Flashcards in Muscle Physiology 1 Deck (53):
Where is the first place a signal is transmitted to in a muscle?
Why do transverse tubules need to be all over a muscle fibre?
So they can be detecting signals all the time.
What are the signals that trigger a response in muscle cells called?
What are the two main types of muscle fibre filament?
What is the sarcoplasmic reticulum similar to (an organelle found in other cells)
Where are terminal cisternae found?
In large parts in the sarcoplasmic reticulum,
Where is calcium stored in muscle fibres?
In terminal cisternae.
What is calcium really important for in muscles?
What is a triad in the sarcoplasmic reticulum?
A pair of terminal cisternae and a t tubule.
What is the functional unit of the muscle?
What causes big movements in muscle fibres for muscle contraction?
How can 'A' bands in sarcomeres be identified?
They are dark bands.
How can 'I' bands in sarcomeres be identified?
They are light bands.
What do 'Z' lines act as in muscle fibres?
A separator between each individual sarcomere.
What is the "H" band made up of?
What is the "M" line?
The middle of the sarcomere where the myosin (thick filaments) are connected together.
What are actin and myosin in terms of muscle contraction?
Actin and myosin are the "main players" in muscle contraction. The interaction between them makes the muscle move/contract.
How many proteins make up actin?
What are the two main proteins that make up actin?
They cover active sites and prevent actin-myosin interaction (in the resting state)
It has three main functions:
Bind to tropomyosin
Keep tropomyosin complex in place
What does myosin look like?
A golf club.
What is myosin formed of?
A myosin pair twisted around each other.
What does the myosin head interact with?
The head interacts with active sites on actin during contraction, forming a cross-bridge.
Where does interaction happen in muscle cells?
Interaction occurs at the "zone of overlap", where there are dark bands and light bands.
Where are triads found?
In the zone of overlap.
What element is there lots of at the zone of overlap?
When calcium is released in a muscle cell, where do you want it to be?
In a place where there's actin and myosin so they can't interact.
What occurs in a contracted sarcomere?
H bands and I bands are narrowed (shorter)
The zone of overlap widens
Z lines move closer together, the A band stays the same.
Give an overview of muscle contraction.
Action potentials travel from the brain or spinal cord to trigger the contraction of skeletal muscles.
The action potential propagates down a motor neuron to a skeletal muscle fibre.
The site where a motor neuron excites a skeletal muscle fibre is called a neuromuscular junction.
What is a neuromuscular junction?
A chemical synapse, consisting of the points of contact between the axon terminals of a motor neuron and the motor end plate of a skeletal muscle fibre.
State the 7 coordinated steps of events at a neuromuscular junction.
1. An action potential travels the length of the axon of a motor neuron to an axon terminal
2. Voltage-gated calcium channels open, and the calcium ions diffuse into the terminal
3. Calcium entry causes synaptic vesicles to release acetylcholine via exocytosis
4. Acetylcholine diffuses across a synaptic cleft and binds to acetylcholine receptors which open ligand-gated cation channels.
5. Ligand-gated cation channels open
6. Sodium enters the muscle fibre and potassium ions exit the muscle fibre. The greater the influx of sodium (compared to the outward flux of potassium), the less negative the membrane potential gets
7. Once the membrane potential reaches threshold, an action potential propagates along the sarcolemma.
When does neurotransmission to a muscle fibre cease?
When acetylcholine is removed from the synaptic cleft.
In what two ways can acetylcholine be removed from the synaptic cleft?
1. Acetylcholine diffuses away from the synapse.
2. Acetylcholine is broken down by the enzyme acetylcholinesterase into acetic acid and choline. Choline is then transported into the axon terminal for the resynthesis of acetyl choline.
What are the components of the neuromuscular junction?
Neuron: axon terminal
Motor end plate: on muscle
Synapse in between
Describe the role of calcium in muscles.
Causes active sites on actin to be exposed
Troponin binds to Calcium when Calcium is released
Troponin binds to tropomyosin and moves it away from the active site. Now the active site is open and ready for the myosin to form a cross-bridge.
How does ATP effect myosin?
It provides the energy for myosin to keep moving.
What does the contraction of a skeletal muscle generate?
The force necessary to move the skeleton.
What is a contraction of a skeletal muscle triggered by?
A series of molecular events, known as the cross-bridge cycle.
When does a sarcomere shorten?
When myosin heads and thick myofilaments form cross-bridges with actin molecules and thin myofilaments.
When is the formation of a cross-bridge initiated?
When calcium ions (released from the sarcoplasmic reticulum) bind to troponin.
What does the binding of calcium to troponin cause?
This causes troponin to change shape.
What must happen to the myosin head before a cross-bridge cycle can begin?
The myosin head must be activated before a cross-bridge cycle can begin.
What activates a myosin head?
When ATP binds to the myosin head and is hydrolysed to ADP and inorganic phosphate. The energy from the hydrolysis of ADP activates the myosin head, forcing it into the "cocked" position.
What 4 steps can the cross-bridge cycle be broken into?
Reactivation of myosin head.
Describe cross-bridge formation.
The activated myosin head binds to actin, forming a cross-bridge. Inorganic phosphate is released and actin becomes stronger.
Describe the power stroke,
ADP is released and the activated myosin head pivots, sliding the thin myofilament toward the centre of the sarcomere.
Describe cross-bridge detachment.
When another ATP binds to the myosin head, the link between the myosin head and actin weakens and the myosin head detaches.
Describe the reactivation of the myosin head.
ATP is hydrolysed to ADP and inorganic phosphate. The energy released during hydrolysis reactivates the myosin head, returning it to the 'cocked' position. As long as the binding sites on the actin remain exposed, the cross-bridge cycle will repeat, and as the cycle repeats, the thin myofilaments are pulled toward each other, and the sarcomere shortens. This shortening causes the whole muscle to contract.
When does cross-bridge cycling end?
When calcium ions are actively transported back into the sarcoplasmic reticulum. Troponin returns to its original shape, allowing tropomyosin to glide over and cover the myosin binding site on actin.
What is ACh?
An excitatory neurotransmitter.
What are the 5 steps that end a muscle contraction?
1. ACh is broken down by acetylcholinesterase (AChE), ending action potential generation
2. Sarcoplasmic Reticulum reabsorbs calcium ions. As they are reabsorbed, their concentration in the cytosol decreases
3. Active sites are covered, and cross-bridge formation ends. Without calcium ions, the tropomyosin returns to its normal position and the active sites are covered again.
4. Contraction ends
5. Muscle relaxation occurs: it returns passively to its resting length.