Control of heart function Flashcards
Which cerebral centre exerts neural control on the heart?
Cardioregulatory centre and vasomotor centre
Where is the cardio regulatory centre located?
Medulla oblongata
Which nervous branch decreases heart rate and exerts negative ionotropy?
Parasympathetic system
What is chronotrophy?
Heart rate
Where do the parasympathetic fibres arise from?
The craniosacral region
Which neurotransmitter is secreted from parasympathetic preganglionic neurones?
Acetylcholine
What type of synapse is present at parasympathetic nerve terminals?
Cholinergic
Which receptors do acetylcholine bind onto at preganglionic nerve junctions?
Nicotinic receptors
Which receptors do acetylcholine bind onto at post-ganglionic nerve junctions?
Muscarinic receptors
Where are muscarinic receptors located?
M2 heart
What effect does stimulation on muscarinic receptors have on the heart?
Negative chronotropy
What effects does sympathetic activity exert onto the heart?
Increase in heart rate (positive chronotropy)- elevates phase 4 slope
Increases force of contraction (Ionotrophy)- increase calcium dynamics
Where do the sympathetic fibres arise from?
Thoracolumbar region
Which neurotransmitter is released from sympathetic preganglionic terminals?
Acetylcholine
Which neurotransmitter is released from sympathetic postganglionic fibres?
Noradrenaline (NA)
Where is the vasomotor centre located?
Bilaterally in reticular substance of the medulla and lower third of pons
Which cerebral centres exert excitatory and inhibitory effects onto the vasomotor centre?
Limbic system (Hypothalamus, cingulate gyrus, amygdala, hippocampus, and thalamus)
Which regions of the brain influence heart rate and contractility?
Lateral regions
What influences does the medial cerebral regions exert on the heart?
Parasympathetic nervous system - negative chronotropy
Which nerve is associated with parasympathetic nervous transmission?
Vagus nerve
What are the three main functions of the vasomotor centre?
Vasoconstrictor
Vasodilator
Cardioregulatory inhibitory area
Which cardiac structure do the parasympathetic and sympathetic branches both innervate?
Sinoatrial node (SAN)
Which cardiac receptors are stimulated by sympathetic activity
beta-1 receptors
What type of receptor are b1 receptors?
GS-linked proteins
Upon sympathetic activity what pathway is triggered within cardiac cell?
Activation of adenyl cyclase, increases the activity of cyclic-amp and protein kinase A, secondary messenger pathway
What effect does cAMP have on cardiac SAN cells?
Molecular regulation of ion channels, influences heart rate and contractility.
What is the relationship between cAMP levels and sympathetic innervation?
Sympathetic innervation positively correlates with cAMP intracellular concentrations
Which type of cardiac receptors are innervated by parasympathetic activity?
M2 receptors
What type of receptors are M2 cardiac receptors?
G-I linked (inhibitory) , reduces levels of cAMP, and adenyl cyclase activity.
What effect is exerted by parasympathetic innervation onto the heart?
Reduces impact of positive chronotropy and ionotropy.
Heart rate and contractility decreases
How does the removal of sympathetic innervation to the heart affect heart rate?
Reduction to heart rate
What is the underlying tonic relationship between parasympathetic and sympathetic control?
Parasympathetic nervous system is more active under tonic/rest conditions.
Both branch removal results in an overall increase in heart rate
What effect does increased sympathetic nerves have on the kidney?
Decrease glomerular filtration rate. reducing sodium secretion
NA release causes afferent arteriole constriction
JG cells secrete renin
How is the glomerular filtration rate (GFR) affected by increased sympathetic activity?
Reduction in sodium secretion into tubular system, therefore favouring sodium retention, this increases water retention and blood volume (blood pressure increases)
What are afferent arterioles?
Afferent arterioles deliver blood via the renal artery, branching within the Bowman’s capsule of the glomerulus
What is the role performed by efferent arterioles?
Efferent arterioles remove blood from nephron, associated with peritubular capillaries
What sympathetic effect is exerted onto afferent arterioles?
Increased sympathetic activity increases the release of noradrenaline from post-ganglionic terminals, hence acting on alpha-1 receptors.
Vasoconstriction on arterioles, reduced blood flow into glomerulus less sodium into the nephron system
Which receptors present on the afferent arteriole are influenced by sympathetic activity?
Alpha-1
What happens to afferent arterioles upon sympathetic activity?
Vasoconstriction
Where do juxta-glomeular cells reside?
Reside proximal to afferent arteriole.
Within the distal convoluted tubule
Which receptors innervated by the sympathetic nervous system affect JG cells?
Beta-1
What effect does increased sympathetic activity on JG cells exert?
Increased renin secretion
What parasympathetic activity is exerted onto the kidney?
None
How is blood volume detected?
Venous volume receptor
What effect does secreted renin exert?
Catalyses the conversion of angiotensinogen into angiotensin-1 –> angiotensin-II –> increased aldosterone release –> increased blood pressure
What effect is exerted by angiotensin-II?
Vasoconstrictor, thus increasing blood pressure
How is blood pressure detected?
Arterial baroreceptors
Where are arterial baroreceptors located?
Aortic arch,
carotid sinus
Which nerves transmit sensory stretch signals?
Glossopharyngeal and vagus nerve
What effect does reduced diastolic filling have?
Reduction volume activates sensory stimulation of baroreceptor, reduction in baroreceptor firing, increases sympathetic activity
What is distention?
Distention: Increased filling and volume
What effect does distension have?
Increased baroreceptor firing, decreased SNS activity
What are the two circulation systems?
Pulmonary and systemic
How is venous volume distribution affected?
Affected by peripheral venous tone, gravity , muscle pump & breathing
What is central venous pressure?
Mean pressure in the right atrium, determines amount of blood return to heart
What effect does venous return have on cardiac output?
Increased diastolic volume increased preload, increasing preload subsequently increases contractility
Starlings law.
What effect does vasoconstriction in veins have?
Reduced compliance
Reduced venous return
What effect does arteriole vasoconstriction have?
Increased arteriole constriction results in elevated BP, therefore influencing afterload
Blood flow to downstream organs
Mean arterial blood pressure
Pattern of blood flow to organs
What are intrinsic mechanisms of blood flow regulation?
Vasodilator and vasoconstrictor release from local endothelial cells.
Name the intrinsic vasodilators
Nitric oxide
Prostacyclin
What effect does prostacyclin have?
Vasodilator expresses anti platelet and anticoagulant effects
What effects does NO have?
Potent vasodilator, diffuses into vascular smooth muscle cells
Name the intrinsic vasoconstrictors:
Thromboxane 2
Endothelin
Where is Thromboxane A2 produced?
Synthesised by cyclo-oxygenase activity from activated platelets
What are systemic mechanisms of blood flow regulation?
Extrinsic to smooth muscle, impacts blood pressure and volume. These include autonomic nervous system and circulating hormones
Name the vasodilator extrinsic mediators:
Kinins,
Atrial natriuretic peptide (ANP)
What are extrinsic mediators?
Non-endothelial derived mediators
What effect do kinins have?
Kinins bind to endothelial cell receptors, stimulates NO synthesis, expressing vasodilator effects
Why is ANP secreted?
Response to high afterload and increased diastolic volume (stretch), attempts to reduce blood pressure
Name the extrinsic vasoconstrictors:
Vasopressin
NA/Adrenaline
Angiotensin-II
Which receptors does ADH bind onto on arterioles?
V1 receptors
Where is the SAN located?
Junction of crest terminals, upper wall of right atrium, opening of the superior vena cava.
Where do the branches form the Bundle of His traverse?
Intraventricular septum
What are Purkinje fibres?
Specialised conducting fibres, transmitting electrical impulses to stimulate ventricular systole
What is the cardiac electrical pattern (action potential)?
Rapid depolarisation –> plateau phase –> Repolarisation
How many phases are associated with the cardiac action potential?
5 (0-4)
What is phase 0?
Upstroke
What is phases 1-4?
Early repolarisation, plateau phase, repolarisation, resting membrane potential
How are action potentials transmitted throughout cardiomyocytes?
Action potentials are transmitted through gap junctions. Cell to cell ion flow.
What occurs during upstroke?
Fast voltage-gated ion channels rapidly open. Causes sodium influx along electrochemical and concentration gradients
What happens during the plateau phase?
Membrane potential declines relatively slow, due to opening of slow Calcium L-type voltage gated channels (L=latent)
Enables calcium influx, potassium channel open, potassium efflux
The influx of which ion is associated with the plateau phase?
Calcium ion
Which type of calcium ion channel is associated with the plateau phase?
L-type voltage gated calcium channels
When do the L-type calcium ion channel close?
When the membrane potential reaches zero
What occurs during repolarisation?
Membrane potential reaches resting potential
When does the absolute refractory period occur?
Occurs during the plateau phase.
What is the absolute refractory phase?
Action potentials are not stimulated in response to stimuli, prevents the possibility of tetany
How long is the absolute refractory period?
200ms
What is the relative refractory period?
Subsequent contraction corresponds to the repolarisation phase. Buffer period, greater action potential required to overcome the RRP. Larger stimulus required to trigger action potential
