AP Biology
    Skeleton/muscles

A. Types of Skeletons

        1. Three types of skeletons exist in animal kingdom.
        2. A hydrostatic skeleton occurs in cnidarians, flatworms, roundworms and annelids.
        3. An exoskeleton includes arthropods as well as mollusc with calcium carbonate shells.
        4. An endoskeleton is found in echinoderms and vertebrates.

    B. Hydrostatic Skeleton

        1. A fluid-filled gastrovascular cavity or coelom can act as a hydrostatic skeleton.
        2. It offers support and resistance to the contraction of muscles for motility.
        3. Many animals use hydroskeletons.
            a. Hydras use a fluid-filled gastrovascular cavity to support tentacles that rapidly contract.
            b. Planaria easily glide over substrate with muscular contractions of body walls and cilia.
            c. Nematodes have a fluid-filled pseudocoelom and move when muscles contract against it.
            d. Earthworms are segmented with septa dividing coelom into compartments; muscle contractions in
                each segment allow coordinated elongation and contraction.

    C. Exoskeletons and Endoskeletons

        1. An exoskeleton is an external skeleton.
            a. Corals and some mollusks have exoskeletons that are predominantly calcium carbonate (CaCO3).
            b. Insects and crustacea have jointed exoskeletons composed of chitin.
            c. Exoskeleton provides protection against damage and enemies and keeps tissues from drying out.
            d. Although stiffness provides support for muscles, exoskeleton is not as strong as an endoskeleton.
            e. Clam and snail exoskeletons grow with the animals; thick non-mobile CaCO3 shell is protection.
            f. Chitinous exoskeleton of arthropods is jointed and moveable.
            g. Arthropods molt when exoskeleton becomes too small; a molting animal is vulnerable to predators.
        2. Vertebrates have endoskeleton composed of bone and cartilage that grows with the animal.
            a. Endoskeleton does not limit space available for internal organs and can support greater weight.
            b. Soft tissues surround endoskeleton to protect it; injuries to soft tissue are easier to repair.
            c. Usually endoskeleton has elements that protect vital internal organs.
            d. Jointed exoskeleton of arthropods and endoskeletons of vertebrates allow flexibility and helped
                arthropods and vertebrates colonize land.

II. The Human Skeletal System

    A. Human Skeletal Functions

        1. Large, heavy leg bones support the body against pull of gravity.
        2. Skeletons protect organs: skull (brain), vertebral column (spinal cord), and rib cage (heart and lungs).
        3. Leg and arm bones permit flexible body movement.
        4. Flat bones of skull, ribs, and breastbone contain red bone marrow, which manufactures blood cells.
        5. All bones store inorganic calcium and phosphorous salts.

    B. Bone Growth and Renewal

        1. Prenatal human skeleton is cartilaginous; cartilage structures serve as "models" for bone construction.
            a. Cartilaginous models are converted to bones when calcium salts are deposited in matrix, first by
                cartilaginous cells and later by bone-forming cells called osteoblasts.
            b. Conversion of cartilaginous models to bones is called endochondral ossification.
            c. Some bones (e.g., facial bones) are formed without a cartilaginous model.
        2. During endochondral ossification, there is a primary ossification center at middle of a long bone; latter
            secondary centers form at ends.
        3. A cartilaginous disk remains between primary and secondary ossification centers.
        4. As long as a growth plate remains between the two centers, bone growth occurs.
        5. Rate of growth is controlled by hormones, including growth hormones and sex hormones.
        6. Eventually plates become ossified and bone stops growing; this determines adult height.
        7. In adults, bone is continually being broken down and built up again.
            a. Bone-absorbing cells called osteoclasts break down bone, remove worn cells, and deposit calcium in blood.
            b. Osteoblasts form new bone, taking calcium from blood.
            c. Osteoblasts become entrapped in the bone matrix and become osteocytes in the lacunae of osteons.
            d. This continual remodeling allows bones to gradually change in thickness.
            e. Osteoclasts also determine calcium level in blood; calcium level is important for muscle contraction
                and nerve conduction and levels are controlled by hormones PTH and calcitonin.
        8. Adults need more calcium in the diet than children do to promote the work of osteoblasts.

    C. Anatomy of a Long Bone

        1. A long bones illustrates bone anatomy.
            a. A long bone consist of a central medullary cavity surrounded by compact bone.
            b. Ends are composed of spongy bone surrounded by a thin layer of compact bone and covered with cartilage.
            c. Compact bone contains many osteons (Haversian systems or canals); bone cells in tiny chambers
                (lacunae) are arranged in concentric circles around central canals.
            d. Central canals contain blood vessels and nerves.
            e. Lacunae are separated by matrix that contains protein fibers of collagen and mineral deposits.
        2. Spongy bone has numerous plates and bars separated by irregular spaces.
            a. Spongy bone is lighter but designed for strength; solid portions follow lines of stress.
            b. Spaces are often filled with red bone marrow, a specialized tissue that produces blood cells.

    D. Bones Make Up the Skeleton

        1.Axial skeleton lies at midline of the body and consists of skull, vertebral column, sternum and ribs.
            a. The Skull
                1) The skull is formed by the cranium and facial bones.
                2) Newborns have membranous junctions called fontanels that usually close by age of two.
                3) Cranium bones contain sinuses, air spaces lined with mucous membrane that reduce weight of skull
                     and give resonant sound to the voice.
                4) Two mastoid sinuses drain into middle ear; mastoiditis inflammation can lead to deafness.
                5) Cranium is composed of eight bones: frontal, two parietal, occipital, two temporal, sphenoid, and ethmoid.
                6) Spinal cord passes through foramen magnum opening at base of skull in occipital bone.
                7) Each temporal bone has an opening that leads to the middle ear.
                8) Sphenoid bone completes the sides of the skull and forms floors and walls of eye sockets.
                9) Ethmoid bone is in front of sphenoid, part of orbital wall, and a component of nasal septum.
                10) Fourteen facial bones include: mandible, two maxillae, two palatine, two zygomatic, two lacrimal,
                      two nasal, and vomer.
                11) Mandible or lower jaw is only movable portion of the skull; contains tooth sockets.
                12) Maxilla forms the upper jaw and anterior of hard palate; also contain tooth sockets.
                13) Palatine bones make up posterior portion of hard palate and floor of nasal cavity.
                14) Zygomatic gives us our cheekbone prominences.
                15) Nasal bones form bridge of nose.
                16) Other bones make up nasal septum which divides nose cavity into two regions.
                17) Ears are elastic cartilage and lack bone; nose is a mixture of bone, cartilage, and fibrous connective tissue.
            b. The Vertebral Column and Rib Cage
                1) Vertebral column supports head and trunk and protects spinal cord and roots of spinal nerves.
                2) Vertebral column serves as an anchor for all other bones of skeleton.
                3) Seven cervical vertebrae are located in the neck.
                4) Twelve thoracic vertebrae are in the thorax or chest.
                5) Lumbar vertebrae are in the small of the back.
                6) One sacrum is formed of five fused sacral vertebrae.
                7) One coccyx is formed of four fused coccygeal vertebrae.
                8) The column has four normal curvatures that provide strength and resiliency in posture.
                9) Scoliosis is abnormal sideways curvature; hunchback and swayback are also abnormal.
                10) Intervertebral disks between vertebrae act as a padding to prevent vertebrae grinding against each
                      other, and to absorb shock during running, etc.; they weaken with age.
                11) Disks allow motion between vertebrae for bending forward, etc.
                12) All twelve pairs of ribs connect directly to thoracic vertebrae in back; seven attach directly to sternum.
                13) Three pairs connect via cartilage to sternum at front.
                14) The two ribs totally unattached to sternum are called "floating ribs."
                15) Rib cage protects heart and lungs, yet is flexible to allow breathing.
        2. The Appendicular Skeleton
            a. Appendicular skeleton consists of pectoral girdles and upper limbs and pelvic girdles and lower limbs.
            b. Pectoral girdle is specialized for flexibility; pelvic girdle is built for strength.
            c. Pectoral girdle bones are only loosely linked by ligaments.
                1) Clavicle or "collarbone" connects with sternum in front and scapula in back.
                2) Scapula connects with clavicle, but is freely movable and held in place only by muscles.
            d. Humerus is long bone of upper arm; its smoothly rounded head fits into socket of scapula.
            e. Radius is more lateral of the bones of lower arm; it articulates with humerus at elbow joint, a hinge
                joint, and radius crosses in front of ulna for easy twisting.
            f. Ulna is more medial of the two bones of lower arm; its end is the prominence in elbow.
            g. Many hand bones increase its flexibility.
                1) Wrist has eight carpal bones which look like small pebbles.
                2) Five metacarpal bones fan out to form framework of palm.
                3) Phalanges are bones of fingers and thumb.
            h. Pelvic girdle consists of two heavy, large coxal (hip) bones.
                1) Coxal bones are anchored to sacrum; together with sacrum they form a hollow cavity that is wider
                    in females than in males; transmit weight from vertebral column via sacrum to legs.
                2) Femur is largest bone of the body; it is limited in amount of weight that it can support.
                3) Tibia has a ridge called the "shin"; its end forms inside of ankle.
                4) Fibula is smaller of the two bones; its end forms outside of ankle.
                5) Seven tarsal bones are in each ankle; one receives weight and passes it to heel and ball of foot.
                6) Metatarsal bones form arch of the foot and provide a springy base.
                7) Phalanges are bones of toes, stouter than fingers.

    E. Classification of Joints

        1. Bones are joined at joints, classified as fibrous, cartilaginous, or synovial.
        2. Fibrous joints, such as those between cranial bones, are immovable.
        3. Cartilaginous joints, such as those between vertebrae, are slightly moveable; the two hipbones are slightly
            movable because they are ventrally joined by cartilage and respond to pregnancy hormones.
        4. Synovial joints are freely movable.
            a. Most joints are synovial joints, with the two bones separated by a cavity.
            b. Ligaments are fibrous connective tissue that bind bones to bone, forming a joint capsule.
            c. In a "double-jointed" individual, ligaments are unusually loose.
            d. Joint capsule is lined with a synovial membrane that produces a lubricating synovial fluid.
            e. Knee represents a synovial joint.
                1) Bones are capped by cartilage; a crescent-shaped pieces of cartilage, the menisci, is between.
                2) Athletes who injure the meniscus have torn cartilage.
                3) Knee joint also contains 13 fluid-filled sacs called bursae to ease friction between tendons.
                4) Inflammation of bursae is bursitis; this includes "tennis elbow."
                5) Knee and elbow are hinge joints; shoulder and hip are ball-and-socket joints.
            f. Synovial joints are subject to arthritis.
                1) In rheumatoid arthritis, synovial membrane becomes inflamed and thickens.
                2) The joint degenerates and becomes immovable and painful.
                3) This is likely due to an autoimmune reaction.
                4) In osteoarthritis from old age, cartilage at bone ends disintegrates; bones become rough and irregular.

III. The Human Muscular System

    A. Muscle Tissue Function

        1. Skeletal muscle contraction assists homeostasis by helping maintain constant body temperature.
        2. Skeletal muscle contraction causes ATP breakdown, releasing heat that is distributed about the body.

    B. Macroscopic Anatomy and Physiology

        1. Skeletal muscles attach to skeleton by tendons made of fibrous connective tissue.
        2. When muscles contract, they shorten; therefore, skeletal muscles must work in antagonistic pairs.
            a. One muscle of an antagonistic pair bends the joint and brings limb toward the body.
            b. The other one straightens the joint and extends limb.
        3. If a muscle is given rapid series of stimuli, it responds to next stimulus before completely relaxing.
        4. Muscle contraction summates until maximal sustained contraction, called tetanus.
        5. Even at rest, muscles maintain tone by some fibers contracting; essential to posture.

    C. Microscopic Anatomy and Physiology

        1. A whole skeletal muscle consists of muscle fibers.
        2. Each muscle fiber is a cell with special features.
            a. A plasma membrane called sarcolemma forms a T (transverse) system.
                1) Transverse (T) tubules penetrate down into cell and contact with, but do not fuse with, modified
                    endoplasmic reticulum (sarcoplasmic reticulum).
                2) Expanded portions or sacs of sarcoplasmic reticulum are modified for Ca2+ ion storage; encases
                    hundreds and sometimes thousands of myofibrils.
            b. Myofibrils are contractile portions of fibers that lie parallel and run length of fiber.
            c. A light microscope shows light and dark bands called striations.
            d. An electron microscope shows striations of myofibrils are formed by placement of protein filaments
                within sarcomeres.
            e. Protein filaments are thick (made of myosin) and thin (made of actin).
            f. Sarcomere has repeating bands of actin and myosin that occur between two Z lines in myofibril.
                1) I band contains only actin filaments.
                2) H zone contains only myosin filaments.
        3. Sliding Filament Model
            a. As a muscle fiber contracts, sarcomeres within myofibrils shorten.
            b. As sarcomere shortens, actin filaments slide past myosin; I band shortens and H zone disappears.
            c. Sliding filament theory: actin filaments slide past myosin filaments because myosin filaments have
                cross-bridges that pull actin filaments inward, toward their Z line.
            d. Contraction process involves sarcomere shortening, filaments themselves remain same length.
            e. ATP supplies energy for muscle contraction.
            f. Myosin filaments break down ATP to form cross-bridges that attach to and pull actin filament.
        4. ATP Is Needed
            a. Muscle cells contain myoglobin that stores oxygen that does not supply all ATP needed.
            b. Muscle fibers rely on supply of stored creatine phosphate (phosphocreatine).
            c. Creatine phosphate does not participate in muscle contraction directly but regenerates ATP rapidly:

            d. This reaction occurs in midst of sliding filaments and is speedy.
            e. When all creatine phosphate is depleted, and if O2 is in limited supply, fermentation produces a small
                amount of ATP, but this results in buildup of lactate that is toxic.
            f. Buildup of lactate partially accounts for muscle fatigue and represents oxygen debt.
            g. Lactate is transported to liver; 20% is completely broken down to CO2 and H2O in aerobic respiration.
            h. ATP gained from this respiration is then used to reconvert 80% of the lactate to glucose.
            i. In persons who train, the number of mitochondria increases, reducing need for fermentation.

    D. Muscle Innervation

        1. Muscles are stimulated to contract by motor nerve fibers.
        2. Neuromuscular junction is region where an axon bulb is in close association with sarcolemma of a
            muscle fiber.
        3. Axon bulb contains synaptic vesicles filled with a neurotransmitter.
        4. When nerve impulses travel down a motor neuron to axon bulb, vesicles merge with presynaptic membrane
            and acetylcholine molecules are released into synaptic cleft.
        5. Acetylcholine rapidly diffuses to and binds with receptors on sarcolemma.
        6. Sarcolemma generates impulse spreading down T tubule system to sarcoplasmic reticulum where it
            triggers release of Ca2+ ions out amongst the myofilaments.
        7. The Ca2+ ions then initiate muscle contraction.

    E. Contraction

        1. Ca2+ ions bind to troponin, which causes tropomyosin threads to shift position.
        2. Change in structure of tropomyosin exposes myosin heads with ATP binding sites.
        3. The myosin heads function as ATPase enzymes, splitting ATP into ADP and P.
        4. After attaching to actin filaments, myosin cross-bridges bend forward, actin filament is pulled along.
        5. While ATP and Ca2+ ions are available, cross-bridges attach; as ADP and P are released, the cross-
            bridges change their positions and cause a power stroke as filaments pull together.
        6. When another ATP molecule binds to myosin head, cross-bridge detaches and the cycle begins again.
        7. When nerve impulses cease, active transport proteins in the sarcoplasmic reticulum pump calcium ions
            back into storage sites.