Case 9 - Stress fractures Flashcards
What is the role of the menisci?
- Deepen articular surface of tibia (increases stability of knee joint)
- Act as shock absorbers by increasing the surface area
What is the patellar ligament a continuation of? Where does it attach?
Quadriceps femoris tendon, attaches to the tibial tuberosity
What is the role of the collateral ligaments?
Stabilise the knee by preventing excessive medial or lateral movements
Where do the 2 collateral ligaments attach?
Tibial (medial) collateral: from medial epicondyle of femur to medial condyle of tibia
Fibular (lateral) collateral: from lateral epicondyle of femur to fibular head
Where do the ACL and PCL attach?
ACL: anterior intercondylar region of tibia to femur intercondylar fossa
PCL: posterior intercondylar region of tibia to anteromedial femoral condyle
What are transverse tubules?
Invaginations of the sarcolemma
How is skeletal muscle derived embryologically?
Paraxial mesoderm > somite > myotome > skeletal muscle
What is a myofibril composed of?
Thin filaments = actin, bounded with troponin and tropomyosin
Thick filaments = myosin (chain, 2 globular heads)
What is epimysium, endomysium and perimysium?
Epimysium = Membrane surrounding muscle
Endomysium = Membrane surrounding individual muscle fibres
Perimysium = membrane surrounding a fasicle (group of muscle fibres)
What are I bands?
Light bands, only contain actin (and Z discs).
What are A bands?
Dark bands, contain myosin and actin (where actin overlaps myosin)
What are H bands?
Zone of thick filaments with no actin.
Which band disappears with contraction?
I bands, as actin overlaps myosin
What is the sarcoplasm comprised of?
It is intracellular fluid:
- Lots of myoglobin (oxygen-carrying molecule)
- Potassium, magnesium and phosphate ions
- Sarcoplasmic reticulum
- Protein enzymes
What is the role of titin and where does it attach?
Has an elastic end attaching to the Z disc (acts as a spring and changes length as the sarcomere contracts/ relaxes), and the other part attaches to myosin.
Role = provides strength to sarcomere
Describe the structure of actin
1 molecule of ADP = active site. Tropomyosin is wrapped around the active helix; at rest this covers the active sites. Troponin (I, T, C) is attached intermittently, forming a complex.
What are the 3 types of troponin and what do they have strong affinity for?
Troponin I: strong affinity for actin
Troponin T: strong affinity for tropomyosin
Troponin C: strong affinity for calcium
How many troponin complexes are there per actin filaments (roughly)? What is the significance of this?
1 complex per 7 actin
Allows for cooperativity, i.e. when troponin C binds to calcium, the signal is relayed and not every troponin complex requires calcium to switch actin ‘on’
What is a motor unit?
Muscle unit plus its motor neurone (axon)
What is a muscle unit?
Muscle fibres innervated by a single motor neurone
What is a motor neurone pool?
Collection of neurones innervating a single muscle
What is an innervation ratio?
The number of fibres innervated by a motor unit
What is the resting membrane potential of skeletal fibres?
around -80 to -90mV
Which neurone transmits the impulse to a motor neurone of the muscle?
Alpha motor neurone
Explain how the arrival of an impulses causes an end-plate potential
Impulse at muscle = opens voltage-gated Ca2+ channels, leading to calcium ion influx. Vesicle exocytosis of ACh into cleft
Opens ligand-gated ACh channels = Na+ in and K+ out
End-plate membrane depolarises, leading to an end plate potential
What enzyme breaks down ACh?
Acetylcholinesterase
How does an end plate potential lead to intracellular calcium release?
1: Depolarisation wave moves along the sarcolemma to the transverse tubules (close tot the sarcoplasmic reticulum)
2: Opens voltage-gated Ca2+ channels within the SR, leading to release of Ca2+ into sarcoplasm
3: Ca2+ influx activates ryanodine receptors in the SR (an intracellular Ca2+ store)
4: Increased intracellular Ca2+
Explain the process of a power stroke
1: Ca2+ binds to troponin C, which shifts tropomyosin and exposes the binding site for myosin
2: myosin can form a cross bridge with actin
3: ATPase in myosin head splits ATP into ADP + Pi + energy (power stroke)
4: ATP is used to break the bridge, so myosin can move further down
5: Ca2+ channels in SR close, Ca2+ active transport pumps (SERCA) use ATP to restore low levels of Ca2+ in sarcoplasm
6: troponin-tropomyosin rebinds to actin
What is hypertrophy and what is it caused by?
When muscle cells increase in size, due to satellite cells fusing and increased protein synthesis
What are satellite cells? What do they prevent?
Known as ‘skeletal stem cells’, which stay asleep until needed. They are post-mitotic so cannot proliferate, but self-renew and differentiate into new muscle.
They prevent nuclear dilution
Describe type 1 muscle fibres
Oxidative fibres: contracts slowly but never really fatigue due to its myoglobin store, which captures O2 and releases it when haemoglobin is insufficient
Describe the following characteristics for type 1 muscle fibres:
- Myosin
- Ca2+ pump rate
- Diameter
- Oxidative capacity
- Glycolytic capacity
- Fatigue
- Slow myosin
- Moderate Ca2+ pump rate
- Moderate diameter
- High oxidative capacity
- Moderate glycolytic capacity
- Resistant to fatigue
Describe type 2A muscle fibres
These are fast oxidative fibres, which are glycolytic. They don’t have as fast a contraction as type 2B but are fairly resistant to fatigue. Have a very high oxidative capacity too
Describe the following characteristics for type 2A muscle fibres:
- Myosin
- Ca2+ pump rate
- Diameter
- Oxidative capacity
- Glycolytic capacity
- Fatigue
- Fast myosin
- High Ca2+ pump rate
- Small diameter
- very high oxidative capacity
- High glycolytic capacity
- resistant to fatigue
Describe type 2B muscle fibres
Fastest muscle fibre, all glycolytic as they cannot deliver efficient oxygen in time. Produces lactic acid and fatigues fairly quickly
Describe the following characteristics for type 2B muscle fibres:
- Myosin
- Ca2+ pump rate
- Diameter
- Oxidative capacity
- Glycolytic capacity
- Fatigue
- Fastest myosin
- High Ca2+ pump rate
- Large diameter
- Low oxidative capacity
- high glycolytic capacity
- Non-resistant to fatigue
Which type fibres have a faster nerve conduction and why?
Type 2 = have a thicker myelin sheath
What is the effect of high-intensity aerobic training on:
- Alpha motor neurones
- Hypertrophy
- Mitochondria
- Muscle fibres
- Activation of muscle fibres
- Increased alpha-motor neurone firing frequency and conduction velocity
- increased hypertrophy so increased force
- More mitochondria = increased oxidative capacity
- Increased muscle fibres activated by each alpha neurone
- Preferential activation of type 2A
- Increased number of type 2a fibres