Heart Physiology Vid 1 Part 2
Cellular junctions overview
Gap junctions
Transmembrane proteins that span the plasma membranes of two adjacent cells.
Comprised of subunits that form tiny tunnels/passageways, allowing solutes to pass from one cell to the next.
Tight junctions
Transmembrane proteins between the plasma membranes of two cells.
Function: seal the space between cells to keep certain substances like liquids confined to a lumen or cavity (compartmentalization).
Examples: within blood vessels to keep blood components inside the vessel walls; in the urinary bladder to prevent urine from seeping between epithelial cells until urination.
Adherens junctions
Found in epithelial tissues with microvilli.
Provide structural integrity and resist pulling forces, preventing cells from pulling apart under stress.
Desmosomes
Comprised of a thick protein plaque linked to keratin (intermediate filaments) via cadherin proteins.
Button-like junctions that keep cells together in tissues subjected to stretching and twisting to maintain tissue integrity.
Hemidesmosomes
Not cell-to-cell junctions.
Anchor the basal surface of epithelial cells to the basement membrane.
Distinguish from other junctions by connecting cells to the extracellular matrix rather than to neighboring cells.
Cardiac muscle tissue: linking junctions to function
Cardiac muscle fibers are also called cardiomyocytes; they are striated and contract via the sliding filament mechanism.
Sarcomere: the basic contractile unit, defined from one Z disc to the next Z disc (Z disc = Z line).
Thin filaments: actin, tropomyosin, troponin, nebulin.
Thick filaments: myosin.
Shape and size Differences
Skeletal muscle fibers: long and cylindrical; multinucleated.
Cardiomyocytes (cardiac muscle fibers): short, branched, usually uninucleated with a single central nucleus; extensive capillary supply.
Metabolic apparatus
Cardiomyocytes contain many mitochondria (about of the cell volume) to support continuous aerobic respiration for ATP production.
This high mitochondrial content reflects the heart’s constant demand for oxygen.
Blood supply
The heart has an extensive capillary network to sustain continuous function.
Ultrastructural differences in calcium handling
Cardiomyocytes: T-tubules are wider and fewer in number.
Sarcoplasmic reticulum (SR) is simpler and lacks large terminal cisternae.
No triads in cardiomyocytes; instead, the dyadic arrangement (T-tubule with a nearby SR segment) is present, and not a classic triad as in skeletal muscle.
No triads in cardiomyocytes
In skeletal muscle, the triad consists of a transverse tubule flanked by terminal cisternae on both sides:
In cardiomyocytes, the SR does not form large terminal cisternae around T-tubules, so classic triads are not present.
Intercalated discs and cell-to-cell coupling in the heart
Intercalated discs
Unique to cardiac muscle; interdigitating folds of the sarcolemma increase surface area for cell-to-cell contact.
Help neighboring cardiomyocytes interlock and communicate more effectively.
Junctions within intercalated discs
Desmosomes: provide strong mechanical linkage to keep cells bound during contraction and mechanical stress.
Gap junctions: allow ions and small solutes to pass directly between cells, promoting electrical coupling and synchronized contraction (the heart acts as a functional syncytium).
Fascia adherens (a type of adherens junction variant in cardiac tissue)
Actin filaments anchor to fascia adherens to reinforce cell–cell adhesion.
Fascia adherens contribute to the secure interaction of cardiomyocytes within the myocardium (the middle layer of the heart wall).
Terminology recap and functional significance
Sarcolemma: plasma membrane of a muscle fiber.
Sarcoplasm: cytoplasm of a muscle fiber.
Sarcoplasmic reticulum (SR): stores Ca²⁺ for muscle contraction; in skeletal muscle, terminal cisternae are prominent around T-tubules, forming a classic triad.
Transverse tubules (T-tubules): invaginations of the sarcolemma that penetrate into the sarcoplasm to quickly propagate action potentials.
Terminal cisternae: large SR storage sacs adjacent to T-tubules in skeletal muscle; form the triad.
Triad: specialized arrangement in skeletal muscle consisting of a T-tubule flanked by terminal cisternae on both sides; not present as such in cardiomyocytes.
Z disc (Z line): boundary between adjacent sarcomeres; defines the start and end of a sarcomere.
M line: center line in the sarcomere that holds thick filaments together.
Titin: elastic protein that anchors to the Z disc and helps center-and-stabilize the thick filaments during contraction.
Nebulin: helps stabilize the thin filament length.
Actin, tropomyosin, troponin: components of thin filaments that regulate contraction.
Myosin: major component of thick filaments that drives contraction via cross-bridge cycling.
Summary of key contrasts to study for exam readiness
Cardiac muscle features
Short, branched cardiomyocytes with single nucleus and extensive capillary network.
High mitochondrial content (~ of cell volume) for constant ATP production via aerobic respiration.
Intercalated discs containing desmosomes, gap junctions, and fascia adherens for mechanical and electrical coupling.
No classic triads; T-tubules are wider and fewer; SR is simpler.
Skeletal muscle features (for comparison)
Long, cylindrical, multinucleated fibers with well-developed triads (T-tubule + two terminal cisternae).
Distinct terminal cisternae arrangement around T-tubules in the SR.
Shared features
Both have sarcomeres from Z disc to Z disc, contain actin and myosin in thin and thick filaments, and rely on the sliding filament mechanism for contraction.
Practical implications
Intercalated discs enable rapid electrical coupling through gap junctions, essential for synchronized cardiac contraction.
Fascia adherens and desmosomes help cardiac tissue withstand mechanical stress during repeated cycles of contraction and relaxation.
The heart’s metabolism is tightly coupled to oxygen supply due to the high mitochondria content, underscoring the importance of coronary circulation.