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1. Muscles
The Three Types of Muscle
Contractility is a fundamental property of cells and the majority
of them contain essentially the same contractile machinery as
that found in muscle cells. In muscle cells, however, a larger
proportion of the cells' resources are given over to this function
than in other cell types.
These are the three types of muscle:
Skeletal Muscle :
Contractions move part of the
skeleton. Also called 'voluntary'
because usually its contractions are
under your control.
It has a stripy appearance, because
of the repeating structure of the
muscle: there are many myofibrils (fibers), each one of which is
made up of repeating units called muscle sarcomeres. Each
sarcomere is 2.5 m long.

2. Cardiac Muscle:
Cardiac muscle makes up the muscular
walls of the heart (myocardium). It is
'involuntary' because its contractions are
not under your control. However, it has a
similar ultrastructural organisation to
skeletal muscle. So, it too has a stripy
appearance because of the repeating units
called muscle sarcomeres.
Smooth Muscle:
Found in the walls of most blood vessels
and tubular organs such as the intestine. It
is also 'involuntary'. However, it does
NOT have a stripy appearance, because it
does not have repeating sarcomeres. The
contractile proteins, myosin and actin are
much more randomly arranged than in
skeletal or cardiac muscle

3. Skeletal Muscle and Cardiac Muscle are also called 'striated
muscle', because they have dark and light bands running across
the muscle width when they are looked at under the microscope.
Confusingly the prefixes myo- and sarco- (respectively from the
Latin and Greek, both meaning muscle) are often used when
naming structures and organelles associated with muscle.
Thus the plasma membrane of muscle cells is sometimes called
the sarcolemma and their cytoplasm sarcoplasm.
Their endoplasmic reticulum is called sarcoplasmic reticulum
and their mitochondria are sometimes called sarcosomes.
The contractile fibres that lie in the sarcoplasm are known as
myofibrils and the embryonic precursors of skeletal muscle cells
are called myoblasts.