Protection, Support, and Movement
All animals have some kind of protection, support, or movement that is noticeable in some way or another.† In order to have protection, support, and movement, animals need certain structures, such as muscles, skin, and skeletons.† Without these, there could not be any protection, support, or movement.
I. Integumentary System
A. The outer covering of animal bodies is called the integument.
1. For most species, the integument is a tough yet pliable barrier against the environment.
2. In arthropods, it is a hardened cuticle made of chitin and protein.
B. Vertebrate Skin and Its Derivatives
1. In vertebrates, the integument consists of skin and the structures derived from epidermal cells, such as scales, feathers, hair, beaks, horns, nails, and so forth.
2. The skin consists of an outer epidermis and an underlying dermis; a deeper hypodermis anchors the skin to the body.
C. Functions of Skin
1. The skin covers and protects the body from abrasion, bacterial attack, ultraviolet radiation, and dehydration.
2. It helps control internal temperature.
3. Its vessels serve as a blood reservoir for the body.
4. The skin produces vitamin D.
5. Its receptors are essential in detecting environmental stimuli.
II. A Closer Look at Human Skin
A. Structure of the Epidermis and Dermis
1. Epidermis is a stratified epithelium.
a. Keratinocyte cells produce keratin, a tough, water-insoluble protein that accumulates in the cells.
b. Melanocyte cells produce melanin pigment that darkens the skin and protects against the sunís rays; hemoglobin and carotene also contribute to skin color.
c. The outermost layer (stratum corneum) consists of flattened, dead cells filled with keratin.
2. The dermis lies beneath the epidermis.
a. Its dense connective tissue cushions the body against everyday stretching and mechanical stresses.
b. Blood vessels, lymph vessels, and receptors of sensory nerves are embedded in the tissue.
B. Sweat Glands, Oil Glands, and Hairs
1. Sweat glands produce a fluid that is release in response to stress (overheating, and fright, for example).
2. Oil glands (sebaceous) lubricate and soften the skin plus they produce secretions that reduce bacterial populations on the skin.
3. Each hair is a flexible structure rooted in the skin and projecting above it.
III. Types of Skeletons
A. Operating Principles for Skeletons
1. Animals move by the action of muscles, which need some medium or structural element against which the force of contraction can be applied.
2. There are three main types of skeletons in animals:
a. In hydrostatic skeletons, the force of contraction is applied against internal fluids.
b. In an exoskeleton, the force is against rigid external body parts, such as shells or plates.
c. In an endoskeleton, the force is applied against rigid internal cartilage and bones.
B. Examples From the Invertebrates
1. Sea anemones and earthworms use the fluids in their body cavities as resistance against which the muscles can act to cause varying degrees of movement.
2. The rigid exoskeletons of arthropods provide support for bodies deprived of waterís buoyancy; plus they provide sites for muscle attachments that maximize leverage.
C. Examples From the Vertebrates
1. Some fishes have a flexible skeleton made of an elastic, translucent form of cartilage.
2. The cartilage in sharks is opaque and hardened with calcium salts.
3. Vertebrate skeletons are made primarily of bone.
IV. Characteristics of Bone
A. Functions of Bone
1. Bones interact with muscles to maintain or change the position of body parts.
2. Bones support the skin and soft organs.
3. Bones form compartments that enclose and protect soft internal organs.
4. Bone tissue acts as a depository for calcium, phosphorus, and other ions.
5. Parts of some bones are sites of blood cell production.
B. Bone Structure
1. There are four types of bones: long (arms), short (wrist), flat (skull), and irregular (vertebrae).
2. Bone is a connective tissue with living cells and collagen fibers distributed throughout a ground substance that is hardened by calcium salts.
a. Compact bone tissue forms the boneís shaft and the outer portion of its two ends.
1. Concentric layers (lamellae) form Haversian systems around canals that contain blood vessels and nerves.
2. The living bone cells reside in the ground substance.
b. Spongy bone tissue has areas of red marrow that produces blood cells; cavities in most mature bones contain yellow marrow, which can be converted to red marrow if blood cell production needs to be increased.
3. How Bones Develop
a. Osteoblasts secrete material inside the shaft of the cartilage model of long bones.
b. Calcium is deposited; smaller cavities merge to form the larger marrow cavity.
c. Eventually osteoblasts become trapped within their own secretions and become osteocytes (mature bone cells).
4. Bone Tissue Turnover
a. Bone is renewed constantly as minerals are deposited and withdrawn during the growth and development processes as well as in maintenance of body calcium levels.
b. Bone turnover helps to maintain calcium levels for the entire body; enzymes from bone cells dissolve bone tissue and release calcium to the interstitial fluid and blood.
c. Osteoporosis (decreased bone density) is associated with decreases in osteoblast activity, sex hormone production, exercise, and calcium uptake.
V. Human Skeletal System
A. The 206 bones of a human are distributed in two portions:
1. The axial skeleton includes the skull, vertebral column (individual bones separated by cartilaginous intervertebral disks), ribs, and sternum.
2. The appendicular skeleton consists of the pectoral girdle with attached upper limbs, and the pelvic girdle with lower limbs.
B. Skeletal Joints
1. Joints are areas of contact or near-contact between bones.
a. Fibrous joints have no gap between the bones and hardly move; flat cranial bones are an example.
b. Cartilaginous joints, such as intervertebral disks, permit slight movement.
c. Synovial joints move freely; they are stabilized by ligaments; a capsule of dense connective tissue surrounds the bones of the joint; synovial fluid lubricates the joint.
2. Joints are vulnerable to stress.
a. Stretching or twisting a joint may result in a strain; tearing ligaments or tendons is a sprain.
b. In osteoarthritis, the cartilage at the end of the bone has worn away.
c. In rheumatoid arthritis, the synovial membrane becomes inflamed, the cartilage degenerates, and bone is deposited into the joint.
VI. Skeletal-Muscular Systems
A. How Muscles and Bones Interact
1. Each skeletal muscle contains several bundles of perhaps hundreds or thousands of muscle cells (fibers).
a. Tendons, cordlike straps of dense connective tissue, attach muscle to bone.
b. Skeletal muscles, often arranged in antagonistic pairs, interact with one another and with bones.
2. There are 3 types of muscle tissues: skeletal, cardiac, & smooth (digestive tract).
B. Human Muscle System
1. The human body has more than 600 skeletal muscles.
2. The major muscles are depicted in Figure 38.18.
VII. Muscle Structure and Function
A. Functional Organization of a Skeletal Muscle
1. A muscle shortens because the contraction units (sarcomeres) are shortening.
2. Muscle cells (fibers) are composed of myofibrils, which are composed of two kinds of filaments: actin and myosin.
a. Actin is a thin filament composed of two beaded strands twisted together.
b. Myosin is thicker; each molecule has a bulbous head and long tail making it resemble a golf club.
B. Sliding-Filament Model of Contraction
1. Muscles shorten because sarcomeres can shorten within each cell by the sliding-filament model.
2. Each sarcomere consists of two sets of actin filaments attached to opposite sides of the sarcomere (at Z-lines) and one set of myosin filaments extending unattached between the actin filaments.
a. The myosin filaments slide along and pull the actin filaments toward the center of the sarcomere.
b. Cross-bridges form between the heads of myosin molecules and actin filaments.
c. The cross-bridges are then activated and tilt inward; then the heads detach and reattach.
d. ATP supplies the energy for both attachment and detachment.
VIII. Control of Muscle Contraction
A. The Control Pathway
1. Skeletal muscles contract in response to signals from the nervous system that trigger action potentials along the plasma membrane and into the interior of the muscle cell.
2. Eventually the signal reaches the sarcoplasmic reticulum (internal tubes), which responds by releasing stored calcium ions.
B. The Control Mechanism
1. When contraction is not occurring, calcium blocks the binding sites on the troponin-tropomyosin complex.
2. Under stimulation, the sarcoplasmic reticulum releases calcium ions, which will bind to the troponin component of actin allowing cross-bridges to form.
3. A muscle relaxes when calcium ions are actively taken up after contraction to be stored in the sarcoplasmic reticulum.
C. Sources of Energy for Contraction
1. During periods (few seconds) of intense muscle activity, creatine phosphate is the source of phosphate to remake ATP.
2. When muscle action is moderate, most of the ATP is provided by aerobic electron transport phosphorylation, which is dependent on oxygen supply and number of mitochondria present.
3. During intense and prolonged muscle action, anaerobic glycolysis produces low amounts of ATP but also results in an oxygen debt.
IX. Properties of Whole Muscles
A. Muscle Tension and Muscle Fatigue
1. The cross-bridges that form during contraction exert muscle tension.
2. When muscle tension is greater than the forces opposing it, contracting muscle cells shorten; when opposing forces are stronger, muscle cells lengthen.
3. A motor neuron and the muscle cells under its control are a motor unit.
a. A single, brief stimulus to a motor unit causes a brief contraction called a muscle twitch.
b. Repeated stimulation without sufficient interval causes a sustained contraction called tetanus.
B. Effects of Exercise and Aging
1. With regular exercise, muscle cells do not increase in number; however, they do increase in size and metabolic activity and become resistant to fatigue.
a. Aerobic exercise, not intense but long in duration, increases the number of mitochondria and blood capillaries.
b. Strength training affects fast-acting muscle cells by forming more myofibrils and more enzymes of glycolysis.
2. Muscle tension decreases as adult humans age but exercise remains beneficial in improving blood circulation and preventing loss of muscle tissue.