5.2 - Muscle Microstructure and Contraction Flashcards
1
Q
What are the three main types of muscle?
A
- 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
2
Q
What are the different arrangements of muscle fibres?
A
- parallel - generally contract quickly
- fusiform - bulge in middle, spindle shaped
- triangular - e.g. deltoid
- pennate means feathery
- unipennate
- bipennate
- multipennate
3
Q
What is the structure of skeletal muscles from macroscopic to microscopic?
A
- 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
4
Q
What is the structure of myofibres like?
A
- 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
5
Q
What is the structure of myofibrils like?
A
- 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
6
Q
What is the structure of myofilaments like?
A
- 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
7
Q
What is the structure of actin and myosin together like?
A
- 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
8
Q
What is the structure of myosin like?
A
- two globular heads
- a single tail formed by two alpha helices
- tails of several hundred molecules form one filament
9
Q
What is the structure of actin like?
A
- 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
10
Q
What is the sliding filament theory in terms of bands and zones?
A
- 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)
11
Q
Describe the steps of the initiation of muscle contraction
A
- AP opens voltage-gated calcium channels
- Ca2+ enters presynaptic terminal
- Ca2+ triggers exocytosis of vesicles
- acetylcholine diffuses across cleft
- binds to acetylcholine receptors and induces AP in muscle
- local currents flow from polarised region and adjacent region and AP spreads along surface of muscle fibre membrane
- ACh broken down by acetylcholine esterase and muscle fibre response to that molecule of ACH ceases
12
Q
Describe the steps of activation of muscle contraction
A
- AP propagates along surface and into T-tubules
- 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
- Ca2+ binds to troponin and tropomyosin moves out of the way
- reveals myosin binding site on actin and crossbridges attach to actin
- Ca2+ is actively transported into SR continuously while APs continue (ATP-driven pump where uptake rate < or = release rate)
13
Q
Describe the steps of excitation contraction coupling
A
- Ca2+ binds to troponin and tropomyosin moves out of the way
- movement exposes myosin binding site on surface of actin chain
- ‘charged’ myosin heads bind to exposed site on actin filament
- 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)
- ATP binds and releases myosin head from actin chain
- ATP hydrolysis provides energy to recharge the myosin head
- this process happens repeatedly across many sarcomeres and myofibres and the muscle shortens
14
Q
Describe neuronal control/pathway of muscle contraction
A
- 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
15
Q
What is a motor unit?
A
- 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
16
Q
What are the three types of motor unit?
A
- slow (S, type I)
- fast, fatigue resistant (FR, type IIA)
- fast, fatiguable (FF, type IIB)
17
Q
What are the features of type I motor units?
A
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
18
Q
What are the features of type IIA motor units?
A
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
19
Q
What are the features of type IIB motor units?
A
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
20
Q
How are muscle fibre types distributed through muscle?
A
- 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
21
Q
By which two ways does the brain regulate muscle force?
A
- 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)
22
Q
How does recruitment work?
A
- 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
23
Q
How does rate coding work?
A
- 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
24
Q
What are neurotrophic factors?
A
- 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
25
Q
What are the three types of muscle contraction?
A
- 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
26
Q
What is the plasticity of motor units/muscle fibres and give examples?
A
- 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)
