Page 79 - Understanding NCERT Science 09

Muscles contain special proteins called [Fig. 6.11(a)]. These muscles are also called
contractile proteins, which contract and relax skeletal muscles as they are mostly attached
to cause movement. to bones and help in body movement. Under
the microscope, these muscles show alternate
Nuclei
light and dark bands or striations when
Striations stained appropriately. As a result, they are
also called striated muscles. The cells of this
tissue are long, cylindrical, unbranched and
multinucleate (having many nuclei).
(a) The movement of food in the alimentary
canal or the contraction and relaxation of blood
vessels are involuntary movements. We cannot
Spindle shaped
muscle cell really start them or stop them simply by
wanting to do so! Smooth muscles [Fig.
6.11(b)] or involuntary muscles control such
movements. They are also found in the iris of
the eye, in ureters and in the bronchi of the
lungs. The cells are long with pointed ends
(spindle-shaped) and uninucleate (having a
single nucleus). They are also called unstriated
muscles – why would they be called that?
Nucleus
The muscles of the heart show rhythmic
(b)
contraction and relaxation throughout life.
These involuntary muscles are called cardiac
muscles [Fig. 6.11(c)]. Heart muscle cells are
cylindrical, branched and uninucleate.
Striations
Activity 6.5
Compare the structures of different
types of muscular tissues. Note down
their shape, number of nuclei and
Nuclei position of nuclei within the cell in
the Table 6.1.

Table 6.1:

Features Striated Smooth Cardiac

Shape
Number of nuclei
(c)
Position of nuclei
Fig. 6.11: Types of muscles fibres: (a) striated muscle,
(b) smooth muscle, (c) cardiac muscle
6.3.4 NERVOUS TISSUE
We can move some muscles by conscious
will. Muscles present in our limbs move when All cells possess the ability to respond to
we want them to, and stop when we so decide. stimuli. However, cells of the nervous tissue
Such muscles are called voluntary muscles are highly specialised for being stimulated and

68 SCIENCE

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