L15 muscle 1 Flashcards
What is most skeletal muscle attached to?
Bone, via Tendons
What are the 2 main functions of skeletal muscle?
- Movement
2. Generation of heat
What are individual muscle cells called?
Myocytes (also known as muscle fibres)
What are myocytes covered with?
A layer of connective tissue called the endomysium. (note: it’s distinct from the muscle cell membrane, the sarcolemma).
How is each muscle fibre formed?
By the fusion of cells during development. (each cell is nucleated)
What are muscle fibres grouped into?
Bundles of fibres termed fascicles. (related to fascist which is a bundle of sticks - in the emblems). The fascicles are in turn covered with another layer of connective tissue called the perimysium).
What surrounds the fascicles and groups them together?
The perimysium covers one fascicle. The epimysium groups the fascicles together and covers everything. This is the muscle.
What are muscle fibres made up of?
Bundles of protein filaments (called myofilaments) that make up myofibrils.
Bundles of myofibrils = muscle fibre.
What are T tubules?
Tunnels which lead off the sarcomela (cell membranes). T tubules lead to the interior of the muscle fibre and mean that the membrane has a high surface area.
Where is the sarcoplasmic reticulum in muscles?
It wraps around the myofibrils.
Specialised parts of the SR, called the terminal cistemae, interact with the T tubules to form a structure called a triad.
What is the triad and what is it key for?
The terminal cistemae of the SR, interacting with the T tubules. It is key to coupling excitation of the muscle membrane to contraction.
From the smallest to the biggest, what is the structure of the muscle?
Myofilaments, myofibrils, muscle fibre/myocite, fascicles, muscle.
Muscle fibres are made up of repeating what?
Sarcomeres
What part of the sarcomere is actin & myosin?
myosin head….myosin tail… actin
What is the structure of myosin?
It is an hexamer - 2 heavy chains coiled around each other with ATPase heads, and 4 light chains.
What 2 forms does actin exist in?
A globular form - G actin
A filamentous form - F actin. (form found in muscles)
How are Tropomyosin and Troponin involved in actin?
1 - Tropomyosin runs along the actin chain.
-> it consists of 2 alpha helical chains that coil around each other
-> 1 tropomyosin interacts with 7 actin monomers.
2 - Troponin is a complex of 3 proteins - Troponin T, I + C.
-> 1 troponin trimer per tropomyosin molecule
-> T- binds to tropomyosin
-> I - binds to actin and inhibits contraction.
->C - binds calcium.
What do Tropomyosin & Troponin allow?
Muscle contraction to be regulated by calcium.
What are the 6 stages of muscle movement?
1 - Myosin is bound to actin (attached state)
2 - Myosin head binds ATP. Detaches from actin.
3 - Myosin head hydrolyses ATP. Conformation changes and it enters “ cocked state”
4 - Myosin forms new cross-bridge with actin and actin monomers further down the chain. ( no muscle movement has happened yet)
5 - the phosphate is released and the power stroke occurs, because myosin head goes back to uncocked state.
6 - ADP is released and the system is back at start except 2 monomers movement. (the muscle has contracted).
At rest how does the troponin trimer sit?
So that the troponin - I inhibits the formation of actin-myosin cross-bridge, it covers up the myosin binding site.
What happens to troponin when [Ca 2+] increases?
Ca2+ binds to troponin C. This triggers a conformation change in troponin and tropomyosin. Tropomyosin moves deeper into the actin groove and troponin reveals the myosin binding sites on actin.
How do neuromuscular junctions/end plates work?
- Motor neurones release ACh into the synapse
- ACh binds to nicotinic ACh receptors on the muscle.
- This opens Na channels which flood into the muscle and depolarise the membrane.
- The depolarisation triggers the calcium signal which causes the muscle contraction.
What tunnels of plasmamembrane (sarcolemma) burrow into the muscle fibres?
Transverse tubules (T tubules)
How are the sarcoplasmic reticulum and plasma membrane linked in the muscle?
The terminal cistemae of the SR associate with the T tubules. (2 terminal cistemae for each T tubule). This is known as a triad. (Note: the SR is not continuous with the T tubule).
