PBL 9

Question 1

What are the 3 layers of connective tissue that skeletal muscle is arranged within?

Answer 1

Epimysium - collagen fibres surrounding the entire muscle
Perimysium - divides the skeletal muscle into fasicles - bundles of fibre
Endomysium - surrounds the individual skeletal muscle fibres

Question 2

How are skeletal muscle fibres specialised for their function?

Answer 2

• Large
• Multinucleated
• T tubules - allow action potential to reach whole cell at once so whole region can contract at the same time

Question 3

What is a muscle cell triad?

Answer 3

Terminal cisternae plus T tubercle
Terminal cisternae - is an expanded area at the end of each sarcoplasmic reticulum in muscle cells. It contains calcium stores which are released at the beginning of muscle contraction

Question 4

What does the sarcomere A band contain?

Answer 4

M line - where thick myosin filaments connect
H band - contains only thick myosin filaments
Zone of overlap - a dark region where actin filaments are loacted between the thick myosin filaments

Question 5

What does the sarcomere I band (light band) contain?

Answer 5

Actin - extending from the A band of one sarcomere to the A band of the next sarcomere
Z line - marks the boundary between adjacent sarcomeres, constipation of proteins which interconnect the thin filaments
Titin - elastic protein that extends from the thick filaments in the A band to the Z line

Question 6

Which proteins comprise a thin filament?

Answer 6

F actin - twisted strand composed of two rows of G actin
Nebulin - exerts along F actin between G actin strands to hold them together
Tropomyosin - covers active sites on G actin (7 active sites)
Troponin - bound to tropomyosin

Question 7

What are the 3 subunits of troponin and what do they bind to?

Answer 7

Troponin T - binds to tropomyosin
Troponin I - binds to G-actin
Troponin C - has receptors that bind to calcium ions

Question 8

What is a motor unit?

Answer 8

A motor unit is a single lower motor neurons and all of the muscle fibres that it innervates

Question 9

What are the two types of lower motor neurons?

Answer 9

Alpha neruons - innervate extrafusal muscles (involved in strength and power)
Gamma neurons - innervate intrafusal muscles (involved in tone and tension)

Question 10

Explain ‘calcium induced calcium release’ within muscle cells

Answer 10

Action potentials arriving at motor end plate pass into T tubules which cause voltage gated calcium channels to open in T tubules, so calcium flows into sarcoplasm
Influx of calcium activates ryanodine receptor channels on the sarcoplasmic reticulum membrane, which then release further stored calcium into the sarcoplasm

Question 11

Explain the contraction cycle?

Answer 11

Arriving calcium at zone of overlap binds to troponin causing it to change shape and pull tropomyosin away from actin
Myosin with charged ADP+P binds to actin to form cross bridge
Power stroke occcurs where myosin head swings towards M line, ADP + P are lost
Binding of another ATP to myosin causes the cross bridge to detach
Myosin is reactivated by ATP hydrolysis ready restart the cycle
Cycle is inhibited when calcium is no longer present

Question 12

What causes the contraction cycle to end?

Answer 12

Ach is broken down by acetylcholinesterase
Action potential is no longer fired
Calcium is taken back up by the sarcoplasmic reticulum
Troponin can reattach to actin bringing tropomyosin with it to recover the actin binding sites

Question 13

What is the size principle of motor unit activation?

Answer 13

Small, slow motor fibre units are recruited first followed by fast large motor fibres:

Slow > fast resistant > fast fatigue

Question 14

What is fused tetanus

Answer 14

Where all the motor units are activates so all the muscle fibres are contracting at the same time. This is peak tension.

Question 15

What happens to motor units during sustained contraction

Answer 15

They are activating on a rotating basis, so some are resting and recovering while others are actively contracting

Question 16

What is muscle tone?

Answer 16

This is the contraction of some motor units even when the muscle is at rest. The contractions do not produce enough tension to produce movement, but they tense and firm the muscle

Question 17

What is the point in muscle tone?

Answer 17

Helps stabilises positions of bones and points
Helps maintain body posture when producing tension in muscles involved with balance and posture
Prevents sudden uncontrolled changes in positions of bones and joints

Question 18

What is the difference between an isotonic and isometric contraction?

Answer 18

Isotonic - where tension rises and skeletal muscle length changes
Isometric - where tension rises but skeletal muscle length doesn’t change

Question 19

What are the two types of isotonic contraction?

Answer 19

Concentric - shortening of the muscle

Eccentric - lengthening of the muscle

Question 20

What are type I muscle fibres?

Answer 20

• Slow oxidative fibres
• Red in colour
• Good supply of nutrients
• get ATP from oxidative phosphorylation (oxygen)
• Resistant to rapid depletion of energy reserves
• Found in marathon runners

Question 21

What are type IIA muscle fibres?

Answer 21

• Fast twitch oxidative fibres
• Fast resistant
• have faster contraction speed
• still relatively resistant to fatigue
• get ATP from oxidative phosphorylation
• need more energy to operate than type I

Question 22

What are type IIB muscle fibres

Answer 22

• Fast twitch glycolytic
• White in colour
• become short of energy very quickly “fast fatigue”
• larger cells which have limited blood supply and nutrient supply
• generate ATP via substrate level phosphorylation (glucose>lactic acid) (aerobic)
• also can use creative kinase as immediate source of ATP (requires neither glucose or oxygen)
• good for sprinting

Question 23

How does mitochondrial biogenesis occur?

Answer 23

• Exercise increases demand for ATP
• Activation of AMPK (adenosine monophosphate activated protein kinase)
• AMPK inhibits anaerobic energy consumption and stimulating energy producing, catabolic pathways e.g, oxidative phosphorylation

Question 24

What signalling molecules can lead to muscle cell atrophy?

Answer 24

Akt, mTOR - decrease protein synthesis rate

Glucocorticoids, myostatin NF kappaB, ROS - increase protein degradation and decrease protein synthesis

Question 25

How is a stress fracture caused?

Answer 25

Excessive and repeated trauma in a bone which leads to lots of microscopic fractures.
Overtime, these exceed the capacity of normal healing process, resulting in a macro fracture

Question 26

What is an osteophyte?

Answer 26

Bony projections that occur at sites of cartilage degeneration or destruction near joints and IV discs

Form due to an increase in damage at surface area

Question 27

What is the female athlete triad?

Answer 27

• Eating disorder leads to energy imbalance
• This triggers amenorrhoea (stop menstruation)
• Decreased estrogen leads to osteoporosis

Higher risk of stress fractures

Question 28

What are shin splints?

Answer 28

Medial tibial stress syndrome
Due to over pronation during running
Pain due to disruption of sharpey’s fibres

Question 29

Why are females more at risk of an anterior cruciate ligament tear?

Answer 29

Females have a wider pelvis
Vastus lateralis develops more than vastus medialis, pulling the patella more laterally
This causes a tear in the ligament

Question 30

What are the stages of the gait cycle?

Answer 30

Stance phase (60%) - heel strike, support, toe-off
Swing phase (40%) - leg lift, swing

Question 31

What are the stages of the gait cycle?

Answer 31

Stance phase (60%) - heel strike, support, toe-off
Swing phase (40%) - leg lift, swing