 We know that our heart can beat on its own. It has nodal tissues that can generate impulse to create the heart beats, right? So the normal heart beat is 70 to 75 times per minute. Well, this rate is only for the resting relaxed state, okay? But all of a sudden if there is an emergency situation, maybe a dog is chasing you, okay? You will notice that your heart beat will increase. Now the question is, how does your heart know that a dog is chasing you? I mean the heart don't have any processing unit in here, right? So it is our brain that informs the heart. Okay, and how does it inform? Well first the brain processes the the situation and then sends a signal through the lower part of the brain which is the medulla, okay? The medulla has a center, actually a group of neuron, okay, which we call the cardiovascular center. So from the cardiovascular center, there arises a nerve that goes directly to the heart and this nerve will secrete certain chemicals onto the heart and the chemicals secreted by the nerves are called neurotransmitters and the neurotransmitter here is called epinephrine and this epinephrine, it increases the heart rate. Now what is the need of increasing the heart rate during an emergency situation? Well if the heart rate increases, there will be more blood flow and if there is a situation when you need to run, your muscles will demand more blood in there, right? Because it will need more oxygen to produce energy, right? That's the reason the heart rate increases. Now the nerve that sends epinephrine to the heart is a part of sympathetic nervous system. NS is nervous system, okay, and this operates only during emergency. But what happens after that? How does our heart get back to its normal beating condition? Well, this again is conveyed by our brain. So the brain sends a signal saying, hey, there is no danger anymore, you can calm down and to bring down the heart rate, there is another nerve that is extended from the medulla again, which secretes acetylcholine, another neurotransmitter onto the heart. Well, now when I'm saying secret neurotransmitter on the heart, I actually mean they secreted on the nodal tissues, the tissues that can stimulate impulses and can contract and relax the heart, the tissues that controls the heart rate. So these neurotransmitters are secreted on those and acetylcholine, they stimulate the nodal tissues to calm down. To bring down the heart rate. And nerve that brings the acetylcholine to the heart is a part of parasympathetic nervous system. And this operates during resting state. Well, now if you're wondering, what is this sympathetic and parasympathetic nervous system now? Well, these two are a part of nervous system called the autonomic nervous system. Now, what is this autonomic now? Well, this is a part of nervous system that takes care of all the body organs that is involuntary. Okay, we don't have voluntary control over it. Like our heart, our stomach, our intestine, we cannot control those, right? So the nervous system that takes care of all those involuntary organs or parts of our body is called the autonomic nervous system and sympathetic and parasympathetic are the two parts of the autonomic nervous system. One works during emergency and another during the normal resting state. Now apart from this neural control over heart, we also get to see hormonal control over the heart rate. So hormones secreted by the adrenal medulla, a gland which sits over the kidneys, it secretes two hormones. One is the epinephrine and the other is the nor epinephrine. They're also called adrenaline or noradrenaline, okay? Now if you're wondering that, hey, we just learned that epinephrine is a neurotransmitter. Now you're saying that it's a hormone. Well, epinephrine acts both as a neurotransmitter and as a hormone in our body, okay? So what is the difference when it becomes a hormone? Well, hormones are secreted into the blood stream, right? And when something has to travel through blood to the heart, it is slow. So the epinephrine that is a neurotransmitter will have immediate action over the heart rate. But the one that is moving through the blood, it will not have an instant effect. But it will have a long-lasting effect because, you know, it will be secreted onto the blood and it will stay in the blood for some time. So even after the person is out of the danger zone, even then the heart will continue to beat fast because of the effect of the hormone. So epinephrine and nor epinephrine, again, these hormones are also called the fight or flight hormone. Well, by this you can guess that these hormones are the ones which are secreted during an emergency situation where either you need to fight or you need to run away, that is flight. That is why it is called the fight or flight response hormones. And that's the reason both these hormones, epinephrine and nor epinephrine, it increases the heart rate. Now, by the name, many gets confused that maybe epinephrine and nor epinephrine, they work opposite to each other. They are antagonistic, but they mostly work in a similar fashion. So here we saw how the hormones has impact over the heart rate. But again, it is not necessary that our heart will increase its beating rate only during an emergency. You must have heard people saying that the infants have faster heart beat than us adults because the heart rate also depends on the heart size as well. But apart from the size, there are other hormones that impact your metabolism. And when the metabolism is really very high, the heart rate increased then as well. So one best example is the thyroxin hormone released by the thyroid gland, a butterfly shaped gland that sits in your neck area. And the hormone it produces increases the BMR, that means the basal metabolic rate. When the metabolism is increased, it again increases the heart rate. So here we saw how the nervous system, the autonomic nervous system through its sympathetic and parasympathetic pathway, impacts the heart rate. One increases and the other decreases the heart rate. Okay, when we say increase, how much does it increase? Well, it can increase the heart beat up to 150 beats per minute. Well, this is not normal. Okay, it is only during an extreme emergency condition. But then again, we have the acetylcholine, a neurotransmitter secreted by the parasympathetic nervous system that brings it down to 70 to 75 beats per minute. And again, we have learned about thyroxin epinephrine and norepinephrine, which also has the capacity to regulate the heartbeat. So even though our heart is myogenic, that means it has tissues that can generate its own impulse and the contraction happens on its own without the help of any other body part. Even then, we saw that there is both neural control and hormonal control over the activity of our heart.