Control Of Heart Function

Question 1

What are the three categories of the main anatomical components of the heart?

Answer 1

1. Muscle cells (cardio myocytes) - contract and relax in response to electrical stimuli. Essential for pumping blood around the body
2. Specialised electrical cells - cells that create spontaneous currents and those that transmit currents exist within the heart. Essential for regulating contraction of the cardio myocytes. These control heart function
3. Vessels - The major blood vessels are responsible for transporting blood in and out of the heart, while the coronary blood vessels are responsible for supplying blood to the heart

Question 2

What are the electrical nodes of the heart?

Answer 2

Sinoatrial node:

Pace maker of the heart, 60-100 bpm

Located at junction of the crista terminalis; upper wall of right atrium and opening of superior vena cava

Atrioventricular node:

Has pacemaker activity, slow calcium mediated action potential

Located at triangle of Koch at the base of the right atrium

Question 3

What are the electrical tracts of the heart?

Answer 3

Internodal tracts:

Connect the SA node and the AV node. Specialised myocytes

Bundle of His and bundle branches:

Specialised myocytes, AV node: His bundle -> branches at intraventricular septum -> apex

Purkinje fibres:

Specialised conducting fibres. Start at the apec and spread up the sides of the ventricles

Question 4

What is the nodal cell Action potential?

Answer 4

In SA or AV node

Has three phases:

0: upstroke. Membrane potential goes from -40 to +20
3: repolarisation. MP from +20 to -60
4: pre potential. MP from -60 to -40

This cycle occurs spontaneously and continuously

The upstroke is due to calcium influx

Repolarisation is due to potassium efflux

Nodal cells don’t have a resting potential. Only a pre potential that is due to sodium influx through a ‘funny’ channel

Question 5

What are the action potential profiles in the heart like?

Answer 5

Different parts of the heart have different action potential shapes

This is caused by different ion currents flowing and different ion channel expression in cell membranes

The SA and AV nodes have pretty similar shapes though

Question 6

What is an cardiac muscle action potential like?

Answer 6

Compared to nerves, cardiac AP is long (200-300ms vs 2-3)

Duration of AP controls duration of contraction of heart

Long, slow contraction is required to produce an effective pump

AP has 5 phases numbered 0-4:

Phase 0: upstroke. -90 to +30 mv

Phase 1: early repolarisation. +30 to +20

Phase 2: plateau. Maintains cell at a value of depolarisation around +20. This is relatively long

Phase 3: repolarisation. +20 to -90

Phase 4: resting membrane potential. -90mv

Google this to see what it looks like

Question 7

What are the refractory periods in cardiac muscle?

Answer 7

Absolute refractory period: time during which no AP can be initiated regardless of stimulus intensity. 200ms, during phase 0,1,2 and a bit of 3

Relative refractory period: period offer ARP where action potential can be triggered but only with larger stimulus strength. 200-250ms

So the ARP limits the maximum rate the heart can beat at

Question 8

What ions cause each phase of cardiac muscle action potential?

Answer 8

Phase 0: depolarisation. Sodium influx

Phase 1: early repolarisation. Potassium efflux.

Phase 2: plateau. Calcium influx

Phase 3: Repolarisation. Potassium efflux

Question 9

What organ systems play the main roles in modulating the activity of the heart?

Answer 9

Brain/CNS - immediate changes through nerve activity, or slower changes through hormonal activity

Kidneys - the heart and kidneys share a bi-directional regulatory relationship usually through indirect mechanisms

The blood vessels - By regulating the amount of blood that goes to and from the heart the blood vessels are able to influence cardiac activity

Question 10

What component of the CNS has the biggest role in the control of heart rate?

Answer 10

The autonomic nervous system - cardio-regulatory centre and Vasomotor centres in the medulla.

Parasympathetic- rest and digest. Vagus nerve from medulla to heart. Slows heart rate by decreasing the slope of phase 4 in nerves (pre potential)

Sympathetic nervous system- fight or flight. Raises Heart rate (chronotropy) by increasing the slope of phase 4. Increases the force of contraction (inotropy) by increasing calcium dynamics

Both of these have a basal rate of activity that has a constant effect (if you cut the parasympathetic nerves, the heart rate will be basally higher)

Question 11

How does the parasympathetic nervous system work?

Answer 11

Rest and digest

Preganglionic fibres use acetylcholine as a NT

PNSpost ganglion if NT is acetylcholine

Parasympathetic nerve fibres originate from the cranial and sacral segments of the spinal cord

Question 12

How does the sympathetic nervous system work?

Answer 12

Sympathetic nerve fibres originate from the thoracic and lumbar vertebra

Fight or flight

Pre ganglionic fibres use acetylcholine

Post ganglionic use noradrenaline

The nerves tend to synapse shortly after they leave the spinal cord in the paravertebral ganglia

Question 13

What is the vasomotor centre?

Answer 13

Located bilaterally in reticular substance of medulla and lower third of pins

Composed of:

Vasoconstrictor (pressor) area

Vasodilator (depressor) area

Cardio-regulatory inhibitory centre

Transmits impulses distally through spinal cord to almost all blood vessels

Many higher centres of the brain such as the hypothalamus can exert power for excitatory or inhibitory effect on the vasomotor centre

Lateral portions of the vasomotor centre controls heart activity by influencing heart rate and contractility

Medial portions of the vasomotor centre transmit signals via the vagus nerve to the heart that tend to decrease the heart rate

Question 14

What are the receptors involved in the parasympathetic control of the heart?

Answer 14

The acetylcholine from parasympathetic nerves acts on M2-receptors (g protein coupled)

Gi acts on and inhibits adenylyl Cyclase which inhibits the conversion of ATP to protein kinase

Question 15

What are the receptors involved in the parasympathetic control of the heart?

Answer 15

Noradrenaline released by sympathetic nerves acts on B1-receptors (beta 1) (G protein coupled)

Gs stimulates adenylyl cyclase and stimulates the conversion of ATP to protein kinase

Question 16

What is the renal system?

Answer 16

Controlled by sympathetic nerves - these increase activity in the renal system

This involves: decreasing glomerular filtration, leading to less sodium excretion and therefore an increase in blood volume (also with aldosterone)

The increase in blood volume is detected by venous blood volume receptors

Sympathetic nerves also lead to increased renin activity, leading to increased angiotensin II production. This causes vasoconstriction and therefore an increase in blood pressure

To summarise kidneys:

• increase blood volume with less sodium excretion
• increase blood pressure with increased ang-II production

Blood pressure is detected by arterial baroreceptors

Question 17

Where do the sympathetic nerves act on the renal system? The

Answer 17

Nerve fibres innervate Both afferent and efferent arterioles of the glomerulus

The afferent arterioles are the primary site of sympathetic activity

Alpha1-adrenoceptor-> vasoconstriction

This decreases the glomerular filtration rate -> less Na+ is filtered -> blood volume increases

Beta1-adrenoceptor -> renin secretion (at juxtaglometular cells

This also increase blood volume

Question 18

How is the cardiopulmonary circuit involved in control of the heart?

Answer 18

Cardiopulmonary circuit:

Large pulmonary vessels

Volume sensors (also in atrium and right ventrcle): send signals through vagus and glossopharygeal nerves

Decrease In filling -> decreased baroreceptor firing -> increased sympathetic nerve activity (increased heart rate and force)

Distension (full) -> increased Baroreceptor firing -> decreases sympathetic nerve activity

Question 19

How is the arterial circuit involved in control of the heart?

Answer 19

Aortic arch, carotid sinus, afferent arterioles of kidney

Pressure sensors: send signals through glossopharyngeal and vagus nerves

Decrease in pressure -> decreased baroreceptor firing -> increased sympathetic activity (heart rate up)

Increase in pressure -> increased baroreceptor firing -> decreased sympathetic activity (heart rate down)

Question 20

How are the veins involved in heart control?

Answer 20

Venus volume distribution affected by peripheral venous tone, gravity, skeletal muscle pump and breathing

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

Amount of blood flowing back to the heart determines stroke volume (using starlings law)

In veins, constriction reduces compliance and increases venous return

In arterioles, constriction determines:

A) blood flow to downstream organs

B) mean arterial blood pressure

C) the pattern of blood flow to organs

Question 21

What are some local mechanism for controlling the heart?

Answer 21

Endothelium derived mechanisms:

Nitric oxide - potent vasodilator

Prostacyclin- vasodilator, also anti platelet and anticoagulant effect

Thromboxane A2: vasoconstrictor

Endothelins: vasoconstrictors

Systemic mechanisms:

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

Atrial naturetic peptide - secreted from the atria in response to stretch - vasodilator effect to reduce BP

Vasopressin - from pituitary gland, binds to V1 receptors to cause vasoconstriction

Noradrenaline - from adrenal gland ,cause vasoconstriction

Angiotensin II - potent vasoconstrictor from the renin-angiotensin-aldosterone axis. Also stimulate ADH (vasopressin) secretion