Chapter 19: Musculoskeletal
System
19-1
Anatomy and Physiology of
Bones
The bones provide attachment sites for
muscles, enabling complex movement.
Bones also support and protect internal
organs.
The organs of the skeletal system are
largely composed of connective tissues,
including bone and cartilage.
Connective tissue contains cells
separated by matrix that contains fibers.
19-2
Structure of Bone
The matrix of bone contains mineral salts.
Bone cells are osteocytes and they lie in
tiny chambers called lacunae.
Compact bone is highly organized into
tubular osteons, each with a central canal.
Spongy bone has an unorganized
appearance but is designed for strength.
Spaces in spongy bone contain red bone
marrow that produces blood cells.
19-3
Anatomy of a bone
19-4
Tissues Associated with Bones
Cartilage
Cartilage has a gel-like matrix with
collagen and elastin fibers; it lacks blood
vessels.
Hyaline cartilage is glassy and is found in
the nose, ends of ribs, and in the larynx.
Fibrocartilage is stronger with thicker
collagen fibers and is found in the disks
between vertebrae.
Elastic cartilage has mainly elastin fibers
and is in the ear flaps and epiglottis.
19-5
Dense Fibrous Connective Tissue
Dense fibrous connective tissue contains
fibroblasts are separated by bundles of
collagen fibers.
This type of tissue is found at the flared
sides of the nose, in ligaments that
bind bone to bone, and in tendons that
connect muscles to bone.
19-6
Structure of a Long Bone
Bone is covered by fibrous connective
tissue called the periosteum.
The diaphysis (shaft) of a long bone has
a medullary cavity of yellow bone
marrow containing fat.
Hyaline articular cartilage covers the
ends of bones at the joint.
Epiphyses of bones have spongy bone.
19-7
Bone Growth and Repair
Remodeling of Bones
Bone is a living tissue that is constantly
broken down and built up.
Osteoclasts are derived from monocytes
and break down bone and deposit
calcium in the blood.
Osteoblasts then rebuild the bone and
some become osteocytes in lacunae.
19-8
Bone Development and Growth
The embryonic human skeleton is at first
hyaline cartilage, but it is later replaced
by a bony skeleton in a process of
endochondral ossification.
Osteoblasts form a primary ossification
center.
A band of cartilage called a growth plate
separates it from the secondary
ossification center.
19-9
Endochondral ossification of a
long bone
19-10
Bones of the Skeleton
The skeleton:
supports the body;
protects soft body parts;
permits flexible movement;
produces blood cells; and
serves as a storehouse for mineral salts,
particularly calcium phosphate.
19-11
Classification of the Bones
The 206 bones of the human may be
classified according to their shape or
whether they are in the axial skeleton
or appendicular skeleton.
Shapes include long bones, short cubeshaped bones, flat bones, round bones,
and irregular bones such as vertebrae.
The bones are not smooth but have
knobs and processes where muscles
attach.
19-12
The skeleton
19-13
The Axial Skeleton
The axial skeleton lies in the midline of
the body and consists of the skull, the
hyoid bone, the vertebral column, and
the rib cage.
The Skull
The skull contains the cranium, which
protects the brain, and also includes
the facial bones.
19-14
Newborns have incomplete skull bones
with membranous fontanels that grow
closed by 16 moths.
Some skull bones contain sinuses.
Infections in the mastoid sinuses can
lead to mastoiditis, an inflammation
that can lead to deafness.
The major bones of the cranium include
the frontal, parietal, temporal, occipital,
ethmoid, and sphenoid bones.
19-15
Bones of the skull
19-16
19-17
At the base of the occipital bone is the
foramen magnum through which the
spinal cord attaches to the brain.
The Facial Bones
The facial bones include the mandible
(lower jaw), maxillae (upper jaw and
anterior hard palate), zygomatic bones
(cheek bones), and the nasal bones.
Ears are only elastic cartilage.
The nose is a mixture of bones, cartilage,
and fibrous connective tissue.
19-18
Bones of the face
19-19
The Hyoid Bone
The hyoid bone located above the larynx
is the only bone in the body that does
not articulate with another bone.
The hyoid bone anchors the tongue and
serves as the site of attachment for the
muscles associated with swallowing.
19-20
The Vertebral Column
The vertebral column consists of 33
vertebrae, and supports the head and
trunk, protects the spinal cord and
roots of spinal nerves, and serves as a
site for muscle attachment.
Scoliosis is a sideways curvature of the
spine.
The first and second cervical vertebrae
are the atlas and axis that allow the
head to pivot.
Intervertebral discs act as padding.
19-21
The vertebral column
19-22
The Rib Cage
The rib cage is composed of the thoracic
vertebrae, the ribs with their associated
cartilages, and the sternum.
The rib cage protects the heart and
lungs, and expands during inhalation.
The Ribs
There are 12 pairs of ribs attached to the
thoracic vertebrae.
19-23
The upper seven pairs of the ribs attach
to the sternum (true ribs); the next
three pairs connect indirectly to the
sternum by means of common cartilage
(false ribs), and the last two pairs are
called floating ribs because they have
no connection at all to the sternum.
The Sternum
The sternum consists of the manubrium,
the body, and the xiphoid process that
fuse during fetal development.
19-24
Thoracic vertebrae and the rib
cage
19-25
The Appendicular Skeleton
The appendicular skeleton consists of the
bones of the pectoral girdle, arms,
pelvic girdle, and legs.
The Pectoral Girdle and Arm
The pectoral girdle includes the clavicle
(collarbone) and scapula (shoulder
blade).
The arm is made up of the humerus
(upper arm), and ulna and radius
(forearm).
19-26
Tendons forming a socket for the
humerus are the rotator cup.
Vigorous rotations of the arm can
damage the rotator cuff.
The glenoid cavity of the scapula also
articulates with the humerus.
The bones of the hand are: eight carpal
bones, five metacarpal bones, and
phalanges of the fingers and thumb.
19-27
Bones of a pectoral girdle and
arm
19-28
The Pelvic Girdle and Leg
The pelvic girdle is made of two coxal
bones; the pelvis is composed of the
pelvic girdle, sacrum, and coccyx.
In the leg, the femur is the longest and
strongest bone; the femur articulates
with the coxal bones at the acetabulum.
The patella is the kneecap and the tibia
and fibula form the lower leg.
Bones of the foot are: tarsal bones,
calcaneus (heel), metatarsal bones, and
phalanges.
19-29
A coxal bone and the bones of a
leg
19-30
Articulations
There are three types of joints
(articulations):
Fibrous joints such as the sutures of the
cranium, are immovable.
Cartilaginous joints, like those between
the ribs and sternum or the vertebral
discs, are slightly movable.
Synovial joints consist of a membranelined synovial capsule that is freely
movable.
19-31
The knee, which is a synovial joint, also
has pads of cartilage called menisci
that add stability to uneven surfaces
within the knee, along with fluid-filled
sacs called bursae that ease friction
between the tendons and ligaments.
There are different kinds of synovial
joints based on the movements they
permit.
Most movable are the ball-and-socket
joints, such as the shoulder or hip
joints.
19-32
Knee joint
19-33
Skeletal Muscles
Humans have three types of muscle tissue:
Smooth muscles lack striations and
comprise involuntary muscle in internal
organs.
Cardiac muscle cells are striated,
cylindrical and branched; fibers are
intercalated to allow contractions to
spread quickly.
Skeletal muscle fibers are striated,
multinucleate, and voluntary.
19-34
Skeletal Muscles Work in Pairs
Skeletal muscle is covered in layers of
fibrous connective tissue called fascia.
A skeletal muscle has an origin on the
stationary bone; the end of the muscle
that moves is the insertion.
Prime movers do most of the work but are
assisted by synergists.
Whole muscles work in antagonistic
pairs; for example, the biceps flexes the
lower arm and the triceps extends it. 19-35
Attachment of skeletal muscles
19-36
Nomenclature
Skeletal muscles are named according
to:
muscle size,
muscle shape,
location,
direction of fibers,
number of attachments, and
action of the muscle.
19-37
Human musculature, anterior
19-38
Human musculature, posterior
19-39
Mechanism of Muscle Fiber
Contraction
Overview of Muscular Contraction
The sarcolemma (plasma membrane) of a
muscle fiber forms transverse tubules
(T tubules) that extend into the fiber
and almost touch the sarcoplasmic
reticulum which stores calcium ions.
The sarcoplasmic reticulum encases
hundreds up to thousands of
myofibrils, the contractile portions of
muscle fibers.
19-40
Contraction of a muscle
19-41
19-42
19-43
19-44
Myofibrils and Sarcomeres
Myofibrils that run the length of a muscle
fiber are divided into contractile units
called sarcomeres.
A sarcomere extends between two dark
lines called Z lines.
The arrangement of myosin (thick)
filaments and actin (thin) filaments in a
sarcomere accounts for striations or
banding patterns of myofibrils.
19-45
Light micrograph of skeletal
muscle
19-46
Sliding Filaments
Impulses travel through T tubules to the
sarcoplasmic reticulum, which releases
Ca2+, and the muscle fiber contracts.
When sarcomeres shorten, actin filaments
slide past myosin filaments.
The movement of actin filaments in relation
to myosin filaments is called the sliding
filament theory of muscle contraction.
During the sliding process, the sarcomere
shortens, but the filaments remain the
same length.
19-47
Muscle Innervation
The motor neuron axon bulb is separate
from the sarcolemma at a synaptic cleft
within the neuromuscular junction.
Synaptic vesicles in the axon bulb release
the neurotransmitter acetylcholine (Ach)
that binds to protein receptors on the
muscle fiber sarcolemma.
Next, impulses to travel down T tubules
and calcium leaves the sarcoplasmic
reticulum, resulting in myofibril
contraction.
19-48
Neuromuscular junction
19-49
Two other proteins are associated with
the actin filament: tropomyosin, that
winds about the actin filament, and
troponin that occurs at intervals along
the tropomyosin threads.
Calcium ions bind to troponin, allowing
tropomyosin to shift position to expose
myosin binding sites.
A myosin filament is composed of many
myosin molecules, each containing a
head with an ATP binding site.
19-50
Function of Ca2+ in muscle
contraction
19-51
Myosin heads function as ATPase
enzymes, and once they break down ATP,
the myosin heads are ready to attach to
the next set of myosin binding sites on
actin myofilaments.
The release of ADP + (P) causes the head
to change its position; this is the power
stroke that causes the actin filament to
slide toward the center of a sarcomere.
When the myosin head catalyzes another
ATP, the head detaches from actin, and
the cycle begins again.
19-52
Function of cross-bridges in
muscle contraction
19-53
Whole Muscle Contraction
Basic Laboratory Observations
In the laboratory, muscle contraction can
be studied by using an excised frog
muscle (gastrocnemius) and
stimulating it with electricity.
Muscle contraction is recorded as a
myogram and is described in terms of a
single muscle twitch or sustained
contraction called tetanus.
19-54
A muscle twitch is divided into three
stages: the latent period, or time between
stimulation and when the contraction
begins; the contraction period, during
which the muscle shortens; and the
relaxation period, when the muscle
returns to its former length.
A muscle fiber contracts in an all-or-none
fashion.
The contraction of a whole muscle varies
in strength depending on the number of
muscle fibers contracting.
19-55
Physiology of skeletal muscle
contraction
19-56
Muscle Tone in the Body
In the body, muscles exhibit tone, in
which some fibers within a muscle are
always contracting.
Maintenance of muscle tone requires
muscle spindles.
Recruitment and the Strength of
Contraction
As the intensity of nervous stimulation
increases, more and more motor units
are activated; this is recruitment.
19-57
Energy for Muscle Contraction
A muscle fiber has three ways to acquire
ATP after muscle contraction begins:
(1) creatine phosphate, built up when a
muscle is resting, donates phosphates
to ADP, forming ATP;
(2) fermentation with the concomitant
accumulation of lactic acid quickly
produces ATP; and
(3)oxygen-dependent aerobic respiration
that occurs within mitochondria.
19-58
The three pathways for acquiring ATP
work together during muscle
contraction.
Myoglobin, an oxygen carrier similar to
hemoglobin, is synthesized by muscle
cells and accounts for the reddishbrown color of skeletal muscle.
Myoglobin serves as an extra source of
oxygen during aerobic respiration in
muscles.
19-59
Oxygen Debt
When a muscle uses up its available
supplies of oxygen, oxygen debt occurs,
and the muscle cells switch to anaerobic
means of supplying energy.
Fermentation results in oxygen debt
because oxygen is needed to complete
the metabolism of lactate; lactate builds
up in muscle tissue in the absence of O2.
Repaying the oxygen debt requires
replenishing creatinine phosphate and
disposing of lactate.
19-60
Energy and Muscle Contraction
Exercise and Size of Muscles
Lack of exercise causes atrophy or
shortening of muscle fibers.
Frequent exercise can cause hypertrophy
or increase in muscle size.
Regular exercise has many health
benefits, including enhancing mood
and relieving depression.
19-61
Slow-Twitch and Fast-Twitch
The muscles of some individuals have
many slow-twitch fibers.
These fibers are aerobic and have steady
power and endurance, enhancing
performance at a sport such as crosscountry running.
Muscles of others have many fast-twitch
fibers.
These fibers are anaerobic, have explosive
power but fatigue easily, enhancing
sports like weight lifting.
19-62
Slow- and fast-twitch muscle
fibers
19-63
Chapter Summary
Bone is an active living tissue that grows
and undergoes repair.
The fetal skeleton is cartilaginous and is
soon replaced by bone.
Bones are constantly being broken down
and rebuilt by two specialized cells.
Skeletal bones are divided into those of
the axial skeleton and those of the
appendicular skeleton.
19-64
Joints are classified according to
anatomy; only one type is freely
movable.
Skeletal muscles work in antagonistic
pairs to move bones in opposite
directions.
Muscles permit movement but have other
functions as well.
19-65
A chain of events lead from nervous
stimulation to muscle fiber contraction.
At the neuromuscular junction, the
nervous stimulus is passed from nerve
fiber to muscle fiber.
In muscle fiber contraction, the protein
myosin breaks down ATP.
In the body, muscles have tone, and vary
in the strength of contraction.
Muscle fibers contract in an all-or-none
fashion.
19-66
The three sources of ATP for muscle
contraction are aerobic respiration,
creatine phosphate breakdown, and
fermentation.
Muscle fibers differ in capabilities; some
are better for one function or sport than
others.
Exercise has many health benefits aside
from increasing the strength and
endurance of muscles.
19-67