human physiology 100 - musculoskeletal system

What is the haversian system?

It allows bone to be arranged around a blood supply (blood vessel)

How to long bones form?

Endochondral ossification:
1. Chondrocytes (stem cells) make cartilage
2. Growth of cartilage: cells in centre burst cause pH shift, triggering calcification
3. Primary ossification centre: nutrient artery penetrates centre of cartilage - bone mineral matrix covers the calcified cartilage forming spongey bone
4. Medullary cavity formation
5. Secondary ossification centre: blood vessels enter the epiphyses. Cancellous bone is formed but no medullary cavity
6. Formation of cartilage on the joints and epiphyseal plate

How do long bones grow?

- the growth plate forms new cartilage (mitosis) which the becomes bone in the metaphysis (underside) making the bone longer
- epiphysis is shifted up and metaphysis is shifted down to elongate the bone

How to flare bones grow?

Via intramembranous ossification

- form directly form osteoblasts and so not have a growth plate or cartilage stage

What components make up bone?

- organic collagen mix 10%
- mineral hydroxyapatite (insoluble salt of calcium and phosphorus) 65%
- water 25%
- grace amounts of magnesium, sodium and bicarbonate

What is collagen formed from?

- chains (shirt pieces of thread)
- triple helices ( strings, 3 chains)
- fibrils (like ropes)

What are the four bone cell types?

- osteoblasts
- osteoclasts
- osteocytes
- bone lining cells

What are osteoblasts?

- bone forming cells
- make the collagen chains that will form the mature organic collagen matrix
- control the deposition of mineral around new collagen matrix

What do osteoblasts do once they have finished making new bone?

- some become surrounded with matrix and differentiate into osteocytes
- some remain in the surface of new bone and become lining cells
- the rest undergo apoptosis

What are osteoclasts?

- are bone break down / reabsorbing cells
- large with many nuclei (50)
- form sealed compartments next to the bone surface and secrete acids and enzymes which degrade bone
- then undergo apoptosis

What are osteocytes?

- long lived cells (half life of 25 years)
- live inside the bone in cave called lucunae
- extend long branches (cannuliculi) to communicate
- sense any mechanical strain and micro fractures
- respond to remodel bone by secreting hormonal factors which regulates osteoblasts and osteoclasts

Why is it necessary to remodel bone?

- to repair micro fractures
- to heal major fractures
- to adapt the skeleton to the demands of the structure
- to supply mineral as required

What are the hormones that control bone metabolism?

- parathyroid hormone (PTH): released in response to low blood calcium
- calcitriol (active vitimin D): synthesised in response to high PTH levels

Both increase bone reabsorption

How to bones change with age?

- spongey bone structure heavily decreases in density
- bone formation declines: decrease in number of osteoblasts
- bone reabsorption increases: increase in number of osteoclasts

What is osteoporosis?

- condition in which the holes in the bones become larger
- imbalance of bone formation and bone reabsorption

What are the functions of the muscular system?

- body movement
- maintenance of posture
- respiration
- production of body heat
- communication
- control of blood flow in vessels

What is the overview of a muscle contraction?

1. Neural control: stimulus arrives at the neuromuscular junctions as an action potential
2. Production of an action potential in the sarcolemma (excitation)
3. Release of calcium ions
4. Contraction cycle begins
5. Sacromere shortening
6. Generation of muscle tension

What is the role of calcium in the contraction cycle?

- calcium ions cause exposure of myosin binding sites on actin
- calcium ions act as the 'on switch' for muscle contraction

What is the role of ATP in the contraction cycle?

- ATP provides the energy for the filaments to slide over one another
- ATP causes myosin heads to bind and swivel dragging the filaments along

What are the steps that initiate muscle contraction?

1. ACh is released at the neuromuscular junction and binds to ACh receptors on the sarcolemma
2. Action potential is generated and spreads across the membrane surface of the muscle fibre and along the T tubules
3. Sarcoplasmic recticulum releases stored calcium ions
4. Calcium binds to troponin, exposing the active sites on the thin filaments. Cross bridges form when myosin heads bind to those active sites
5. Contraction cycle begins as repeated cycles of cross bridge binding, pivoting and detachment occur- all powered by ATP

What are the six steps of the contraction cycle?

1. Contraction Cycle Begins
2. Active-Site Exposure
3. Cross-Bridge Formation: energised myosin heads bind to active sites
4. Myosin Head Pivoting
5. Cross-Bridge Detachment: another ATP binds to myosin head
6. Myosin Reactivation: free myosin head splits ATP —> ADP + Pi, the energy is used to recock the myosin head

What are the steps that end a muscle contraction?

6. ACh is broke down by acetylcholinesterase (AChe)
7. Sarcoplasmic recticulum reabsorbs calcium ions
8. Active site covered by tropomyosin
9. Contraction ends
10. Muscle relaxation occurs

What are the skeletal muscle contraction?

- isotonic: muscle length shortens to live load
- isometric: tension in muscle, no change in muscle length

What are the skeletal muscle fibres?

Slow twitch
- less powerful
- fewer myofibrils
- more mitochondria and myoglobin

Fast twitch
- most fibres
- powerful
- lots of myofibrils
- few mitochondria

What is a motor unit?

- somatic motor neuron plus all the muscle fibres it stimulates that function together
- one axon terminal on each fibre
- each motor neuron has terminals on many fibres

What are the effects of exercise on muscle?

Endurance
- increased number and size of mitochondria
- increased number of blood capillaries
- increased myoglobin in muscle tissue

Strength
- increased size of muscle fibres (increased number of myofibrils not muscle fibres)
- increased motor unit recruitment, increased force

What are the effects of exercise on bone?

- compression on bone increases bone formation

- osteocytes act as mechano sensors by responding to compression

- fluid that surrounds osteocytes and within canuliculi is compressed creating fluid shear stress in cells

- nitric oxide produced osteocyte that triggers osteoblasts to form collagen and bone mineral

What is wolfs law?

Bone reacts to the stresses put on it