Control of Heart Function Flashcards
What components of the autonomic nervous system are found in the medulla?
Cardio-Regulatory centre and vasomotor centres
What does the parasympathetic nervous system do to the heart?
‘Rest & digest’
Lowers heart rate (HR) – decreases the slope of phase 4
Always active
What does the sympathetic nervous system do to the heart?
‘fight or flight’
Increases HR (chronotropy) – increases the slope of phase
Increases force of contraction (inotropy) – increases Ca2+ dynamics
Only active in certain situations
What does the sympathetic nervous system do to the renal system?
Increases activity
Decreases glomerular filtration
Decreases Na+ excretion and increases blood volume
Increases blood pressure
How is blood volume detected?
Venous volume receptors
What else does the sympathetic nervous system do to the renal system?
Increases renin secretion
Increases angiotensin II production
What does angiotensin do?
Potent vasoconstrictor
Increases blood pressure
How is blood pressure detected?
Arterial baroreceptor
Where are volume sensors in the heart?
Large pulmonary vessels
Atria
Right ventricle
How can volume sensors in the cardiopulmonary circuit control the heart?
send signals though glossopharyngeal & vagus nerves
Decrease in filling Decreases baroreceptor firing
Activated sympathetic nerve (SNS) activity
Distention leads to increased baroreceptor firing
Reduces SNS activity
How can the arterial circuit control the heart?
Pressure sensors: send signals though glossopharyngeal & vagus nerves
Decrease in pressure Decreased baroreceptor firing Increased SNS activity
Increase in pressure
Increased baroreceptor firing
Decreased SNS activity
What comprise the autonomic nervous system?
Parasympathetic
Sympathetic
What are the main features of the parasympathetic nervous system?
rest and digest’
Pre-ganglionic fibres use ACh as neurotransmitter
PNS post ganglionic NT = ACh
PNS is important for controlling the heart rate
What are the main features of the sympathetic nervous system?
‘fight or flight’
Pre-ganglionic fibres use ACh as their neurotransmitter
SNS post ganglionic NT = NA
SNS is important for controlling the circulation
Where is the vasomotor centre?
bilaterally in reticular substance of medulla & lower third of pons
What is the vasomotor centre comprised of?
Vasoconstrictor (pressor) area
Vasodilator (depressor) area
Cardio-regulatory inhibitory area
Where does the VMC transmit impulses?
Transmits impulses distally through spinal cord to almost all blood vessels
What does the VMC do?
Lateral portions of VMC controls heart activity by influencing heart rate and contractility
Medial portion of VMC transmits signals via vagus nerve to heart that tend to decrease heart rate.
How is the VMC controlled>
Many higher centers of the brain such as the hypothalamus can exert powerful excitatory or inhibitory effects on the VMC.
How is the sympathetic innervation in the heart involved in control?
Increases activity of SAN
Cyclic AMP activated by beta-1 receptors
Regulates different ion channels that control heart rate and force of contraction
How is the parasympathetic innervation in the heart involved in control?
Decreases activity of SAN
Cyclic AMP activated by beta-1 receptors
Regulates different ion channels that control heart rate and force of contraction
How do these systems work under normal resting conditions?
Tonic underlying activity of both sympathetic and parasympathetic
Parasympathetic has more activity in resting conditions
If all nerves were cut there would be an overall increase in heart rate
How do sympathetic nerves innervate the renal system?
Sympathetic nerve fibres innervate afferent & efferent arterioles of the glomerulus (& nephron tubule cells)
More of effect of afferent than efferent arteriole
What is the role of afferent arterioles in control of the heart?
Primary site of sympathetic activity
Relasese of noradrenaline
alpha 1-adrenoceptor Increased vasoconstriction (more so in the afferent arteriole)
Decrease in glomerular filtration rate
Decreased Na+ filtered
More sodium in body increases blood volume
What is the role of juxtaglomerular cells in the control of the heart?
Noradrenaline acts on beta-1 receptors
Juxtaglomerular cells are the site of synthesis, storage & release of renin
Increases renin secretion
Increase in angiotensin II
Vasoconstriction increases blood pressure
What are the two circulatory systems?
Pulmonary
Systemic
What is venous volume affected by?
peripheral venous tone, gravity, skeletal muscle pump & breathing
What does central venous pressure determine?
Determines amount of blood flowing back to heart
What is central venous pressure?
Mean pressure in the right atrium
What determines the stroke volume?
Amount of blood flowing back to the heart
What does vasoconstriction in veins result in?
Reduces compliance and venous return
What does vasoconstriction in arteries result in?
Blood flow to downstream organs
Mean arterial blood pressure
The pattern of blood flow to organs
What are the main vasodilators in the body?
Nitric oxide Prostacyclin (Endothelium-derived) Kinins ANP (Non-endothelium derived)
What are the main vasoconstrictors in the body?
Thromboxane Endothelins (Endothelium-derived) Vasopressin Noradrenaline/Adrenaline Angiotensins (Non-endothelium derived)
B1 receptor activation in renal afferent arterioles will have which initial affect?
Secretion of renin
What are the main component of the electrical signalling of the heart?
SAN
AVN
Bundles of His
Purkinje fibres
How does the cardiac action potential differ from the vast majority of nerve cells?
cardiac action potential (AP) is very long (200-300 ms vs. 2-3 ms)
Duration of AP controls duration of contraction of heart
Long, slow contraction is required to produce an effective pump
How many phases are there of the cardiac AP?
AP has 5 phases numbered 0-4
What happens in phase 0 +1?
0: Depolarisation phase that leads to the initial contraction in the cardiac myocyte
1: Early depolarisation
What happens in phase 2?
Plateaus
Important in production og prolonged contraction
What happens in phase 2?
Repolarisation
What happens in phase 3?
Resting membrane potential
What is the ARP?
During plateau phase
Absolute refractory period (ARP) = time during which no AP can be initiated regardless of stimulus intensity
What is RRP>
Relative refractory period (RRP) = period after ARP where an AP can be elicited but only with larger stimulus strength
What determines the RMP?
K+ efflux
What is the upstroke determined by?
By large increase in membrane to Na+ permeability
What is essential for contraction that occurs in phase 2?
Ca2+ influx required to trigger Ca2+ release from intracellular stores
Ca2+ current (ICa) activates rapidly (within a few milliseconds) but the upstroke is more dependent on INa
What occurs if phase 3?
Gradual activation of K+ currents (K+ moving outward) that balance, then overcome, inward flow of Ca2+
Large K+ current (IK1) that is inactive during the plateau starts to flow once the cells have partially repolarised
IK1 is responsible for fully repolarising the cell
How is the RMP re-established?
IK1 is large and flows during diastole. It stabilises the resting membrane potential
What is different about different parts of the heart?
Different parts of the heart have different action potential shapes
Caused by different ion currents flowing and different ion channel expression in cell membrane
Describe the phases of SA node AP
SA node AP only has phase 0, 3 & 4
No early repolarisation or plateau phase
Resting membrane potential becomes ‘pre-potential’ due to T-type Ca2+ channels & ‘funny’ current (If)
What is a T-type Ca2+ channel?
Transient
‘quicker’
Why do pacemaker cells not have a phase 4?
They do not rest for long
