Muscles

Question 1

What are the properties of muscle cells (4)

Answer 1

1. extensible
2. contractile
3. excitable
4. elastic

Question 2

What are functions of skeletal muscles (5)

Answer 2

• maintain posture
• maintain stability and structure of joints
• move substances through the digestive tract
• generate heat to maintain internal body temperature
• locomotion

Question 3

What are motor units

Answer 3

the motor nerve and the muscle fibres it innervates

Question 4

What are motor units

Answer 4

the motor nerve and the muscle fibres it innervates

Question 5

What is the difference between a smaller motor unit and a large motor unit

Answer 5

Small - motor neurone innervates few muscle fibres
- allow for finer and more precise control of movement and produces a smaller force

Large - motor neurone innervates thousands of muscle fibres
- produce a much larger force

Question 6

What is a motor pool

Answer 6

all the motor units which work together to innervate a single muscle

Question 7

How is a transverse-tubule (T-tubule) formed?

Answer 7

sarcolemma extends deep into centre of muscle fibres

Question 8

What is titin and what’s its function?

Answer 8

• extends from z-line to m-line
• attaches to myosin filament -> keeps myosin centered
• allows for muscle to have its passive elasticity

Question 9

What is nebulin and what is its function

Answer 9

• extends from z-line and acts as a scaffold for actin filaments
• determines the length of actin filaments in sarcomeres

Question 10

What is excitation-contraction coupling?

Answer 10

conversion of an electrical stimulus into a mechanical response

Question 11

what occurs after depolarisation of motor-endplate

Answer 11

• Local depolarisation of motor end plate initiates an action potential
  
  • travels along sarcolemma and down the t-tubule
• this causes conformational change in VG Ca2+ channels
  
  • open up
• Ca2+ releasing channels also open up and stored Ca2+ moves out from SR into the sarcoplasm
• cytoplasmic [Ca2+] increases
• initiates a sequence which leads to muscle contraction (cross-bridge cycle)
• once nerve stimulation stops, muscles relax
• Ca2+ is pumped back into the SR by the SERCA
  
  • ATP is needed for energy

Question 12

What are the stages in the cross-bridge cycle?

Answer 12

• Ca2+ binds to troponin to cause a conformational shape change
  
  • tropomyosin moves away from actin binding site (binding site is exposed)
    
    1. ATP binds to myosin head
       1. breaks the link between actin and myosin
    2. ATP is hydrolysed by ATPase → ADP + Pi
       1. ADP + Pi remain attached to myosin
       2. myosin head aligns with the new actin binding site that is further along the actin filament
    3. Pi dissociates from myosin
       1. allows myosin head to bind to binding site on actin with a higher affinity
       2. myosin head attaches to binding site on actin filament
       3. ADP still attached to myosin head
    4. Powerstroke occurs
       1. myosin neck rotates myosin head which causes myosin to pull on actin filament towards the center of the sarcomere
       2. muscle shortens and a force in generated in the muscle
    5. ADP dissociates from myosin
       1. myosin is still attached to actin in a rigid state
       2. myosin awaits another ATP molecule to break the link between actin and myosin so that another cross-bridge cycle can begin

Question 13

what is a muscle twitch

Answer 13

one complete cycle of muscle fibre contraction and relaxation in response to a single action potential

Question 14

What generates force of muscle contraction?

Answer 14

cycle of cross-bridge formation and breaking

Question 15

Why is muscle contraction so smooth?

Answer 15

cross-bridges form independently to one another

- at any one time, some cross-bridges are bound while others are not

Question 16

What is muscle recruitment

Answer 16

process where the motor units in a muscle are gradually being activated

Question 17

what factor is motor units fired based on

Answer 17

size of motor units

Question 18

how does the twitch force increase

Answer 18

• increase in frequency of action potential

- increase in the number of motor units recruited

Question 19

What is tetanus

Answer 19

tetanus - state of sustained maximal muscle contraction

Question 20

what is the latent period and what happens during this period?

Answer 20

• time before the muscle fibres generate a tension

- formation of cross-bridges occur in this period + influx of calcium ions into sarcoplasm -> bind to troponin

Question 21

What is the contraction period

Answer 21

time from when muscle first generates tension -> the peak tension generated

Question 22

What is the relaxation period

Answer 22

time in which tension decreases back to 0

• calcium ions unbind from troponin and cross-bridges detach
• calcium ions are taken back up into the sarcoplasmic reticulum

Question 23

in concentric contraction, what happens as the load increases? (5)

Answer 23

• latent period increases
• sarcomere shortening distance decreases
• shortening velocity decreases
• total twitch duration decreases
• relaxation period decreases

Question 24

What are the 5 reasons for muscle fatigue?

Answer 24

• inhibition of cross-bridge cycling
• lactic acid build-up
• conduction pathway failure
• depletion of energy sources
• central command fatigue

Question 25

What happens after ACh binds to the recceptors

Answer 25

• ACh is broken down by acetylcholinesterase -> Acetate + choline
• choline is taken back into the nerve terminal through the choline carrier
• choline acetyltransferase catalyses the reaction to synthesise more ACh (choline + acetyl coA -> ACh + coA)

Question 26

How is the neuromuscular junction maintained?

Answer 26

• muscle cells synthesize ACh receptors, LPR4 and MuSK which cluster at the centre of the cell
• LRP4 and MuSK form a complex
• at the presynaptic nerve terminal, ACh and agrin are synthesised
• agrin binds to the LPR4 and MuSK complex -> trigger a series of intracellular events
• Rapsyn is formed which stabilises the ACh receptor!