ments necessary for all body mobility from sitting
to running.
Muscle cells form collective structures, muscle
fibers, that are the functional units of movement.
NERVEimpulses from motor neurons (nerve cells
that direct movement) travel from the NERVOUS
SYSTEMto the muscle fibers. The NEUROTRANSMITTER
acetylcholine facilitates the transfer of the impulse
from the NEURONto the muscle fiber. Some muscle
fibers remain in a state of partial contraction, pro-
viding muscle tone that supports posture. Other
muscle fibers contract and relax in rapid sequence,
providing muscle strength.
Connecting structures: tendons, ligaments, and
fasciaeSpecialized structures of connective tissue
join the bones and the muscles. Tendons, fibrous
bands that arise from muscle, join muscle to bone.
Ligaments are tough and sinewy; they connect
bones to each other. Sheetlike FASCIAcovers the
muscles, connecting muscle to muscle and muscle
to SKIN.
Articulating interfaces: the joints The ends of
the bones come together in various ways that
facilitate their movement. Hinge joints, such as
the knee and the elbow, allow flexion and exten-
sion. Ball and socket joints, such as the hips and
shoulders, allow rotational movement. The joints
of the cranium—called sutures—are fused, allow-
ing no movement at all. The vertebrae—the bones
of the spine—have slight movement between each
but collectively allow the body to bend in half.
Most joints contain cartilage, the body’s most
dense type of connective tissue, to cushion and
protect the bones. Cartilage is very smooth, almost
slick, permitting movement with minimal resist-
ance. Synovial fluid lubricates larger joints, fur-
ther reducing friction. A thin coat of cartilage
covers the caps of the long bones in the arms and
the legs. Thick pads of cartilage cushion the knees
and the vertebrae, joints that bear considerable
force with movement such as walking.
Biomechanics of movement Movement is a
function of leverage and resistance that represents
a complex and intricate interaction among the
nerves, muscles, connective tissues, and bones.
The cerebral cortex coordinates the numerous
processes that make movement possible, integrat-
ing external sensory data with internal messages.
A specialized sensory process, PROPRIOCEPTION,
establishes unconscious awareness of the body’s
location within its physical environment. Proprio-
ception helps the brain interpret and respond to
the myriad messages about the body’s relation to
gravity and speed.
Harder than bone: the teeth The TEETHare the
hardest structures in the body, formed of calcium
and other minerals with a nearly impermeable
enamel coating. The jaw bones—the maxilla
(upper jaw) and the mandible (lower jaw)—
anchor the teeth. Like most mammals, humans
have two sets of teeth, the deciduous (sometimes
called primary, milk, or baby teeth) and the per-
manent. Deciduous teeth begin to erupt through
the gum line at about age four months; they drop
out and permanent teeth replace them starting at
about age six or seven years. The adult mouth
contains 32 permanent teeth, generally occurring
in pairs on each side of the mouth. They are of
three major structures:- Incisors and cuspids have sharp surfaces for cut-
ting; these teeth are in the front of the mouth. - Molars have flat surfaces for grinding and crush-
ing; these teeth are in the back of the mouth. - Bicuspids, sometimes called premolars, function
somewhat as transitional structures, capable of
secondary biting and preliminary chewing; they
are in the middle of the jaw line.
Within the calcium cap is the tooth’s living tis-
sue, the pulp. Hollow extensions penetrate deep
into the bones of the jaw, their protective canals
encasing the nerves and blood vessels that supply
the pulp. Chips and cracks in the enamel occur
over time, weakening its protection and allowing
BACTERIAto penetrate and begin to destroy the cal-
cium cap, exposing the inner pulp. This kind of
damage—dental caries (cavities)—is the leading
oral health challenge. The teeth also facilitate
speech, functioning like reflective walls to amplify
sound and providing resistance for the tongue as it
reshapes sound into words.Health and Disorders
of the Musculoskeletal System
The musculoskeletal system carries the human
body hundreds of thousands of miles in the course
of a typical lifetime. Trauma notwithstanding, it294 The Musculoskeletal System