L16 - The Cardiac Conduction system

Question 1

Summarise the metabolic control concept

Answer 1

• Increase in metabolic activity leads to…
• Increased release of metabolic byproducts…
• Causes increased resistance vessel dilation
• Permitting increased BF
• Resulting in increased Oxygen and nutrient supply

Question 2

Resistance vessels are sensitive to metabolites found where?

Answer 2

Extracellular fluid and oxygen.

Vascular smooth muscle cells dilate, increasing blood flow

Question 3

Myogenic control concept

Answer 3

IF

• intraluminal pressures rise
• then resistance vessels may constrict
• contraction mediated by stretch activated Ca2+ channels in VSMC membranes

Question 4

IF arterial pressure were to rise suddenly…

Answer 4

Flow will increase

• metabolites washed away faster than produced
• resistance vessels constrict reflexively

Question 5

Sudden drop in arterial pressure may lead to…

Answer 5

Reflex vasodilation

Question 6

Reactive hyperaemia

Answer 6

Period of increased BF that follows transient ischaemia, caused by arterial occlusion.

Question 7

What hormones help control blood flow? (4)

Answer 7

• Epinephrine
• atrial natriuetic peptide
• angiotensin II
• ADH

Question 8

Briefly, what modulated release of ADH?

Answer 8

• released from posterior pituitary

- when tissue osmolarity rises or blood volume falls.

Question 9

Where does ADH bind?

Answer 9

Binds to vasopressin V1a receptors n VSMC

V1a receptor couple to IP3 pathway

Question 10

Angiotensin II

Answer 10

Potent vasoconstrictor.
Appears in blood stream when renal artery pressure falls.
Binds to angiotensin AT1 receptor on VSMC, which couple to the IP3 signaling pathway

Question 11

Epinephrine

Answer 11

Adrenal medulla, SNS response.

Increase myocardial contractility and HR.

Binds:

• A1 adrenergic receptor VSMC –> V. CCONSTRICTION
• B2 adrenergic receptor –> V.DILATION

Question 12

What is released by atrial myocytes if they are stretched by high blood volume?

Answer 12

Atrial myocytes will release ANP.

Question 13

Role of ANP

Answer 13

• vasodilation
• binds to natriuretic peptide receptors, NPR1 on VSMCs
• NPR1 is a guanyl cyclase that relaxes VSMCs by activating protein kinase G.

Question 14

Summarise what chemicals contribute to endothelial control? (4)

Answer 14

• nitric oxide
• prostaglandins
• endothelins
• endothelium derived hyperpolarising factor

Question 15

Vasodilators produced by the endothelium

Answer 15

• NO
• Prostacyclin
• Endothelium-derived hyperpolarizing factors

Question 16

Vasoconstrictors produced in the endothelium

Answer 16

• endothelin
• thromboxane A2
• superoxide

Question 17

Describe how NO is produced and its action?

Answer 17

• eNOS (constitutive endothelial NO synthase) produces NO.
• diffuses into VSMC
• activated guanylyl cyclase
• causes release of cGMP
• causes protein kinase G activation
• PKG phosphorylated thereby inhibiting myosin light chain kinase
• phosphorylates and increases activity of sarcoendoplasmic retriculum ATPase
• VASODILATION and increased BF

Question 18

Angina

Answer 18

Caused by inadequate myocardial oxygen supply.

Question 19

What could be used as treatment for angina and why?

Answer 19

• nitroglycerin (nitrate)
• nitrates break down to release NO in vivo
• causing arerial and venous vasodilation to lower ventricular afterload and preload

Question 20

Describe action of prostaglandins on vasculature

Answer 20

• many PGE relax VSMC in vascular beds

- PGF and thromboxane A are vasoconstrictors

Question 21

Describe role of endothelium derived hyper polarising factor

Answer 21

• EDHF opens K+ channels in the plasma membrane of VSMCs
• ensuing membrane hyperpolarisation reducing calcium permeability
• causing intracellular calcium levels to fall
• vasodilation

Question 22

Action of endothelin

Answer 22

• ET-1 = potent vasoconstrictor binds to ETa receptor on VSMC membrane
• triggers intracellular Ca2+ release via IP3 pathway.

Question 23

Action of oestrogen on NO?

Answer 23

Oestrogen stimulates NO release from endothelium.

Question 24

How is Mean arterial pressure mediated?

Answer 24

• changes in circulating BV by modifying renal function.

Question 25

What are Baroreceptor and where are they found? HINT what layer of carotid sinus?

Answer 25

Cluster of bare sensory nerve endings. buried within elastic layer of the carotid sinus.

Question 26

How are information from baroreceptors relayed to the brain?

Answer 26

- information to brain via sensory afferent travelling in sinus nerve - sinus nerve will join with glossopharyngeal nerve

Question 27

What occurs to the carotid sinus during systole?

Answer 27

Carotid sinus swells during systole. Stretches the sinus wall and is embedded with nerve endings. If there is no stretch, baroreceptors are inactive

Question 28

What occurs to aorta walls and carotid sinus when MAP increases?

Answer 28

- aorta walls and carotid sinus expand - embedded nerve endings are stretched - if degree of deformation is sufficiently high, receptor potentials trigger spikes in sensory nerve.

Question 29

Compare the threshold values for carotid and aortic baroreceptors?

Answer 29

Carotid baroreceptors have a lower threshold that aortic baroreceptors, hence usually are inactive under resting conditions.

Question 30

Where are cardiopulmonary receptors found? Where are the sensory nerve endings embedded?

Answer 30

- found in low pressure regions of CVD system | - bare sensory nerve endings embeddded in walls of the vena cavae, the pulmonary artery and vein.

Question 31

Two types of baroreceptors found in atria

Answer 31

A-receptor | B-receptor

Question 32

A-receptor

Answer 32

Responds to tension that develops in the atrial wall during contraction

Question 33

B-receptor

Answer 33

Sensitive to atrial wall stretching during filing. | Also involved in raising HR when central venous pressure (CVP) is high - known as the Bainbridge reflex.

Question 34

The medulla contains a collection of nuclei known as the Cardiovascular centre. Describe the different subdivisions?

Answer 34

1. Vasomotor centre - cells cause vasoconstriction when active 2. Cardioinhibitory system - slows heart rate 3. Cardioaccelatory centre - cells increase HR - increase myocardial inotrophy

Question 35

Describe the phases of cardiac action potential

Answer 35

``` Phase 4 - maintained by ATP dependent Na-K pumps Phase 0 - AP arrives at neighbouring cell and triggers a rapid influx of Na+ Phase 1 - closure of the Na+ channels - brief opening of some K+ Phase 2 - plateau phase - some K+ out some Ca2+ in Phase 3 - Ca2+ channels close and various K+ channels open leading to repolarisation. ```