Cardiac Muscle Tissue and Cardiomyocytes
Cardiac muscle tissue is a highly specialized type of muscle found exclusively in the heart. It is one of the three main types of muscle tissue, distinguished by its unique structural and functional characteristics, including cross-striations and involuntary contraction.
Cardiomyocytes (Cardiac Muscle Cells) - Structure
Cardiomyocytes are the individual cells that make up cardiac muscle. They are specialized for strong, rhythmic, involuntary contractions.
- Cell Shape and Size:
- Cardiomyocytes are cylindrical, measuring approximately 15 μm in diameter and 85 to 100 μm in length.
- They are frequently branched, allowing them to interweave in a complex arrangement within fascicles, which provides for efficient contraction of the heart ventricles.
- Nucleus:
- Each cardiac muscle cell typically possesses one or two centrally located, pale-staining nuclei. This contrasts with the peripherally located, multinucleated skeletal muscle cells.
- Striations:
- Cardiac muscle exhibits a cross-striated banding pattern, similar to skeletal muscle. These striations are caused by the regular arrangement of actin (thin) and myosin (thick) protein filaments.
- However, the myofibrils in cardiac muscle are generally less dense and less well-organized compared to skeletal muscle.
- Intercalated Discs:
- A unique and distinguishing characteristic of cardiac muscle is the presence of dark-staining transverse lines called intercalated discs.
- These discs represent the interface between adjacent muscle cells and contain numerous junctional complexes.
- Transverse regions of the discs contain many desmosomes and fascia adherentes (adherent junctions) that firmly bind cardiac muscle cells together, preventing them from pulling apart during continuous contractile activity.
- Longitudinal regions of the discs contain numerous gap junctions, which provide ionic continuity between cells. These act as "electrical synapses," enabling cardiac muscle cells to function like a multinucleated syncytium, where contraction signals spread in a wave from cell to cell.
- Sarcoplasmic Reticulum (SR) and T-Tubules:
- The T-tubule system in cardiac muscle is more numerous and larger than in skeletal muscle.
- The sarcoplasmic reticulum is less abundant and less regularly arranged.
- Unlike the triads (T-tubule flanked by two terminal cisterns of SR) found in skeletal muscle, cardiac muscle typically has dyads (a T-tubule associated with only one small terminal cistern of SR).
- Mitochondria and Energy Metabolism:
- Cardiomyocytes are rich in mitochondria, which can occupy 40% or more of the cytoplasmic volume. This high density reflects their constant need for aerobic metabolism to produce ATP for continuous contraction.
- Fatty acids, transported by lipoproteins and stored as triglycerides in lipid droplets, are the major fuel source for the heart. Glycogen granules are also present.