Autonomic nervous system control of the heart Lecture 15 Flashcards
Right Sympathetic Fibers innervate…
innervate the S-A node and have a great effect on heart rate
Left Sympathetic Fibers Innervate…
innervate the Ventricle muscles and have a great effect on muscle contractility
Right (parasympathetic) Vagal fibers innervate…
innervate the S-A node and greatly effect heart rate.
Innervate the coronary vessels do not have a major effect on heart rate
Left (parasympathetic) Vagal fibers innervate
A-V node and leads to a great effect on AV node conduction
Does the Parasympathetic or Sympathetic nervous system have a greater effect on heart contractility?
The sympathetic nervous system because there are not many nerve fibers from the vagus (parasympathetic system) that innervate the ventricles
Sympathetic Alpha Adrenergic Receptors
Location?
Purpose?
Located on coronary vessels
the coronary vessels are regulated by metabolic factors so the sympathetic regulation is overridden and it does not have much of an effect on the heart
Sympathetic Beta-1 Adrenergic Receptors
Location?
Function?
Located mainly on cardiac muscles and nodal cells.
they are the SINGLE most IMPORTANT in regulating the minute to minute cardiac function
Sympathetic Beta-2 Adrenergic Receptors
Location?
Function?
Located mainly on Vascular smooth muscle
they do not have a large effect on cardiac function because of local regulation of coronary vessel diameter- so minimal effect
there are a few Beta -2 receptors on cardiac muscles
Sympathetic Beta -3 adrenergic receptors
Location?
Function?
Located on Endothelial cells within the coronary vessels
they are involved in the release of nitric oxide and have a minimal effect on cardiac function
Mechanism for sympathetic nerve activity the process……
if a beta agonist like Norepinephrine is present it binds to Beta adrenergic receptor.
The beta receptor is coupled by a G-Protein that activates adenylate cyclase
Adenylate cyclase stimulates/ converts cAMP
cAMP acts as a second messenger and binds to protein kinase A
Protein kinase A phosphorolates Ca+ channels in the cell membrane
regulatory proteins on the SR
and contractile proteins
What is the final response/ outcome of NE binding to Beta adrenergic receptors
- Phosphorylation of Ca+ channels on cell membrane
- Phosphorylation of regulatory proteins on Sarcoplasmic reticulum
- Phosphorylation of one contractile protein.
Calcium channels response to sympathetic activity (NE)
Phosphorylation of L-type Ca+ channels increases the ca+ influx during an action potential leading to an INCREASE IN CONTRACTILITY
Sarcoplasmic Reticulum response to sympathetic activity (NE)
Phosphorylation of phospholamban increases the uptake of Ca+ into the SR leading to an increase rate of relaxation.
When you increase the Ca+ uptake into theS R it meaNs there will be more Ca+ available to be released by the SR which increases contractility. bc more Ca+ is being released.
INCREASE IN RATE OF RELAXATION AND INCREASE IN CONTRACTILITY.
Contractile proteins response to sympathetic activity (NE)
phosphorylation of troponin leads to reduced sensitivity of calcium of contractile proteins. this could lead to a minor effect on the rate of relaxation (decreasing it) and decrease contractility
but it is so minor that it doesn’t affect cardiac function because calcium channels and SR cause a greater effect that overrides the effect on contractile proteins
how does the sympathetic nervous system effect ventricular contraction and relaxation
the rate of contraction and relaxation occur faster and increase the heart rate if the heart rate is increased than leads to a shorter time available for relaxation.
- the rate of contraction and rate of relaxation occur faster meaning contraction starts sooner and ends sooner
Why is faster relaxation important
it allows the heart to fill up between contractions
what are the important parasympathetic receptors for cardiac function
Muscarinic M2 subtype receptors
What is the mechanism for parasympathetic nerve activity
stimulation of M2 receptors leads to the activation of pathways through G proteins.
the parasympathetic mechanism acts in opposition of sympathetic mechanism
IT……
inhibits adenylate cyclase
stimulates guanylate cyclase
activate specific potassium channels
During parasympathetic inhibition of adenylate cyclase and activation of Guanylate cyclase what is occuring
inhibiting adenylate cyclase decreases the cAMP levels
stimulating guanylate cyclase increases cGMP levels which inhibits the increase in calcium that cAMP causes
both stimulation of guanylate cyclase and inhibition of adenylate cyclase contribute to accentuated antagonism
what is concept of accentuated antagonism?
the negative effect of parasympathetic nervous system on the ventricular depends on the sympathetic nervous system.
EX: if a pt is taking a beta blocker so there is no sympathetic stimulation and you stimulate the vagus nerve(parasympathetic) there will be no effect on heart contractility
what is the mechanical response to parasympathetic activity
there will not be a major effect on the heat it reduces the likelihood that the sympathetic effects are going to be to large and act as a safety mechanism.
what are the effects of parasympathetic nervous system on the slope of pacemaker potential of an SA nodal cell
the pacemaker potential is less steep and takes longer to reach threshold the max diastolic value is less than –60 it will also take longer to reach potential because the cell is hyperpolarized so its further from threshold
what effect does the sympathetic nervous system have on slope of the pacemaker potential of SA nodal cells
during sympathetic stimulation you reach the pacemaker potential quick and the slope is more steep you reach the -40 threshold potential quicker.
why is the slope and threshold potential important in SA nodal cells?
The quicker you reach threshold potential the higher the heart rate is.
What is the sympathetic effects on heart rate caused by?
stimulation of L-type Ca+ currents and Funny currents in SA nodal cells cause an INCREASE heart rate
What are the sympathetic effects on AV conduction velocity?
Similar to the sympathetic effects on heart rate the stimulation of L-type currents and funny currents in AV nodal cells INCREASE the speed of conduction of AV node
What are the Parasympathetic effects on heart rate due to?
Opening of the IKach (acetylcholine activated potassium channels) in the SA node DECREASE the rate of depolarization of the pacemaker potential opposing the depolarization and Hyperpolarize the cell taking it further away from threshold DECREASING slope of the pacemaker potential DECREASING heart rate
What are the parasympathetic effects on AV conduction?
Opening of IKach channels and the inhibition of L-type Ca+ channels DECREASE conduction through the AV node
