section
2
i.i
Muscle Systems
455
TABLE 21-2
Comparison o f Four Types o f Mammalian Cells Specialized for Contraction
Cell Type
Structure
Contractile Properties
Skeletal
muscle
Cardiac
muscle
Smooth
muscle
Myoepithelia
Long syncytial, multinucleated
cells; orderly arrangement
of myosin and actin filaments
gives striated appearance,
each fiber is directly innervated
by a motor neuron.
Similar to skeletal muscle
but extrinsic innervation is
only at the specialized
nodal pacemakers; the
action potential is conducted
from cell to cell via gap
junctions (nexuses).
Elongated, tapering cells;
mononuclear; no striations;
occurs singly, in small
clusters, or in sheets enclosing
organs; innervated by local
plexuses and extrinsically
by autonomic nerves.
Basket-shaped, mononuclear
cells surrounding the acini
of exocrine glands; have
cytoplasmic fibrils resembling
smooth muscle; derived
from ectoderm rather
than mesoderm.
Rapid, powerful contractions;
can shorten to 60 -80% of
resting length; contraction
is initiated by the central
nervous system under
voluntary control.
Similar to skeletal muscle
but contraction is initiated
by automatic firing of
pacemaker cells;
contraction is slower
and more prolonged
than in skeletal muscle.
Slow contractions under
involuntary control; can
shorten to 25% of resting
length.
Contraction stimulated by
hormones (e.g., oxytocin)
and presumably by
autonomic nerves; may
have noncontractile functions
such as pressure transduction
in the renal cortex.
Function
Movement of the bony parts
across joints.
Movement of blood by
repetitive rhythmic
contraction; beats about
3 billion times during a
normal lifetime.
Control of shape and size of
hollow organs such as the
digestive, respiratory, genital,
and urinary tracts and the
vascular system.
Contraction to expel contents
of exocrine glands (salivary,
sweat, mammary); form the
dilator muscle of the iris; may
be the pressure transducers
in juxtaglomerular cells.
myoblasts.
Some stem cell precursors of myoblasts re-
main in an adult animal; they are located between the
sarcolemma and basement membrane of mature muscle
cells, and are called satellite cells. Since each myoblast
contributes its nucleus to the muscle cell, skeletal muscle
fibers are all multinucleated, the longest having
2 0 0
or
more nuclei.
In the past decade, several growth and differentiation
factors have been identified which play a role in caus-
ing embryonic stem cells to become committed to the
muscle cell lineage, and influence the rate and extent of
their proliferation and differentiation. These include four
“myogenic regulatory factors” (or MRFs) of the helix-
loop-helix (HLH) family of DNA-binding transcription
factors, called MyoD, Myf5, Mrf4 and myogenin. There
are also inhibitory factors such as myostatin, the absence
of which causes substantially greater than normal mus-
cle mass in animals with the corresponding gene deletion.
The MRFs interact with the promoter regions of many
muscle-specific genes, and with other transcription factors
(especially MEF2). A summary of major myogenic regu-
lators and their actions is given in Table 21-3.
Figure 21-1 schematically illustrates the structure of
skeletal muscle. Individual muscle cells, or fibers, are
elongated, roughly cylindrical, and usually unbranched,
with a mean diameter of 10-100 /xm. The plasma mem-
brane of muscle fibers is called the
sarcolemma,
and fibers
are surrounded by structural filaments of the extracellular
matrix which are often described as forming a basement
membrane.
Within each fiber is a longitudinal network of tubules
called the sarcoplasmic reticulum (SR), analogous to
the endoplasmic reticulum of other cells (Figure 21-2).
Release of Ca2+ from the SR is a the key step in coupling
