- striated muscle of mononuclear cells
- electrically and mechanically coupled at intercalated discs
- Tissue operates as functional syncitium
- nuclei centrally located
- less powerful than skeletal muscle
- capable of generating repetitive, fatigue resistant rhythmic contractions
- Contractility generated by actin and myosin II sliding filaments
- cardiac myocytes develop from precursor cells of pericardial cavity and not from somites
- Structure:
- branched myocytes, cross striations visible but less conspicuous than in skeletal muscle
- intercalated discs
- attach cells together at desmosomes
- connect actin filaments of adjacent myocytes
- contain gap junctions which allow an action potential to spread from one cell to another
- myofilament and T-tubules less well delineated or organised than skeletal muscle
- Mitochondria numerous
- contraction is myogenic
- activated by increasing levels of free cytosolic Ca++
- Myocardial contraction is asynchronous
- autonomic nerve system acts on cardiac pacemaker region
- SA node > AV node > bundle of His > Purkinje fibre system
- No stem cells remain.
- ischemia leads to myocyte necrosis, replacement by scar tissue
6.8.2 Cardiac Muscle [Phys]
| Contrast with skeletal muscle. Absence of motor units and
twitch-summation; control of pumping via frequency and chemical (autonomic) input (ACh,
noradrenaline, adrenaline). Starling’s law of the heart. Electrical syncitium for
cardiac muscle: nexuses. Action potential in heart; ionic basis, Na+, Ca2+,
Na-Ca exchange currents and K+ currents. Functional advantage of long action
potential. Heterogeneity of cardiac action potential coupled with functional pathways of
conduction in the heart. Mechanism of excitation-contraction coupling; role of Ca2+. |
Length–tension curve. |
|