Control of Heart Function

Question 1

What are the three main anatomical components of the heart

Answer 1

Muscle cells (Cardio-myocytes) - contract and relax

Specialised eletrical cells - create spontaneous currents, essential for regulating contraction

Vessels - major blood vessels responsible for transporting blood in and out

Question 2

Where is the sinoatrial node located

Answer 2

Junction of the crista terminalis: upper wall right atrium and opening of superior vena cava

60-100bpm

Question 3

Where is the atrioventricular node

Answer 3

Located at the triangle of koch at base of right atrium

Has pacemaker activity: slow calcium mediated action potential - normally SA node

Question 4

What are the tracts of the heart

Answer 4

Internodal tracts

Bundle of HIS - bundle brunches - purkinje fibres - apex

Specialised conducting fibres

Question 5

What is the nodal cell action potential

Answer 5

3 phases - 0,3,4

Phase 0 - Uptake calcium influx

Phase 3 - repolarisation due to K efflux

Phase 4 - Nodal cells don’t have a resting membrane potential - due to Na influx through a funny channel

Question 6

Why do the action potential profiles have differen action potential shapes

Answer 6

Caused by different ion current flowing and differen ion channel expression

Question 7

What is the time difference between cardiac muscle and nerve muscle

Answer 7

Cardiac AP is long

Question 8

Why does cardiac muscle AP have a long slow contraction

Answer 8

Required to produce an effective pump

Question 9

What is the cardiac muscle action potential

Answer 9

Phase 0 - upstroke

Phase 1 - early repolarisation

Phase 2 - plateau phase

Phase 3 - repolrisation

Phase 4 - membrane potential

Absolute refractory period - heart cannot start another action potential

Relative refractory period - can be elicitied if stimulus strength is high enough

Question 10

What is the diagram of ventricular cell

Answer 10

Question 11

What part fo the CNS controls the heart

Answer 11

Autonomic nervous system (cardio-regulatory centre & vasomotor centres in medulla)

parasymthatetic nervous system - vagus nerve

decrease slope of phase 4 - reduce activity of SAN

sympathetic nervous system

increase in heart rate (chronotropy) - increase slope of phase 4

increase force of contraction (inotropy) - increases Ca dynamics

Question 12

Where is the vasomotor centre located

Answer 12

Bilaterally in reticular substance of medulla and lower third of pons

Question 13

What are facts of the vasomotor centre

Answer 13

Composed of vasoconstrictor area

Composed of vasodilator area

Cardio-regulatory inhibitory area

Transmits impulses distally through spinal cord to almost all blood vessels

Higher centres of the brain can exert powerful excitatory or inhibitory effects on VMC

Lateral VMC - influencing heart rate and contractility

Medial VMC - trasmits signals via vagus nerve to heart that decreases heart rate

Question 14

Parasympathetic action at the SAN

Answer 14

Ach act on M2 receptor of SAN

Gi protein causes inhibition fo adenylyl cyclase which converts ATP to protein kinase A

Question 15

Sympathetic action at the SAN

Answer 15

Noradrenaline acts on B1-receptors

Stimulate increase in andenylyl cyclase

Increase in protein kinase A

Question 16

Why does the heart rate dip when sympathetic nerves are cut

Answer 16

Already some level of sympathetic activity, occurs all the time

Question 17

What do sympathetic nerves do to the kidney

Answer 17

Decrease glomerular filtration - decrease Na excretion - increase in blood volume

Blood bolume detected by venous volume receptors

Increase in renin secretion - increase angiotensin-11- increase in vasoconstriction and increased blood pressure

blood pressure detected by arterial baroreceptors

Question 18

What happens in the glomerulus during sympathetic stimulation

Answer 18

Sympathetic nerve fibres innervate afferent and efferent arterioles

noradrenaline acts on a1-adrenoceptor causes vasoconstriction

decrease in glomerular filtration rate and amount of sodium filtered

b1-adrenoceptor causes renin secretion, this increases aldosterone which raises blood volume

Question 19

Cardiopulmonary cicurti

Answer 19

Pulmonary vessels - volume sensors - send signals trhough glossopharyngeal and vagus nerves

Decrease in filling (less blood coming to the heart) - decrease in baroreceptor firing - increase in sympathetic nerve (SNS) activity

Distention (heart is full) - increase baroreceptor - decrease SNS

Question 20

Arterial circuit - aortic arch, carotid sinus and afferent arterioles of kidneys

Answer 20

Pressure sensors: signals through glossopharyngeal and vagus nerves

Decrease in pressure - decrease in baroreceptor firing - increase SNS activity

Increase in pressure - increase baroreceptor firing - decrease SNS activity

Question 21

How is venous pressure and volume affected

Answer 21

Venous volume distribution - peripheral venous tone, gravity, skeletal muscle pump and breathing

Central venous pressure - determines amount of blood flowing back to heart

Amount of blood flowing back to the heart determines stroke volume

Veins - constriction reduces compliance and increases venous return

Arterioles - constriction determines blood flow to downstream organs, mean arterial blood pressure, the pattern of blood flow to organs

Question 22

Local mechanisms that affect blood vessels

Answer 22

Nitric oxide: vasodilator

Prostacylin: vasodilator - antiplatelet and anticoagulant effects

Thromboxane A2 (TXA2): vasoconstrictor that is heavily synthesised in platelets

Endotehlins: vasoconstrictors generated from nucleus of endothelial cells

Local blood flow regulation within an organ

Question 23

Systemic mechanisms that affect blood vessels

Answer 23

Extrinsic to the smooth muscle

Autonomic nervous system and circulating hormones

Kinins: bind to receptors on endothelial cells and stimulate NO synthesis

Atrial natriuretic peptide: ANP - secreted from atria in response to stretch - vasodilator to reduce BP

Vasopressin (ADH) - binds to V1 receptors to cause vasoconstriction

Noradrenaline/adrenaline: secreted from adrenal gland - causes vasoconstriction

Angiotensin 2 - vasoconstrictor from renin-angiotensin-aldosterone axis, stimulates ADH secretion