4.3 Autonomic Control of the Heart Flashcards
What is the primary regulator of heart rate and stroke volume
The autonomic nervous system
What is the symmetrical synergy of the autonomic nervous system when controlling the heart
The 2 sub systems of the autonomic nervous system that act with opposing functions to keep a balance. The sympathetic nervous system acts to increase heart rate, blood pressure, and redirect blood flow to muscles. The parasympathetic nervous system acts to decrease heart rate, blood pressure and redirect blood flow to the digestive system.
Where do the sympathetic and parasympathetic pathways that innervate the heart originate from
The sympathetic pathways originate from the superior cervical ganglion and the parasympathetic pathways originate from the brain stem.
Describe the differences in the areas of heart innervation between the sympathetic and parasympathetic pathways
The parasympathetic nerves primarily innervate nodal tissue (such as the SAN and AVN), affecting pacemaker cells which influence heart rate.
But the sympathetic nerves innervate the ventricular myocardium and nodal tissue so it can influence the strength of heart contractions as well as the heart rate.
What can occur during excessive parasympathetic stimulation of the heart
Too much parasympathetic stimulation can lead to vasovagal episodes where heart rate drops quickly and too much causing a significant drop in blood pressure that can lead to fainting
Describe how the resting base heart rate is set.
The intrinsic heart rate set by the SAN is 100bpm and without autonomic regulation, it would stay at 100. However, the heart is continuously under parasympathetic activity that modulates the heart rhythm to keep lower than the intrinsic rate at rest. At rest, the heart is primarily under parasympathetic control, but sympathetic activation can rapidly increase heart rate during stress or exercise.
How does sympathetic stimulate increase the heart rate
Sympathetic stimulation accelerates the heart rate by increasing the rate of depolarisation of the pacemaker cells, causing an increase in the rate of action potentials being transmitted
How does parasympathetic stimulation decrease the heart rate
Parasympathetic (aka vagal) stimulation causes the pacemaker cells to take longer to reach the threshold potential and causes the action potentials to become less frequent.
What is Isoproterenol
Isoproterenol is a B-adrenergic receptor agonist, mimicking the sympathetic stimulation effects. It causes an increase in intracellular calcium and hence an increase in cardiac muscle cell contraction.
Describe the effects of sympathetic stimulation on Calcium
By activating calcium channels and increasing calcium influx, hence causing a higher intracellular calcium concentration. The calcium transient (the amount of calcium released during each heart beat) is increased, causing stronger muscle contractions (as more actin-myosin bonds form)
Also decreases the myofilament calcium sensitivity. This causes the troponin to be less sensitive to the calcium ions. This prevents excessive contraction during times of high calcium concentration.
At higher heart rates, the combination of the increased calcium concentration and the decreased ca sensitivity causes efficient contraction and prevents overstimulation.
Describe Starling’s Law in relation to heart contraction
Starling’s law states that the heart muscle’s contraction strength is directly proportional to its initial stretch thus, a higher filling pressure in the heart chambers (known as the preload) results in a stronger contraction (which results in how forcefully the heart pumps blood). The afterload is the pressure opposing the blood ejection, e.g resistance from high blood pressure, that makes it harder for the heart to eject blood which reduces stroke volume
The mechanism behind this law is the length/tension relationship of the cardiomyocytes. Increase in stretch of the cardiomyocytes increases their sensitivity to calcium (up to a point) thus increasing contractile force. Additionally increase in stretch of the cardiomyocytes produces a more optimal alignment of the sarcomeres so that the actin can be pulled further along the myosin at produce a greater contractile force.
