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Structure
and organization
The
heart is a network of highly branched cardiac cells 110
micrometres in length and 15 micrometres in width, which
are connected end to end by intercalated disks. The cells
are organized into layers of myocardial tissue that are
wrapped around the chambers of the heart. The contraction
of the individual heart cells produces force and shortening
in these bands of muscle with a resultant decrease in the
heart chamber size and the consequent ejection of the blood
into the pulmonary and systemic vessels. Important components
of each heart cell involved in excitation and metabolic
recovery processes are the plasma membrane and transverse
tubules in registration with the Z lines, the longitudinal
sarcoplasmic reticulum and terminal cisternae, and the mitochondria.
The thick (myosin) and thin (actin, troponin, and tropomyosin)
protein filaments are arranged into contractile units (that
is to say, the sarcomereextending from Z line to Z line)
that have a characteristic cross-striated pattern similar
to that seen in skeletal muscle.
