Bone Structure and Growth (Quick Review)

A. Bone is a living tissue that renews itself.

B. Structure: Compact and Spongy
i. Compact bone contains bone cells (osteocytes) in lacunae, which are tiny chambers arranged in concentric circles around Haversian canals. The canals contain blood vessels and nerves.
ii. A matrix that has protein fibers of collagen and mineral deposits of calcium and phosphorus salts separates lacunae.
iii. Spongy bone has bony bars and plates separated by irregular spaces.
iv. Spongy bone is lighter than compact bone.
v. Spaces in spongy bone are filled with red bone marrow. Located in hipbone and sternum of adult.
vi. Bone marrow is where RBC's are made.
vii. The cavity of long one has yellow bone marrow where fat is stored.
viii. Bone covered by periosteum (outer membrane).
ix. Osteocytes have canaliculi.
A. Bone growth/constant renewal
i. During prenatal development skeleton is made of cartilage.
ii. Replaced by bone. Osteoblast (bone forming cell) does this.
iii. Primary ostification at middle of long bone, and secondary centers form at the ends of the bones.
iv. A cartilaginous disk between primary and secondary centers which can increase in length. Growth controlled by hormones.
v. Disks disappear at adulthood.
vi. In adult bones broken down and built up.
vii. Osteoclast-bone absorbing cell.
viii. Osteoblast-repair bone damage (eventually becomes osteocyte once trapped in matrix).

A. Skeleton functions
i. Movement-Bones provides sites for muscle attachment and permit flexible body movement.
ii. Protection- Protects soft body parts.
iii. Support- Heavy bones of legs support the body against gravity.
iv. Mineral storage- Storage area for calcium and phosphorus salts.
v. Blood cell formation- Flat bones and ribs have marrow that produces RBC.

A. Axial Skeleton- one of two divisions. Provides body support and protection
i. Skull (cranial bones and facial bones), Rib cage (sternum and 12 pairs of ribs), vertebral column (26), and disks.
ii. Cranium protects the brain and has 8 bones in adult. In newborns the bones are joined by fontanels that close by 16 months.
iii. Bones of cranium are named after the lobes of the brain: frontal, parietal, temporal and occipital.
iv. The skull
a. Frontal bones are the forehead, eye sockets, sinuses (air spaces that drain into respiratory tract).
b. Temporal bones surround the ear canal, bones for hearing, sphenoid bone (eye socket) and ethmoid bone (eye socket and nose)
i. Parietal bones sweep up and meet at back of the head.
i. Occipital bones are at the back and base of the skull
A hole called the foramen magnum is where spinal cord passes to the brain stem. The jugular vein carries blood from the brain. It leaves from an opening between the occipital and temporal bones.
i. Facial bones:
a. Lower jaw is the mandible
b. Upper jaw is the maxillary
c. Cheekbone is called xygomatic bones (eyes)
d. Inner eye socket is lacrimal
e. Nasal cavity is palantine
f. Septum is the vomer.

A. Appendicular Skeleton. Provides movement.
i. Includes the pelvic girdle, femur, patella, tibia, fibula,7 tarsals, metatarsals (which form arches of the foot), phalanges, clavicle, scapula, humerus, raduis (thumb side, to elbow), ulna(, carpals, metacarpals and phlanges.

B. Joints
i. Synovial joint- freely movable, adjoining bone separated by cavity, capsule of dense connective tissue that secrete synovial fluid. Most joints are synovial joints. Two bones separated by cavity, ligaments bind the bones to form a capsule. Capsule is lined by a synovial membrane , which makes synovial fluid.
ii. Knee is an example of a synovial joint. Bones are capped by cartilage, pieces of cartiage between bones are called menisci. Athletes suffer injury of menisci. 13 sacs called bursae, ease friction between tendons and ligaments and tendons and bones. Inflammation of the bursae is called bursitis, tennis elbow. Synovial joints subject to arthritis. Rheumatoid arthritis is when the synovial membrane is inflamed and thickens. Joint becomes immovable and painful. Due to an autoimmune reaction. When you age the cartilage at the ends of bones disintegrates causing osteoarthritis.
iii. Ball and socket –hips, slightly movable, more flexible during late pregnancy. Femur, movement in all planes.
iv. Hinge like-knee and elbow. One direction only.
v. Cartilaginous-vertebrae, breastbone and ribs
vi. Fibrous-fibrous connective tissue, flat skull bones of fetus, fontanels
vii. Ligaments join bones at joints
viii. Tendons join muscles to bones.

1. Skeletal muscles
A. Muscles are effectors. They attach to skeleton, contract and cause voluntary movement. Involuntary muscles are smooth muscle and cardiac muscle.
i. Muscles attach to bone by tendons. Tendons are a fibrous connective tissue.
i. ii. When a muscle contracts one bone remains stationary and one moves.
i. Origin of the muscle is on the stationary bone.
ii. Insertion of the muscle is on the bone that moves.
iii. Muscles shorten when they contract.
iv. Muscles work in antagonistic pairs such as the bicep and tricep.

A. Physiology
i. Myogram is the pattern made when contractions are studied.
ii. When a stimulus reaches a threshold level the individual fiber contracts in an all or none way.
iii. The number of fibers recruited determines the total strength of contraction.
iv. Muscles contract and then relax. This is called a twitch.
v. A myogram of a twitch has a latent period, a contraction period, and a relaxation period. Finally there is a refractory period where another twitch can’t happen.
vi. When a rapid series of stimuli hit the fiber, the fiber can respond without relaxing completely. This is called summation until it reaches tetanus, the final maximum sustained contraction.

A. Exercise
i. Can increase endurance, and strength of muscles.
ii. Increased stores of ATP and increased tolerance to lactate.
iii. Muscles enlarge, and increase the protein content in the muscle.
iv. The number of myofibrils composed of actin and myosin increase.
v. Cardiac muscle enlarges, heart can work harder, and resting hear rate decreases.
vi. Lung capacity increases.
vii. Body fat decreased.
viii. Bone density increases.
ix. Blood cholesterol and fat levels decrease as does blood pressure.

A. Structure of Muscle Fiber
i. Muscle fiber is a cell with special features.
a. The membrane is called the sarcolemma.
b. The sarcolemma forms a T system.
c. The T tubules dip into the cell and come into contact with expanded portions of the endoplasmic reticulum.
d. Expanded portions called calcuim storage sacs have calcuim.
e. Endoplasmic reticulum encases 100-1000 myofibrils (contractile units).
i. Myofibrils are cylinders, which run the length of muscle fibers.
a. Myofibrils have striations or light and dark bands.
i. b .Striations are formed by protein filaments called sarcomeres.
a. Sarcomeres have thick filaments made of myosin and thin filaments made of actin.
b. Sarcomere extends between two Z lines.
c. H band is myosin
d. I band is actin
e. Myosin and actin overlap at Z band.

A. Sliding filaments
i. Sarcomeres shorten when contractions occur.
i. ii. Actin slides past myosin.
ii. iii. I band shortens and the H zone disappears.
iii. iv. Movement of actin relative to myosin is called the sliding filament theory.
iv. v. Sarcomere shortens but filaments remain the same length.
v. vi. Myosin filaments do the work.
vi. vii. Sliding Filament theory
vii. a. When CA is present the cross bridge of myosin bends backward to attach to actin filament.
viii. b. The bridge bends forward and actin is pulled along.
ix. c. ATP is broken down by myosin releasing energy.
x. d. Detachment occurs.
xi. e. Creatine phosphate stored in muscle fibers can regenerate the ATP.

A. Oxygen debt
i. When creatine phosphate is depleted and no oxygen is available for aerobic respiration, muscle fibers use fermentation process leading to a buildup of lactate.
i. Lactate causes aches and fatigue.
ii. You need to breath to complete the metabolism of lactate.
iii. Lactate goes to the liver where 1/5th is broken down to carbon dioxide and water. The ATP generated from this is used to convert lactate to glucose.

A. Innervation
i. Muscles are innervated by nerves. When a nerve dies the muscle dies.
i. Axon branches of nerves go to muscle fibers. This unit is called the motor unit.
ii. Axons contain the neurotransmitter Acetylcholine (Ach)
iii. The neuromuscular junction is the gap between the axon and the muscle fiber.
iv. Axon has the presynaptic membrane, gap is the synaptic cleft, and the muscle fiber has the postsynaptic membrane (part of the sarcolemma).
v. Sequence of events:
a. Impulse goes down the axon and reaches the axon bulb.
b. Synaptic vesicles merge with presynaptic membrane and Ach is released into cleft.
c. Ach binds to receptor on sarcolemma and the membrane is depolarized causing muscle action potential.
d. Depolarization goes over the sarcolemma and down T system to calcium storage sac.
e. Ca ions are released. The Ca diffuses to sarcoplasm, attach to thin filaments and change their structure. Myosin binding sites are exposed. Myosin cross bridges reach out and bind, then pull actin.
f. ATP is involved in getting cross bridges to unbind. Myosin detaches and can then reattach.