skeletal muscle (wk8) Flashcards
fascicles
made of linearly aligned muscle fibers (myofiber)
- The muscle fiber is a single, multi-nucleated, elongated cell
what is each muscle fibre in a facile made of
many sarcomeres, arranged linearlyw
what are sarcomeres made of
myofibrils (organelles)
epimysium
connective tissue sheath surrounding the whole muscle and extending from the tendons
perimysium
sheath surrounding each fascicle
endomysium
heath surround each individual muscle fibe
what is below the endoymysim
the sarcolemma – the cell membrane of the muscle cell.
how many myofibrils in each muscle fiber
Muscle fibers are large, multinucleated cells:
* Contain 1000 – 2000 myofibrils
what are myofibrils composed of
myofilaments
Each myofibril composed of many myofilaments, composed of:
- Contractile proteins
▪ Actin
▪ Myosin - Regulatory proteins
▪ Tropomyosin
▪ Troponin - Additional accessory
proteins
what are myofilaments composed o
- Contractile proteins
▪ Actin
▪ Myosin - Regulatory proteins
▪ Tropomyosin
▪ Troponin - Additional accessory
proteinsw
what is the contractile unit of skeletal muscle
Myofibrils are arranged into a series of sarcomeres, which form the contractile unit of skeletal muscle.
what 2 filaments are in a sarcomere
thick (myosin) and thin (actin)
Consist of interdigitating myofilaments, composed of:
* Thin filaments (made of actin)
* Thick filaments (made of myosin)
▪ The striated appearance of skeletal muscle is due to the overlapping of thick and thin filaments
what binds each sarcomere to a z disk
thin filament
what is in the center of a sarcomere and what’s it made of
M line made myosin/ thick filament
sarcomere: z disk and m line
- Each sarcomere is bound by the Z-disk
▪ Thin filament attaches to the Z-disk
▪ Other proteins involved in stretch sensing and signal communication also attach to Z-disk. - At the center is the M-line
▪ Includes a variety of proteins to bind the myosin molecules within the thick filament
how do you get alternating light and dark bands in sarcomeres
- Thick filaments lie between and partially interdigitate with thin filaments
light bands (I bands) vs dark bands (A bands)
- Light bands (aka I bands) are regions of thin filaments that do not overlap with thick filaments
- At the center of the light bands is the Z disk
- Dark bands (aka A bands) are the region were the thick and thin filaments do overlap
in muscle contraction is it the light bands (I bands) or dark bands (A bands) that shorten
I band shortens (light)
During muscle contraction, the I band shortens, while the A band does not change in length
g actin vs f actin
- Monomer:G-actin
- Polymer: F-actin
thin filaments are made of how much actin?
▪ Thin filaments are composed of two strands of F-actin wound together.
▪ F-actin is the major constituent of the thin filament
how can myosin bind actin?
▪ Each G-actin monomer has a binding site for myosin
how are thick filaments made?
Arranged into thick filaments composed of many myosin units. Each unit is composed of head & tail regions
* Head region forms cross-bridges that interact with adjacent actin filaments
▪ Many myosin units are arranged in staggered position into a thick filament
how is a cross bridge formed
head region of myosin forms cross bridge that interacts with adjacent actin filaments (G actin monomer has binding site)
3 biochemical features of myosin head region
▪ ATPase activity
▪ Actin-binding region
▪ ATP binding region
what does tropomyosin associate with
actin
what are the 2 contractile proteins
▪ Actin
▪ Myosin
what are the 2 regulatory proteins
▪ Tropomyosin
▪ Troponin
When a skeletal muscle is in a relaxed state what does tropomyosin bind to
When a skeletal muscle is in a relaxed state, tropomyosin molecules cover the myosin-binding site on G-actin monomers
* This prevents cross-bridge formation between actin and myosin
what prevents the cross bridge formation between actin and myosin
tropomyosin
=relaxed
and troponin I is also there (inhibtirou)
what does troponin associate with
actin
what are the 3 subunits of the complex troponin forms
troponin I, troponin T, troponin C
what do troponin I, troponin T, troponin C all bind to
- Troponin C - binds to Calcium
- Troponin T - binds to Tropomyosin
- Troponin I – binds to actin and inhibits contraction
Troponin C - binds to..?
calcium
Troponin T - binds to …?
tropomyosin
troponin I- binds to..?
binds to actin and inhibits contraction
what inhibits and what allows for cross bridge formation?
troponinC and T allows for cross bridge
tropomyosin and troponin I inhibits formation
i think
how do the 3 subunits of troponin work?
When calcium binds to Troponin C, The troponin complex undergoes a conformational change and Troponin T “pulls” tropomyosin and Troponin I off of the myosin-binding site of G-actin subunits
where is sarcolemma?
The plasma membrane of the muscle fiber is called the sarcolemma
what does the sarcolemma contain?
Contains invaginations called transverse Tubules (aka T- tubules)
* Allows the action potential to be carried deep into the muscle fiber
* Continuous with the extracellular fluid
use of t tubules in sarcolema
-carry AP deep in muscle fibers
-continuous with ECF
The sarcolemma and T-tubules closely associate with…?
the sarcoplasmic reticulum
what is found in high concentrations of the sarcoplasmic reticulum?
Sarcoplasmic reticulum is a specialized endoplasmic reticulum containing high concentrations of calcium
what are terminal cistern
Terminal cisternae of the sarcoplasmic reticulum are specialized regions that associate with the T-tubules
what is the muscle triad
The junction between T-tubules and sarcoplasmic reticulum cisterna is called the muscle triad
▪ Volume of T-tubule is large compared to the SR cisterna
is [Ca2+] high or low in ECF
high
how is a synapse formed at the neuromuscular junction
A motor nerve axon contacts each muscle fiber near the middle of the fiber, forming a synapse called the neuromuscular junction.
what part of the sarcolemma is close to presynaptic terminal for synapse at neuromuscular junction
The region of the sarcolemma in closest contact with the presynaptic nerve terminal is called the motor end plate
what is released at the motor nerve terminal and what does it bind
The motor nerve terminal releases acetylcholine (Ach), which binds to the nicotinic receptor on the sarcolemma.
what happens when Ach binds nicotinic receptor at neuromuscular junction
▪ This gives rise to a graded, depolarizing end-plate potential.
▪ Activates voltage-gated sodium channels → triggering an action potential that propagates along the sarcolemma
▪ The skeletal muscle fibre action potential “looks” very similar to the neuron action potential (slightly longer duration, ~ 10 msec)
which channels does Ach binding activate
v gated Na+ channels
purpose of T tubule in action potential
- The action potential propagates along the surface of the skeletal muscle fiber and penetrate deeper into the muscle fiber via the T-tubules
▪ This is the purpose of the T- tubule – to “bring” the action potential (travels along the sarcolemma) deep within the very large muscle fibres
▪ The action potential then signals to the sarcoplasmic reticulum
what can action potential alongs T tubules activate in the sarcolemma?
L-type Ca2+ channels
what does activation of the L-type Ca2+ channels in the sarcolemma trigger
triggers mechanical activation of Ryanodine receptors (RYR) on the surface of the SR terminal cisterna within the cell
L-type Ca2+ channels activated?
mechanical activation of Ryanodine receptors (RYR)
Where is most of the calcium that activates the skeletal muscles
Most of the calcium that activates the skeletal muscle sarcomere is from the SR, NOT the extracellular fluid
how does l type ca2_ channel and RYR work
EC coupling
- Opening of L-type Ca2+ channels and RYR allows Ca2+ to flow down its concentration gradient into the cytosol of the muscle fiber
▪ Calcium then binds to troponin (C subunit) to expose the binding sites for myosin
whole muscle contraction
- Typical muscle receives input from ~ 100 alpha-motor neurons
▪ The number of muscle fibers that each motor neuron innervates varies widely - All the muscle fibers innervated by a single nerve fiber is called a motor unit
▪ Often these complexes intermingle extensively - Allows motor units to contract in support of one another rather than entirely individually
motor unit
All the muscle fibers innervated by a single nerve fiber
What is the alpha-motor neuron-muscle fiber complex called?
neuromuscular junction
how do you get a muscle contraction?
alpha motor neurons send many high frequency APs
twitch vs tetanus
twitch is few APs with low frequency; tetanus is high AP with high frequency
summation of skeletal muscle twitches –> tetany
- A single action potential from a motor neuron barely causes any force development in the group of muscle fibres (motor unit) that it innervates
▪ Smooth, continuous, forceful contractions occur when the alpha-motor neuron sends many action potentials to a motor unit at a relatively high frequency
tetany
- Tetany = development of force (tension) in a muscle fibre due to many action potentials→greater and greater release of calcium from the SR
fused tetany
▪Fused tetany→maximal force development in the muscle fibres of a motor unit – no individual “twitches” visible
▪Muscle has no time to “clear” calcium back to the SR
