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Musculoskeletal SystemTypes of Skeletal Systems

1. Hydrostatic skeletons

2. Exoskeletons

3. Endoskeletons

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Hydrostatic Skeletons

Exoskeletons

• Surrounds the body as a rigid hard case.

• Composed of chitin in arthropods.

• Provides protection for internal organs and a site for muscle attachment.

• Must be periodically shed in order for the animal to grow.

• Not as strong as a bony skeleton.

• Respiratory system and gravity set limits on body size.

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Invertebrate Endoskeletons

• Rigid internal skeletons that form the body’s framework and offer surfaces for muscle attachment.

• Echinoderms have calcite skeletons.

• Made of calcium carbonate

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Vertebrate Endoskeletons

• Vertebrate endoskeletons have bone and/or cartilage.

• Bone is much stronger than cartilage, and much less flexible.

• Unlike chitin, bone and cartilage are living tissues.

• Can change and remodel in response to injury or physical stress.

• Vertebrate bone is made of calcium phosphate.

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Bone Structure

• In most mammals, bones retain internal blood vessels and are called vascular bones. • These typically have osteocytes and are also called

cellular bones or “living bones.”• Vascular bone has a special internal organization termed

the Haversian system.

• In birds and fishes, bones are avascular. • Lack osteocytes and are also called acellular bones.

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Bone Structure

• Based on density and structure, bone falls into three categories:

1. Compact bone – outer dense layer.2. Medullary bone – lines the internal cavity.

• Contains bone marrow in vertebrates.• Bird bones are hollow.

3. Spongy bone – forms the epiphyses inside a thick shell of compact bone.

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Compact Bone• Outer dense layer.

• Haversian system = osteon

Spongy (Cancellous) Bone

• Hollow to give space for marrow.

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Osteoporosis

Joints (articulations)

• Joints: Locations where one bone meets another.

• 4 basic joint types:1. Ball-and-socket joints – permit movement in all

directions.2. Hinge joints – allow movement in only one plane3. Gliding joints – permit sliding of one surface over

another.4. Combination joints – movement characteristics of two

or more joint types.

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Ball-and-Socket

Combination Joint

Hinge Joint Gliding Joint

a.

d.

b. c.

Muscles: What they do for you.

• Allow you to stand upright.

• Make it possible for you to move.

• Allow you to digest food.

• Help maintain a normal body temperature.

• Hold your skeleton together.

• Moves blood throughout the body.

Muscle tissue• Most abundant in animals.

• Consists of long cells called muscle fibers.

• Each contain many contractile proteins.

• Three kinds: smooth, skeletal, and cardiac.

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• Tendon:

attaches muscle

to bone.

• Ligament:

attaches bone

to bone.

Skeletal Muscle Movement

• Skeletal muscle fibers are attached to bones.• Directly to the periosteum by a tendon.

• One attachment of the muscle, the origin, remains stationary during contraction.

• The other end, the insertion, is attached to a bone that moves when muscle contracts.

• Muscles can be antagonistic.• One counters the action of the other.

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Extension

Flexion

Flexors

(hamstrings)

Tendon

Tendon

Extensors

(quadriceps)

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← Muscle fiber cells lengthwise view.

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Skeletal Muscle Contraction

• Thick filament • Composed of several myosin subunits packed together.• Myosin consists of two polypeptide chains wrapped

around each other.• Each chain ends with a globular head.

• Thin filament• Composed of two chains of actin proteins twisted

together in a helix.

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←Thin filament

(actin)

← Thick

filament

(myosin)

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Skeletal Muscle Contraction

• Muscle contracts and shortens because the myofibrils contract and shorten.• Myofilaments themselves do not shorten.

• Instead, the thick and thin filaments slide relative to each other.

• Thin filaments slide deeper into the A bands, making the H and I bands narrower.

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• Ca+ binds to

tropomyosin and

exposes actin

binding sites.

• Myosin heads

activated by ATP and

Mg+.

• Bind to actin sites.

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Skeletal Muscle Contraction

• In low Ca2+ levels, tropomyosin inhibits cross-bridge formation.

• In high Ca2+ levels, Ca2+ binds to troponin.• Tropomyosin is displaced, allowing the formation of

actin-myosin cross-bridges.

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Neuromuscular Junction

• What stimulates the muscle to contract?

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Skeletal Muscle Contraction

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• Muscle fiber is stimulated to contract by motor

neurons, which secrete acetylcholine at the neuromuscular junction.

• Membrane becomes depolarized.• Depolarization is conducted down the transverse

tubules (T tubules).• Stimulate the release of Ca2+ from the sarcoplasmic

reticulum (SR).

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Skeletal Muscle Contraction

• Motor unit: • Motor neuron and all of the muscle fibers it innervates. • All fibers contract together when the motor neuron

produces impulses.

• Muscles that require precise control have smaller motor units.

• Muscles that require less precise control but exert

more force, have larger motor units.• Recruitment is the cumulative increase in motor

unit number and size leading to a stronger contraction.

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