Control of heart function (32)

Question 1

What regions of the brain control the heart?

Answer 1

cardioregulatory centre and vasomotor centre in medulla

Question 2

What type of nervous control is involved in the heart?

Answer 2

autonomic nervous system

• parasympathetic–> decreases HR (prolongs phase 4)
• sympathetic–> inc. HR (+ve chronotropy) and inc. slope of phase 4 bc cAMP activates If AND inc. force of contraction (+ve inotropy) through inc. calcium influx etc.

Question 3

How does the sympathetic nervous system act on the kidneys to affect blood volume?

Answer 3

• inc. sympathetic activity–> dec. GFR–> less sodium excretion, so more Na+ and H2O retention–> inc. blood volume–> inc. BP

also
- inc. symp. activity–> inc. renin secretion–> inc. angiotensin-2 production–> vasoconstriction–> inc. BP

N.B. no parasympathetic to kidney

Question 4

What are volume sensors?

Answer 4

• type of baroreceptors
• present in large pulmonary vessels, atria and right ventricle
• sense change in volume

Question 5

How do baroreceptors regulate SNS activity?

Answer 5

• decrease in filling/volume/pressure–> dec. baroreceptor firing–> inc. sympathetic nerve activity
• distention/inc. filling/pressure–> inc. baroreceptor firing–> dec. SNS activity

Question 6

What are pressure receptors?

Answer 6

• type of baroreceptor
• present in arterial circuit
• send signals through glossopharyngeal and vagus nerves

Question 7

Where are the pressure sensors of the arterial circuit?

Answer 7

aortic arch, carotid sinus and afferent arterioles of kidneys

Question 8

What are the 3 main functions of the vasomotor centre in medulla/pons?

Answer 8

• vasoconstrictor (pressor) area
• vasodilator (depressor) area
• cardioregulatory inhibitory area

Question 9

What is the general split of functions of the VMC in terms of lateral/medial portions?

Answer 9

• lateral portion: controls heart activity by influencing HR and contractility (sympathetic)
• medial portion: signals via vagus nerve to heart–> dec. HR (parasympathetic)

Question 10

How does the sympathetic branch increase activity at the SAN?

Answer 10

via beta-1 receptors linked to Gs protein–> activated AC

–> inc. cAMP

Question 11

How does the parasympathetic branch reduce activity at the SAN?

Answer 11

via M-2 muscarinic receptors

• Gi linked (inhibitory)
• -> reduce cAMP levels

Question 12

What type of nerves innervate the afferent/efferent arterioles at the glomerulus and what receptors receive the activity?

Answer 12

• sympathetic: release noradrenaline bc post-synaptic
• greater effect on afferent arteriole:

NA acts on alpha-1 adrenoceptor–> vasoconstriction–> less blood goes into glomerulus–> dec. GFR–> less sodium filtered–> ETC… inc. blood volume

Question 13

What receptors on the juxtaglomerular cells lead to renin secretion?

Answer 13

NA acts on beta-1 adrenoceptors

–> renin secretion

Question 14

What are the distributions of blood in the body?

Answer 14

61% veins and venules
11% arteries
7% arterioles and capillaries
9% heart
17% pulmonary circulation

Question 15

What blood flow mediators are released from the endothelium/smooth muscle?

Answer 15

intrinsic; important for reflex local blood flow regulation in an organ

• nitric oxide (vasodilator)
• prostacyclin (vasodilator)
• thromboxane A2 (vasoconstrictor)
• endothelins (vasoconstrictors)

Question 16

What are the non-endothelium-derived mediators?

Answer 16

extrinsic to smooth muscle; systemic mechanisms

• kinins (vasodilator)
• atrial natriuretic peptide (ANP) (vasodilator)
• vasopressin (ADH) (vasoconstrictor)
• noradrenaline/adrenaline (vasoconstrictor)
• angiotensin 2 (vasoconstrictor)

Question 17

What are the 5 phases of the cardiac action potential?

Answer 17

phase 0: upstroke- depolarisation–> contraction
phase 1: early repolarisation
phase 2: plateau
phase 3: repolarisation
phase 4: resting membrane potential
N.B. duration of AP determines duration of contraction of heart
- long, slow contraction is required for an effective pump

Question 18

What is the absolute refractory period?

Answer 18

period during plateau phase where you cannot initiate another heartbeat regardless of stimulus intensity

Question 19

What is the relative refractory period?

Answer 19

period after ARP where an AP can be elicited but only with larger stimulus strength

Question 20

What are the phases of the SAN action potential?

Answer 20

phase 0, 3 and 4

• no early repolarisation or plateau phase
• upstroke due to calcium influx through T-type voltage-gated calcium channels
• no fast Na+ channels, instead there are slow Na+ currents at the end of repolarisation called ‘funny’ currents (If)
• no resting membrane potential, instead is ‘pre-potential’

Question 21

What are the differences between the effect of constriction on veins vs arteries?

Answer 21

• in veins, constriction–> less blood returns to heart–> dec. preload
• in arteries–> constriction causes inc. afterload

Question 22

What ion channel movements occur in a cardiac myocyte?

Answer 22

• Na+ influx in
• then Ca2+ in to maintain plateau phase/depolarisation
• K+ flows out then more out
• return to resting membrane potential (some K+ efflux)

Question 23

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

Answer 23

different ion currents flow and there is different ion channel expression in the cell membranes