Microstructure And Contraction Flashcards
What type of control is smooth muscle under?
Involuntary control
What part of the nervous system does smooth muscle act under?
Autonomic nervous system
What type of control is the skeletal muscle under?
Voluntary control
What does skeletal muscle do?
contract to bring about movement
What connects bone to muscle
A tendon
What are skeletal muscles usually attached to?
Bones
What is the microstructure of a muscle? - 5
Myofilaments => Myofibril => Myofibre => Fascicles => Muscle
What are muscle fasciculus surrounded by?
Perimysium
What are myofibres/muscle fibres surrounded by?
Endomysium
What surrounded the bundles of muscles fascicles to hold them together?
Epimysium
What is the nucleus state of muscle fibers?
Multinucleate
What is the plasma membrane that covers the myofibres called?
Sarcolemma
What is found in the sarcoplasm?
myoglobin and mitochondria
What is the sarcoplasmic reticulum?
Network of fluid filled tubules regulating intracellular calcium concentration for contraction and relaxation
What is one repeating unit of a myofibril called?
A sarcomere
What two proteins do myofibril consist of?
Actin and myosin
What is the thin filament called
Actin
Thick filaments are
Myosin
What makes up the dark portion of the light and dark bands?
Myosin
What separates sarcomere?
Dense Z protein discs
What is the I band?
The light bands with thin actin filaments
Do myofilaments extend the length of the myofibres?
No
What causes muscle contraction?
Movement of actin filaments over the myosin filaments, which shortens the length of the sarcomere
How many heads does myosin have?
Two globular heads
What forms the single tail of myosin?
Two alpha helices
What forms a myosin filament?
The tails of several hundred molecules
What two other molecules do actin filaments contain?
Troponin and tropomyosin
What shape are actin molecules twisted into?
Helix
What happens to the I band when the muscle contracts?
Shortens
What happens to the A band during contraction?
Stays the same length
What is the A band?
The dark band
What happens to the distance between the Z discs during muscle contraction?
Shortenes
Describe the initiation of muscle contraction - 7 steps
- Action potential arrives at end of motor neurone
- Voltage gated Ca2+ channels open
- Ca2+ enters pre-synaptic terminal
- Triggers the exosytosis is of vesicles
- ACh released into the cleft
- Binds to receptors => generates action potential
- local currents flow from depolarised region to adjacent region, allowing AP to spread along the surface of the muscle fibre membrane
How is ACh broken down?
Using acetylcholine esterase
Describe the activation of muscle contraction - 8 steps
- AP propagates along surface and into T tubules
- Dihydropyridine (DHP) receptors in T tubule membranes sense change in voltage and changes shape of the protein linked to ryanodine receptor
- Ryanodine receptor then opens and releases Ca2+ out into space around filaments
- Ca2+ binds to troponin => causes tropomyosin to move, exposing the myosin head binding site
- Ca2+ is then actively transported into the sarcoplasmic reticulum
- Charged myosin head binds to exposed site on actin filament => binding causes the myosin head to pivot pulling the actin filament towards centre of sarcomere
- ATP binds which causes the myosin head to be released
- ATP hydrolysis provides energy to recharge the myosin head
What receptor detects the change in voltage when an action potential arrives at the T tubule
Dihydropyridine
What is the DHP receptor bound to?
Ryanodine receptor
What does Ca2+ bind to on the actin filament?
Troponin
What is covering the myosin binding site?
Tropomyosin
What is the power stroke?
The pivoting of the the myosin head to slide the actin chain over the myosin filament
What binds to the myosin head to charge the myosin head to move?
ATP
What is used to “recharge” the myosin head for more contraction?
ATP hydrolysis to make ADP + Pi
Where is neural control of muscle contraction - 2
Upper motor neurone in the brain to lower motor neurones in the brainstem or spinal cord
What is a motor unit?
A single motor neurone together with all the fibers that it innervates
How many muscle fibres do each motor neurone supply on average?
600
What happens when a single motor neurone is stimulated?
All the muscle fibers in that motor unit will contract
What is the best innervation ratio for high levels of fine control?
Neurones which innervate fewer muscle fibers
What is a type one motor unit?
Slow
What are type IIa muscle fibers?
Fast, fatigue resistant
What are type IIB motor units?
Fast and fatiguable
Which motor units have small cell body diameter?
Slow, type I
Which motor unit type have thicker axons?
Type IIA and IIB
What is the size difference in dendritic trees of the different motor units ?
Larger dendritic trees in Type IIA and Type IIB, smaller in type I
How are muscle fibers distributed throughout the muscle?
Randomly
Which muscle fiber type are rich in myoglobin?
Slow type I and type IIB (fast, fatiguable)
Which type of muscle fibers are used for aerobic activity?
Slow twitch fibres
Which muscle fibre type are used for anaerobic functions?
Fast and fatiguable muscle fibers
Which muscles have a high proportion of slow twitch fibers?
Muscles involved in postural control
What three factors are motor units classified by? - 3
Amount of tension, speed of contraction and fatiguability
What are the 2 mechanisms by which the brain regulates the forces that a single muscle produces?
Recruitment and rate coding
Describe the recruitment mechanism?
The more motor units that are recruited, the larger the size of the force
What is the size principle?
The smaller motor units are recruited first, then as more force is required, more units are recruited
What happens during recruitment when more forces is needed?
More muscle units are recruited
Describe the rate coding principle?
The firing rate down the motor neurones to the motor unit changes as more force is needed to be produced
What is ramp contraction?
When the force of contraction increases over time due to more recruitment
How does recruitment regulate the stimulation of motor units?
By regulating the number of motor units that are recruited
Which motor unit is the first to be derecruited?
The motor unit which was recruited last - therefore this motor unit will be the one largest in size
What are neurotrophic factors?
Type of growth factor which prevent neuronal death and promote growth of neurones after injury
What does the motor unit and fibre characteristics depend on?
The nerve which innervates them
What happens to fast and slow twitch muscle fibers is they are cross innervated?
The fast becomes slow and slow becomes fast
What are the three main types of muscle contraction?
Isometric
Concentric
Eccentric
What is isometric contraction?
When the muscle unit produces forces but doesn’t change length
What is a concentric muscle contraction?
When the muscle shortened in order to produce movement
What is eccentric contraction?
When force is produced but the muscle fibers is getting longer
What is an example of eccentric muscle contraction?
When a person is lifting up a dumbbell which is too heavy for them
Which type of muscle contraction produces the most force?
Eccentric
Which type of muscle contraction can damage and tear muscle fibers?
Eccentric
Following training, what is the most common type of muscle fiber change?
Type IIB to IIA (FAST FATIGUABLE TO FATIGUE RESISTANT)
What can cause a change from type 1 to type 2 muscle fibers?
Severe deconditioning or spinal cord injury
What type of fibers are found in aged muscle and why?
Larger proportion of type 1 fibers, due to loss of type II fibres
What provides evidence for the larger proportion of type 1 fibers in aged muscle?
Slower contraction time
What shift in muscle fibers type is associated with microgravity!
Slow to fast
Why is there a slow to fast shift in microgravity?
Less need for postural muscle innervation (slow)
Where is the sarcolemma in relation to the endomysium?
The sarcolemma lies under the endomysium which surrounds the muscle fibre
What is meant by a pennate arrangement of muscle fibres?
When muscle fibres are placed at angles to the tendon
What is a unipennate arrangement of muscle fibres?
When the muscle is going in the same direction as the tendon
What is a bipennate arrangement of muscle fibres?
When the muscle fibres are spread out in two direction from the tendon
What is a multipennate arrangement of muscle fibres?
When there are many tendons and many fibres
What is a parallel muscle fibre?
When the muscle fibre is straight across
What is meant by a fusiform muscle fibre?
When the middle of the fibre is thicker and larger in size than the two outer edges
What are the three shapes of muscle fibres?
parallel
fusiform
triangular
what part of the nervous system does skeletal muscles act under?
somatic nervous system
what is a triangular arrangement of muscle fibre?
converge at one end (typically at a tendon) and spread over a broad area at the other end in a fan-shape
Label A
bone
Label B
tendon
label C
muscle
label D
Fascicles
label E
Myofibre
label F
myofilaments
label G
myofibril
do myofibrils extend along the entire length of myofibres?
yes
what two myofilaments overlap
actin and myosin
what is the dark band also called
A-band
what is the light bands also called
I band
What happens to the H zone when the muscle contracts?
narrowed or disappears
What theory is used to describe muscle contraction?
sliding filament theory
When acetylcholine is broken down by acetylcholine esterase, what happens to the muscle fibre response?
ceases contraction
during muscle contraction, when action potentials continue where is Ca2+ actively transported?
into the SR
during muscle contraction, when action potentials continue where is Ca2+ actively transported?
into the SR
during muscle contraction, when action potentials continue where is Ca2+ actively transported?
into the SR
what neurones supply muscle contraction for voluntary neural control?
upper and lower motor neurones
Which motor unit has the slowest conduction velocity?
Type I
Which motor unit has the FASTEST conduction velocity?
Type IIB
Which motor unit has the thinnest axons
Type I
when does summation occur?
when units fire at frequency too fast to allow the muscle to relax between arriving action potentials
what is the relationship between muscle force regulation and number of motor units?
muscle force is regulated by the no of motor units recruited
what is the relationship between muscle force regulation and number of motor units?
muscle force is regulated by the no of motor units recruited
