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CHAPTER 19: MUSCULOSKELETAL SYSTEM
LECTURE OUTLINE
19.1 Anatomy and Physiology of Bone
Together, the bones and muscles comprise the musculoskeletal system.
Organization of Tissues in the Skeleton
Bones are classified by their shape. Bone tissues include the periosteum, medullary
cavity, and epiphyses. Both red and yellow bone marrow can be found in long bones.
Structure of Bone and Associated Tissues
The primary connective tissues of the skeleton are bone, cartilage, and dense fibrous
connective tissue. All connective tissues contain cells separated by a matrix that contains
fibers.
Bone
Bone tissue is strong because the matrix contains mineral salts, notably calcium
phosphate. Compact bone is highly organized, while spongy bone has an
unorganized appearance. The spaces of spongy bone are often filled with red
bone marrow.
Cartilage
Cartilage is not as strong as bone, but it is more flexible because the matrix is
gel-like and contains many collagenous and elastic fibers.
Dense Fibrous Connective Tissue
Ligaments bind bone to bone while tendons connect muscle to bone at joints.
Bone Growth and Remodeling
Bones are composed of living tissues.
Bone Development and Growth
The bones of the human skeleton, except those of the skull, first appear during
embryonic development as hyaline cartilage. The cartilaginous structures are
then gradually replaced by bone.
Remodeling of Bones
In the adult, bone is continually being broken down and built up again, a process
called remodeling.
19.2 Bones of the Skeleton
The functions of the skeleton include: supporting the body, protecting soft body parts, producing
blood cells, storing minerals and fat, and permitting flexible body movement.
Classification of the Bones
The approximately 206 bones of the skeleton are classified as either axial (along the
midline) or appendicular (the limbs along with their girdles).
The Axial Skeleton
The axial skeleton consists of the skull, hyoid bone, vertebral column, rib cage, and
ossicles.
The Skull
The skull is formed by the cranium and the facial bones.
The Cranium
The cranium protects the brain. It is composed of eight bones fitted
tightly together.
The Facial Bones
The most prominent facial bones are the mandible, the maxillae, the
zygomatic bones, and the nasal bones.
The Hyoid Bone
The hyoid bone is the only bone in the body that does not articulate with another
bone.
The Vertebral Column
The vertebral column consists of 33 vertebrae.
Types of Vertebrae
The various vertebrae are named according to their location in the
vertebral column.
Intervertebral Disks
Between the vertebrae are intervertebral disks composed of fibrocartilage
that act as a kind of padding.
The Rib Cage
The rib cage is composed of the thoracic vertebrae, the ribs and their associated
cartilages, and the sternum.
The Ribs
There are twelve pairs of ribs, and all twelve connect directly to the thoracic
vertebrae in the back. The upper seven pairs of ribs connect directly to the
sternum by means of costal cartilages.
The Sternum
The sternum, or breastbone, helps protect the heart and lungs.
The Appendicular Skeleton
The appendicular skeleton consists of the bones within the pectoral and pelvic girdles and
their attached limbs.
The Pectoral Girdle and Upper Limb
A pectoral girdle consists of a scapula and a clavicle. The upper limb consists of
the humerus in the arm and the radius and ulna in the forearm. The wrist has
eight carpal bones, while the palm is composed of five metacarpal bones. The
bones of the digits are called phalanges.
The Pelvic Girdle and Lower Limb
The pelvic girdle consists of two heavy, large coxal bones. The pelvis is a basin
composed of the pelvic girdle, sacrum, and coccyx. The femur or thighbone is
the longest and strongest bone in the body. The kneecap is called the patella and
the lower leg is formed from the fibula and the tibia. The ankle and foot contain
tarsal and metatarsal bones, respectively. The toes are made of phalanges.
Articulations
Bones are jointed at the joints, which are classified as fibrous, cartilaginous, or
synovial based on their structure and their ability to move.
19.3 Skeletal Muscles
Humans have three types of muscle tissue: smooth, cardiac, and skeletal.
Skeletal Muscles Work in Pairs
Skeletal muscles contract and move bones at a joint. The origin of a muscle is on the
stationary bone, and the insertion of a muscle is on the bone that moves. Most muscles
have antagonists that bring about movement in the opposite direction.
Major Skeletal Muscles
There are approximately 650 skeletal muscles in the human body.
Nomenclature
Skeleton muscles are named based on size, shape, location, direction of muscle fibers,
number of attachments, and/or action.
19.4 Mechanism of Muscle Fiber Contraction
Skeletal muscle tissue has alternating light and dark bands due to the arrangement of
myofilaments in the muscle fiber.
Muscle Fiber
The components of muscle fibers often have names beginning with the prefix “sarco-.”
Myofibrils and Sarcomeres
Myofibrils are cylindrical in shape and run the length of the muscle fiber. The
striations of skeletal muscle fibers are formed by the placement of myofilaments
within units of myofibrils called sarcomeres. A sarcomere contains two types of
protein myofilaments: myosin and actin.
Myofilaments
Thick Filaments
A thick filament is composed of myosin, which is shaped like a golf
club.
Thin Filaments
A thin filament consists of two intertwining strands of actin.
Sliding Filaments
When a sarcomere shortens, the actin filaments slide past the myosin
filaments and approach one another.
Skeletal Muscle Contraction
Muscle fibers are stimulated to contract by motor neurons whose axons are in nerves. A
branch of an axon ends in an axon terminal that lies in close proximity to the sarcolemma
of a muscle fiber.
The Molecular Mechanism of Contraction
Two other proteins, tropomyosin and troponin, are associated with actin filaments. When
calcium ions are released from the sarcoplasmic reticulum, they combine with troponin,
and this causes the tropomyosin threads to shift their position, revealing ATP binding
sites.
Energy for Muscle Contraction
ATP produced previous to strenuous exercise lasts a few seconds, and then muscles
acquire new ATP in three different ways.
Creatine Phosphate Breakdown
Creatine phosphate is a high-energy compound built up when a muscle is resting.
Cellular Respiration
Cellular respiration usually provides most of a muscle’s ATP.
Fermentation
Fermentation provides ATP without consuming oxygen. However, the
accumulation of lactate makes the cytoplasm of muscle fibers more acidic.
Oxygen Debt
When a muscle uses creatine phosphate or fermentation to supply its energy
needs, it incurs an oxygen debt. Oxygen debt is obvious when a person continues
to breathe heavily after exercising.
19.5 Whole Muscle Contraction
In the Laboratory
A muscle fiber contracts completely or not at all. A muscle twitch is customarily divided
into three stages: the latent period, the contraction period, and the relaxation period.
Unlike the contraction of a muscle fiber, a whole muscle has degrees of contraction, and
a twitch can vary in strength depending on the degree of stimulation. Maximal sustained
contraction is called tetanus.
In the Body
A nerve fiber along with all the muscle fibers it innervates is called a motor unit. A
motor unit obeys the all-or-none law. Even when muscles appear to be at rest, they
exhibit muscle tone, in which some of their fibers are always contracting.
Athletics and Muscle Contraction
Athletes and the general public are interested in staying fit by exercising.
Exercise and Size of Muscles
Muscles that are not used or that are used for only very weak
contractions decrease in size, or atrophy.
Slow-Twitch and Fast-Twitch Muscle Fibers
Slow-twitch fibers have a steadier tug and more endurance, and tend to
be aerobic. Fast-twitch fibers tend to be anaerobic and seem designed
for strength.
19.6 Disorders of the Musculoskeletal System
Disorders of the Skeleton and Joints
Bones are susceptible to being broken, or fractured. Osteoporosis is a condition in which
bone loses mass and mineral content. There are many different types of arthritis, or
inflammation of the joints. Rheumatoid arthritis is considered an autoimmune disease.
Disorders of the Muscles
Muscle cramps and twitches don’t usually rise to the level of a disorder. Fibromyalgia is
a disorder that causes chronic pain in the muscles and ligaments. Muscular dystrophies
are a group of genetic diseases that affect the muscles.