MUSCLE
Motion, as a reaction of
multicellular organisms to changes in the internal and external environment, is
mediated by muscle cells.
The basis for motion mediated by
muscle cells is the conversion of chemical energy (ATP)
into mechanical energy by the contractile apparatus of muscle cells. The
proteins actin and myosin are part of the contractile apparatus. The
interaction of these two proteins mediates the contraction of muscle cells.
Actin and myosin filaments, each composed of many action and myosin molecules,
form myofibrils arranged parallel
to the direction of cellular contraction.
A further specialisation of
muscle cells is an excitable cell membrane which
propagates the stimuli which initiate cellular contraction.
Three structurally and
functionally distinct types of muscle are found in vertebrates:
- smooth muscle,
- skeletal muscle and
- cardiac muscle.
Smooth Muscle
- Smooth muscle consists of spindle
shaped cells of variable size.
The largest smooth muscle cells occur in the uterus
during pregnancy (12x600 µm). The smallest are found around small
arterioles (1x10 µm).
- Smooth muscle cells contain one
centrally placed nucleus.
The chromatin is finely granular and the nucleus contains
2-5 nucleoli.
- The innervation of smooth muscle is provided by the autonomic nervous system.
- Smooth muscle makes up the visceral or
involuntary muscle.
Structure of smooth muscle
In the cytoplasm, we find
longitudinally oriented bundles of the myofilaments actin and myosin. Actin
filaments insert into attachment plaques located
on the cytoplasmic surface of the plasma membrane. From here, they extend into
the cytoplasm and interact with myosin filaments. The myosin filaments interact
with a second set of actin filaments which insert into intracytoplasmatic
dense bodies. From these dense bodies further actin filaments extend to
interact with yet another set of myosin filaments. This sequence is repeated
until the last actin filaments of the bundle again insert into attachment
plaques.
In principle, this organisation
of bundles of myofilaments, or myofibrils, into
repeating units corresponds to that in other muscle types. The repeating units
of different myofibrils are however not aligned with each other, and myofibrils
do not run exactly longitudinally or parallel to each other through the smooth
muscle cells. Striations, which reflect the alignment of myofibrils in other
muscle types, are therefore not visible in smooth muscle.
Smooth endoplasmatic reticulum
is found close to the cytoplasmatic surface of the plasma membrane. Most of the
other organelles tend to accumulate in the cytoplasmic regions around the poles
of the nucleus. The plasma membrane, cytoplasm and endoplasmatic reticulum of
muscle cells are often referred to as sarcolemma,
sarcoplasm, and sarcoplasmatic
reticulum.
During contraction, the tensile
force generated by individual smooth muscle cells is conveyed to the
surrounding connective tissue by the sheath of reticular fibres. These fibres
are part of a basal lamina which surrounds muscle cells of all muscle types. Smooth muscle cells can remain in a state of contraction for
long periods. Contraction is usually slow and may take minutes to develop.
![thumbnail]( "Structure of smooth muscle")
Title:
Structure of smooth muscle
by:
om
at
2013-02-15T03:06:00+07:00
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Structure of smooth muscle