre-ganglionic fibres use ACh
• Post ganglionic fibres use NA
• Synapse onto paravertebral ganglia
• Important for controlling circulation

Question 12

What type of receptor on the SA nodal cell receives the post-ganglionic fibre? SNS

Answer 12

beta-1-receptor

Stimulate adenylyl cyclase and increase levels of protein kinase A through second messenger pathway

Question 13

Where is vasomotor centre located

Answer 13

Bilaterally in reticular substance of medulla & lower third of pons

Question 14

What is VMC composed of

Answer 14

• Vasoconstrictor area
• Vasodilator area
• Cardio-regulatory inhibitory area

Question 15

What does VMC do

Answer 15

Transmits impulses distally through spinal cord to almost all blood vessels

Many higher centres of brain such as hypothalamus can exert powerful excitatory or inhibitory effects on the VMC

• What do the lateral VMC portions do?Lateral portions of VMC controls heart activity by influencing heart rate and contractility
• What does the medial VMC portion do?Medial portion of VMC transmits signals via vagus nerve to heart that tend to decrease HR

Question 16

What does sympathetic innervation to kidney do?

Answer 16

• Decrease glomerular filtration → reduced Na+ excretion
  
  • This causes increase in blood volume (also done through aldosterone)
  • Change in blood volume is detected by venous volume receptors
• Increase in renin secretion which increases angiotensin II production
  
  • Causes vasoconstriction and increase in blood pressure which is detected by arterial baroreceptors
  • Also causes aldosterone release which impacts blood volume

Question 17

What part of the kidney do sympathetic nerves innervate?

Answer 17

Afferent and efferent arterioles of glomerulus and nephron tubule cells

Question 18

What happens at afferent arterioles due to sympathetic activity (2)?

Answer 18

• Primary site of sympathetic activity
• Activation of alpha-1-adrenoreceptors by noradrenaline (NA) causes vasoconstriction
  
  • This reduces glomerular filtration rate which decreases Na+ filtered
• Juxtaglomerular cells are site of synthesis, storage and release of renin
  
  • Activation of beta-1-adrenoceptor in JGA cells causes renin secretion which increases blood volume (and increases blood pressure)

Question 19

• What does the cardiopulmonary circuit consist of and what do they sense?

Answer 19

• Large pulmonary vessels
• Are volume sensors (also atria and right ventricle)- send signals through glossopharyngeal and vagus nerves

Question 20

Explain how the cardiopulmonary circuit responds to a decrease and increase in volume?

Answer 20

• Decrease in filling (due to less blood returning to heart) → decreased baroreceptor firing → increased SNS activity
• Distention (more filling of heart) → increased baroreceptor firing → decreased SNS activity

Question 21

What does the arterial circuit consist of (3)?

Answer 21

Aortic arch, carotid sinus and afferent arterioles of kidneys

Question 22

Explain how the arterial circuit responds to a decrease and increase in pressure?

Answer 22

• Pressure sensors- send signals through glossopharyngeal and vagus nerves
• Decrease in pressure (detected by baroreceptors in carotid sinus & aortic arch) → decreased baroreceptor firing → increased SNS activity (received by beta-1-receptor of SAN on heart, which increases blood pressure)
• ## Increase in pressure → increased baroreceptor firing → decreased SNS activity (which decreased blood pressure)

Question 23

What are the 2 circulations of the blood?

Answer 23

• Pulmonary and systemic
• Right heart → lungs → left heart → body

Question 24

What is venous volume distribution affected by

Answer 24

Veins and venules contain 61% of blood

• Peripheral venous tone
• Gravity
• Skeletal muscle pump
• Breathing

Question 25

What is central venous pressure and what does it determine?

Answer 25

Mean pressure in the right atrium

Determines amount of blood flowing back to heart, which in turn determines stroke volume (Starling’s Law)

Question 26

How does constriction of veins affect compliance and venous return?

Answer 26

Reduces compliance and increases venous return

Question 27

• What does constriction in arteries determine?

Answer 27

• Blood flow to downstream organs
• Mean arterial blood pressure
• The pattern of blood flow to organs

Question 28

What are some local mechanisms which regulate local blood flow in an organ? (4)

Answer 28

Are intrinsic to smooth muscle and are endothelium-derived mediators:

• Nitric oxide (NO): potent vasodilator, which diffuses into vascular smooth muscle cells.
• Prostacylin: vasodilator that also has antiplatelet & anticoagulant effects
• Thromboxane A2 (TXA2): vasoconstrictor that is also heavily synthesised in platelets
• Endothelins (ET): vasoconstrictors generated from nucleus of endothelial cells

Question 29

What are some systemic mechanisms which regulate blood flow? (5)

Answer 29

These include autonomic NS and circulating hormones and non-endothelium-derived mediators:

• Kinins: bind to receptors on endothelial cells & stimulate NO synthesis – vasodilator effects
• Atrial natriuretic peptide (ANP): secreted from the atria in response to stretch – vasodilator effects to reduce BP
• Vasopressin (ADH): secreted from pituitary gland. Binds to V1 receptors on smooth muscle to cause vasoconstriction
• Noradrenaline/Adrenaline: secreted from adrenal gland (& SNS); causes vasoconstriction
• Angiotensin II: potent vasoconstrictor from the renin-angiotensin-aldosterone axis. Also stimulates ADH secretion.