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Cardiac muscle cells are autorhythmic, meaning they can contract spontaneously and rhythmically without external stimulation. This is due to their ability to initiate their own electrical impulses, which is a unique property of cardiac muscle. Cardiac muscle cells are physically and electrochemically interconnected through intercalated discs, allowing them to synchronize their actions and contract as a coordinated unit to pump blood through the body. This autorhythmicity is a key characteristic of cardiac muscle, distinguishing it from skeletal and smooth muscle.
| Characteristics | Values |
|---|---|
| Ability | Can contract spontaneously and rhythmically |
| Interconnection | Physically and electrochemically connected through intercalated discs |
| Function | Pumping blood through the body |
| Control | Under involuntary control |
| Electrical impulses | Can initiate their own electrical impulses without external stimulation |
| Contraction | Contract as a coordinated unit |
| Cells | Myocardial contractile cells and myocardial conducting cells |
| Cell composition | 99% myocardial contractile cells and 1% myocardial conducting cells |
| Cell size | Myocardial conducting cells are generally much smaller than contractile cells |
| Myofibrils | Myocardial conducting cells have fewer myofibrils for contraction |
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What You'll Learn
Cardiac muscle is composed of a syncytium of connected fibres
Cardiac muscle, or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the heart wall. The cardiac muscle forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
Cardiac muscle tissue is only found in the heart. The highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Cardiac muscle fibres are shorter than skeletal muscle fibres and usually contain only one nucleus, which is located in the central region of the cell. Cardiac muscle fibres also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibres are extensively branched and are connected to one another at their ends by intercalated discs. An intercalated disc allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.
Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibres that allow the depolarising current produced by cations to flow from one cardiac muscle cell to the next. This joining is called electric coupling, and in cardiac muscle, it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium.
The cardiac syncytium is a network of cardiomyocytes connected by intercalated discs that enable the rapid transmission of electrical impulses through the network, enabling the syncytium to act in a coordinated contraction of the myocardium. There is an atrial syncytium and a ventricular syncytium that are connected by cardiac connection fibres. Electrical resistance through intercalated discs is very low, thus allowing free diffusion of ions. The ease of ion movement along cardiac muscle fibre axes is such that action potentials are able to travel from one cardiac muscle cell to the next, facing only slight resistance.
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Cardiac muscle cells can be branched, linear or longitudinal
Cardiac muscle cells, also called cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are involuntary striated muscles that constitute the main tissue of the heart wall. Cardiac muscle cells are unique in that they can be branched, linear, or longitudinal.
Cardiac muscle cells are connected by intercalated discs, which appear as thin, dark-staining lines under a light microscope. These discs run perpendicular to the direction of the muscle fibres. The intercalated discs facilitate the transport of molecules and electrical signals from myocyte to myocyte. This allows for a coordinated contraction of the heart, termed systole.
The cardiac muscle is composed of sheets of muscle that wrap around the ventricles, with the sheets closest to the endocardium oriented perpendicularly to those closest to the epicardium. When these sheets contract in a coordinated manner, they allow the ventricles to squeeze in multiple directions simultaneously: longitudinally (becoming shorter from apex to base), radially (becoming narrower from side to side), and with a twisting motion. This allows the heart to squeeze out the maximum amount of blood with each heartbeat.
The contraction of cardiac muscle cells is triggered by electrical stimulation in the form of a cardiac action potential, which causes the release of calcium from the cell's internal store, the sarcoplasmic reticulum. This rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling. The sino-atrial (SA) node, a small region of tissue with the fastest rate of spontaneous depolarization, functions as the overriding pacemaker of the heart.
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Cardiac muscle cells are autorhythmic, like neurons
Cardiac muscle cells are indeed autorhythmic, and they share this feature with neurons. Autorhythmicity is a property of cardiac muscle cells that allows them to self-excite and generate action potentials independently, resulting in the contractions that pump blood through the body. This is similar to the function of neurons, which transmit electrical impulses.
The autorhythmic nature of cardiac muscle cells is due to the presence of pacemaker cells, which are a specialized group of myocardial conducting cells. These cells initiate and propagate the action potential that triggers contractions in the heart muscle. The contractions of the heart muscle are essential for pumping blood and distributing it throughout the body.
While cardiac muscle cells share some similarities with skeletal and smooth muscle cells, they exhibit unique characteristics that set them apart. Cardiac muscle cells, also known as cardiomyocytes, are shorter and have smaller diameters compared to skeletal muscle cells. They also demonstrate autorhythmicity, which is not a characteristic of skeletal or smooth muscle cells.
Additionally, cardiac muscle cells have a distinct structure. They are composed of individual muscle fibers that contain thick and thin contractile filaments, giving them a striated appearance. This arrangement of filaments is similar to that found in fast skeletal muscle fibers. However, the overall organization of cardiac muscle differs from skeletal muscle, allowing it to perform its specialized function in the heart.
In conclusion, cardiac muscle cells exhibit autorhythmicity, a property that enables them to initiate and regulate contractions independently. This autorhythmic behaviour is similar to the function of neurons in transmitting electrical impulses. The unique characteristics of cardiac muscle cells, including their structure and autorhythmicity, make them specially adapted for their crucial role in the heart's functioning.
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Cardiac muscle cells are smaller than contractile cells
Cardiac muscle cells, also called cardiomyocytes, are considerably smaller than contractile cells. They are shorter in length and have smaller diameters. Cardiac muscle cells are responsible for the contractility of the heart and, therefore, the pumping action. The contractile forces of the cardiac muscle and the frequency at which they are activated determine what is known as cardiac output, which is defined as heart rate multiplied by stroke volume.
The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The cardiac action potential lasts approximately 200 ms and is divided into five phases: resting, upstroke, early repolarization, plateau, and final repolarization. The resting membrane potential is approximately −90 mV, and during the resting phase, the Na/K ATPase pump exchanges three sodium ions for two potassium ions to maintain a negative intracellular potential.
Cardiac muscle cells are also structurally different from contractile cells. They demonstrate striations, an alternating pattern of dark A bands and light I bands attributed to the precise arrangement of the myofilaments and fibrils that are organized in sarcomeres along the length of the cell. These contractile elements are virtually identical to skeletal muscle. T (transverse) tubules penetrate from the surface plasma membrane, the sarcolemma, to the interior of the cell, allowing the electrical impulse to reach the interior. The T tubules are only found at the Z discs, whereas in skeletal muscle, they are found at the junction of the A and I bands. Therefore, there are only half as many T tubules in cardiac muscle as in skeletal muscle.
Additionally, the sarcoplasmic reticulum of cardiac muscle cells stores few calcium ions, so most of the calcium ions required for contraction must come from outside the cells. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess. These calcium channels play a crucial role in the electrical impulses that trigger contractions.
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Cardiac muscle cells have a distinctive striated appearance
Cardiac muscle cells, also known as cardiomyocytes, have a distinctive striated or striped appearance under a microscope. This unique structure is characterised by alternating dark and light bands, known as A bands and I bands, respectively. These bands are composed of myosin and actin filaments, with the thick, dark bands indicating myosin filaments and the thin, lighter bands containing actin. The precise arrangement of these filaments and fibrils contributes to the striated pattern observed in cardiac muscle cells.
The sarcolemma, a specialised cell membrane surrounding each cardiomyocyte, plays a crucial role in the function of these distinctive cells. It contains voltage-gated calcium channels, which are absent in skeletal muscle cells. These calcium channels facilitate the entry of calcium ions into the cell, triggering contraction. Additionally, intercalated discs connect cardiac muscle cells, enabling them to contract synchronously and work together as a pump.
The cardiac muscle, or myocardium, is one of the three layers of the heart, along with the pericardium and endocardium. It is composed of specialised cardiac muscle cells that contract and expand involuntarily in response to electrical impulses from the nervous system. These rhythmic contractions generate the heartbeat, ensuring the continuous pumping of blood throughout the body.
The structure and function of cardiac muscle cells differ from those of skeletal and smooth muscle cells. Unlike skeletal muscle, which is under voluntary control, cardiac muscle cells contract involuntarily. Smooth muscle fibres, found in the walls of hollow visceral organs, have a spindle shape and are also under involuntary control. In contrast, cardiac muscle cells exhibit the distinctive striated appearance, contributing to their unique functional properties.
In summary, cardiac muscle cells, or cardiomyocytes, possess a distinctive striated appearance due to the alternating arrangement of myosin and actin filaments. This structural characteristic is integral to their function, enabling them to contract and relax in a coordinated manner, ultimately driving the rhythmic contractions of the heart and facilitating the continuous circulation of blood throughout the body.
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Frequently asked questions
Yes, cardiac muscles are autorhythmic. This means they can contract spontaneously and rhythmically without needing a stimulus from a nerve source.
Cardiac muscles share characteristics with skeletal and smooth muscles but have some unique properties. One such property is autorhythmicity, or the ability to initiate an electrical potential at a fixed rate that spreads from cell to cell to trigger the contractile mechanism.
The two types of cardiac muscle cells are myocardial contractile cells and myocardial conducting cells. Myocardial contractile cells are autorhythmic and make up 99% of the cells in the atria and ventricles. Myocardial conducting cells are much smaller and make up 1% of the cells.
Autorhythmic cardiac muscles can initiate their own electrical impulses without external stimulation. They are interconnected physically and electrochemically through intercalated discs, allowing them to synchronize their actions and contract as a coordinated unit to pump blood through the body.
