5.2 - Muscle Microstructure and Contraction

Question 1

What are the three main types of muscle?

Answer 1

• smooth muscle - under involuntary control from the autonomic nervous system
• cardiac muscle - can contract autonomously but is under the influence of the autonomic nervous system and circulating chemicals
• skeletal muscles - under voluntary control from the somatic nervous system, usually attached to bones and contract to bring about movement

Question 2

What are the different arrangements of muscle fibres?

Answer 2

• parallel - generally contract quickly
• fusiform - bulge in middle, spindle shaped
• triangular - e.g. deltoid
• pennate means feathery
• unipennate
• bipennate
• multipennate

Question 3

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

Answer 3

• bone –> tendon –> muscle –> fascicles (bundles of muscle fibres/myofibres) –> myofibre –> myofibril –> myofilaments

Membranes:

• epimysium
• muscle fascicles bounded by perimysium
• muscle fibre/myofibre surrounded by endomysium

Question 4

What is the structure of myofibres like?

Answer 4

• sarcolemma - plasma membrane
• transverse-tubules (T-tubules) tunnel into centre
• cytoplasm in fibres is sarcoplasm - myoglobin and mitochondria present
• sarcoplasmic reticulum - network of fluid-filled tubules
• composed of myofibrils

Question 5

What is the structure of myofibrils like?

Answer 5

• myofibrils extend along entire length of myofibre
• composed of thin myofilaments made of actin (light band) and thick filaments of myosin (dark band) that do not extend along the length of the myofibre
• myofilaments overlap and are arranged in repeating units called sarcomeres from Z disc to Z disc (middle of light band)
• light and dark bands give skeletal muscle striated appearance

Question 6

What is the structure of myofilaments like?

Answer 6

• dense protein Z-discs separate sarcomeres
• dark bands are A bands made of thick myosin
• light bands are I bands made of thin actin
• myosin and actin filaments overlap

Question 7

What is the structure of actin and myosin together like?

Answer 7

• myosin attached to M line (middle of sarcomere) and spreads in both directions
• comes into contact with actin filaments attached to Z disc
• movement of actin over myosin causes force generation and contraction

Question 8

What is the structure of myosin like?

Answer 8

• two globular heads
• a single tail formed by two alpha helices
• tails of several hundred molecules form one filament

Question 9

What is the structure of actin like?

Answer 9

• actin molecules are twisted into a helix
• each molecule has a myosin binding site
• actin filaments have troponin and tropomyosin associated which move and uncover binding sites when calcium is present
• Ca2+ binds to troponin –> conformational change causing tropomyosin to move –> actin-myosin binding sites exposed

Question 10

What is the sliding filament theory in terms of bands and zones?

Answer 10

• Z discs get closer together (sarcomere shortens)
• A-band remains the same length - as actin is being pulled while myosin does the pulling so does not move
• I-band becomes shorter
• H-zone narrowed or disappeared (where only myosin present)

Question 11

Describe the steps of the initiation of muscle contraction

Answer 11

1. AP opens voltage-gated calcium channels
2. Ca2+ enters presynaptic terminal
3. Ca2+ triggers exocytosis of vesicles
4. acetylcholine diffuses across cleft
5. binds to acetylcholine receptors and induces AP in muscle
6. local currents flow from polarised region and adjacent region and AP spreads along surface of muscle fibre membrane
7. ACh broken down by acetylcholine esterase and muscle fibre response to that molecule of ACH ceases

Question 12

Describe the steps of activation of muscle contraction

Answer 12

1. AP propagates along surface and into T-tubules
2. dihydropyridine (DHP) receptor in T-tubule membrane senses change in voltage and changes shape of protein linked to a ryanodine receptor –> ryanodine receptor Ca2+ channel in sarcoplasmic reticulum opens –> Ca2+ released from SR into space around filaments
3. Ca2+ binds to troponin and tropomyosin moves out of the way
4. reveals myosin binding site on actin and crossbridges attach to actin
5. Ca2+ is actively transported into SR continuously while APs continue (ATP-driven pump where uptake rate < or = release rate)

Question 13

Describe the steps of excitation contraction coupling

Answer 13

1. Ca2+ binds to troponin and tropomyosin moves out of the way
2. movement exposes myosin binding site on surface of actin chain
3. ‘charged’ myosin heads bind to exposed site on actin filament
4. this binding and discharge of ADP causes myosin head to pivot (the ‘power stroke’) which pulls the actin filament towards centre of sarcomere (M line)
5. ATP binds and releases myosin head from actin chain
6. ATP hydrolysis provides energy to recharge the myosin head

• this process happens repeatedly across many sarcomeres and myofibres and the muscle shortens

Question 14

Describe neuronal control/pathway of muscle contraction

Answer 14

• upper motor neurones are in the brain
• these synapse onto lower motor neurones in brainstem or spinal cord
• we have voluntary neural control from upper and lower motor neurones

Question 15

What is a motor unit?

Answer 15

• a single neurone and all of the muscle fibres it innervates (around 600 fibres per neurone on average)
• the same muscle fibre cannot be innervated by >1 neurone
• stimulation of one motor unit causes contraction of all the muscle fibres in that unit
• in muscles with fine control there are fewer fibres per neurone so we can finely adjust force

Question 16

What are the three types of motor unit?

Answer 16

• slow (S, type I)
• fast, fatigue resistant (FR, type IIA)
• fast, fatiguable (FF, type IIB)

Question 17

What are the features of type I motor units?

Answer 17

Nerve:

• smallest diameter cell bodies
• small dendritic trees
• thinnest axons
• slowest conduction velocity, low force, fatigue resistant

Muscle:

• high myoglobin content
• high aerobic capacity
• low anaerobic capacity
• slow twitch

Question 18

What are the features of type IIA motor units?

Answer 18

Nerve:

• larger diameter cell bodies
• larger dendritic trees
• thicker axons
• faster conduction velocity, moderate force, fatigue resistant

Muscle:

• high myoglobin content
• moderate aerobic capacity
• high anaerobic capacity
• fast twitch

Question 19

What are the features of type IIB motor units?

Answer 19

Nerve:

• larger diameter cell bodies
• larger dendritic trees
• thicker axons
• faster conduction velocity, high force, high fatigue (produce less force as time goes on)

Muscle:

• low myoglobin content
• low aerobic capacity
• high anaerobic capacity
• fast twitch

Question 20

How are muscle fibre types distributed through muscle?

Answer 20

• randomly distributed throughout muscle
• muscles have different proportions of slow and fast twitch muscles e.g. back has a lot of slow twitch muscle fibres as it is involved in posture

Question 21

By which two ways does the brain regulate muscle force?

Answer 21

• recruitment
• rate coding
• at low levels of force, slow motor units come into play - as force increased, we recruit from slow to FR to FF (recruitment) but also the motor units already recruited increase their firing rates (rate coding)

Question 22

How does recruitment work?

Answer 22

• motor units are not randomly recruited - there is an order
• governed by ‘size principle’ - smaller units are recruited first (generally slow twitch units)
• as more force required, more units are recruited, allowing fine control (e.g. when writing), under which low force levels are required
• slow –> FR –> FF
• reverse happens when contraction is lessened - the last motor unit to be recruited is derecruited first

Question 23

How does rate coding work?

Answer 23

• a motor unit can fire at a range of frequencies - slow units fire at a lower frequency
• as firing rate increases, the force produced by the unit increases
• summation occurs when units fire at a frequency too fast to allow the muscle to relax between arriving APs

Question 24

What are neurotrophic factors?

Answer 24

• a type of growth factor that prevents neuronal death
• they promote growth of neurones after injury
• even with a good blood supply, if a nerve to a muscle is severed, the muscle wastes away because of the lack of neurotrophic factors provided to it
• motor unit and fibre characteristics are dependent on the nerve innervating them
• if a fast and slow twitch muscle are cross innervated, the slow one becomes fast and vice versa
• the motor neurone has some effect on the properties of the muscle fibres it innervates

Question 25

What are the three types of muscle contraction?

Answer 25

• concentric - muscle shortens to produce movement
• eccentric - muscle produces force but is getting longer e.g. holding something heavier than can be managed
• isometric - muscle produces force but does not change in length

Question 26

What is the plasticity of motor units/muscle fibres and give examples?

Answer 26

• fibre types can change properties under many different conditions
• following training, type IIB to IIA is most common
• type I to II is possible in cases of severe deconditioning or spinal cord injury
• microgravity in spaceflight results in shift from slow to fast muscle fibre types as not much postural control needed
• ageing associated with a loss of type I and II fibres but preferential loss of type II –> results in larger proportion of type I fibres in aged muscle (evidence from slower contraction times)