Muscle and Nonmuscle
Contractile Systems
All cells produce movement internally, and many are capa-
ble of motility or of changing shape. In some, movement is
related to the function of individual cells, as in the migra-
tory and engulfing movements of phagocytic cells or the
swimming movements of sperm cells. In other cases, cells
generate movement as one aspect of tissue function, as in
the ciliary transport of mucus by the bronchial epithelium.
Cells specialized for changing the dimensions or shape of
anatomical structures or for movement of body parts with
respect to each other are called muscle cells.
The ability of a cell to hold or to change shape and
to move organelles within it depends on the existence
of a cytoskeleton, comprising
actin filaments
intermediate filaments
capable of transmitting force. Actin filaments and micro-
tubules contain predominantly actin and tubulin, respec-
tively. (Table 21-1). The diameter of intermediate fila-
ments (10 nm) is between that of actin filaments (6-7 nm)
and microtubules (25 nm). They are structural proteins not
directly involved in motion.
Five classes of intermediate filament proteins have been
described, generally referred to as types I through V. Types
I and II are the acidic and basic polypeptides, respectively,
which comprise the keratins and cytokeratins, a family of
heteropolymers that are abundant in epithelia. Type III
consist of vimentin, desmin, glial fibrillary acidic protein,
and peripherin. Of these, desmin has specific importance
in muscle cells. Desmin links adjacent Z-disks to one an-
other and is involved in linking Z-disks to cell membrane
integrins at the costameres. It may also help bind filaments
into myofibrils. Type IV includes the neurofilament pro-
teins NF-L, NF-M, and NF-H plus another protein from a
similarly organized gene called nestin.
In neurons, microtubules are responsible for axonal
transport and longitudinal axon growth, while neurofila-
ments are related to radial growth, so that axonal diameter
(and thus conduction velocity) are roughly proportional
to neurofilament content. Type V IFs are the lamins as-
sociated with the nuclear membrane. Actin, tubulin, and
several of the intermediate filament proteins are found in
all cells.
Controlled transformation of the chemical energy of
nucleoside triphosphates into mechanical energy is called
chemomechanical transduction.
In addition to the actin
filaments and microtubules, the “motor” proteins
are needed for chemomechanical
transduction. Several other proteins are associated with
these, including regulatory proteins that control contractile
activity and enzymes involved in maintaining the supply
of high-energy phosphate.
This chapter discusses muscle, an actin filament sys-
tem, and cilia, a microtubule system. These do not typify
all contractile systems. In some actin filament systems,
movement occurs in the absence of myosin, being driven
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