These muscles are found in the heart of animals (vertebrates). The fibres of these muscles are involuntary in nature as their activities are not under the control of will.
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The fibres of these muscles are both longitudinally and transversely striated as are striated or skeletal muscles but its fibres exhibit branching and contain myofibrils and filaments of actin and myosin are arrayed similarly to those in skeletal muscles.
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Likewise, the mechanisms of contraction are essentially the same as those in skeletal muscle but these muscles differ histologically from striated muscle of vertebrates in some ways (i) these muscles are uninucleated whereas striated muscles are multi- nucleated, (ii) these are automatically innervated, while striated muscles are innervated by peripheral fibres whose cell bodies lie within the central nervous system and they interdigitate to form what are known as intercalary discs, these are actually cell membranes that separate individual cardiac muscle cells from each other.
Thus cardiac muscle is not a syncytium. Despite the lack of syncytium cardiac muscle fibres transmit impulses from one fibre to another throughout the muscle without the need of booster stimuli from nerves.
This suggests that cardiac muscle fibres have pseudosyncytial arrangement through which cardiac muscle is allowed to contract sequentially from fibre to fibre and, thus, the cardiac muscle is said to be- functional syncytium.
In the end we can say that the cardiac muscles have one character of voluntary (striated) muscles being striated and one character of involuntary muscles, being not under the control of will.