Chapter 6 The Skeleto-Muscular System
Movement
 Movement is a defining characteristic of animals
 Humans move by applying tension to the bones and joints of the skeletal system
 The tension is applied by the muscular system
 Therefore, the skeletal and muscular systems work together as a unit
Functions of skeleto-muscular system
 Support the body allowing us to stay upright
 Allow for movement and locomotion
 Help maintain a constant body temperature
 Protect internal organs and stabilize joints
 The skeleton produces blood cells (hematopoiesis) and stores and releases minerals
such as calcium and phosphorus (essential for muscular contraction)
Movement
 Movement depends on the unique arrangement of muscle and bone
 All human skeletal muscles have a similar function and structure
o
They contract (shorten) to produce movement
o
They relax to their original (resting length)
Bone formation
 Bones are a form of connective tissue
 Formed by immature bone cells called osteoblasts
 Ossification (bone formation) can be endochondral (formed within a cartilage frame)
or intramembranous (formed between sheets of fibrous connective tissue)
 Most bones are endochondral; intramembranous bones are flat bones of the skull,
clavicle, mandible
Bones grow thicker and longer
 Growth occurs at the outer surface of the bone
 Cells within the periosteum differentiate into osteoblasts and begin to add matrix to
the exterior
 The accumulating matrix entraps these osteoblasts, which mature into osteocytes,
creating new bone tissue around the exterior of the bone
Bone tissue comes in two forms
 Compact (dense) bone usually occurs at the edges of bones and is composed of many
individual osteons
o
Each osteon has a central canal that houses blood vessels and nerves
 Spongy bone is less organized and lacks osteons
o
Has trabeculae (struts) that form in response to stress
Bones grow thicker and longer
 The ends of the bones (epiphyses) include the epiphyseal plate
o
Area of cartilage where long bones continue to grow during childhood and
adolescence
o
Eventually the bones cease growing, the cartilage is replaced by bone,
leaving an epiphyseal line
 Wherever two bones meet, there will be a layer of hyaline cartilage preventing the
bones from rubbing against each other
Bone marrow
 The central canal of the long bone houses the marrow
 Blood cells form in the red marrow found in the spongy bone at the bone ends
 Energy is stored in the yellow marrow found in the medullary cavity in the shaft
Bone remodeling
 Bones are highly dynamic tissues, constantly being remodeled to better suit the needs
of the body
 At maturity, there is no further lengthening of bones but there is still continued
remodeling
 Bone renewal may occur at rates as high as 18%
 Bone remodeling occurs for two basic reasons
o
In response to physical stress to strengthen or weaken the bones
o
They are the body’s main store of calcium used to maintain homeostatic
levels of calcium in the blood
 Calcium is required for nerve transmission and muscle contraction
 If blood calcium levels are low, osteoclasts dissolve the matrix,
releasing calcium
 If blood calcium levels are high, osteoblasts produce new matrix,
removing calcium from the blood
Bone repair
 Steps involved in bone repair
o
Hematoma - blood clot forms (6-8 hours)
o
Fibrocartilaginous callus (~3 weeks)
o
Bony callus - cartilaginous callus to spongy bone (3-4 months)
The human skeleton
 206 bones
The axial skeleton
 Along the central axis of the body
o
Skull - cranial and facial bones
o
Hyoid bone
o
Vertebral column - vertebrae and intervertebral disks
o
Rib cage - ribs and sternum
 Cranial bones protect the brain
o
Consists of 8 tightly-fitting bones held together by fixed joints called
sutures
 Facial bones protect the entrances of the respiratory and digestive systems and some
of the sensory organs
o
Mandible, maxillae, zygomatic bones, nasal bones
 Hyoid - only bone that does not articulate with another
Vertebral column
o Types of vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 1 sacrum (5 fused), coccyx (3-5
fused into tailbone)
o Intervertebral disks
o
Fibrocartilage pads preventing the vertebrae from rubbing against one
another
o
Allow limited motion between vertebrae
Ribs and the sternum
o 7 pairs of true ribs and 5 pairs of false ribs
o
True ribs either attach directly to the sternum or attach to costal cartilage
that is directly attached to the sternum
o
False ribs either attach to costal cartilage that is “indirectly” attached to
the sternum or has unattached ends
The appendicular skeleton
o Includes all of the bones that are attached to the axial skeleton
o
Pectoral girdle and upper limbs
o
Pelvic girdle and lower limbs
o Pectoral girdle
o Scapula and clavicle
o Upper limb
o Arm and hand bones
o
o Pelvic girdle
o Coxal (hip) bones
o Lower limb
o Leg and foot bones
Joints
o Places where bones meet
o Joints are classified based on function or structure
o Function is determined by degree of movement
o Synovial joints provide the greatest movement
o Structure is determined by the composition of the joint
Muscle tissue
Attachments:
o Tendon – connective tissue that connects muscle to bone
o Origin – attachment of a muscle on a stationary bone
o Insertion – attachment of a muscle on a bone that moves
o Muscles are complex structures made up of many muscle cells and accompanying
connective tissue
o Each muscle cell is long, slender, and fragile
o The connective tissues prevent the individual cells from ripping apart under tension
We have over 700 muscles in our body
o They are named based on a number of criteria - shape, size, attachment sites, number
of attachment sites, location, direction of fibers, and action
Muscle cells
o Terminology for cell structure
o
The plasma membrane is called the sarcolemma
o
The cytoplasm is called the sarcoplasm
o
The SER of a muscle cell is called the sarcoplasmic reticulum and stores
calcium
Muscle fibers
o Terminology for structure within a whole muscle
o
Muscle cells (fibers) are arranged in bundles called fascicles
o
Myofibrils are within the cells and are bundles of myofilaments that run
the length of a fiber
o
Myofilaments are proteins (actin and myosin) that are arranged in
repeating units
o
Sarcomeres are the repeating units of actin and myosin found along a
myofibril
The beginning of muscle contraction: The sliding filament model
o Nerve impulses travel down motor neurons to a neuromuscular junction
o Acetylcholine (ACh) is released from the neurons and binds to the muscle fibers
o This binding stimulates the fibers causing calcium to be released from the
sarcoplasmic reticula
The sarcomere
o Made of two protein myofilaments
o Myosin are the thick filaments shaped like a golf club
o Actin are the thin filaments
o These filaments slide over one another during muscle contraction
o The released calcium combines with troponin, a molecule associated with actin
o This causes the tropomyosin threads around actin to shift and expose myosin binding
sites
o Myosin heads bind to these sites forming cross-bridges
o ATP bind to the myosin heads and are used as energy to pull the actin filaments
towards the center of the sarcomere = contraction now occurs
So the contractions occurring in all of those muscle cells cause the whole muscle to contract
Lots of motor neurons, lots of impulses, lots of cells contracting
o Rigor mortis?
o
ATP is needed to attach and detach the myosin heads from actin
o
After death, muscle cells continue to produce ATP through fermentation
and muscle cells can continue to contract
o
When ATP runs out some myosin heads are still attached and cannot
detach = rigor mortis
o
Body temperature and rigor mortis help to estimate the time of death
Muscle fibers come in two forms
Fast-twitch fibers
o Rely on creatine phosphate and fermentation (anaerobic)
o Designed for strength
o Light in color
o Few mitochondria
o Little or no myoglobin
o Fewer blood vessels than slow-twitch
Slow-twitch fibers
o Rely on aerobic respiration
o Designed for endurance
o Dark in color
o Many mitochondria
o Myoglobin present
o Many blood vessels