chapter 47 musculoskeltal Flashcards
Types of Skeletal Systems
Changes in movement occur because muscles pull against a support structure Zoologists recognize three types: Hydrostatic skeletons Exoskeletons Endoskeletons
Hydrostatic Skeletons
Found primarily in soft-bodied invertebrates (terrestrial and aquatic)
Locomotion in earthworms
Involves a fluid-filled central cavity (hydrostatic skeleton) and surrounding circular and longitudinal muscles
A wave of circular followed by longitudinal muscle contractions move fluid down body
Chaetae prevent slipping backward
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
Endoskeletons
Rigid internal skeletons that form the body’s framework and offer surfaces for muscle attachment
Echinoderms have calcite skeletons
Made of calcium carbonate
Vertebrate bone is made of calcium phosphate
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
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
Bones form within a layer of connective tissue
Flat skull bones and jaw bones
Endochondral development
Begin as tiny cartilaginous model
Intramembranous development
usually begins in dermis of skin when mesenchyme cells differentiate
Osteoblasts initiate bone development—lay down matrix of calcium phosphate
Some cells become trapped in the bone matrix that they have produced
Change into osteocytes
Reside in tight spaces called lacunae
The cells communicate through little canals termed canaliculi
Osteoclasts break down the bone matrix
Endochondral development
Typically bones that are deeper in the body
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
Trapped osteoblasts transform into osteocytes
Osteoclasts remodel bone
Endochondral development
Increase in length unlike intramembranous bone
Limb bones have a shaft (diaphysis) with epiphyses
Epiphyseal growth plates separate epiphyses from shaft
Plates are cartilage in growing bone
Growth pushes epiphysis away from shaft
Cartilage becomes calcified
Growth in length ends by late adolescence
Growth in width does not
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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
Bird bones are hollow
Spongy bone – forms the epiphyses inside a thick shell of compact bone
Skeletal Muscle Movement
Skeletal muscle fibers are attached to bones
Directly to the periosteum
Through a tendon attached to the periosteum
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
Muscle contraction
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
Under a microscope, the myofibrils have alternating dark and light bands – striated
Sarcomere – distance between two Z lines
Smallest subunit of 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
Sliding filament mechanism
Thin filaments slide deeper into the A bands, making the H and I bands narrower
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
Cross-bridge cycle
Hydrolysis of ATP by myosin activates the head for the later power stroke
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
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
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
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)
Excitation–contraction coupling
Release of Ca2+ that links excitation by motor neuron to contraction of the muscle
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
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)
Excitation–contraction coupling
Release of Ca2+ that links excitation by motor neuron to contraction of the muscle
Ca2+ binds to Troponin and Troponin moves Tropomyosin
off of myosin binding sites on the actin
ATP hydrolyzes into ADP and Pi
This cocks the myosin head and the myosin head attaches to
the binding sites on the actin
Power stroke of myosin head moves actin toward the middle
of the sarcomere
ADP and Pi are released
ATP binds to myosin head which causes it to release from the
actin
If ATP hydrolyzes again, it will start over
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