Control of heart function Flashcards
What is the sinoatrial node?
Pacemaker of the heart: 600-100bpm
Junction of crista terminalis; upper wall of right atrium and opening of superior vena cava
What is the atrioventricular node?
Has pacemaker activity: slow calcium mediated AP
Triangle of koch at base of right atrium
What is the bundle of his and bundle branches?
Specialised myocytes
What are purkinje fibres?
Specialised conducting fibres
What are the different phases of SA node AP?
Nodal AP has 3 phases (0,3,4):
Phase 4: pre-potential- Na+ influx through funny channel. Theres no resting membrane potential
Phase 0: Upstroke due to Ca2+ influx
Phase 3: Repolarisation due to K+ efflux
How does AP profiles change across the heart?
Different parts of the heart have different AP shapes
This is caused by different ion currents flowing and different ion channel expression in cell membranes
What are the different phases of cardiac muscle AP?
Compared to nerves, cardiac AP is long Duration of AP controls duration of contraction of heart Long, slow contraction is requires to produce effective pump AP has 5 phases (0-4) Phase 0: upstroke Phase 1: early repolarisation Phase 2: Plateau Phase 3: Repolarisation Phase 4: Resting membrane potential
There’s an absolute refractory period where no AP can be initiated
There’s also a relative refractory period where AP can be initiated but needs a larger stimulus strength
How does the parasympathetic nervous system affect cardiac function?
Decreases heart rate - decreases slope of phase 4
Long pre-ganglionic fibre from spinal cord synapses with post-ganglionic fibre via nicotinic receptor that uses ACh
Short post-ganglionic fibre will synapse with target organs using ACh but on muscarinic receptors
M2 receptor are Gi protein linked- ACh acts on M2 receptor to cause inhibition of adenyl cyclase and prevent conversion of ATP to protein kinase A
This leads to decreased cAMP so decreases the opening of LTCCs so less Ca2+ influx and less forceful contractions so decreased heart rate
How does the sympathetic nervous affect cardiac function?
Increases heart rate- increases slope of phase 4
Short pre-ganlionic fibre from spinal cord synapses with long post-ganglionic fibre via nicotinic receptor that uses ACh
Long post-ganglionic fibre synapses at target organ and uses noradrenaline and alpha or beta receptor depending on organ (at adrenal medulla direct noradrenaline is received)
Noradrenaline acts at beta-1 receptors (Gs protein linked) and stimulates adenyl cyclase production. This increases levels of pKA and cAMP
This increases LTCC opening so theres increases Ca2+ influx and increased force of contraction (inotropy) so increased heart rate
Where is the vasomotor centre?
The vasomotor centre is located bilaterally in reticular substance of medulla and lower third of pons
What is the vasomotor centre?
Composed of:
- vasoconstrictor (pressor) area
- vasodilator (depressor) area
- cardio-regulatory inhibitory area
Transmits impulses distally through spinal cord to almost all blood vessels
Many higher areas such as the hypothalamus can exert powerful excitatory or inhibitory effects on the VMC
Lateral portions of the VMC control heart activity and influence heart rate and contractility
Medial portion of VMC transmits signals via vagus nerve to heart that tend to decrease heart rate
How do the kidneys affect blood volume and pressure at afferent arterioles ?
The kidneys have sympathetic innervation to afferent and efferent arterioles of glomerulus
Sympathetic nerves act on alpha-1 receptors in afferent arterial causing vasoconstriction
Vasoconstriction causes less blood flow so lower glomerular filtration rate (GFR) so less Na+ enters filtrate therefor less water is lost
This increases water remaining in the blood so increased blood volume and blood pressure
How do the kidneys affect blood pressure and volume at juxtaglomeluar cells?
Sympathetic fibres act on beta-1 receptors in juxtaglomerular cells and cause the release of renin.
Renin is also released due to lower pressure in afferent arteriole detected by baroreceptors and low sodium in afferent arteriol detected by macula densa
Renin activates renin-angiotensin-aldosterone system which creates angiotensin II. Angiotensin II is a vasoconstrictor of blood vessels.
Angiotensin II also causes release of aldosterone which increases Na+ retention so increases water retention
What is the role of the cardiopulmonary circuit in cardiac control?
Cardiopulmonary circuit involves large blood vessels
Volume sensors send signals through glossopharyngeal and vagus nerves
When theres decrease in filling theres decreased cardiac output
This leads to decreased baroreceptor firing so increased SNS activity
This leads to increased HR and vasoconstriction and increased blood volume/ pressure
When there is dissension (heart is full), theres increased baroreceptor firing so decreased SNS activity
This leads to vasodilation, decreased HR, and decreased blood pressure/ volume
What is the role of the arteriole circuits in cardiac control?
Arteriol circuit includes aortic arch, carotid sinus, and afferent arterioles of kidneys)
Pressure sensors send signals through glossopharyngeal and vagus nerve
A decrease in pressure causes decrease in baroreceptor firing which increases SNS activity
This leads to vasoconstriction so increases in blood pressure and blood volume
