HOW IS THE HEART REGULATED ?

In this article, we will study how the brain is linked with the control of the heartbeat. Indeed, we all have already noticed that our heart has not the same rhythm when we are sleeping than when we are doing a physical activity. Nonetheless, this control of the heart rate occurs even if we don’t want to or even if we don’t think about it. Then our will has no link with this control and it is not a reflex either because if the heart rate increases or decreases, it is in fact in response to a physiologic demand from the organism. So we want to understand how the nervous system can act according to the circumstances, and to do so, we will study the autonomic nervous system because it controls this unconscious mechanism.

  

To begin with, we discovered that in embryonic stages, the heart is created from the third to the seventh week. Its rhythm is 90 beats per minutes and at this stage, the nervous cells multiply but they are linked neither each other nor to the heart.

At the end of the third month, all the organs are correctly placed and the nervous system takes the control of the heart.

Moreover, someone who undergo a cardiac transplant and whom heart has been disconnected from the nervous system, has a heart rate of 100 beats per minutes (either during an activity or at rest).

These two examples show that the “ordinary” frequency is about 90 or 100 beats per minutes when the brain does not control the heartbeat.

Nevertheless, the nervous system is essential to the heart because it permits to adapt the cardiac rhythm according to the energetic demand of the organism. Indeed, when we sleep, the frequency goes down until 50 beats per minutes whereas during a physical effort (or because of the stress), it can go up to 180 beats per minutes.

In 1921, Loewi created an experiment based on an isolated heart which showed the role of the brain on the heart rate. In fact, the scientist immersed two frog hearts in two jars filled with a liquid and related each other by a pipe in order to let the liquid of the first jar go into the second jar. Then, Loewi stimulated the vagus nerve of the first heart whom heart rate slowed down. He also observed that the heartbeat of the second heart decreased too (whereas its vagus nerve has not been stimulated). Loewi deduced that the cardiac rhythm is controlled by a chemical substance (which was in the liquid) which is secreted by the vagus nerve. Therefore, this experiment shows that the nervous system controls the cardiac rhythm thanks to chemical secretions.

 In addition, we discovered the two main molecules which control the heart rate: noradrenalin and acetylcholine.

The first one is a neurotransmitter which takes part to the first subsystem of the autonomic nervous system: the sympathetic nervous system. Therefore, the noradrenalin is released when the sympathetic fibers are stimulated and its effect is excitatory: it permits to accelerate the cardiac rhythm. Thus, the sympathetic system takes part in case of physical activity (in this case the organism needs more energy and oxygen so it is essential that the heart rate increases to respond to the need). It intervenes also when we are stressed (in this case, the adrenalin, which is a hormone secreted by the adrenal medulla, plays the same role than the noradrenalin on the cardiac rhythm since it has a sympathomimetic action. However, if it has the same role than the noradrenalin [an inotropic and chronotropic effect], the adrenalin (blog adrenalin) is not a neurotransmitter because it is not secreted in a synapse. Indeed, it is secreted in the blood and it amplifies the noradrenalin effect when it reaches the heart).

The second molecule, the acetylcholine; takes part to the second subsystem of the autonomic nervous system: the parasympathetic nervous system. This system has an effect rather opposed to the previous system since it has an inhibitory effect. The noradrenalin is a neurotransmitter which permits to decrease the cardiac rate and it is secreted when the parasympathetic nerve (or vagus nerve) is stimulated. Hence the parasympathetic system intervenes for example when we are sleeping, or when the body is immersed (indeed, the heartbeat slows down in order to preserve the oxygen to stay longer in the water), or when the arterial pressure is too high (the baroreceptors of the carotid sinus stimulate the parasympathetic center and this release the acetylcholine which decreases the heart rate). 

Despite the fact that we have seen that the heart beats at 90-100 beats per minutes without the intervention of the nervous system, the average frequency is 70-75 beats per minutes. This diminution of 20% is due to the fact that actually, the brain never stops to control the heart. Indeed, the parasympathetic and sympathetic systems send impulses to the sinoatrial node without stopping and since the parasympathetic fibers send more impulses, the heartbeat is globally decelerated (in comparison to what it should be). We call this phenomenon the vagal tone because it is due to the vagus nerve.

All these examples permit us to understand that the cardiac control is governed by the brain which never stops to control the heart (either while we are sleeping or while a physical activity).

Heart rate is regulated by the autonomic nervous system which occurs unconsciously, but this control requires the coordination of several actors. In fact, the first are the receptors (baro-chemoreceptors) which capture the blood pressure variations or the blood composition (When its parameters values are too far from its references values because of the situation or the external environment).

Then, the information which is received by the receptor is transmitted to the nervous system thanks to the sensory nerve fibers (which are the Hering nerves and Cyon-Ludwig nerves for the sinus and the cardiovascular fibers for the right auricle).

This information is transmitted to the parasympathetic nervous system or to the sympathetic nervous system. It depends on the situation. The parasympathetic nervous system is located in the medulla oblongata. It’s the “inhibitory” system. The second is located in the spinal cord. It’s the “excitory” system.

Between these two nervous centers, there are inhibitory neurons which weaken one of the two systems when the other is stimulated.

Then, the nervous center analyzes the received information and sends a response to the target tissues by the intermediate of the motor nerve fibers (which are in the vagus nerve and the cardiac nerve. The first are connected to the sinus node and the second to the ventricle).

The blood pressure stimulation involves the sympathetic nervous system with the aortic and carotid baro-receptor, the bulbar center, effectors organs (here the heart) and vessels. When the heart needs to be stimulated, the sympathetic nervous system releases noradrenalin which will fix to the myocardial ß1 receptors. This fixing activates the adenylyl cyclase which causes an increase of AMPc and this leads to the activation of the kinase protein. This kinase protein will increase the Ca²⁺ cell permeability and the Ca²⁺ will enter plentifully in the “sarcoplasma”. Then, the Ca²⁺ will bind with the troponin, and this protein complex changes its physical structure to allow the bond between the myosin heads and actin. This bond activates the mechanism of the contractility. That’s why the systolic ejection volume increases (heart rate and strength of ventricular contraction are increasing so the heart pumps more blood).

 The contractility is determinated by the cardiac capacity to supply a given pressure.

That’s why if the strength and heart rate increase, the heart will manage a higher pressure and pump a greater blood volume.

The intervention of the parasympathetic system isn’t the same that the sympathetic system because the parasympathetic and the sympathetic innervations are different. Indeed, the parasympathetic is linked to the auricles and the nodal tissue whereas the sympathetic is linked to the auricles, the nodal tissue and the ventricles. 
 

To conclude, this explains why when you burn yourself, your heart rate increases in few milliseconds.