Control Of Heart Function

Question 1

What are the nodes of the heart and where are they found

Answer 1

Sinoatrial node, pacemaker, junction of crista terminalis; upper wall of right atrium and opening of superior vena cava

Atrioventricular node, has pacemaker activity; slow calcium mediated action potential, triangle of Koch at base of right atrium

Question 2

What tracts are found in the heart and where are they located

Answer 2

Internodal tracts
Bundle of his and bundle branches
Specialised myocytes,
SA node-> internodal tracts -> AV node -> His bundle -> branches at intraventricular septum -> apex

Purkinje fibres are specialised conducting fibres

Question 3

What phases does a nodal cell action potential have

Which phase is due to Ca2+ influx
Which phase is due to K+ efflux
Which phase is due to Na+ influx

Answer 3

Phase 4 prepotential, phase 0 upstroke, phase 3 repolarisation

Upstroke
Repolarisation
Prepotential; no resting potential, Na through funny channel

Question 4

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

How does a cardiac muscle AP differ to a nerve cell AP

Answer 4

Different ion currents flowing and different ion channel expression in cell membrane

Longer, as it controls duration of contraction, long, slow contraction needed for effective pump

Question 5

What are the phases of a cardiac muscle AP, and what ion activities cause them

When do the absolute and relative refractory periods occur

Answer 5

Phase 0 upstroke, Na influx
Phase 1 early repolarisation, little K efflux
Phase 2 plateau (maintains depolarisation), Ca influx
Phase 3 repolarisation, K efflux
Phase 4 resting membrane potential , K efflux

ARP between upstroke and end of phase 2
RRP between phase 3 (requires larger stimulus strength)

Question 6

What area of the brain is involved in exogenous heart control, and which nerves bring about a response

Answer 6

Cardio regulatory centre and vasomotor centres in medulla

Parasympathetic nerves carried by vagus
Sympathetic nerves

Question 7

How is the heart rate affected by parasympathetic and sympathetic nerves

Answer 7

Reduced HR - decreases slope of phase 4

Increased HR (chronotropy) increases slope of phase 4
Increased force of contraction (inotropy) increases Ca2+ dynamics

Question 8

Compare Parasympathetic and sympathetic nerves
where they arise from
Neurotransmitter
Post ganglionic fibre length
What it controls (cardiovascular)

Answer 8

Cranial part of spinal cord and sacral part of spinal cord, vs thoracic and lumbar vertebra
Ach, Vs Ach preganglionic and NA postganglionic
Short vs long
Heart rate vs circulation

Question 9

Where do sympathetic nerves tend to synapse

Answer 9

At sympathetic ganglia (paravertebral ganglia)

Question 10

What is the vasomotor centre composed of

Answer 10

Vasoconstrictor (pressor) area
Vasodilator (depressor) area
Cardio-regulatory inhibitory area

Question 11

In the parasympathetic system, what receptors does Ach act on and what does this cause

In the sympathetic system, what receptors does NA act on and what does this cause

Answer 11

M2 muscarinic receptors, Gi protein causes inhibition of adenyl Cyclase, prevents conversion of ATP to protein Kinase A

B1 receptors, Gs protein stimulates adenyl cyclase, catalyses conversion of ATP to protein Kinase A

Question 12

How do the kidneys affect the control of the heart indirectly
Blood volume and pressure

Answer 12

Sympathetic nerves innervate kidney, reduce glomerular filtration -> red Na+ excretion: increase in blood volume (aldosterone)
Increased Blood volume detected by venous volume receptors,

Sympathetic nerves increase renin secretion -> increased angiotensin-II production: vasoconstriction and increased blood pressure
Angiotensin also causes release of aldosterone (blood volume)
Increased blood Pressure detected by arterial baroreceptors

Question 13

How does the brain affect glomerular filtration

Answer 13

Sympathetic nerve fibres innervate afferent and efferent arterioles of glomerulus
Afferent- primary site of sympathetic activity
a1 adrenoceptor -> vasoconstriction = lowered GFR -> less Na+ filtered

Question 14

What cells synthesise, store and release renin and what receptor is involved in renin secretion

Answer 14

Juxtaglomerular cells

B1 adrenoceptor

Question 15

What occurs when volume sensors detect
decreased filling of the heart
Distention

Answer 15

Reduced baroreceptor firing -> increased sympathetic NS activity

Increased baroreceptor firing -> reduced SNS activity

Question 16

Where are volume sensors found, and what nerves do they send signals down

Answer 16

Large pulmonary vessels, atria, right ventricle

Glossopharyngeal and vagus nerves

Question 17

How is the heart controlled through the arterial circuit, and what vessels does this involve

Answer 17

Aortic arch, carotid sinus, afferent arterioles of kidneys
Pressure sensors: send signals through glossopharyngeal and vagus nerves
Decrease in pressure -> reduced baroreceptor firing -> increased SNS activity
And vice versa

Question 18

What affects venous volume distribution
What is central venous pressure
How does this relate to stroke volume

Answer 18

Volume distribution affected by venous tone, gravity, skeletal muscle pump and breathing

Central venous pressure (mean pressure in right atrium) determines amount of blood flowing back to heart

Amount of blood flowing back to heart determines stroke volume (Starling’s law)

Question 19

How do you increase venous return in veins

Answer 19

Constriction reduces compliance and increases venous return

Question 20

What does constriction determine in arterioles

Answer 20

Blood flow to downstream organs
Mean arterial blood pressure
Pattern of blood flow to organs

Question 21

What mechanisms regulate local blood flow within an organ

Answer 21

Intrinsic to or closely related to smooth muscle, endothelium derived mediators
Vasodilator- Nitric oxide and prostacyclin
Vasoconstrictors- thromboxane and endothelins

Question 22

What are the systemic mechanisms of regulating blood flow

Answer 22

Extrinsic to smooth muscle, include ANS and circulating hormones
Non endothelium derived mediators

Vasodilators- kinins stimulate NO synthesis, atrial natriuretic peptide (ANP) which is secreted from atria in response to stretch

Vasoconstrictors- ADH, noradrenaline and adrenaline, angiotensin II (renin-angiotensin-aldosterone axis, also stim ADH release)