Muscles constitute roughly 50% of the total body mass. Their job is to contract causing either movement (shortening - isotonic contraction) or force generation without shortening (maintenance of tone or posture - isometric contraction). Three main types of muscle exist:
Skeletal - fibres 10 - 80
mm in diameter; up to several cm in length. Multinucleate (syncitial)with the nuclei normally lying beneath the sarcolemma. Characteristic pattern of
cross-striations (light or I bands alternating with dark or A bands). Running down
the middle of each light band is a dark line, the Z-line. The segment of the fibre in
between successive Z-lines is termed a sarcomere. Activated by somatic nerves.
Cardiac - fibres 10 - 20 mm in diameter; about 50 - 100 mm in length (1/4 the diameter of
skeletal fibres). They are also cross-striated. The fibres are uninucleate, branched
structures. Individual fibres are joined together by special junctions -
the intercalated discs. These have characteristics tight junctions holding the
cells firmly together to form a strong, meshwork basket, and gap junctions,
allowing electrical continuity between all the joined cells. The cells function as
a syncitium although they are anatomically uninuclear. Spontaneous periodic discharges
are modulated by hormones and the autonomic nervous system.
Smooth - fibres 2 - 5 mm in diameter; about 20 - 500 mm in length (1/4 the diameter of cardiac
fibres). Arranged in sheets around hollow organs of the body for the most part. May
show sustained activity using little ATP. May display spontaneous basal rhythmicity.
Modulated by endocrine and paracrine secretions and by the ANS.