Electron
micrographs of thin sections of muscle fibres reveal groups
of filaments oriented with their axes parallel to the length
of the fibre. There are two sizes of filaments, thick and
thin. Each array of filaments, called a myofi
, is shaped
like a cylindrical column. Along the length of each myofi
alternate sets of thick and thin filaments overlap, or interdigitate,
presenting alternate bands of dark regions (with thick filaments
and overlapping thin ones) and light regions (with only
thin filaments). Within a fibre all the myofi
s are in
register, so that the regions of similar density lie next
to each other, giving the fibre the characteristic striated
appearance it shows in the phase-contrast or polarized light
microscope. Each light region is divided in two by a dark
band. The unit between two dark bands is known as a sarcomere.
Each
myofi
is about one or two micrometres (or microns; one
micrometre = 10-6 metre) in diameter and extends the entire
length of the muscle fibre. The number of myofi
s per
fibre varies. At the end of the fibre the myofi
s are
attached to the plasma membrane by the intervention of specialized
proteins.
Forty to 80 nanometres usually separate adjacent myofi
s
in a fibre. This space contains two distinct systems of
membranes involved in the activationof muscle contraction
(Figure 7). One system is a series of channels that open
through the sarcolemma to the extra-fibre space. These channels
are called the transverse tubules because they run across
the fibre. The transverse tubular system is a network of
interconnecting rings, each of which surrounds a myofi
.
It provides an important communication pathway between the
outside of the fibre and the myofi
s, some of which are
located deep inside the fibre. The exact spatial relationship
of the tubulesto the filaments in the myofi
depends
on the species of animal.
The
other membrane system that surrounds each myofi
is the
sarcoplasmic reticulum, a series of closed saclike membranes.
Each segment of the sarcoplasmic reticulum forms a cufflike
structure surrounding a myofi
. The portion in contact
with the T tubule forms an enlarged sac (terminal cisterna).
In
most vertebrates each transverse tubule has two cisternae
closely associated with it, forming a three-element complex
called a triad. The number of triads per sarcomere depends
onthe species: in frog muscle there is one per triad; in
mammalian muscle there are two. In fishes and crustaceans
only one cisterna is associated with each transverse tubule,
thus forming a dyad. The sarcoplasmic reticulum controls
the level of calcium ions in the sarcoplasm. The terminal
cisternae apparently are the sites from which the calcium
ions are released when the muscle is stimulated, and the
longitudinal tubules are the sites at which calcium ions
are effectively removed from the sarcoplasm. The removal
of calcium ions (Ca2+) from the sarcoplasm is accomplished
by a protein that catalyzes the breakdown of ATP making
the free energy of hydrolysis available for the energy-requiring
process of Ca2+ transport.
