Glossary of Biology - Musculoskeletal System

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What is the primary function for the skeleton?
Functions in physical support of an organism
Do vertebrates have an internal or external skeleton?
They have in internal skeleton
What is it called?
The sarcoplasmic reticulum
How many skeleton parts is the mammalian skeleton divided into?
Two, the axial and appendicular skeletons
What is the axial skeleton?
It is the basic framework of the body, consisting of the skull, the vertebral column, and the rib cage
What is the appendicular skeleton?
It consists of the limb bones and the pelvic and pectoral girdles
What are the two major components of the skeleton?
Cartilage and bone
What is cartilage?
It is a type of connective tissue that is softer and more flexible than bone
What is it composed of?
Sparsely scattered population of cells contained in an amorphous ground substance which may be liquid, jelly-like, or solid
What secretes chondrin?
Cartilage is the principal component of what?
Embryonic skeletons in higher animals
Where is cartilage retained in adults?
Where firmness and flexibility are needed
Where is it located in humans?
In the external ear, the nose, the walls of the larynx and trachea, and the skeleton joints
Is most cartilage vascular or avascular?
It is avascular
Does it have nerves or not?
It does not have nerves
Where does it receive nourishment?
From capillaries located in nearby connective tissue and bone via diffusion through the surrounding fluid
What is bone?
A mineralized connective tissue that has the ability to withstand physical stress
What is it designed for?
Body support. Bone tissue is hard and strong, while at the same time, somewhat elastic and lightweight
What are the two basic types of bone?
Compact bone and spongy bone
What is compact bone?
Dense bone that does not appear to have any cavities when observed with the naked eye
What is spongy bone also called?
Cancellous bone
Is it as dense as compact bone?
No, much less dense
What does it consist of?
A latent period, a contraction period, and a relaxation period
What are the cavities in between the spicules filled with?
Yellow and or red bone marrow
What is yellow marrow?
It is an inactive marrow infiltrated by adipose tissue
What is red marrow?
It is involved in blood cell formation
What are the bones of the appendages called?
The long bones
What are they characterized by on the ends and shaft?
The cylindrical shaft is called a diaphysis and the dilated ends are called epiphyses
What is the diaphysis composed primarily of?
Compact bone surrounding a cavity containing bone marrow
What are the epiphyses composed of?
Spongy bone surrounded by a thin layer of compact bone
What is the epiphyseal plate?
A disk of cartilaginous cells separating the diaphysis from the epiphysis
What does it function in?
The site of longitudinal growth
What is the fibrous sheath that surrounds the long bone called?
What is its function?
It is the site of attachment to muscle tissue
What can some periosteum cells differentiate into?
Bone-forming cells
What is compact bone composed of?
A dense, hardened bone matrix, which contains both organic and inorganic components
What do the organic components include?
Proteins like collagen fibers and glycoproteins, while the inorganic components include calcium, phosphate, and hydroxide
What can hydroxide combine and harden into?
Hydroxyapatite crystals
What else is in there?
Sodium, potassium, and magnesium ions
What gives bone its characteristic strength?
The association of hydroxyapatite crystal with collagen fibers
What are the structural units that the bony matrix is deposited into?
Osteons or Haversian systems
What does each osteon consist of?
A central microscopic channel called a Haversian canal, surrounded by a number of concentric circles of bony matrix called lamellae
What else are in the Haversian canals?
Blood vessels, nerve fibers, and lymph, as well as vascularizing and innervating bone tissue
What is interspersed within the matrix?
Spaces called lacunae
What do lacunae house?
Mature bone cells called osteocytes
What are osteocytes involved in?
Bone maintenance
What radiates from each lacuna?
A number of minute canals called canaliculi
What do they do?
They interconnect with each other and with the Haversian canals, allowing for exchange of nutrients and wastes
What are two other types of cells found in bone tissue?
Osteoblasts and osteoclasts
What do osteoblasts do?
They synthesize and secrete the organic constituents of the bone matrix
What happens once they have become surrounded by their matrix?
They mature into osteocytes
What are osteoclasts?
Large multinucleated cells involved in bone resorption
What are the two ways in which bone formation occurs?
It occurs by either endochondral ossification or by intramembranous ossification
What happens in endochondral ossification?
Existing cartilage is replaced by bone
In intramembranous ossification, what occurs?
Mesenchymal (embryonic, undifferentiated) connective tissue is transformed into and replaced by bone
Which method do long bones primarily arise?
Through endochondral ossification
What does it mean when the bone matrix is called dynamic?
It is continuously and simultaneously degraded and reformed
What happened during bone reformation?
Inorganic ions like calcium and phosphate are absorbed from the blood for use in bone formation
In the process of bone resorption, what occurs?
These ions are released into the blood
What are these two processes collectively known as?
Bone remodeling
What are some of the hormones and minerals involved with bone remodeling?
Vitamin D, and hormones such as parathyroid hormone and calcitonin
What does bone use and stress during exercise do to influence bone remodeling?
They are also factors in it
What are joints?
They are connective tissue structures that join bones together
Bones that do not move relative to each other are held in place by what?
Immovable joints
Bones that do move relative to one another are held together by what?
Movable joints
What are they additionally supported and strengthened by?
What do movable joints consist of?
A synovial capsule
What does it enclose?
A joint cavity
What is movement facilitated by?
Synovial fluid
How does it work?
It lubricates the joint
What does articular cartilage do?
It is on the apposing bone surfaces, which makes for a smooth surface in order to reduce tension during movement
What are the three types of muscles in mammals?
Skeletal muscle, smooth muscle, and cardiac muscle
What is skeletal muscle responsible for?
Voluntary movements
What is it innervated by?
The somatic nervous system
What is a muscle?
A bundle of parallel fibers
What is each fiber?
A multinucleated cell created by the fusion of several mononucleate embryonic cells
Where are the nuclei usually found in muscle cells?
On the periphery
What is embedded in the fibers?
Filaments called myofibrils. Myofibrils are further divided into contractile units called sarcomeres
What are they myofibrils enveloped by?
A modified endoplasmic reticulum that stores calcium ions
What is the cytoplasm of a muscle fiber called?
A sarcoplasm
What is the cell membrane called?
The sarcolemma
What is the sarcolemma capable of?
It can propagate an action potential
What is it connected to a system of?
Transverse tubules called the T system, which is oriented perpendicularly to the myofibrils
What does the T system provide?
It provides channels for ion flow throughout the muscle fibers, and can also propagate an action potential
Why is skeletal muscle striated?
Because it has striations of light and dark bands
What can skeletal muscle fibers be characterized as?
Either red or white
What are red fibers?
They are slow-twitch fibers
What do they have a high content of?
What else do they have?
Many mitochondria
How do they derive their energy?
They get their energy primarily from aerobic respiration and are capable of sustained and vigorous activity
White fibers are aerobic or anaerobic?
They are anaerobic
What are they known as?
Fast-twitch fibers
What does this imply?
It means they contain less myoglobin and fewer mitochondria than red fibers
Which fiber has a greater rate of contraction?
White fibers
Which fatigues more easily?
White fibers
What a long distance runner have more red fibers than white fibers?
What two things is the sarcomere composed of?
Thin and thick filaments
What are the thin filaments?
Chains of globular actin molecules
What two proteins are actin molecules associated with?
Troponin and Tropomyosin
What are thick filaments composed of?
Organizing bundles of myosin molecules
What does each myosin molecule have?
A head region and tail region
What do Z lines define?
They define the boundaries of a single sarcomere and anchor the thin filaments
What is the M line?
A line that runs down the center of a sarcomere
What is the I band?
The region containing thin filaments only
What is the H zone?
The region containing thick filaments only
What does the A band span?
The entire length of the thick filaments and any overlapping portions of the thin filaments
During contraction, what does and doesn’t reduce in size?
The A band does not reduce, while the H zone and I band are reduced in size
How is muscle contraction stimulated?
By a message from the somatic nervous system
What is it sent via?
A motor neuron
What is the link between the nerve terminal (synaptic bouton) and the sarcolemma of the muscle fiber?
The neuromuscular junction
The space between the two is called what?
The synapse or synaptic cleft
What does depolarization of the motor neuron result in?
The release of neurotransmitters such as acetylcholine from the nerve terminal
What does the neurotransmitter do?
It diffuses across the synaptic cleft and binds to special receptor sites on the sarcolemma
What happens if a lot of these receptors are stimulated?
The permeability of the sarcolemma will be altered and an action potential is generated
What happens once an action potential is generated?
It is conducted along the sarcolemma and the T system, and into the interior of the muscle fiber
What does this cause?
It causes the tropomyosin strands to shift
What does the Ca2+ bind to?
The troponin molecules
What does this expose?
The myosin-binding sites on the actin filaments
What do the free globular heads of the myosin molecules do?
They move toward and bind to the exposed binding sites on the actin molecules
What does this form?
It dorms actin-myosin cross-bridges
What do the cross-bridges cause?
They cause the myosin to pull on the actin molecules, drawing the thin filaments toward the center of the H zone and shortening the sarcomere
What does ATPase activity in the myosin head provide?
Energy for the power stroke that results in the dissociation of the myosin head from the actin
Then what happens?
The myosin returns to its original position and is now free to bind to another actin molecule and repeat the process, thus further pulling the thin filaments towards the center of the H zone
What happens when the sarcolemmic receptors are no longer stimulated?
The Ca2+ is pumped back into the sarcoplasmic reticulum
What happens to the products of ATP hydrolysis via ATPase?
The products are released from the myosin head, and a new ATP binds to the head, resulting in the dissociation of the myosin from the thin filament, and the sarcomere returns to its original width
What happens in the absence of Ca2+?
The myosin-binding sites on the actin are again covered by tropomyosin molecules, thereby preventing further contraction
What happens after death?
No ATP is produced, so the myosin heads can’t detach from actin, and therefore the muscle cannot relax
What is this known as?
Frequency summation
What do individual muscle fibers exhibit?
An all-or-none response
What elicits contraction?
A stimulus above a minimal value called the threshold value
Can the strength of the contraction of a single muscle fiber be increased?
Does whole muscle exhibit an all-or-none response?
Is there a minimal threshold value needed to elicit a muscle contraction?
Can the strength of the contraction increase as stimulus strength is increased?
How does this happen?
By involving more fibers
What is a maximal response?
The response reached when all fibers have reached the threshold value and the muscle contracts as a whole
What is simple twitch?
The response of a single muscle fiber to a brief stimulus at or above the threshold stimulus
What is the latent period?
The time between stimulation and the onset of contraction
During this time lag, what occurs?
The action potential spreads along the sarcolemma and Ca2+ ions are released
What happens following the contraction period?
There is a brief relaxation period in which the muscle is unresponsive to a stimulus
What is this period known as?
The absolute refractory period
What is this followed by?
A relative refractory period
What is this?
The period when a greater-than-normal stimulus is needed to elicit a contraction
What happens when a muscle is exposed to very frequent stimuli?
The muscle cannot fully relax
What happens then?
The muscle contractions begin to combine, becoming stronger and more prolonged
When do the contractions become continuous?
When the stimuli are so frequent the muscle cannot relax
What is this type of contraction known as?
Is it stronger than simple twitch of a single fiber?
What happens if tetanization is prolonged?
The muscle will begin to fatigue
What is smooth muscle responsible for?
Involuntary reactions
What innervates it?
The autonomic nervous system
Where is smooth muscle found?
In the digestive tract, bladder, uterus, and blood vessel walls, among other places
How many nuclei do smooth muscle posses?
Where is it located?
Centrally in the muscle cell
Does smooth muscle cell contain actin and myosin filaments?
What do they lack?
The smooth organization of skeletal sarcomeres
What is the result of this?
They lack the striations of skeletal muscles
How do smooth muscle contractions result?
From the sliding of actin and myosin over one another
How is it regulated?
By an influx of calcium ions
Are smooth muscle contractions faster or slower than skeletal?
Can they be sustained longer than skeletal muscle contractions?
Does smooth muscle have inhibitory, excitatory or both types of synapse?
They have both
What do they regulate?
They regulate contraction via the nervous system
What is myogenic activity?
A property of smooth muscle enabling it to reflexively contract without nervous stimulation
What is the muscle tissue of the heart composed of?
Cardiac muscle fibers
What characteristics do these fibers possess?
Those of smooth and skeletal fibers
Do they have actin and myosin fibers arranged in sarcomeres?
What appearance does this give cardiac muscle?
A striated appearance
How many nuclei do cardiac muscle cells contain?
One or two centrally located nuclei
How is cardiac muscle innervated?
By the autonomic nervous system
What does this serve to do?
Modulate its inherent beat, since cardiac muscle, like smooth muscle, is myogenic
What can high-energy compounds such as fatty acids, glycogen, and glucose be degraded in muscle cells into?
Energy can temporarily be stored in high-energy compounds called what?
Creatine phosphate
When is creatine phosphate produced and how?
During resting periods its produced via a reaction that transfers a high-energy phosphate group from ATP to creatine
What happens during exercise?
The reaction works in reverse, re-synthesizing ATP from creatine phosphate and ADP
What does this do?
Replenishes the ATP supply without the need for additional oxygen
What is myoglobin?
It is a hemoglobin-like protein found in muscle tissue
Does myoglobin have a high or low O2-affinity?
A very high, higher than hemoglobin
What happens during strenuous exercise?
Muscle cells rapidly run out of available O2, so myoglobin releases its O2
In this way myoglobin acts as what?
An additional oxygen reserve for active muscle
During strenuous exercise, the oxygen supply to muscles may be insufficient o meet its energy demands, despite the extra O2 supplied by myoglobin. What happens then?
During this period the muscle obtains additional energy via anaerobic respiration
What does this result in?
The build-up of lactic acid
What is the function of connective tissue?
To bind and support other tissue
What is loose connective tissue, where is it found?
It binds epithelium to underlying tissues and is the packing material that holds organs in place. It is found all over the body.
What 3 types of proteinaceous fibers does it contain?
Collagenous fibers, elastic fibers, and reticular fibers
What are collagenous fibers?
They are composed of collagen and have great tensile strength
What are elastic fibers?
They are composed of elastin and endow connective tissue with resilience
What are reticular fibers?
Highly branched, tightly woven fibers that join connective tissue to adjoining tissue
What are two major cell types in loose connective tissue?
Fibroblasts and macrophages
What are fibroblasts?
They secrete substances that are components of extracellular fibers
What are macrophages?
They engulf bacteria and dead cells via phagocytosis
What is dense connective tissue?
Connective tissue with a very high proportion of collagenous fibers
How are the fibers organized?
Into parallel bundles that give the fibers great strength
What does dense connective tissue form?
Tendons and ligaments
What do tendons do?
They attach muscle to bone
What do ligaments do?
They hold bones together at joints
What is locomotion dependent on?
The interactions between the skeletal and muscular systems
If a given muscle is attached to two bones, will contraction of a muscle cause only one of the two bones to move or both?
Only one will move
The end of the muscle attached to the stationary bone is called what?
The origin
In limb muscles it corresponds to what end?
The proximal end
The end of the muscle attached to the bone that moves during contraction is called what?
The insertion
In limb muscles, the insertion corresponds to what end?
The distal end
What are antagonistic muscle pairs?
One muscle will relax while the other contracts
What is an example of this?
Your biceps and triceps
When you move your hand toward your shoulder, what happens?
The bicep contracts and the tricep relaxes
When you move your hand down again, what happens?
The bicep relaxes and the tricep contracts
What are synergistic muscles?
They assist the principal muscle during movement
What is a flexor muscle?
It will contract to decrease the angle of a joint
What is an extensor muscle?
It contracts to straighten a joint
What is an abductor?
It moves a part of the body away from the body’s midline
What is an adductor?
It moves a part of the body toward the midline

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