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Cardiac muscle, also known as the myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. Cardiac muscle cells are short cylindrical, branching cells about 80 µm in length and 15 µm in diameter with a single, centrally placed nucleus (or occasionally two nuclei). The intercalated discs that connect cardiac muscle cells to each other contain desmosomes and gap junctions, which allow action potentials to propagate between cells and facilitate a synchronized heartbeat.
| Characteristics | Values |
|---|---|
| Cardiac muscle cells | Short cylindrical, branching cells |
| Length | 50–100 μm |
| Diameter | 10–20 μm |
| Nucleus | Single, centrally placed |
| Interconnection | Intercalated discs |
| Contractility | Involuntary |
| Contraction | Coordinated |
| Action potential | 200 ms |
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What You'll Learn
- Cardiac muscle cells are short, cylindrical, and branched
- Intercalated discs facilitate a synchronized heartbeat
- Cardiac muscle is innervated via neurons in the thoracic paravertebral ganglia
- Cardiac muscle cells are smaller than skeletal muscle cells
- Cardiac muscle is made up of sarcomeres that allow for contractility
Cardiac muscle cells are short, cylindrical, and branched
Cardiac muscle, also known as myocardium, is one of the three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. The myocardium forms the inner muscular layer of the heart wall and is responsible for the heart's contractility.
The branched nature of cardiac muscle cells and the presence of gap junctions allow for the rapid propagation of electrical impulses across the myocardium. This electrical coupling ensures that the muscle cells beat in synchrony, enabling the heart to contract and relax as a single unit, known as a functional syncytium. The coordinated contractions of the cardiac muscle generate sufficient force to pump blood into circulation and meet the metabolic demands of the entire body.
The sarcolemma, or the cardiomyocyte plasma membrane, contains transverse tubules (t-tubules) that play a crucial role in excitation-contraction coupling (ECC), action potential initiation and regulation, maintaining resting membrane potential, and signal transduction. Approximately 50% of the sarcoplasm in cardiac muscle cells is occupied by mitochondria, arranged parallel to the longitudinal axis of the cell, along with a significant amount of the oxygen-bearing protein myoglobin.
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Intercalated discs facilitate a synchronized heartbeat
Cardiac muscle, or myocardium, is one of three major categories of muscles in the human body, the other two being smooth and skeletal muscle. The primary function of the cardiac muscle is to pump blood into circulation by generating sufficient force. The cardiac muscle must contract with enough force to supply blood to the metabolic demands of the entire body.
The cardiac muscle consists of individual heart muscle cells, or cardiomyocytes, connected by intercalated discs to work as a single functional syncytium. Intercalated discs are complex structures that connect adjacent cardiac muscle cells. The three types of cell junctions recognized as making up an intercalated disc are desmosomes, fascia adherens junctions, and gap junctions. Desmosomes are intercellular structures that anchor cardiac muscle fibers together and are vital in maintaining the structural integrity of the heart. Gap junctions allow the muscle cells to be electrically coupled, so that they beat in synchrony.
The intercalated disc (ID) is a highly specialized structure that connects cardiomyocytes via mechanical and electrical junctions. The ID is a highly organized structure that is arranged both transversely and longitudinally in a staircase-like fashion with "steps" and "risers". The "steps" are transverse or plicate segments of the membrane that run in a zigzag arrangement with three-dimensional finger-like microprojections. The ID's extensive three-dimensional structure with many folds and projections increases the surface area of connections between cardiomyocytes, allowing for the propagation of electrical stimuli.
The intercalated disc mediates the coupling of neighboring myocytes through intercellular signaling. Gap junctions directly couple the cytosol of neighboring myocytes and enable the propagation of electrical stimuli through low-resistance pores. A single gap junction is composed of 12 connexin proteins, 6 from each neighboring myocyte. Six connexin molecules form a hemi-channel and are trafficked to the ID membrane as a single unit. Once at the ID, two hemi-channels from opposing myocytes couple to form a gap junction. An assembly of 5–500 gap junctions constitutes a gap junction plaque and facilitates electrical coupling between myocytes.
Overall, intercalated discs support the synchronized contraction of cardiac tissue in a wave-like pattern so that the heart can work like a pump.
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Cardiac muscle is innervated via neurons in the thoracic paravertebral ganglia
Cardiac muscle, or myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. The primary function of the cardiac muscle is to pump blood into circulation by generating sufficient force. The cardiac muscle must contract with enough force to supply blood to the metabolic demands of the entire body. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses. These contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
The network of nerves supplying the heart is called the cardiac plexus. It receives contributions from the right and left vagus nerves, as well as the sympathetic trunk. The cardiac plexus is a network of nerves including both the sympathetic and parasympathetic systems. The parasympathetic portions of the cardiac plexus receive contributions from the vagus nerve only. The preganglionic fibres, branching from the right and left vagus nerves, reach the heart and enter the cardiac plexus by synapsing with ganglia within this plexus and the walls of the atria. The sympathetic part of the cardiac plexus, on the other hand, is composed of fibres from the sympathetic trunk, arising from the upper segments of the thoracic spinal cord. Fibres from the sympathetic trunk reach the cardiac plexus via cardiac nerves. The preganglionic fibres branch from the upper thoracic spinal cord and synapse in the lower cervical and upper thoracic ganglia. Postganglionic fibres extend from the ganglia to the cardiac plexus.
The intrinsic cardiac ganglia reside on the epicardium and receive post-ganglionic sympathetic and pre-ganglionic parasympathetic connections. In the thorax, the extra-cardiac but intrathoracic ganglia such as the stellate ganglia, the middle cervical ganglia, and the thoracic ganglia of T2–T4 also process neural information, controlling sympathetic outflow to the heart. Sympathetic innervation of the heart originates from preganglionic neurons located in the spinal cord's thoracic segments (T1-T4 or T5). These neurons project to postganglionic neurons in sympathetic ganglia, such as the stellate ganglia, which then innervate cardiac tissues.
Cardiac muscle cells are often branched and are tightly connected by specialised junctions called intercalated discs. These intercalated discs contain gap junctions, adhering junctions, and desmosomes. Gap junctions allow the muscle cells to be electrically coupled, so that they beat in synchrony.
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Cardiac muscle cells are smaller than skeletal muscle cells
Cardiac muscle cells are rectangular in shape and are often branched. They are tightly connected by specialised junctions called intercalated discs, which contain gap junctions, adhering junctions, and desmosomes. These intercalated discs allow for the electrical coupling of muscle cells, enabling them to beat in synchrony and facilitating the pumping action of the heart.
Skeletal muscle cells, on the other hand, are long and cylindrical in appearance. When viewed under a microscope, skeletal muscle tissue has a striped or striated pattern due to the regular arrangement of contractile proteins actin and myosin. Skeletal muscle cells are incredibly large, with diameters of up to 100 µm and lengths of up to 30 cm.
The difference in size between cardiac and skeletal muscle cells can be attributed to their distinct functions and locations in the body. Cardiac muscle tissue is only found in the heart, and its primary function is to pump blood into circulation by generating sufficient force and maintaining blood pressure. On the other hand, skeletal muscle is attached to bones and facilitates body movements through voluntary contractions.
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Cardiac muscle is made up of sarcomeres that allow for contractility
Cardiac muscle, also called the myocardium, is one of three major categories of muscles found within the human body, the others being smooth muscle and skeletal muscle. Cardiac muscle is made up of sarcomeres that allow for contractility. The sarcomere is the fundamental unit of striated muscle and is directly responsible for most of its mechanical properties. It generates active or contractile forces and determines the passive or elastic properties of striated muscle.
Sarcomeres are composed of thick (myosin) and thin (actin) filaments, the interactions between which form the basis of the sliding filament theory. The regular organization of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when viewed under a microscope. These striations are caused by lighter I bands composed mainly of actin, and darker A bands composed mainly of myosin.
The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses.
Cardiac muscle cells are striated, branched, and contain many mitochondria. They are under involuntary control, and each contains a single, centrally located nucleus surrounded by a cell membrane called the sarcolemma. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess.
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Frequently asked questions
Yes, cardiac muscle cells are short cylindrical, branching cells about 80 µm in length and 10-20 µm in diameter.
Intercalated discs connect cardiac muscle cells to each other and contain desmosomes and gap junctions, which perform important functions. Desmosomes provide a tight mechanical connection between cells, while gap junctions allow action potentials to propagate between cells.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force.
