Distinctive features of the myocardium

· The myocardium is composed of specialised striated muscle and intervening connective tissue

· Cardiac muscle, like skeletal muscle, is striated, and its contraction is accomplished by the same sliding filament mechanism

However:

A) HISTOLOGY:

1. In contrast to the long, cylindrical, multinucleate skeletal muscle fibres, cardiac cells are short, fat, branched & interconnected. Each fibre contains one or at most two large, pale, centrally located nuclei

2. Unlike skeletal muscle fibres which are independent of one another in both structure & function, adjacent cardiac cells interconnect at junctions called intercalated discs

3. There is a greater abundance of mitochondria in cardiac muscle revealing that it depends more on a continual supply of O₂ for its energy metabolism than does skeletal muscle (heart relies almost exclusively on aerobic metabolism) . Mitochondria account for 25% of the volume of cardiac muscle fibres compared to only 2% in skeletal muscle

Intercalated Discs:

- contain anchoring desmosomes and gap junctions

- Desmosome: sites of attached between adjacent cardiac cells

- Gap Junction: allow ions to pass freely between cells allowing direct transmission of the depolarising current across the entire heart. Because all cardiac fibres are electrically coupled via the gap junctions, the entire myocardium behaves as a single unit or functional syncytium

- Myocardial fibres are organised into two discreet networks of interconnecting cells — one in the atrial walls and the other in the ventricular walls. Impulses originating at any point in a given network (syncytium ) will spread rapidly & uniformly, causing each muscle mass to contract as a unit

B) PHYSIOLOGY

1)All or none law

· In skeletal muscle the all-or-none law applies to the contraction of each individual muscle cell only, impulses do not spread to adjacent cells. In cardiac muscle, this law applies to the organ level, either the heart contracts as a unit or it does not contract at all. This occurs by the transmission of impulses via specialised cell junctions [gap junctions] which tie all the cardiac muscle cells into a single contractile unit [syncytium]

2)Means of stimulation

· Each skeletal muscle cell must be individually stimulated to contract by a nerve ending. In cardiac muscle, some cells are self excitable and can initiate their own depolarisation thereby depolarising the rest of the heart in a spontaneous & rhythmic way [automaticity/autorythmicity]

3)Source of activating Calcium

In skeletal muscle, the sarcoplasmic reticulum is the sole source of intracellular calcium for binding to regulatory protein- troponin thereby initiating muscle contraction. Both the sarcoplasmic reticulum & extracellular calcium contribute to elevating intracellular calcium in cardiac muscle cells.

4)Length of absolute refractory period

The absolute refractory period (the unexcitable period when potassium ions are leaving the cell) last 250 ms in cardiac muscle cells which is nearly as long as the contraction. In skeletal muscle, a very short refractory period of 1 to 2 ms associates with a duration of contraction of 20 - 100 ms. This long refractory period prevents wave summation & tetany which would stop the hearts pumping ability.