synthesize fibers and ground substance.Hyaline cartilage covers articular
surfaces and forms the cartilage model in long bone development.Elastic
cartilageis found in the pinna of the ear and in the epiglotlis, whilefibro-
cartilageis an intermediate form found in the intervertebral disc, pubic sym-
physis, and connecting tendon and bone. Hyaline cartilage contains matrix
comprised of type II collagen electrostatically bound to proteoglycans. Elas-
tic cartilage contains type II collagen and elastic fibers, and fibrocartilage, like
bone, contains type I collagen.
MUSCLE AND CELL MOTILITYSkeletal and cardiac (striated) muscle contract by sliding myosinandactin
filaments past each other in a process facilitated by ATP. Myosin contains a
motor that interacts with the actin filament and allows myosin to ratchet along
the actin. The filaments are arranged in a banded pattern in individual sar-
comeres, which act in series. Specialized invaginations of the plasma mem-
brane(T tubules)spread the surface depolarization to the interior of the cell
to release calcium from the sarcoplasmic reticulum,initiating contraction.
Troponinandtropomyosinare specialized proteins that permit contraction
of skeletal and cardiac muscle to be regulated by calcium. Skeletal muscle is a
syncytium, while cardiac muscle consists of individual cells connected by
intercalated disks. The organization of striated muscle is shown below:
22 Anatomy, Histology, and Cell Biology
Extrafusal muscle fibers generate the force of contraction while intra-
fusal fibers (both IA and II-type fibers (receptors) serve as sensory detectors
signaling muscle location (proprioception) and the rate of contraction.