Lecture 4 Flashcards
What is the microstructure of muscle?
- Sarcoplasmic reticulum
- T-tubules
- Sarcoplasm
- Myofibrils
- Sarcomere
What is the Sarcoplasmic Reticulum?
- It is the surrounding of each myofibril.
- It regulates the intercellular levels of calcium (store Ca2+ and releases it on demand when the muscle fibre is stimulated to contract)
- It also has end sacs called terminal cisternae
What are the T-tubules
The T-tubules are a continuation of the sarcolemma (muscle plasma membrane) and are closely related to the terminal cisternae.
What shape does a muscle fibre look like?
A muscle fibre is a long cylindrical cell
What does the sarcoplasm contain?
The sarcoplasm contains glycosomes (stored glycogen) and myoglobin (red pigment that store O2 in the muscles
What are myofibrils?
They are rod-like structures that contain a series of bands, dark ‘A’ bands and light ‘I’ bands, these bands give the myofibril a striated appearance.
What is a sarcomere?
A sarcomere is the smallest contractile unit of muscle fibre and is in the region of the z-discs.
Each sarcomere is made of myofilaments
- thin = actin
- thick and centeral = myosin
What are the two types of myofilaments?
Thick filaments = myosin
Thin filaments = actin
What is the purpose of think myosin filaments?
A myosin molecule has a rod-like tail with two globular heads. During muscle contraction, these globular heads attach to the actin-binding site.
What are the two main proteins that the thin actin filaments contain and what are they for?
- tropomyosin - blocks the actins active binding sites during muscle relaxation so that the myosin cannot bind to the actin.
- troponin - when calcium binds to the troponin it changes shape and moves tropomyosin away from the myosin-binding sites so that muscle contraction can occur.
What happens in muscle contraction phase 1?
Motor neuron action potential goes to the neuromuscular junction and causes the release of acetylcholine (ACh) into the synaptic cleft. The ACh then binds to the receptors on the motor endplate which causes the opening of sodium channels. The increase in sodium causes depolarisation and sends an action potential along the muscle cell membrane (sarcolemma). These impulses are conducted down the T-tubules.
What happens in muscle contraction phase 2?
The impulses that have been sent down the T-tubules cause them to depolarise which causes the release of Ca2+ from the cisternae of the sarcoplasmic reticulum.
The Ca2+ that has been released then binds to troponin, which then exposes the myosin binding site.
Actin combines with myosin ATPase to hydrolise ATP into ADP + Pi and energy which produces crossbridge movement.
ATP binds to the myosin crossbridge causing the cross bridge to detach from actin. Crossbridge attachment and detachment will continue if Ca2+ and ATP levels are high. When muscle stimulation ceases, Ca2+ moves back into the sarcoplasmic reticulum to inhibit the action of the toponin-tropomyosin complex.
Explain a single muscle twitch
There is a latent period, where the sarcolemma and T-tubules are depolarised, Ca2+ is being released, and cross bridges begin to cycle but there is no evidence of contraction.
Contraction Phase is the shortening of the sarcomere as a result of myosin cross-bridges cycling. The more Ca2+ and ATP the more cross bridges that form.
Relaxation Phase. is when the Ca2+ is transported back into the terminal cisternae, and into the sarcoplasmic reticulum. Because of this the cross-bridge cycling slows and ends, tension is reduced and the muscle returns to its original length.
What is a treppe?
A treppe is a type of muscle contraction where there is enough time for the muscle to fully relax before another contraction occurs.
What is temporal summation?
Temporal summation is when another stimulus is applied to the muscle before it has completely relaxed, resulting in a stronger contraction. (looks like a wave or a staircase)
