Chapter 48 Outline and Terms

48.1. Comparing Skeletons (p. 856)

A. Different Types of Skeletons

1. Three types of skeletons exist in animal kingdom: a hydrostatic skeleton, an exoskeleton, and an endoskeleton.

B. A Water-Filled Cavity

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 so that motility results. (Fig. 48.1)

3. Many animals use hydroskeletons.

a. Hydras use a fluid-filled gastrovascular cavity to support tentacles that rapidly contract.

b. Flatworms such as planaria easily glide over substrate with muscular contractions and cilia.

c. Nematodes have a fluid-filled pseudocoelom and move in a whiplike fashion when muscles contract.

d. Annelids such as earthworms are segmented with septa that divide the coelom into compartments; muscle contractions in each segment allow coordinated elongation and contraction.

C. Exoskeletons or 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. (Fig. 48.2)

c. Besides providing protection against mechanical damage and enemies, exeskeleton keeps tissues from drying out.

d. Although the stiffness provides support for muscles, exeskeleton is not as strong as an endoskeleton.

e. In clams and snails, exoskeleton grows with the animal; the thick nonmobile CaCO3 shell is for protection.

f. Chitinous exoskeleton of arthropods is jointed and moveable.

g. Arthropods molt when the exoskeleton becomes too small; molting makes the animal vulnerable to predators.

2. Vertebrates have an endoskeleton composed of bone and cartilage that grows with the animal. (Fig. 48.3)

a. An endoskeleton does not limit the space available for internal organs and can support greater weight.

b. Soft tissues surround the 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 anthropods and vertebrates colonize land.

48.2. Humans Have an Endoskeleton (p. 859)

A. Human Skeletal Functions

1. The large, heavy bones of the legs support the body against the pull of gravity.

2. A skeleton protects vital organs: skull for brain, vertebral column for spinal cord, and rib cage for heart and lungs.

3. Flat bones of the skull, ribs, and breastbone contain red bone marrow, which manufactures blood cells.

4. All bones serves as storage areas for inorganic calcium and phosphorous salts.

5. The long bones, particularly those of the legs and the arms, permit flexible body movement.

B. How Bones Differ and Grow

1. Bone is living tissue that constantly renews itself.

2. A long bones illustrates bone anatomy. (Fig. 48.4) [transp. 269]

a. A long bone consist of a central medullary cavity surrounded by compact bone; ends are composed primarily of spongy bone surrounded by a thin layer of compact bone, and are covered with cartilage.

b. Compact bone contains many osteons (Haversian systems) in which bone cells in tiny chambers called lacunae are arranged in concentric circles around central canals.

c. The central canals contain blood vessels and nerves.

d. The lacunae are separated by a matrix that contains protein fibers of collagen and mineral deposits.

3. Spongy bone contains numerous plates and bars separated by irregular spaces.

a. Spongy bone is lighter but designed for strength; solid portions follow lines of stress.

b. The spaces are often filled with red bone marrow, a specialized tissue that produces blood cells.

4. The cavity of a long bone is often filled with yellow bone marrow; it functions in fat storage.

C. Bones Grow and Are Renewed

1. Prenatal human skeleton is cartilaginous; cartilaginous structures serve as "models" to future 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 in absence of a cartilaginous model.

2. During endochondral ossification in a long bone, there is a primary ossification center at the middle of a long bone and latter secondary centers form at the ends.

3. A cartilaginous disk remains between primary ossification and secondary centers, which increase in length.

4. Rate of growth is controlled by hormones, including growth hormones and sex hormones.

5. Eventually the disks become ossified and the bone stops growing; this determines adult height.

6. In the adult, 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. After three weeks they disappear and osteoblasts form new bone, taking calcium from the blood.

c. Osteoblasts become entrapped in the bone matrix and become osteocytes in the lacunae of osteons.

7. This continual remodeling allows bones to gradually change in thickness.

8. Adults need more calcium in the diet than children do to promote the work of osteoblasts.

D. Bones Make Up the Skeleton (Fig. 48.5) [transp. 270]

1. The human skeleton is divided into two parts: the axial skeleton and the appendicular skeleton.

2. The axial skeleton lies in the midline of the body and consists of skull, vertebral column, sternum and ribs.

a. Skull is formed by the cranium and the facial bones. (Fig. 48.6) [transp. 271]

1) The cranium protects the brain and is composed of eight bones: the frontal, two parietal, the occipital, two temporal, the sphenoid, and the ethmoid that fit tightly together in adults.

2) Newborns have membranous junctions called fontanels that usually close by age of two.

3) The bones of the cranium provide a protective case for the brain; contains the sinuses that are spaces lined with mucous membrane that reduce the weight of the skull and give a resonant sound to the voice.

4) Two mastoid sinuses drain into middle ear; mastoiditis is an inflammation that can lead to deafness.

5) A foramen magnum is an opening at base of skull in occipital bone, through which spinal cord passes.

6) Each temporal bone has an opening that leads to the middle ear.

7) The sphenoid bone completes the sides of the skull and forms floors and walls of eye sockets.

8) The ethmoid bone lies in front of the sphenoid, is part of orbital wall, and is a component of the nasal septum.

9) Fourteen facial bones include mandible, 2 maxillae, 2 palatine, 2 zygomatic, 2 lacrimal, 2 nasal, and vomer.

10) The mandible or lower jaw is the only movable portion of the skull; contains tooth sockets.

11) The maxilla form the upper jaw and the anterior of the hard palate; also contain tooth sockets.

12) The palatine bones make up the posterior portion of hard palate and the floor of the nasal cavity.

13) The zygomatic gives us our cheekbone prominences.

14) The nasal bones form the bridge of the nose.

15) Thin, scale-like lacrimal bones lie between an ethmoid bone and maxillary bone.

16) A thin, flat vomer joins with the perpendicular plate of the ethmoid to form the nasal septum.

b. The vertebral column supports and extends from the skull to the pelvis.

1) It has four curvatures that provide more resiliency and strength than a straight column could.

2) Vertebrae are named for their location: 7 cervical vertebrae, 12 thoracic vertebrae, 4 lumbar vertebrae, one sacrum of 5 fused sacral vertebrae, and one coccyx of 4 fused coccygeal vertebrae.

3) Joined, they form a canal for the spinal cord.

4) The spinous processes of the vertebrae can be felt as bony projections along the midline of back.

5) 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.

6) Disks allow motion between vertebrae for bending forward, etc.

7) Vertebral column serves as an anchor for all other bones of skeleton.

8) All 12 pairs of ribs connect directly to thoracic vertebrae in back and all but two pairs connect via cartilage to sternum; the two unattached to sternum are called "floating ribs."

3. Appendicular Skeleton is Girdles and Limbs

a. The appendicular skeleton consists of bones within the pectoral girdles and upper limbs, and the pelvic girdles and lower limbs.

b. The pectoral girdle is specialized for flexibility; the pelvic girdle is built for strength.

c. The pectoral girdle bones are only loosely linked by ligaments. (Fig. 48.7) [transp. 272]

1) The clavicle or "collarbone" connects with the sternum in front and the scapula in back.

2) The scapula connects with the clavicle, but is freely movable and held in place only by muscles.

d. Humerus is long bone of the upper arm; it has a smoothly rounded head that fits into socket of the scapula.

e. The radius is the more lateral of the bones of the lower arm; it articulates with the humerus at the elbow joint, a hinge joint, and the radius crosses in front of the ulna for easy twisting.

f. The ulna is the more medial of the two bones of the lower arm; its end is the prominence in the elbow.

g. The many hand bones increase its flexibility.

1) The wrist has eight carpal bones which look like small pebbles.

2) Five metacarpal bones fan out to form framework of the palm.

3) The phalanges are the bones of the fingers and the thumb.

h. The pelvic girdle consists of two heavy, large coxal (hip) bones. (Fig. 48.8) [transp. 273]

1) The coxal bones are anchored to the sacrum; together with the sacrum they form a hollow cavity that is wider in females than in males; transmit weight from the vertebral column via the sacrum to the legs.

2) The femur is the largest bone of the body; it is limited in the amount of weight that it can support.

3) The tibia has a ridge called the "shin"; its end forms the outside of the ankle.

4) The fibula is the smaller of the two bones; its end forms the outside of the ankle.

5) Seven tarsal bones are in each ankle; only one receives weight and passes it on to heel and ball of foot.

6) The metatarsal bones form the arch of the foot and provide a springy base.

7) The phalanges are the bones of the toes.

E. Joints Join Bones

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 by becoming more flexible.

4. Synovial joints are freely movable.

a. Most joints are freely movable 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, the ligaments are unusually loose.

d. The joint capsule is lined with a synovial membrane that produces a lubricating synovial fluid.

e. The knee represents a synovial joint. (Fig. 48.9) [transp. 274]

1) The bones are capped by cartilage; a crescent-shaped piece of cartilage, the menisci, is between.

2) Athletes who injure the meniscus have torn cartilage.

3) The 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."

f. Synovial joints are subject to arthritis.

1) In rheumatoid arthritis, the synovial membrane becomes inflamed and thickens.

2) The joint degenerates and becomes immovable and painful.

3) It is likely this is due to an autoimmune reaction.

4) In osteoarthritis from old age, cartilage at bone ends disintegrates; bones become rough and irregular.

48.3. How Muscles Function (p. 865)

A. Muscle Tissues Vary

1. Skeletal muscle contraction is voluntary; it allows an animal to move using the skeleton.

2. Smooth muscle contraction is involuntary; it regulates blood pressure, causes motility in digestive and urinary systems.

3. Cardiac muscle contraction enables the heart to pump blood.

B. On a Macroscopic Level

1. Skeletal muscles comprise more than 40% of body's weight; 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 the limb toward the body.

b. The other one straightens the joint and extends the limb. (Fig. 48.10) [transp. 275]

3. Mechanical force of contraction is transduced to electrical current recorded on a Physiograph; pattern is a myogram.

C. On a Microscopic Level (Fig. 48.12) [transp. 276]

1. A whole skeletal muscle consists of muscle fibers (myofibrils).

2. Each muscle fiber is a cell with special features.

a. Myofibrils are cylindrical, multinucleated cells of skeletal muscle that lie parallel to one another.

b. A plasma membrane called the sarcolemma forms a T (transverse) system.

1) Transverse (T) tubules penetrate down into cell and contact with, but do not fuse with, 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.

c. A light microscope shows light and dark bands called striations.

d. An electron microscope shows the 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. The sarcomere is the fundamental unit of contraction in skeletal muscle; it has repeating bands of actin and myosin that occur between two Z lines in a myofibril.

1) The I band contains only actin filaments.

2) The H zone contains only myosin filaments.

3. Filaments Slide

a. As a muscle fiber contracts, sarcomeres within myofibrils shorten.

b. When sarcomere shortens, actin filaments slide past myosin; I band shortens and H zone nearly 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. (Table 48.1)

d. The contraction process involves sacromere shortening, but filaments themselves remain same length.

e. ATP supplies energy for muscle contraction.

f. Myosin filaments break down ATP and form cross-bridges that attach to the actin filament, and pull actin filament along.

4. ATP Is Needed

a. To ensure a ready supply of ATP for contraction, muscle fibers contain creatine phosphate (phosphocreatine).

b. Creatine phosphate does not participate in muscle contraction directly; it regenerates ATP rapidly: creatine P + ADP ATP + creatine

c. When all of the creatine phosphate is depleted, and if O2 is in limited supply, fermentation produces a small amount of ATP, but this results in the buildup of lactate that is toxic.

d. The buildup of lactate partially accounts for muscle fatigue and represents oxygen debt.

e. Lactate is transported to liver where 20% is completely broken down to CO2 and H2O in aerobic respiration.

f. The ATP gained from this respiration is then used to reconvert 80% of the lactate to glucose.

D. At the Neuromuscular Junction

1. Muscle fiber contraction occurs on stimulus from motor neurons; neuromuscular junction is a region where an axon bulb is in close association with sarcolemma of a muscle fiber. (Fig. 48.13) [transp. 277] [micro. slide 99]

2. The synaptic bulb contains synaptic vesicles filled with a neurotransmitter.

3. There is a small gap between synaptic bulb and sarcolemma; neurotransmitter transmits signal across this cleft.

4. When nerve impulses travel down a motor neuron to the bulb, the vesicles merge with the presynaptic membrane and acetylcholine molecules are released into the synaptic cleft.

5. Acetylcholine rapidly diffuses to and binds with receptor sites on the sarcolemma.

6. The action potential moves along the sarcolemma and down the T tubule system to endoplasmic reticulum where it triggers release of Ca2+ ions out amongst the myofilaments.

7. The Ca2+ ions then initiate muscle contraction.

E. As Contraction Occurs (Fig. 48.14)

1. The Ca2+ ions bind to troponin, which causes tropomyosin threads to shift position.

2. Change in structure of tropomyosin exposes myosin binding sites on actin filaments. (Fig. 48.14)

3. After attaching to actin filaments, myosin cross-bridges bend forward, actin filament is pulled along.

4. While ATP and Ca2+ ions are available, cross-bridges attach and detach as thin filaments pull toward the center.

5. When nerve impulses cease, active transport proteins in the sarcoplasmic reticulum pump calcium ions back into storage sites.


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