Flashcards in Human Bone, Muscle and Joints Deck (215):
What are the two meanings of 'bone'?
It can refer to bone the organ: they are made up of different types of tissue
It can refer to bone the tissue: one of the tissues found within bone the organ
What are the 6 functions of the skeletal system?
- Calcium and Phosphate reserve
- Fat Storage
Why is the skeletal system necessary for support?
Softer tissue would deform on its own
How does the skeletal system offer protection?
Ribs etc. keep internal organs safe from harm
How does the skeletal system help wth movement?
It provides attachments for muscles, which pull on the bones to move them
Why is the skeletal system a crucial Calcium and Phosphate store?
It maintains the baseline Ca and P levels of tissue. 99% of calcium is stored in the skeletal system, for strength in hydroxyapatite, muscle contraction etc. while P is necessary for formation of cell membranes, DNA and energy (ATP)
Where does haemopoiesis occur and why is it important?
It occurs in bone marrow, to manufacture red blood cells. It is mainly found in the axial skeleton, and in the spongy bone of some bones close to the axial skeleton, like the top of the femur and humerus, as well as the hips.
How does the skeletal system help with fat storage?
Yellow bone marrow, normally within appendicular skeleton, is within the bone cavities.
How does the number of bones we possess change over our lifecourse?
We are born with 270 bones, which drops to an average of 206 when we reach adulthood. From there, it decreases further as we age.
What is included in the axial skeleton?
Spine, head, ribs
What is the approx. number of bones in the axial skeleton?
About 80, some of which are paired
What is the function of the axial skeleton?
What is included in the appendicular skeleton?
Hips, legs, shoulders and arms
What is the approx. number of bones in the appendicular skeleton?
126 (all paired)
What is the function of the appendicular skeleton?
What are the 3 areas of long bones?
What is the epiphysis?
The part of the bone that articulates with a neighbouring bone
2 on each side of long bones
Contains spongy bone as it experiences force from many directions, able to get away with a thinner shell as there's less direct pressure on it, but it needs trabeculae to support it.
What is the epiphyseal line, where is it and what is it made of?
It is found between the epiphysis and the metaphysis, and is made of hyaline cartilage. It's where the bone used to grow from before it sealed.
What is the metaphysis?
The area of bone between the epiphysis and the diaphysis. Each long bone has 2.
What is the diaphysis?
The main shaft of the long bone, with only 1 per bone. It has a very thick layer of compact bone as its forces run parallel down the shaft of the bone, trying to compress it.
What are the two types of bone in long bone?
Where is compact bone found?
Mainly within the diaphysis, where it is thickest.
A very thin layer surrounds the spongy bone in the ends of the bone.
What are the components of bone in the diaphysis?
- Sharpey's fibres
- Bone layer
- Medullary Cavity
+ Blood vessels & nerves
What is the periosteum?
A fibrous connective tissue sheath which surrounds the outside of the bone, and contains blood vessels and nerves.
It covers almost all bone surfaces- except articulating areas of bone and some hand and foot bones
What are sharpey's (perforating) fibres?
They are collagen fibres which blend with the collagen in the bone, which strongly link the periosteum and the compact bone. When a bone breaks, sharpey's fibres do not: that's how strong they are!
What role does compact bone play in a bone?
It forms the outer shell of the cylinder, as this shape makes it strong, but light
What is the endosteum?
The thin fibrous connective tissue layer lining the medullar cavity on the inner surface of compact bone. All inner bone structures are covered with it, and it is the same material as periosteum
What is the medullary cavity?
It's the cavity within the diaphysis, which holds the bone marrow. The position of the bone within the body determines which type of bone marrow it will hold.
Where do blood vessels pass in compact bone?
They are within the periosteum, and pass through sharpey's fibres into the haversian canals of osteons in the compact bone. They open into the medullary cavity.
What are the components of bone in the epiphysis?
Spongy Bone with trabeculae and medullary cavity
+ blood vessels and nerves
Where do the blood vessels reach within epiphyseal bone? Why is it important to have blood vessels here?
Inside the compact bone and between trabeculae. If the tissue is haemopoietic, it is necessary for transporting the new RBCs out of the bone and into blood flow.
What are trabeculae?
A component of spongy bone covered with endosteum. They form a network of little rods.
Why are trabeculae important?
They are crucial for withstanding pressure from all directions
They aren't randomly arranged: they radiate from the thinner cortex to the diaphysis along lines of most common pressure.
What is bone (tissue) made of?
What is the function of cells within bone tissue?
Put down and maintain bones
What is the function of the ECM in bone tissue?
It determines the properties of bone.
What are the two components of the ECM in bone?
Fibres (organic) and ground substance (inorganic)
Fibres make up approx. 1/3rd of the dry weight of bone, and ground substance about 2/3rds.
What determines whether something is organic or inorganic?
Organic is H2O, fibres and cells
Inorganic is everything else
What are the fibres in bone tissue?
Collagen: Types 1 (very strong) and 4
What is the function of collagen fibres?
They are like string: They resist tension in terms of stretch and pulling
What makes up the ground substance of bone tissue?
Hydroxyapatite: a crystalline salt (with large amounts of Ca and P, thus their storage in bone tissue)
What is the function of hydroxyapatite?
Like concrete: resists compression in terms of squeezing and crushing
How do collagen and hydroxyapatite work together in terms of bone properties?
One resists stretching, while the other resists crushing: together, they resist torsion, a combination of compression and stretching (when you try to bend something to break it). Therefore, bone is VERY strong
What are the 4 different cell types in bone?
What is the most prevalent cell type in bone?
What is the precursor of osteogenic cells?
Unspecialized stem cells- mesenchyme from embryonic connective tissue- some remains in red bone marrow
Where are osteogenic cells found?
The surface of bone under the periosteum/endosteum
Also found in the canals of compact bone
What is the function of osteogenic cells?
They are normally dormant, but may divide and form bone-forming cells due to chemical signals- however, some remain as stem cells.
What is the precursor cell of an osteoblast?
Where are osteoblasts found?
Only in active bone tissue- usually a layer under the peri/endosteum, wherever new bone is being formed
What is the function of an osteoblast?
Synthesis, deposition and mineralization of osteoid
Which cell transition can be reversed in bone?
Osteogenic cells can become osteoblasts and vice versa
What is osteoid?
The organic part of the extracellular matrix (collagen fibre), synthesized by osteoblasts before they add the ground substance. It accounts for 70% of osteoid, with the remainder being proteoglycans and other proteins.
What is calcification?
The infiltration of osteoid with bone salts (hydroxyapatite) by osteoblasts. It is similar to pouring concrete over a network of steel fibres.
This displaces any water within the matrix, making the bone strong and dense. As it is so dense, fluids can't diffuse through it easily, relying on osteocytes
What is the precursor of osteocytes?
What is the anatomical structure of an osteocyte?
The main cell body sits within gaps called lacunae, and have processes called canaliculi.
Where are osteocytes found?
They are trapped within the lacunae of bone, and can communicate with other cells only through their cellular processes (canaliculi).
What is the function of the osteocyte?
Maintenance of bone tissue
How do osteoblasts become osteocytes?
When they become trapped by the lamellae (layers) of bone they are forming).
What is the precursor of the osteoclast?
Monocytes and progenitor cells, which fuse together into a large mass. As they have so many cells fused, they have many nuclei and are very large
Where are osteoclasts found?
Sides where bone resorption is occurring, mainly in the endosteum and periosteum.
What is the anatomy of the osteoclast?
They sit in the areas they have dissolved, called howship's lacunae.
They have a ruffled border and a clear zone on the edges of their lacunae
What is the function of an osteoclast?
It secretes acid to dissolve hydroxyapatite, and enxymes to destroy collagen, in order to dissolve bone.
What is the lifespan of an osteoclast?
Short, as they don't want to be running rampant
What is the function of the osteoclast's ruffled border?
Increases the surface area for secretion
What is the function of the clear zone of an osteoclast?
It suckers to the edges of howship's lacunae, to prevent enzymes escaping to where they aren't needed
What is a syncytium?
The fusion of many cells to form a single cell
How can you tell whether a bone is resting or active?
If osteoblasts are present beneath the osteogenic cells, it is active. If not, it is resting.
How does bone grow?
Appositional growth: adding new bone onto an existing surface
Why does bone grow via appositional growth?
bone tissue is too rigid to grow by interstitial growth (where cells divide and secrete more ECM, growing the tissue from within.
What is bone remodelling?
Comprised of 2 processes: Appositional growth and bone resorption
These processes occur independently of each other
Where can appositional growth occur?
Peri and Endosteum, as well as in the haversian canals of compact bone
What is bone resorption?
The dissolution of bone
How do bones grow in diameter?
Appositional growth on the outer surface
Bone resorption on the inner surface
Why does bone resorption occur on the inner surface when a bone grows in diameter?
It is done to remove unnecessary weight.
What are the steps of appositional growth in bone?
1. Tissue is resting, no osteoblasts
2. Periosteum becomes active, and osteogenic cells divide into osteoblasts. These settle on the surface of bone and are very plump with secretory cells, ready to deposit osteoid. They then start putting down osteoid.
3. Some osteoblasts become trapped in their lacunae, eventually becoming osteocytes. They maintain contact with osteogenic cells and the bone surface via canaliculi.
4. Growth stops and osteoblasts remaining revert to osteogenic cells or die. The osteoid becomes fully calcified over time and the tissue returns to resting.
What are the steps of bone resorption?
1. Bone tissue is resting
2. Osteocytes and chemical signals cause Monocyte precursor cells to leave nearby blood vessels and fuse together on the bone surface.
3. Osteoclasts form, and dissolve bone
4. Osteoclasts die via apoptosis, stopping resorption
5. Blood vessels grow into the new spaces created by osteoclasts, the tissue returns to its resting state.
How does bone diameter growth change as we age?
We get less appositional growth, and more resorption.
How do long bones grow in length?
Called endochondral ossification
Hyaline cartilage of the epiphyseal plate grows, but cartilage adjacent to the epiphysis forms bone: like they race each other. When they meet, the epiphyseal plate is closed and growth stops
What is rickets?
Bones don't calcify fast enough due to a vitamin D deficiency, meaning Ca2+ can't be absorbed through the gut wall. This makes bones rubbery. When they're calcified, they're often bent.
What is lamellar bone?
Also called mature bone
What are lamellae?
Sheets of bone tissue in which the collagen fibres all extend along the same orientation. However, when putting down bone tissue, osteoblasts change the direction of the collagen's arrangement periodically- they can rotate up to 90 degrees from the previous sheet
Why are lamellae rotated?
It means the bone is able to withstand forces from different directions, making it stronger. As collagen is the only fibre resistant to tension, if they were all oriented in the same way they could easily be snapped along their parallel surface
What is woven bone and its properties?
It is immature bone, made of collagen in wavy and random patterns. It's not very strong and not good at resisting forces. It's prevalent from development to 3years old
How far away are cells from the nearest blood supply and why? What does this mean for bone width?
Osteocytes can never be more than 2 mm away from a blood vessel as the nutrients have to be shared from outer osteocytes to inner osteocytes via canaliculi- the bone is too dense for diffusion
This means that trabeculae in spongy bone are never more than 4mm wide. Compact bone can be wider as it has blood vessels within it.
How much of the skeletal system is spongy bone?
About 10% (but this differs depending on where in the body the bone is.
What separates lamellae?
Lines of osteocytes
What are some features of spongy bone?
It is better for compression, and changes its composition depending on the function of the bone
It has a high blood supply
This means that osteoclasts have easy access to spongy bone, and the high SA of trabeculae means it is often degraded- 5x more frequently than compact bone!
What causes osteoporosis?
Estrogen is necessary to inhibit the action of osteoclasts. When women go through menopause, their estrogen drops, and osteoclasts are less inhibited.
How can you tell the direction of bone growth based on canaliculi?
Canaliculi reach towards the surface they grew from- eg. periosteum or endosteum- as they maintain contact with the surface when they are covered by bone tissue.
What is the defining feature and main unit of compact bone?
What are the structures present in compact bone?
Periosteum + blood vessels
What is interstitial lamellae?
Bone from old osteons which has been partially replaced by new osteons: it now makes up the space between new osteons and is dead
What is circumferential lamellae?
A few layers of bone underneath the periosteum which makes up the outer edge of compact bone and contains no osteons. This is because it hasn't yet had to surround a blood vessel during appositonal growth.
What makes up an osteon?
A central/haversian canal containing blood vessels and nerves, which is lined with endosteum
Concentric (widening) layers of lamellae, in an alternating orientation (it runs parallel to the bone)
What is the purpose of having osteons?
As they contain blood vessels, compact bone can be thicker as there are more points at which osteocytes can still survive
What are perforating/volkman's canals?
Blood vessels meeting up with those in the osteons, but they run perpendicular to the bone
How do collagen fibres change as we grow?
Their orientation is adaptable- as we put new stress directions on our bones, the collagen will change direction (eventually) to cope
What are the two types of osteon?
What is a primary osteon?
An osteon which formed around an existing blood vessel on the surface of the bone (normally periosteum) as the bone grew and tissue was deposited around it
What are the steps of forming a primary osteon?
1. In the active periosteum, osteoblasts put down new tissue either side of a blood vessel, while those underneath the blood vessel slow their growth.
2. As the ridges grow, they come together and fuse, forming a tunnel around the blood vessel. The periosteum around the vessel is now endosteum
3. Osteoblasts in the new endosteum build concentric lamellae onto the tunnel walls, filling it in toward the vessel
4. The bone itself continues to grow above the vessel in circumferential, becoming concentric as they form around new blood vessels
What are secondary osteons?
They are created through existing bone tissue.
What are the steps of creating secondary osteons?
1. A group of osteoclasts gather where bone needs to be remodelled, and begin boring their way through bone. This area is called the cutting cone, creating a tunnel.
2. Osteoblasts move in where the osteoclasts were, lining the tunnel wall with endosteum, and putting down osteoid.
3. Concentric lamellae are put down, and a small blood vessel grows into the tunnel to supply the osteoblasts. This is called the closing cone, moving behind the cutting cone. As osteoblasts get trapped, they become osteocytes
4. The tunnel is now a regular haversian canal, with remaining osteoblasts reverting or dying.
How is secondary osteon formation initiated?
Osteocytes release signals that they can't reach a blood supply causing vessels in bone to release monocytes.
What is a cement line?
The line at the junction between the outermost lamella of a secondary osteon and the old bone, consisting of glycoproteins.
In which direction do new osteons form?
Parallel to the direction of force on the bone.
What is the main unity of spongy bone?
How is the main unit of spongy bone formed?
Grows outwards into the medullary cavity
Where is spongy bone found?
Inside bones, in the epiphysis of long bones
How does spongy bone get its blood supply?
From the outside in: it is surrounded by blood vessels in the medullary cavity
What is the function of spongy bone?
To support the outer cortex of compact bone where forces occur from multiple directions
Reduction in bone weight
Rapid Ca and P turnover for homeostasis, as it's remodelled 5x faster than compact
What is the main unit of compact bone?
How are the main units of compact bone formed?
Grows inward to form a haversian canal
Where is compact bone located?
Outer shell of bones
Diaphysis of long bones
How does compact bone get its blood supply?
From the inside out: Blood from Haversian and Volkmann's canals
What is the function of compact bone?
Provides strong, dense shell of bone on the outside, thickening in areas exposed to large forces.
What is the other name for joint?
What is the prefix for joint things?
What is a joint?
Any point at which two or more bones interconnect
What are the functions of joints?
- Movement: determined by the bone shape, and its surrounding tissue and fat
- Force transmission: they are the skeletal system's weak point
- Growth: Eg. fusion of cranial bones in infancy
What are the 3 classifications of joints?
What is a synarthrosis?
Commonly axial (eg. skull)
What is an amphiarthrosis?
Slightly moveable joint
Eg. vertebral discs (but movement decreases the further down the column)
What is a diarthrosis?
A freely moveable joint with low stability and high movement. 80% of appendicular skeleton is diarthrosis
What is the issue with diarthroses?
They are most injured, as they can't have high stability
What do synarthroses and amphiarthroses have in common?
their flexibility is determined by how loose the tissue holding them together is (diarthroses are not determined by tissue)
What is ankylosis?
Bone fusion, most commonly done in synarthroses
What are synovial joints?
A structural classification of diarthrosis, not restricted by the properties of the tissue holding the bones together
The ends of articulating bones are mostly free, to allow much motion
What are the 4 common features of all synovial joints?
1. Articular cartilage: smooth and deformable
2. Articular capsuse (fibrous later and synovial membrane)
3. Joint cavity (containing):
4. Synovial fluid (v. small amount)
What is articular cartilage?
A specialized hyaline cartilage, which protects the ends of bones
It is thin, 1-7mm thick, attached to the bone
Highly specialized, lasts most of a lifetime
What is the function of articular cartilage?
Absorbs microshock from movements
Supports heavy loads for long periods of time
Smooth, near frictionless surface with synovial fluids
What is arthritis?
The degradation of articular cartilage
What is the makeup of cells vs. ECM in articular cartilage, and what does this mean for the tissue?
Only 5% cells, so very slow building and repair
What are the cells in articular cartilage called, and what is their function?
- Build, repair, maintain cartilage
- Live in lacunae
- Depending on zone, they occur alone or in groups (nests)
What is the ECM of articular cartilage made up of?
Ground substance: Water & soluble ions, GAGs and Proteoglycans. Water & ions make up 75% of tissue
Fibres: Type II collagen (firm and supple). Makes up 75% of dry weight
What is the function of H2O and ions in articular cartilage's ECM?
It makes up the fluid phase, able to move in and out of the tissue and take dissolved ions and gasses with it
What is the function of the collagen fibres in articular cartilage's ECM?
Provides structural integrity
Divides it into zonation patterns
Part of the solid phase fixed within the tissue
What is the function of the GAGs and PGs in articular cartilage's ECM?
Provides swelling and hydrating mechanism for the function of cartilage, as they're hydrophobic
Forms part of the solid phase fixed in the tissue, as they're bound to collagen.
What are the layers of articular cartilage?
What proportion of cartilage does the surface zone make up?
What proportion of cartilage does the middle zone make up?
What proportion of cartilage does the deep zone make up?
What is within the functional layer of cartilage?
Surface, middle and deep zones
What is within the transitional layer of articular cartilage?
Between tide mark to osteochondral junction, with calcified cartilage in between
How does a fibre of cartilage extend through articular cartilage?
It's attached at the osteochondral junction, extending up and curving into the surface zone.
What is the PG content of the different layers of articular cartilage?
M->D: Cartilage increases
Tide Mark: Calcified (low)
Calcified Cartilage: Low pg, high hydroxyapatite
OC junc: cement line
How does the arrangement of collagen change through the layers of articular cartilage?
SZ: Flat, feltlike
Rest: Vertical until cement line
What is the cell type through the layers of articular cartilage?
DZ: Circular in columns (nests)
CC: Circular, in lacunae (calcified) producing hydroxyapatite
Why is there a cement line?
For stronger adherence to bone
What are the details about blood, nerve, lymph in articular cartilage?
There are no blood vessels, lymphs or nerves. Therefore it appears white. Chondrocytes are nourished by diffusion only, leaving greater spaces between cells than in other tissues.
What happens to cartilage as bone moves?
It is compressed as bones are pressed together, and swells when the pressure is relieved.
What are disaccharides?
Double ring sugars, each missing a proton and giving them a negative charge.
They are the fundamental unit of GAGs.
What are GAGs? + 2 examples
A repeating disaccharide unit, with negative charges.
Eg. Chonroitin Sulphate and Keratin Sulphate
What is a proteoglycan?
Many glycosaminoglycans attached to a protein core (eg. aggrecan).
What is the composition of aggrecan?
about 50 Keratin Sulphate and 125 Chondroitin Sulphate GAGs attached to a protein core.
Why are the negative charges of GAGs important in terms of proteoglycans?
The negative charges of the GAGs repel one another, causing them to stick out from the core like bristles. This means it can act like a spring- the more pressure on it, the more they repel, creating resistance and allowing it to bounce back
What is a large proteoglycan complex?
Proteoglycans attached to a long hyaluronic acid chain.
These attach to collagen fibres in the middle an deep cartilage zones
What are the steps in the loading cycle of articular cartilage?
1. Recently unloaded cartilage
2. -ve charges on proteoglycans attract positive ions into the joint space, increasing its ion concentration
3. Due to the increase in osmolarity, water is drawn into the matrix and the cartilage begins to swell
4. The swelling of the cartilage places the collagen under increasing tension, which continues until swelling fource = tensional force, and the cartilage stops swelling (unloaded equilibrium)
5. When load is introduced, the fluid phase of water and +ve ions is squeezed back into the joint space and synovial fluid, as well as other areas of articular cartilage
6. This reduces the volume of the cartilage via creep, pushing the nevative charges closer. Eventually, the load will be supported only by the solid phase and the repulsion of negative charges. This means the cartilage stops shrinking (loaded equilibrium)
What ions enter the articular cartilage due to negative charges?
What enters the articular cartilage during swelling due to the increased osmolarity?
What leaves the articular cartilage when it's squeezed out by a load?
How does articular cartilage lubricate itself?
When the fluid phase is squeezed out of compressed cartilage, it is able to provide some lubrication
How does the loading cycle improve the health of articular cartilage?
Loading and unloading it flushes out the waste and brings in more nutrients to cartilage, allowing to to repair and grow.
What role does the surface zone play in the loading cycle of articular cartilage?
It prevents the fluid phase from immediately rushing out when the cartilage is placed under pressure. It acts like a filter, retarding the flow due to its feltiness.
This allows the initial load to be supported by the fluid phase as well as solid.
(kind of like trying to squeeze air out of an airbed)
What is the articular capsule?
Forms the sleeve around the joint, connecting the ends of the articulating bones.
Needs to be loose enough to allow movement and function, but does become tight at the limits of its motion (protective)
Perforated by blood vessels and nerves, and may be reinforced by ligaments (DICT connecting bones)
What are the two layers of the articular capsule?
What is the fibrous layer of the articular capsule?
Outer layer of dense connective tissue, which can be regular or irregular depending on the direction of its forces
Variable thickness- thick sections may be called capsular or intrinsic ligaments
Made up of parallel and interlacing collagen bundles.
Continuous with periosteum of the bone.
Poor vasculature but many nerves
What is the function of the fibrous layer of the articular capsule?
Resists tension, abnormal joint movement
Supports synovial membrane
Protects SM and whole joint
Thick areas are for heavy-duty support, thin areas just maintain the joint space
Nerves allow you to detect damage and do proprioception
What is the synovial membrane of the articular capsule?
An inner layer of loose connective tissue of varying thickness
Lines all non-articular surfaces within the joint cavity, up to the edge of the articular cartilage
2 layers: Subintima and Intima
Contains fat cells for cushioning and reducing the volume of fluid needed within it
Contains blood vessels directly interfacing with the synovial fluid
Contains macrophages (which can move into synovial fluid)
Fibroblasts to secrete collage and elastic fibres (allow stretching)
Have folds called villi which increase the surface area
What is the intima?
A very thin layer of cells, called synoviocytes.
Secrete Hyaluronic acid, GAGs, proteoglycans for lubrication, filter and release blood plasma
What is the subintima?
A thicker layer of loose collagen and elastic tissue, between the intima and fibrous layer
What is the joint cavity?
The small area between articulating surface, with peripheral margins including the synovial membrane's villi.
It is very small for quicker diffusion from synovial membrane to cartilage
What is synovial fluid?
A clear or yellowish ultrafiltrate of blood plasma that leaks out of the subintima
Also includes secretions from the synoviocytes (PGs, HA, GAGs)
Only about 1ml in large joints
Lubricates, absorbs shock, chondrocyte metabolism and maintains joints.
What is the function of muscle?
To convert ATP into mechanical energy
How does muscle operate?
Moves different parts of the body by pulling on another tissue
What sub-functions does muscle serve?
Control of body openings
How does muscle allow movement?
It pulls on bones, or contracts to move gut and lymph contents or blood
How does muscle allow stability?
It prevents the joint capsules from getting too loose
It is especially important for joints with a wide range of movement
What is active vs passive stabilization?
Muscle performs active stabilization of joints as it is actively using energy to hold them in place.
The articular capsule performs passive stabilization as it doesn't actively use energy
How does muscle allow communication?
Used for facial expression, body language, writing and speech
How does muscle control body openings?
It forms sphincters for admission of light and food and drink
It also controls elimination of waste via urethral and anal sphincters
How does muscle allow heat production?
It produces up to 85% of body heat
What is the anatomy of a skeletal muscle?
Attached to bones by tendons at either end
Muscle belly in the middle
Junction between tendon and bone is osteotendinous junction (OTJ)
Junction between muscle and tendon is Myotendinous junction (MTJ)
What are the origin and insertion when a muscle moves?
Origin is the attachment that moves the least during muscle contraction (Usually more axial)
Insertion is the attachment that moves the most during muscle contraction (susually appendicular)
What are myofibrils?
Organelles made up of many sarcomeres
Attached by their Z discs
Contain A bands (thick filaments- dArk)
Contain I bands (no thick filaments- light)
What is a myocyte?
A muscle fibre made up of a bundle of myofibrils
Contain many nuclei per cell
Sarcoplasm surrounds myofibrils. This contains mitochondria, glycogen, lipids, myoglobin (for O2 storage)
Most sarcoplasm is at the periphery
Sarcolemma (membrane)- specializd for rapid AP conduction
T tubule infolds to conduct APs into the cell
What is a fasicle?
A bundle of myocytes
Surrounded by endomysium (loose connective tissue containing nerves and capillaries to supply myocytes, made of collagens, proteoglycans and fibroblasts
There is a thin basement membrane surrounding muscle fibres that blends with the endomysium, secreted by fibroblasts and myocytes
What is a muscle?
A bundle of fascicles
DICT perimysium surrounds fasicles
DICT Epimysium surrounds and is continuous with the perimysium and the rest of the muscle
What is the order of tissue layers from outer to inner in muscle?
Deep Fascia (not part of muscle, but forms the fibrous cover over it)
What does deep fascia do?
It groups muscles supplied by the lame nerves or which have a similar action into compartments.
It makes up the compartment's outer sleeve and walls of the compartment
The separating walls are called investing fascia
Where investing fascia contacts bone it blends with the periosteum
What is an intermuscular septa?
An investing fascia layer separating two muscle compartments
What is an interosseous membrane?
A layer of investing fascia connecting two bones
How does deep fascia aid venous return?
By confining muscles (and their veins) into compartments, when tension is placed on the muscle it is transferred to the vein, allowing the skeletal muscle pump to operate due to the resistance of the compartment wall
(all muscles in the same compartment are supplied by the same vessels)
What is compartment syndrome?
Continuous swelling of muscles within a compartment causes edema and pain. This can occur due to putting down muscle too fasts, requiring a fasciectomy to relieve its swelling
Why can't muscle fibres divide?
They are too long, and have too many nuclei for DNA division to work.
What is hypertrophy?
The increase in muscle size due to the increase in size of myocyte rather than number of myocytes.
This occurs as more myofibrils are packed into the same myocyte, expanding it.
What stimulates hypertrophy?
Repetitive contraction of muscles to near maximum tension
Anabolic steroids (increase protein synthesis via target tissue in muscle and bone). Hoewver, these can also affect other tissues, causing liver failure, acne, hair loss, shrivelled testes, infertility, coronary artery disease susceptibility and mood swings.
What is atrophy?
A decrease in the size of myocytes due to loss of myofibrils
What stimulates atrophy?
Disuse of muscles
Diseases such as diabetes, cancer, aids (as patient not well enough to be active)
Normal atrophy starts after 20 yo, by the time we are 80 40% is lost, replaced by fat and CT, although some is lost by hypoplasia (death of myocytes)
However, some myocytes can get bigger- ie. if the nerve is cut.
What are satellite cells?
Also called myoblasts.
They lie between the sarcolemma and basement membrane, and when they fuse together (syncytium) they create myocytes.
They can divide and fuse with other statellite cells and existing myocytes to repair damage
They have a limited ability to replace muscle fibres that die from age or injury (although the number of fibres is pretty much set from birth)
What does myostatin do?
Myostatin turns off satellite cells. If they're inhibited, muscle grows so quickly that it can inhibit the lungs, heart, joints etc.
What is the function of the CT in skeletal muscle?
1. Organize and scaffold muscle
2. Provide access for blood vessels and nerves
3. Prevent excessive stretch and damage
4. Distribute forces from muscle contraction
What happens if a myocyte is cut?
It can still exert pulling forces on tendons
How does CT allow a cut myofibril to function?
As the A and I bands line up perfectly, desmin on the Z discs holds all the myofibrils together. Therefore, if one myofibril is cut, desmin transmits the still-contracting force to surrounding fibres
How does CT allow a cut myocyte to function?
Dystrophin (part of a protein complex) holds the peripheral Z lines to the sarcolemma, through the basement membrane to the endomysium. If the myocyte is cut, the force it is still generating is transmitted into the CT and tendon, causing the function to remain.
What is muscular dystrophy?
Disease where dystrophin is non functional or non present. Myocytes have a weaker sarcolemma that tears easily, resulting in eventual cell death
What is Duchenne muscular dystrophy?
The sarcolemma rips with every muscle movement, meaning that the heart and diaphragm eventually fail, leading to death. This is more common in males as the gene is found on the X chromosome
What elements make up a muscle compartment?
Periosteum of relevant bone
What are synarthroses commonly used for?
Growth of bone eg. cranial fusion
What is the innervation/vascularization of the 3 levels of the articular capsule?
Fibrous layer contains nerves and blood vessels
Subintima is highly vascular, but contains limited nerves (if any)
Intima has neither- it is made up of only 1-3 cell layers
What is the most abundant form of cartilage in the body?
Which zone of cartilage contains few proteoglycans, with elongated cells and parallel collagen fibres to the surface?
Which zone of cartilage contains 'nests' of chondrocytes, with fibres perpendicular to the surface and high proteoglycan content?
Which zone of cartilage contains round cells in calcified lacunae, with high hydroxyapatite and low proteoglycans, with perpendicular collagen?