Anatomy & Embryology of Skeletal Muscle Flashcards
Is skeletal muscle striated or non-striated?
Striated
Is skeletal muscle single or multi-nucleated?
Multi-nucleated
Is skeletal muscle branched or unbranched?
Unbranched
What are myonuclei?
The pleural term for the numerous nuclei in each individual skeletal muscle cell.
Where are the nuclei located in skeletal muscle cells?
At the peripheries.
What does syncytial mean?
A cytoplasm controlled by multiple nuclei.
What is a myofibril and where are they found?
They are fibres consisting of multiple adjacent sarcomeres (the contractile units). Myofibirils are found in great volumes in muscle cells.
What are the X3 layers of fascia which surround various sections of the muscles/muscle fibres?
Endo-, peri- and epimysium .
Where do the endo/peri/epimysium fuse to?
What is the name of this junction?
They fuse to tendons at the myotendinous junctions
‘myo’ = myocyte =muscle cell
Explain the components of a muscle starting from myofibrils.
Include the fascia containing each layer.
Multiple myofibrils = myocyte Myocyte = wrapped in endomysium Multiple myocytes = form a fascicle = wrapped in perimysium Multiple fascicles (whole muscle) = wrapped in endomysium
What is the sarcolemma?
The name given to the cell membrane of skeletal muscle.
What is isotonic contraction?
A muscle action in which the muscle fibres change length (either shorten or lengthen)
What is isometric contraction?
A muscle action in which the muscle fibres do NOT change length.
What are the X2 type of isotonic muscle action?
What happens to the muscle fibres in each of these?
Isotonic concentric (muscle fibres shorten)
Isotonic eccentric (muscle fibres lengthen)
What is a synergist muscle?
Complements the prime mover (the agonist muscle doing the main body of the work)
How do skeletal muscle cells form?
What is this process called?
Single myoblast cells proliferate and fuse into a myotube (muscle fibre).
This is called myogenesis.
What are satellite cells and where are they found?
They are found on the surface of skeletal muscle cells and are resident stem cells, which can be activated to form further satellite cells or myoblasts to regenerate further muscle fibres.
What gives skeletal muscle cells their striated appearance?
The presence of contractile sarcomere units
What X2 filament proteins form the sarcomeres?
Which is thick and which is thin in terms of their banding length?
Actin (thin)
Myosin (thick)
Where are the actin and myosin located in terms of the sarcomere? Which is central and which is ‘at the edges’?
Myosin = central
Actin = edges
Which lines separate sarcomeres?
Z lines
What band is the thick myosin band?
The A band
What band is the thin actin band?
The I band
What is the thin central band where no actin overlaps myosin?
The H band
Which lines and bands move during contraction and which do not?
The Z lines move closer together and the H and I bands narrows
The A band never changes
What are the X2 muscle fibre types?
Type 1 slow
Type 2 fast
Which muscle fibre type is further subdivided and what is it subdivided into?
Type 2 fast
= Fast type 2a
= Fast type 2b
= Fast type 2x
How are skeletal muscles innervated in terms of neuron:muscle fibre ration?
It is a 1:1 ratio, each myfibre is innervated individually by a single motor neuron.
Each motor neuron can innervated muscle muscle fibres however.
What is the name of the numerous sarcolemma invaginations into skeletal muscle cells, which are open to interstitial fluid?
T-tubules
What is the function of the T-tubules?
They ensure the action potential spreads evenly and quickly throughout the muscle fibre.
What is the sarcoplasmic reticulum?
A fluid filled system of membranous sacs which encircle each myofibril.
What are the names of the sacs of the sarcoplasmic reticulum where it meets the T-tubules?
Terminal cisterna
Explain the relationship between the T-tubules and the terminal cisterna.
Each T-tubule has a terminal cistern either side in a triad organisation. They are actually called TRIADS.
What is the role of the sarcoplasmic reticulum?
Stores calcium
The sarcoplasmic reticulum components (thick and thin bands) are not hovering in air. Which molecules attach each of these and where do they extend from/to?
Actin is held by NEBULIN, which extends from the Z-line for the length of the actin
Myosin is held by TITIN which extends from the Z-line to the M-line
What is the name of the location on each actin molecule where the myosin heads bind?
Myosin binding site
What are troponin and tropomyosin?
What do they each do?
What is the name of the complex they form together?
They are regulatory proteins.
Tropomyosin = blocks myosin binding sites
Troponin = the complex which holds tropomyosin in place
Together they form the Troponin-tropomyosin complex
What does calcium bind to in order to initiate muscle contraction?
What happens when it binds?
Troponin C
When calcium binds, the Troponin-tropomyosin complex moves, exposing the myosin binding sites on the actin filaments with the myosin heads.
In terms of initiating muscle contraction, what happens when neurotransmitter (released in the synapse where the nerve meets the muscle fibre) comes into contact with the muscle fibre?
How does this eventually lead to muscle contraction?
It stimulates voltage-gates Na channels to open and allow Na into the cell. This depolarises the cell with the depolarisation spreading all over the cell due to the T-tubules. Calcium is then released from the sarcoplasmic reticulum into the cytoplasm (due to voltage induced calcium release in skeletal muscle and CICR in cardiac muscle) which then binds to Troponin C of the Troponin-tropomyosin complex allowing muscle contraction to be initiated.
What causes relaxation to occur in terms of calcium?
The ATPase pump SERCA pumps calcium back into the SR and a Na/Ca exchanger pumps it out of the cell, decreasing the cytoplasmic calcium concentration and causing calcium to unbidden from troponin C.
Explain the X4 stages of sarcomere contraction.
1) ATP binds to myosin which releases it from actin
2) Myosin hydrolysis ATP —-> ADP + Pi which rotates it’s head and binds it weakly to actin
3) THE POWER STROKE! The Pi is released as the head swivels and moves the actin closer towards the m-line of the sarcomere.
4) At the end of the power stroke the ADP is released and the myosin stays bound to the actin in a RIGOR state
Do skeletal muscle cells contain gap junctions?
No, they are each individually innervated by nerve motor axon. Cardiac and smooth muscle are the only types to have gap junctions between cells.
