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).
What does the T tubule do to the surface area/volume ratio?
Increases it. Which facilitates the spread of action potential.
What are the [Ca2+] in the extracellular fluid, the cytoplasm and the sarcoplasmic reticulum?
Extracellular - a few mM
Cytoplasm - VERY LOW 1/10,000 of extracellular and sarcoplasmic reticulum. LOWEST
SR - similar to extracellular s few mM
Why is the [Ca2+] so low in the cytoplasm (in muscle cells)?
- there are calcium ATPase pumps in the SR membrane that actively pump calcium into the SR.
- this pump is known as SERCA. -> (Sarcoplasmic/endoplasmic Reticulum ATPase)
Why is it sensible that the cytoplasmic conc. of calcium is so low in the muscle?
So that the muscle isn’t constantly contracted
How does calcium get into the cytoplasm to contract muscle?
- From the SR
- The SR has calcium channels linked to the VSCC in the T tubules via a physical connection.
- The ryanodine receptor opens, releasing calcium into the cytoplasm.
What are the VSCC at the T tubules called?
- L-type (Voltage sensitive calcium channels - VSCC)
- Dihydropyridine receptors. -> comes from the drug that blocks the channel.
When an action potential travels down the T tubule, what happens?
Changes the conformation of the dihydropyridine receptors and pulls the ryanodine receptor open.
How do DHP receptors interact with ryanodine receptors?
4 DHP receptors (a tetrad) interact with each ryanodine receptor.
How do we reach the starting point when the neuronal signal is no longer present and muscle contraction has happened?
SERCA pumps the calcium back into the SR. Restores the very low [Ca2+] in the cytoplasm
Summarise the muscle contraction from nerve to end.
1 - Motor neurone releases acetylcholine.
2 - Activates nACh receptors
3 - Sarcolemma = depolarised. The Ap spreads to the T-tubules.
4 - DHP receptors activated. Opens ryanodine receptors on the SR.
5 - Calcium ions released from SR.
6 - Troponin C binds Ca2+ and moves out of myosin binding site on actin.
7 - Muscle contraction initiated. Myosin moves 2 actin at a time.
8 - SERCA pumps Ca2+ back into the SR.
