Page 53 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

filaments supported at the centre by M membrane. Light
band (I band) consists of thin actin filaments supported
at the centre by Z membrane. The whole myofibril may be
considered as containing many units called sarcomeres as
shown in Figure 2.52.

Sarcomere Myosin filament Actin filament
2 (thick) (thin)

Z membrane A band I band M membrane H band
Figure 2.52
(b) Cardiac muscle
Cardiac muscle is only found in the heart. It is also striated,
consisting of fibres and myofibrils as in striated muscle but
differs in many ways.
(i) Cardiac muscle consists of individual cells and not syncytial
tissue.
(ii) Unlike striated muscles with straight fibres, cardiac muscle
fibre is branched and is connected to the neighbouring
fibres by bridges.
(iii) Each of the individual cells is separated from its adjacent
ones by intercalated discs so that excitation can be
transmitted effectively across them from cell to cell.
(iv) Cardiac fibre is not supplied with nerves from motor
neurone as it does not require impulses from the brain
before it can contract.
(v) The muscle is myogenic. It has its own pacemaker to
generate excitation that is transmitted across fibres before
they contract.
(vi) The structure of cardiac muscle is shown in Figure 2.53.

Cell surface
membrane
Nucleus

Intercalated disc

Cross bridge

Figure 2.53 Cardiac muscle
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