Structural Support and Movement Chapter 33
33.1 Skeletons and Muscles Most animals move when the force of muscle contraction is applied to skeletal elements
Animal Skeletons Hydrostatic skeleton A confined fluid accepts the force of muscle contraction Exoskeleton Consists of hardened parts at the body surface Endoskeleton Consists of hardened parts inside the body
Invertebrate Skeletons Fig. 33.2, p.546
Vertebrate Endoskeletons Skull bones, vertebral column, rib cage Pelvic girdle, pectoral girdle, and paired limbs
Adaptations of Vertebrate Skeletons Modifications to fins and other skeletal structures accompanied the move from water to land Fins evolved into limbs, pectoral and pelvic girdles got stronger, and a breastbone helped form the rib cage Evolution of an upright posture in human ancestors involved skeletal modifications
Human Skeletal Structure
b Rib cage These bones and some vertebrae enclose, protect heart, lungs; assist breathing: STERNUM (breastbone) RIBS (twelve pairs) c Vertebral column,or backbone VERTEBRAE (twenty-six bones) Enclose, protect spinal cord; support skull, upper extremities; attachment sites for muscles INTERVERTEBRAL DISKS Fibrous, cartilaginous structures between vertebrae; absorb movement-induced stresses; impart flexibility to backbone ligament bridging a knee joint, side view, midsection a Skull bones CRANIAL BONES Enclose, protect brain and sensory organs FACIAL BONES Framework for facial area, support for teeth e Pelvic girdle and lower limb bones PATELLA (kneebone) Protects knee joint; aids leverage Fig. 33.4a, p.547
d Pectoral girdle and upper limb bones Bones with extensive muscle attachments, arranged for great freedom of movement: PELVIC GIRDLE (six fused bones) Supports weight of backbone; helps protect soft pelvic organs CLAVICLE (collarbone) SCAPULA (shoulder blade) HUMERUS (upper arm bone) RADIUS (forearm bone) ULNA (forearm bone) CARPALS (wrist bones) 1 2 3 5 4 METACARPALS (palm bones) PHALANGES (thumb, finger bones) e Pelvic girdle and lower limb bones FEMUR (thighbone) Body s strongest weight-bearing bone; works with large muscles in locomotion and in maintaining upright posture TIBIA (lower leg bone) Major load-bearing role FIBULA (lower leg bone) Muscle attachment sites; no load-bearing role TARSALS (ankle bones) METATARSALS (sole bones) PHALANGES (toe bones) Fig. 33.4b, p.547
SKULL cranial bones facial bones RIB CAGE sternum ribs VERTEBRAL COLUMN vertebrae intervertebral disks PECTORAL GIRDLES AND UPPER EXTREMITIES clavicle scapula humerus radius ulna phalanges carpals metacarpals PELVIC GIRDLE AND LOWER EXTREMITIES pelvic girdle femur patella tibia fibula tarsals phalanges metatarsals Stepped Art Fig. 33.4, p.547
Key Concepts: SKELETAL SYSTEMS Contractile force exerted against some type of skeleton moves the animal body Many invertebrates have a hydrostatic skeleton, which is a fluid-filled body cavity Others have an exoskeleton of hardened structures at the body surface Vertebrates have an endoskeleton, an internal skeleton of cartilage, bone, or both
33.2 Bone Structure Bone matrix Collagen fibers and mineral salts Bone cells Osteoblasts, osteocytes, and osteoclasts Compact bone and spongy bone Red marrow and yellow marrow
Bone Structure Fig. 33.5a, p.548
Bone Functions
Bone Formation and Remodeling In human embryos, bones develop from a cartilage model Hormonally regulated bone remodeling helps maintain blood levels of mineral ions
Long Bone Formation Fig. 33.6, p.549
Osteoporosis
Joints Joints Regions where bones meet Most allow bones to move Held together by ligaments Types of joints Fibrous joint Cartilaginous joint Synovial joint
Key Concepts: VERTEBRATE SKELETONS Bones are collagen-rich organs that help the body move They also protect and support soft organs, and store minerals Blood cells form in some bones Cartilage or ligaments connect bones at joints
33.3 Skeletal-Muscular Systems Muscle fibers Long, cylindrical cells with multiple nuclei Form from groups of embryonic cells that fuse before they differentiate and mature Bundled inside a dense connective tissue sheath Tendons Extensions of connective tissue sheath Attach skeletal muscles to bones
Muscle Contraction and Movement When skeletal muscles contract, they transmit force to bones and move them Fig. 33.8, p.550
Muscle Interactions Some muscles work together, others work as opposing pairs Fig. 33.9, p.500
Human Musculoskeletal System
TRICEPS BRACHII Straightens the forearm at elbow PECTORALIS MAJOR Draws the arm forward and in toward the body SERRATUS ANTERIOR Draws shoulder blade forward, helps raise arm, assists in pushes EXTERNAL OBLIQUE Compresses the abdomen, assists in lateral rotation of the torso RECTUS ABDOMINIS Depresses the thoracic (chest) cavity, compresses the abdomen, bends the backbone ADDUCTOR LONGUS Flexes, laterally rotates, and draws the thighs toward the body SARTORIUS Bends the thigh at the hip, bends lower leg at the knee, rotates the thigh in an outward direction QUADRICEPS FEMORIS Flexes the thigh at hips, extends the leg at the knee TIBIALIS ANTERIOR Flexes the foot toward the skin BICEPS BRACHII Bends the forearm at the elbow DELTOID Raises the arm TRAPEZIUS Lifts the shoulder blade, braces the shoulder, draws the head back LATISSIMUS DORSI Rotates and draws the arm backward and toward the body GLUTEUS MAXIMUS Extends and rotates the thigh outward when walking, running, and climbing BICEPS FEMORIS (Hamstring muscle) Draws thigh backward, bends the knee GASTROCNEMIUS Bends the lower leg at the knee when walking, extends the foot when jumping Achilles tendon Fig. 33.10a, p.551
Fig. 33.10b, p.551
Key Concepts: THE MUSCLE BONE PARTNERSHIP Skeletal muscles are bundles of muscle fibers that interact with bones and with one another Some cause movements by working as pairs or groups Others oppose or reverse the action of a partner muscle Tendons attach skeletal muscles to bones
33.4 Skeletal Muscle Organization Internal organization of a skeletal muscle promotes a strong, directional contraction A skeletal muscle fiber is made up of many myofibrils
Skeletal Muscle Organization A myofibril consists of sarcomeres (basic units of muscle contraction) lined up along its length Each sarcomere has parallel arrays of actin and myosin filaments
Skeletal Muscle
Fig. 33.11a, p.552
outer sheath of one skeletal muscle one bundle of many muscle fibers in parallel inside the sheath one myofibril in one fiber Fig. 33.11a, p.552
one myofibril inside fiber: b Skeletal muscle fiber, longitudinal section. All bands of its myofibrils line up in rows and give the fiber a striped appearance. sarcomere sarcomere Z band Z band H zone Z band c Sarcomeres. Many thick and thin filaments overlap in an A band. Only thick filaments extend across the H zone. Only thin filaments extend across I bands to the Z bands. Different proteins organize and stabilize the array. I band A band I band Fig. 33.11b, p.552
Fig. 33.11c, p.552
Muscle Contraction Sliding-filament model Actin filaments slide past myosin filaments, shortening the sarcomere ATP-driven Muscle contraction Shortening of all sarcomeres in all myofibrils of all muscle fibers
Sliding-Filament Model
Fig. 33.12a p.553
myosin head one of many myosin binding sites on actin c Myosin in a muscle at rest. Earlier, all myosinheads were energized bybinding ATP, which they hydrolyzed to ADP and inorganic phosphate. cross-bridge cross-bridge d A rise in the localconcentration of calcium exposes binding sites for myosin on actin filaments, so cross-bridges form. e Binding makes each myosin head tilt toward the sarcomere s center and slide the bound actin along with it. f ADP and phosphate are released as myosin heads drag actin inward, which pulls the Z bands closer together. ATP ATP g New ATP binds to myosin heads, which detach from actin. ATP is hydrolyzed, which returns myosin heads to original orientations, ready to act. Fig. 33.12b p.553
Key Concepts: HOW SKELETAL MUSCLE CONTRACTS A muscle fiber contains many myofibrils, each divided crosswise into sarcomeres, the basic units of contraction Sarcomeres contain many parallel arrays of actin and myosin filaments ATP-driven interactions between the arrays shorten sarcomeres, which collectively accounts for contraction of a whole muscle
33.5 Signals and Responses Signals from motor neurons result in action potentials in muscle fibers; causes the release of calcium stored in the sarcoplasmic reticulum Calcium floods out and allows actin and myosin heads to interact; muscle contraction occurs
Pathway of Muscle Control
a Messages from spinal cord trigger release of acetylcholine (ACh) from a motor neuron s axon endings. section from spinal cord motor neuron neuromuscular junction b ACh diffuses to the muscle fiber and its binding causes action potentials. section from skeletal muscle Fig. 33.13a-b, p.554
c Action potentials propagate along a muscle fiber s plasma membrane down to T tubules, then to the sarcoplasmic reticulum, which releases calcium ions. The ions promote interactions of myosin and actin that result in contraction. T tubule sarcoplasmic reticulum one myofibril in muscle fiber muscle fiber s plasma membrane Fig. 33.13c, p.554
Motor Units and Muscle Tension A motor neuron and all the muscle fibers it controls are one motor unit Brief stimulation causes a muscle twitch Repeated stimulation causes a sustained contraction (tetanus) A muscle will shorten when muscle tension exceeds an opposing load
Energy for Contraction Muscle fibers produce ATP needed for contraction by three pathways: Dephosphorylation of creatine phosphate Aerobic respiration Lactate fermentation
Exercise, Aging, and Disease Muscle fatigue decreases a muscle s capacity to generate force, despite repeated stimulation Age and muscular dystrophy can weaken muscles
33.6 Muscles and Toxins Nervous system signals that stimulate contraction can be disrupted by toxins Examples: Food poisoning, the disease tetanus
Key Concepts: PROPERTIES OF WHOLE MUSCLES Muscle fibers in a muscle are organized in motor units that contract in response to signals from one motor neuron Cross-bridges form in all the sarcomeres and collectively exert tension
Key Concepts: PROPERTIES OF WHOLE MUSCLES (cont.) A muscle contracts only when the tensile force exceeds other, opposing forces Exercise enhances the properties of whole muscles, and aging and disease diminish them