I can feel your heartbeat...
Did you know that a mammalian heart can survive outside the body for several hours, beating away by itself, as long as it is provided with nutrients and maintained at the correct temperature? You probably did, but have you ever wondered HOW it does this? There is a whole branch of experimental biology that is concerned with isolating various body organs and keeping them viable, in order to conduct experiments that will give an insight into that specific organ. I know it all probably sounds a bit Dr. Frankenstein (!), but such research is essential for understanding our own physiology, and developing drugs that can modify physiological function.
For my undergraduate research project I worked with the Langendorff Isolated Perfused Heart Apparatus. This was named after a famous scientist called Oskar Langendorff, who first established the technique way back in 1895. Briefly, the still-beating heart is excised from a terminally anaesthetized animal (I worked on rats) and is immediately hooked up to an impressive-looking apparatus (I always used to think how strange it was to see a tiny rat heart, the size of my thumbnail, surrounded by all this machinery and pumps and tubing that practically took up an entire room) that supplies the tissue with the correct mixture of oxygen, carbon dioxide, and nutrients, while removing waste products. Once the heart was mounted, we inserted a home-made balloon into the left ventricle that was attached to specific sensors, along with ECG electrodes on the surface (again, remember the size of the heart!), so that we were able to measure heart rate, the pressure inside the ventricle, and obtain an ECG. We could then infuse the tissue with certain drugs and look at the effects on these cardiac parameters. If everything was set up correctly the heart would remain beating, without any kind of external stimulation, for several hours. I still remember sitting in that room, staring at the little heart beating away merrily, without a care in the world, and being absolutely fascinated.
Here, I'm going to write about the mechanisms underlying this autonomy, and hopefully give an insight into the origins of our heartbeat.
For my undergraduate research project I worked with the Langendorff Isolated Perfused Heart Apparatus. This was named after a famous scientist called Oskar Langendorff, who first established the technique way back in 1895. Briefly, the still-beating heart is excised from a terminally anaesthetized animal (I worked on rats) and is immediately hooked up to an impressive-looking apparatus (I always used to think how strange it was to see a tiny rat heart, the size of my thumbnail, surrounded by all this machinery and pumps and tubing that practically took up an entire room) that supplies the tissue with the correct mixture of oxygen, carbon dioxide, and nutrients, while removing waste products. Once the heart was mounted, we inserted a home-made balloon into the left ventricle that was attached to specific sensors, along with ECG electrodes on the surface (again, remember the size of the heart!), so that we were able to measure heart rate, the pressure inside the ventricle, and obtain an ECG. We could then infuse the tissue with certain drugs and look at the effects on these cardiac parameters. If everything was set up correctly the heart would remain beating, without any kind of external stimulation, for several hours. I still remember sitting in that room, staring at the little heart beating away merrily, without a care in the world, and being absolutely fascinated.
Here, I'm going to write about the mechanisms underlying this autonomy, and hopefully give an insight into the origins of our heartbeat.
Introduction to the heart
The heart is composed of specialized muscle cells called cardiac cells. There are three types of muscle cell, and all are excitable, meaning that they can be stimulated electrically, chemically, or mechanically to produce an action potential (remember this?! We looked at action potentials earlier, in the context of the nervous system; an action potential is a communication signal, allowing a message to be initiated and propagated within and between cells). Muscle cells are unique, in that excitation results in contraction of the cell due to the presence of certain proteins and mechanisms that link to the action potential. Cardiac muscle cells are even more unique as they can contract without any external stimulation at all, and are instead driven by a specialized group of cells in the heart known as the pacemaker cells. [Although cardiac muscle can contract independently, its function can be modulated by the autonomic nervous system, which is not under our conscious control. In this way heart rate and ventricular output can be modified as necessary; for example, in order to allow regulation of blood pressure.]
To understand this phenomenon more clearly, we should first of all take a look at blood flow through the heart, and get an overview of how the cardiovascular system functions.
To understand this phenomenon more clearly, we should first of all take a look at blood flow through the heart, and get an overview of how the cardiovascular system functions.
This diagram shows the heart in the centre, with four chambers (I have to point out that this diagram bears absolutely zero anatomical resemblance to an actual heart and is just for explanatory purposes!): two atria, and two ventricles. Blood can pass from each atrium to its respective ventricle, but it is prevented from flowing backwards by one-way valves between the chambers. We can see that the heart acts as a pump, receiving de-oxygenated blood (the blue arrows) into the right atrium via the vena cava (the largest vein in the body), before it enters the right ventricle and is pumped to the lungs for oxygenation. The oxygenated blood (the red arrows) is then brought back to the left atrium, and enters the left ventricle to be pumped all around the body via the aorta.
The reason for this is that oxygen is essential for efficient cellular respiration, which produces energy to fuel biological processes such as digestion, movement, and thermoregulation, and the heart makes sure that cells are provided with this oxygen, which is carried by iron molecules in red blood cells. In addition, blood carries all manner of useful things, such as nutrients from the digestive system or regulatory hormones and other biochemicals, not to mention white blood cells and platelets, which defend us from infection. Dangerous waste products such as carbon dioxide are also carried away from tissues in the blood and eliminated. Put simply, a well-regulated and efficient cardiovascular system is essential for our survival: the heart is therefore crucially important and should be revered as such!
The power of the pacemaker
The heart is constantly at work, pumping blood all around the body (the average heart rate is 70 beats per minute - imagine!), which requires tremendous amounts of energy. It also requires extraordinary synchronicity between the component cardiac muscle cells to bring about ordered blood flow through its four chambers. So how does it all work?
Well, the heart contains a specialized conduction system that spreads electrical impulses throughout all the cardiac cells in a very organized manner (in order for a cardiac muscle cell to contract it must be stimulated. When all the cells contract together, we get a pumping action). This conduction system consists of the 'pacemaker' cells, also called the sinoatrial (SA) node, which initiate the signal, and other structures called the internodal atrial pathways, the atrioventricular node, the bundle of His (best name, ever), and the Purkinje fibres, which transmit the impulses to the muscle cells.
The SA node is located in the right atrium, close to where the vena cava opens into the chamber. It can initiate the signal that triggers our heartbeat because it has a peculiar membrane potential (the charge across a cell, which depends on the flow of charged ions into and out of the cell; see here!). Normally, when an excitable cell produces an action potential, it reverts afterwards to a relaxed state whereby it returns to a relatively stable resting membrane potential until another action potential is triggered. HOWEVER! Cells in the SA node do not have a stable membrane potential, and after an action potential, it drops back down only as far as the firing level so another spike is immediately produced. This is due to differences in ion flow across the cell membrane, with calcium (Ca2+) playing a much more prominent role in SA spiking, while sodium (Na+) takes a back seat. Due to this unstable membrane potential, action potentials in the SA node are self-initiating, and are not dependent on external stimulation. These spikes in the SA node lead to contraction of muscle cells in the atria, before they converge on the atrioventricular node, which passes the signal on to the ventricular muscle cells via the bundle of His (I love that name) and the Purkinje fibres. The impulse moves quickly through the muscle cells because they contain gap junctions, which join cells together and facilitate the transmission of excitation. This allows the cardiac muscle cells to function as a unit and to contract or beat as one cell.
I'm not going to go into a detailed explanation of the electrophysiology of SA node firing, but if you're interested (!), you can take a look at the diagram below, and compare it to the neuronal action potential that we covered previously...
Well, the heart contains a specialized conduction system that spreads electrical impulses throughout all the cardiac cells in a very organized manner (in order for a cardiac muscle cell to contract it must be stimulated. When all the cells contract together, we get a pumping action). This conduction system consists of the 'pacemaker' cells, also called the sinoatrial (SA) node, which initiate the signal, and other structures called the internodal atrial pathways, the atrioventricular node, the bundle of His (best name, ever), and the Purkinje fibres, which transmit the impulses to the muscle cells.
The SA node is located in the right atrium, close to where the vena cava opens into the chamber. It can initiate the signal that triggers our heartbeat because it has a peculiar membrane potential (the charge across a cell, which depends on the flow of charged ions into and out of the cell; see here!). Normally, when an excitable cell produces an action potential, it reverts afterwards to a relaxed state whereby it returns to a relatively stable resting membrane potential until another action potential is triggered. HOWEVER! Cells in the SA node do not have a stable membrane potential, and after an action potential, it drops back down only as far as the firing level so another spike is immediately produced. This is due to differences in ion flow across the cell membrane, with calcium (Ca2+) playing a much more prominent role in SA spiking, while sodium (Na+) takes a back seat. Due to this unstable membrane potential, action potentials in the SA node are self-initiating, and are not dependent on external stimulation. These spikes in the SA node lead to contraction of muscle cells in the atria, before they converge on the atrioventricular node, which passes the signal on to the ventricular muscle cells via the bundle of His (I love that name) and the Purkinje fibres. The impulse moves quickly through the muscle cells because they contain gap junctions, which join cells together and facilitate the transmission of excitation. This allows the cardiac muscle cells to function as a unit and to contract or beat as one cell.
I'm not going to go into a detailed explanation of the electrophysiology of SA node firing, but if you're interested (!), you can take a look at the diagram below, and compare it to the neuronal action potential that we covered previously...
Let's wrap this up...
I've just given a brief explanation of the origin of our heartbeat, and why it's so important. A little group of cells are responsible for initiating a pumping mechanism that ensures the delivery of oxygen and nutrients and all sorts to every tissue and organ in our bodies. Our heartbeat keeps us alive and is the rhythm of our lives (I was going to say something super-cheesy about the scene in Dirty Dancing where Patrick Swayze is teaching Baby to dance and he tells her to move in time to her heartbeat. But I won't)... our hearts pump ceaselessly from the minute we are born until the minute we die, and we can just sit back and enjoy the ride. Let's hear it for the wonderful, under-appreciated heart!