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The Musculoskeletal System

Chapter 47

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Types of Skeletal Systems

Changes in movement occur because muscles pull against a support structure, called the skeletal system -Zoologists recognize three types: -Hydrostatic skeletons -Exoskeletons -Endoskeletons

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Hydrostatic Skeletons

Are found primarily in soft-bodied invertebrates, both terrestrial and aquatic

Locomotion in earthworms -Involves a fluid-filled central cavity and surrounding circular & longitudinal muscles -A wave of circular followed by longitudinal muscle contractions move fluid down body -Produces forward movement

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Hydrostatic Skeletons

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Hydrostatic Skeletons

Locomotion in aquatic invertebrates -Occurs by fluid ejections or jetting -Jellyfish produce regular pulsations in bell -Squeezing some of water contained

beneath it -Squids fill mantle cavity with sea water -Muscular contractions expel water

forcefully through the siphon, and the animal shoots backward 6

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Exoskeletons

The exoskeleton surrounds the body as a rigid hard case -Composed of chitin in arthropods

An exoskeleton provides protection for internal organs and a site for muscle attachment -However, it must be periodically shed, in order for the animal to grow -It also limits body size

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Endoskeletons

Endoskeletons are rigid internal skeletons that form the body’s framework and offer surfaces for muscle attachment -Echinoderms have calcite skeletons, that are made of calcium carbonate -Bone, on the other hand, is made of

calcium phosphate

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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 -They can change and remodel in response to injury or physical stress

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Endoskeletons

The vertebrate endoskeleton is divided into: -Axial skeleton = Forms axis of the body -Supports the body and protects internal

organs -Appendicular skeleton = Set of limb bones and their associated pectoral girdle (forelimbs) or pelvic girdle (hindlimbs)

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Bone

Bone is a hard but resilient connective tissue that is unique to vertebrates

Bones can be classified by the two fundamental modes of development -Intramembranous development (simple) -E.g.: External bones of skull -Endochondral development (complex) -E.g.: Bones that are deep in the body

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Bone

Intramembranous development -Osteoblasts initiate bone development -Some cells become trapped in the bone

matrix that they have produced -Change into osteocytes, which

reside in tight spaces called lacunae -The cells communicate through

little canals termed canaliculi -Osteoclasts break down the bone matrix

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Bone

Endochondral development -Endochondral bones begin as tiny cartilaginous models -Bone development consists of adding bone to the outside of a cartilaginous model, while replacing interior cartilage with bone -Calcification begins with the fibrous sheath, later called the periosteum

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Bone

Endochondral development -Cartilage that remains after the development of epiphyses serves as a pad between bone surfaces -Bones grow by lengthening and widening -Growth in length usually ceases in

humans by late adolescence -Growth in width continues by bone

addition just beneath the periosteum

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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 -Vascular bone has a special internal organization termed the Haversian system

In birds and fishes, bones are avascular -These typically lack osteocytes and are also called acellular bones

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Bone Structure

Based on density and structure, bone falls into three categories

-Compact bone = Outer dense layer -Medullary bone = Lines the internal cavity -Contains bone marrow in vertebrates -Spongy bone = Honeycomb structure -Forms the epiphyses inside a thick shell

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Bone Remodeling

The phenomenon of remodeling is known for all bones -Small forces may not have a great effect -But larger forces – if frequent enough –

can initiate remodeling by osteoblasts

It is possible that mechanical stress in bones deforms the hydroxyapatite crystal, thus producing a piezoelectric effect

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Bone Remodeling

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Joints

Joints are the locations where one bone meets another

-1. Immovable joints = Join bones -2. Slightly movable joints = Involve fibrous connective tissue or cartilage -3. Freely movable joints = Also called synovial joints -Contain a lubricating fluid

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Joints

Movable joints can be divided into four types -Ball-and-socket joints = Permit movement in all directions -Hinge joints = Allow movement in only one plane -Gliding joints = Permit sliding of one surface over another -Combination joints = Allow rotation and side-to-side sliding

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Skeletal Muscle Movement

Skeletal muscle fibers are attached to the periosteum of bones in one of two ways -Directly; or through a strong, fibrous cord called the 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

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Skeletal Muscle Movement

Skeletal muscles occur in antagonistic pairs -Agonist = Muscle group causing an action -Antagonist = Muscle group that counters movement

Isotonic contraction – The force of contraction remains relatively constant as the muscle shorten in length

Isometric contraction – The length of the muscle does not change as force is exerted

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Skeletal Muscle Structure

Each skeletal muscle contains numerous muscle fibers -Each muscle fiber encloses a bundle of 4 to 20 elongated structures called myofibrils -Each myofibril in turn is composed of

thick and thin myofilaments

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Skeletal Muscle Structure

A bands = Stacked thick & thin myofilaments -Dark bands

H bands = Center of the A band, consisting of thick bands only

I bands = Consist only of thin myofilaments -Light bands -Divided into two halves by a disc of protein called the Z line

Sarcomere = Distance between two Z lines -Smallest subunit of muscle contraction 38

Skeletal Muscle Contraction

A 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 -Sliding filament mechanism -Z lines move closer together, as

the I and H bands become shorter -A band does not change in size

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Skeletal Muscle Contraction

A thick filament is composed of several myosin subunits packed together -Myosin consists of two polypeptide chains wrapped around each other -Each chain ends with a globular head

A thin filament is composed of two chains of actin proteins twisted together in a helix

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Skeletal Muscle Contraction

Muscle contraction involves a series of events called the cross-bridge cycle -Hydrolysis of ATP by myosin, activates the head for the later power stroke -The ADP and Pi remain bound to the head, which binds to actin forming a cross-bridge -During the power stroke, myosin returns to its original shape, releasing ADP and Pi -ATP binds to the head which releases actin

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Skeletal Muscle Contraction

When a muscle is relaxed, its myosin heads cannot bind to actin because the attachment sites are blocked by tropomyosin -In order for muscle to contract, tropomyosin must be removed by troponin -This process is regulated by Ca2+ levels

in the muscle fiber cytoplasm

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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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Skeletal Muscle Contraction

A muscle fiber is stimulated to contract by motor neurons, which secrete acetylcholine at the neuromuscular junction -The 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

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Skeletal Muscle Contraction

A motor unit consists of a 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 52

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Types of Muscle Fibers

A muscle stimulated with a single electric shock quickly contracts and relaxes in a response called a twitch

Summation is a cumulative response when a second twitch “piggy-backs” on the first

Tetanus occurs when there is no relaxation between twitches -A sustained contraction is produced

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Types of Muscle Fibers

Skeletal muscle fibers can be divided based on their contraction speed -Slow-twitch, or Type I, fibers -Rich in capillaries, mitochondria and

myoglobin (red fibers) -Sustain action for long periods of time -Fast-twitch, or Type II, fibers -Poor in capillaries, mitochondria and

myoglobin (white fibers) -Adapted for rapid power generation 56

Types of Muscle Fibers

Skeletal muscles have different proportions of fast-twitch and slow-twitch fibers

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Types of Muscle Fibers

Skeletal muscles at rest obtain most of their energy from aerobic respiration of fatty acids -During muscle use, energy comes from glycogen and glucose

The maximum rate of oxygen consumption in the body is called the aerobic capacity

Muscle fatigue is the use-dependent decrease in the ability to generate force -Usually correlated with the production of lactic acid by the exercising muscle

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Modes of Animal Locomotion

Locomotion in large animals involves: -Appendicular locomotion -Produced by appendages that oscillate -Axial locomotion -Produced by bodies that undulate,

pulse or undergo peristaltic waves The physical constraints to movement –

gravity and frictional drag – occur in every environment, differing only in degree

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Locomotion in Water

Water’s buoyancy reduces effect of gravity Some marine invertebrates move about using

hydraulic propulsion All aquatic invertebrates swim

-Swimming involves using the body or its appendages to push against the water -An eel uses its whole body -A trout uses only its posterior half

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Locomotion in Water

Many terrestrial tetrapod vertebrates are able to swim, usually through limb movement -Most birds that swim propel themselves by pushing against water with their hind legs -These typically have webbed feet -Animals that swim with their forelegs usually have these modified as flippers and pull themselves through the water -Sea turtles and penguins

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Locomotion on Land

Terrestrial locomotion deals mainly with gravity Mollusks glide along a path of mucus Vertebrates and arthropods have a raised

body, and move forward by pushing against the ground with jointed appendages – legs -Vertebrates are tetrapods; all arthropods have at least six limbs -Having extra legs increases stability, but

reduces the maximum speed

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Locomotion on Land

The basic walking pattern of quadrupeds generates a diagonal pattern of foot falls -Left hind leg, right foreleg, right hind leg, left foreleg -Allows running by a series of leaps

Some vertebrates are also effective leapers -Kangaroos, rabbits and frogs have powerful leg muscles

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Locomotion on Land

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Locomotion in Air

Flight has evolved among animals four times -Insects, pterosaurs (extinct flying reptiles), birds, and bats -Propulsion is achieved by pushing down

against the air with wings In birds and most insects, wing raising and

lowering is achieved by alternate contraction of extensor muscles (elevators) and flexor muscles (depressors)

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Locomotion in Air

These different vertebrates all have lightened bones and forelimbs transformed into wings