lecture 9 - muscle tension/contraction Flashcards
What are the key components of the myofilament?
Actin and myosin
What is actin in a myofibril?
A thin filament that runs along the myofilament and forms a structural scaffold.
What is myosin in the myofibril?
Myosin is a thick filament that acts as a motor molecule, when attaching to actin, generating a force.
What are the relative positions of Actin and Myosin and rest?
They are not in contact.
What must be supplied to the myofilament for contraction to occur?
Calcium ions (Ca2+)
What is a cross-bridge?
A bond between actin and myosin that allows the sarcomere to contract and shorten towards its centre.
What are the 5 states of the cross bridge cycle?
Attached, Released, Cocked, Cross-bridge, Power-stroke.
What is the attached state of the cross-bridge cycle?
Myofilament has just finished a power stroke (ADP and P has been released from myosin head), and the actin-myosin cross bridges are still present.
What is the released state of the cross-bridge cycle?
A molecule of ATP binds to myosin, causing its head to detach from actin, removing the cross-bridge.
What is the cocked state of the cross-bridge cycle?
The Myosin head burns ATP to make energy and a phosphate group binds to it. Energy is stored by changing the shape of Myosin, preparing it to attach to actin again.
What is the cross-bridge state of the cross-bridge cycle?
If calcium is present, the myosin head binds to the actin myofilament forming a cross bridge.
What is the power-stroke state?
The energised myosin head uses its stored energy to change conformation, releasing P and creating a power stroke that causes filaments to slide past each other to contract the sarcomere.
What are the two factors that muscle tension is dependent on?
The number of muscle fibres recruited and the rate at which the muscle is stimulated.
If a small number of neutrons are active, muscle force will be…?
Low
What is the name of the process of activating more muscle fibres to generate more force?
Recruitment
What will a single action potential generate in muscle fibres?
A pulse of Ca2+ will be released into the cytoplasm causing a brief period of tension - a twitch
What is a twitch in terms of muscle contraction?
When a muscle if briefly stimulated when it is relaxed, causing a small amount of tension to be produced.
What happens when many action potentials are fired to a muscle fibre in rapid sequence?
The muscle will be restimulated before Ca2+ has returned to the SR, meaning it will increase further, creating a longer actin-myosin interaction and a stronger, longer contraction.
What is the state when maximum signalling and contraction of a muscle fibre is reached?
Tetanus
What is the length-tension relationship?
Each muscle has an optimal length where it is strongest, where there is sufficient length for cross bridging AND contraction.
Why are stretched muscles unable to contract effectively?
There is not enough overlap of the myosin and actin fibres, so cross-bridging is limited/does not occur.
Why are slack muscles not effective for contraction?
The myosin and actin fibres overlap completely - there is no room for contraction to occur.
Why do fast muscle fibres get fatigued rapidly?
They provide a lot of energy quickly, generating a short, powerful force.
Which muscle fibre (slow/fast) has a larger fibre diameter?
Fast
Which muscle fibre (slow/fast) has a greater capillary supply?
Slow (they need to produce a lot of energy when in use)
Which muscle fibre (slow/fast) has more mitochondria?
Slow
Why do fast fibres not have mitochondria?
Mitochondria provides energy too slowly, making it ineffective in providing the rapidly released high levels of energy required.
What colour are fast muscle fibres?
White
What colour are slow muscle fibres?
Red
Which muscle fibre (slow/fast) has better fatigue resistance?
Slow
Which muscle fibre (slow/fast) reaches peak tension more rapidly?
Fast
How can muscle contraction be described in terms of the ‘lines’ of the sarcomere?
Myosin contracts, pulling the Z lines of the actin inwards the M line of the myosin, shortening the sarcomere on both sides of the M line.
Is the M line found in the myosin or actin filaments?
Myosin (in the centre of the sarcomere)
is the Z line found on actin or myosin filaments?
Actin
What is the principle neurotransmitter at a neuromuscular junction?
Acetylcholine
How does a synapse between a motor neuron and muscle fibre result in muscle contraction?
The action potential causes acetylcholine to be released from the neuron via the synaptic cleft to receptors in the muscle fibre. Ultimately, this leads to an action a potential which travels along the sarcolemma to the T-tubule, where it triggers the voltage gated receptor (DHPR) causing the release of calcium ions into the cytosol
What is the position of the myosin heads in relation to the M line during the resting state of a muscle fibre?
Heads point away from the M line
What part of the sarcoplasmic reticulum are Calcium ions stored in and released from?
Terminal cisternae
What is the symbol for acetylcholine?
ACh
