Muscle microstructure and contraction (21)

Question 1

What are the 3 main types of muscle?

Answer 1

• smooth muscle: involuntary from autonomic nervous system
• cardiac muscle: involuntary/autonomic
• skeletal muscle: under voluntary control- usually attached to bones- contract to bring about movement

Question 2

Where is smooth muscle found?

Answer 2

in the walls of the airways

Question 3

What are the general features of skeletal muscle?

Answer 3

• under voluntary control from the somatic nervous system
• usually attached to bones
• contract to bring about movement

Question 4

What are the different arrangements of muscle fibres? (not really year 1)

Answer 4

• parallel
• fusiform
• triangular
• multipennate
• bipennate
• unipennate
• pennate

Question 5

What are fascicles?

Answer 5

bundles of muscle fibres (myofibres)

multiple fascicles make up a muscle

Question 6

What is the structure of skeletal muscle from macroscopic to microscopic?

Answer 6

• muscle
• fascicles
• myofibres (muscle fibres)
• myofibrils
• myofilaments

Question 7

What is the name of the connective tissue that surrounds muscle fascicles?

Answer 7

perimysium

Question 8

What is the name of the connective tissue that surrounds muscle fibres?

Answer 8

endomysium

Question 9

What cells give rise to skeletal muscles?

Answer 9

myoblasts- fused such that muscle fibres have many nuclei

Question 10

What plasma membrane covers myofibres?

Answer 10

sarcolemma

Question 11

Why is there such a big blood supply to muscle fibres?

Answer 11

muscles require lots of energy to function

Question 12

What is the function of T-tubes?

Answer 12

conduct impulses from the sarcolemma down into the centre–> to the sarcoplasmic reticulum

Question 13

What 2 main types of protein do myofibrils contain?

Answer 13

actin and myosin- overlap- arranged all the way along myofibres in sarcomeres–> dark and light bands

Question 14

What dense protein areas separate sarcomeres?

Answer 14

Z-discs

Question 15

What is an M line?

Answer 15

halfway mark between adjacent Z-discs

Question 16

What protein causes the dark A bands?

Answer 16

thick filaments- myosin

Question 17

What protein causes the light I bands?

Answer 17

thin filaments- actin

Question 18

What is the structure of a myosin filament?

Answer 18

• 2 globular heads
• single tail formed by 2 alpha helices
• tails of several hundred molecules form 1 filament

Question 19

What is the structure of actin filament?

Answer 19

• actin molecules twisted into helix
• troponin complex cover myosin binding sites
• troponin complexes are on tropomyosin

Question 20

What evidence is there for the sliding filament theory?

Answer 20

• during contraction I band became shorter (actin being pulled)
• A band remained same length
• H zone narrowed

Question 21

What is the mechanism of initiation of muscle contraction (N.B. AP)?

Answer 21

1. AP opens voltage gated calcium channels
2. calcium enters pre-synaptic terminal
3. triggers exocytosis of vesicles
4. ACh diffuses across the cleft
5. binds to ACh receptors and induces AP in muscle
6. local currents flow- AP spreads along surface of muscle fibre membrane
7. ACh broken down- contraction stops

Question 22

What is the mechanism of muscle contraction occur INSIDE the muscle?

Answer 22

1. AP propagates along surface membrane into T-tubule
2. DHP receptor in T-tubule membrane senses change in voltage
3. leads to opening of ryanodine receptor calcium channel in SR (sarcoplasmic reticulum)
4. calcium released from SR into space around filaments
5. calcium binds to troponin on tropomyosin (filament wound around actin)
6. cross bridges attach myosin to actin
7. calcium actively transported into SR continuously whilst APs continue

Question 23

What is the process of excitation contraction coupling? https://www.youtube.com/watch?v=ousflrOzQHc amazing video

Answer 23

• calcium ions bind to troponin on actin molecules
• troponin initiates contraction process by moving tropomyosin molecules off of the myosin binding sites on actin
• allowing myosin heads to attach to actin filaments
  N.B. contraction begins when a bound ATP is hydrolysed to ADP and Pi–> causes myosin head to extend and attach to binding site on actin–> forming cross-bridge
  …an action- ‘power stroke’ is triggered- allowing myosin to pull actin filament towards M-line- shortening sarcomere
  … ADP and Pi are released during ‘power stroke’…myosin remains attached to actin until new ATP binds

Question 24

Where are the upper and lower motor neurons that control voluntary muscle contraction?

Answer 24

• upper motor neurons in primary motor cortex of brain

- lower motor neurons in brainstem or spinal cord

Question 25

What is the arrangement of neurons: muscle fibres?

Answer 25

- 1 neuron can innervate many muscle fibres (all will contract) - no muscle fibre can be innervated by more than 1 neuron

Question 26

What is a motor unit?

Answer 26

a single motor neuron and the muscle fibres it innervates

Question 27

What are the different types of motor units?

Answer 27

- slow (S, type 1) - fast, fatigue resistant (FR, type 2A) - fast, fatiguable (FF, type 2B)

Question 28

What are the characteristics of neurons in slow motor units?

Answer 28

- smallest cell body diameter - small dendritic trees - thinnest axons - slowest conduction velocity

Question 29

What are the characteristics of neurons in fast motor units?

Answer 29

- large diameter cell bodies - large dendritic trees - thick axons - fast conduction velocity

Question 30

What are the characteristics of slow muscle fibres?

Answer 30

- high myoglobin content - red - high aerobic capacity - low anaerobic capacity

Question 31

What are the characteristics of fast, fatigue resistant muscle fibres?

Answer 31

- high myoglobin content - pink - moderate aerobic capacity - high anaerobic capacity

Question 32

What are the characteristics of fast, fatiguable muscle fibres?

Answer 32

- low myoglobin content - white - low aerobic capacity - high anaerobic capacity

Question 33

What 3 features help us classify the motor unit types?

Answer 33

- amount of tension generated - speed of contraction - fatiguability

Question 34

What 2 mechanisms does the brain use to regulate muscle force?

Answer 34

recruitment and rate coding

Question 35

What is the principle behind motor unit recruitment?

Answer 35

varying the number of activated motor units | - as more force is required, more units are recruited

Question 36

What is the principle behind motor unit rate coding?

Answer 36

varying the rate at which each active motor unit generates action potentials - as firing rate inc., the force produced by the unit inc.

Question 37

What are neurotrophic factors?

Answer 37

type of growth factor--> prevent neuronal death and promote growth of neurons after injury

Question 38

What evidence shows that motor unit and fibre characteristics are dependent on the nerve that innervates them?

Answer 38

if a fast and slow twitch muscle are cross innervated, the slow one becomes fast and vice versa

Question 39

What are the 3 types of muscle contraction?

Answer 39

- concentric: muscle gets shorter - eccentric: muscle gets longer - isometric: muscle stays same length, but force is generated

Question 40

Can we change the properties of muscle fibres?

Answer 40

type 2B-->2A most common following training | N.B. type 1-->2 possible in severe reconditioning or spinal cord injury (or microgravity)

Question 41

What muscle fibre changes occur during ageing?

Answer 41

loss of slow and fast, but preferentially fast | --> results in larger proportion of type 1 fibres in aged muscle + slower contract times