Skeletal muscle 1 Flashcards
Structural components of skeletal muscle:
What is skeletal muscle attached to?
Purpose of veins and arterties for muscle?
Skeletal muscles are attached to bone or indirectly attached to bone through ligaments, fascia, cartilage or skin.
Veins and arteries enter and exit muscle to bring nutrients essential for function, and to remove waste products.
Draw the structure of muscle
include:
Skeletal muscle, connective tissue, nerve and blood vessels, muscle fascile, muscle fibre, nucleus
Lecture slide
Myofibre (muscle cell) ultrastructure:
Why are muscles striated?
Muscle cells: nucleus type and myofibrils
Arises from the highly ordered sarcomeric arrangement of intracellular components (contractile protein filaments).
Cells are multinucleated with nuclei lying peripherally.
Cells contain bundles of myofibrils of 1-2 μm diameter which consist of longitudinally-
arranged myofilaments that interdigitate in a strict geometry
Sarcoplasmic reticulum:
Purpose
is an intracellular compartment that acts as a store for Ca2+
what is the terminal cisternae
What is it?
Purpose?
The terminal enlargements of the SR are known as the terminal cisternae, these store the Ca.
It abuts each T-tubule forming a ‘triad’ in longitudinal section.
These are essential for synchronized excitation-contraction coupling.
SR and calcium role
- 3 proteins involved
The SR provides feedback control required to balance intracellular Ca2+ cycling through the concerted action of three major classes of SR calcium-regulatory proteins:
- luminal calcium-binding proteins (calsequestrin, histidine-rich calcium-binding protein, junctate, and sarcalumenin) for calcium storage
- SR calcium release channels (type 1 ryanodine receptor or RyR1 and IP3 receptors) for calcium release
- sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA) pump for calcium reuptake into the SR.
What is the sacromere
- consist of?
The sarcomere is the basic contractile unit of striated muscle. It consists mainly of an array of thick filaments (myosin) which interdigitate with thin filaments that are attached to Z disks at each end of the sarcomere.
Describe the 5 band types of sarcomere and draw a muscle fibre with these bands
- A -band (so called because it is optically anisotropic): contains both thick and thin filaments (longitudinally overlapping).
- I-band: contains thin filaments only (isotropic) which have opposite ‘polarity’ either side of the Z-line.
- Z-line (or disk): an electron-dense region in the middle of the I-band where actin filaments of different ‘polarity’ in each half sarcomere attach. The inter-Z-line distance defines the (variable) sarcomere length.
- H-band: a region in the centre of the A-band which contains thick filaments only (and, hence, is not as dark as the A-band).
- M-line: an electron-dense region in the centre of the H-band where myosin filaments of different ‘polarity’ attach, tail end to tail end)
Thick filaments:
made of?
Where are they found?
The thick filaments comprised mainly of myosin. They form the dark ‘A bands’ of each sarcomere.
Thick filament composition
-100’s molecules myosin
- oriented in opposite directions
-repeating, staggered array of paired heads, 14.3 nm apart
- each pair is displaced one third of way around the filament
-bare zone (no myosin heads) in middle of sarcomere
role of titin
Titin acts as an adjustable molecular spring during contraction, contributing to passive force & maintaining the structural integrity of the sarcomere.
Also holds F actin in place (acts as a spring by coiling up in contraction)
Thin filaments
formation/shape,
associated compounds, nebulin role in thin filaments
two strands of F-actin twisted together into helix
rod-shaped tropomyosin (Tm) molecule lies along the F-actin in a groove
each tropomyosin molecule is associated with 3 other proteins which comprise the troponin
complex of TnT, TnC & TnI
Nebulin helps align the actin filaments and hold F actin in place.
Cross-bridge cycle what is it and the overall process
A high free energy state of the cross-bridges occurs after ATP binding and hydrolysis to form the myosin ADP-Pi complex which has a high affinity for actin.
- Myosin heads in this state bind readily to the thin filament in a preferred 90° orientation.
- With release of bound ADP-Pi, the free energy of the myosin-actin complex has a minimum
at a ~45° rotation.
3.The rotation from the original 90° orientation to the low energy 45° orientation (while the
myosin head is attached to actin) leads to force development and shortening.
4.ATP binding reduces affinity of myosin for actin and crossbridge detaches. In the
absence of ATP the cross bridge cannot detach and this leads to rigor (‘rigor mortis’ in human skeletal muscle).
6 steps of crossbridge cycle
- Absence of ATP there is tight binding in the rigor state. THe crossbridge is at a 45 degree angle relative to the filaments
- ATP binds to the nucloetide binding site on the myosin. Myosin head disssociates from actin
- ATPase activity of myosin hydrolzes the ATP to ADP and inorganic phosphate. Both products remain bound to myosin
- Relaxed state: The myosin head swings over and binds weakly to a new actin molecule. The cross bridge is now at 90 degree relative to the filaments
- Power stroke: Release of Pi initiates the power stroke: In the power stroke the myosin head rotates on its hinge, pushing the assosicated actin filament past it
- End of power stroke, the myosin head releases the ADP and resumes the tight bound rigor state
Contractile process:
Role of calcium
The contractile “machinery” (mainly the thick and thin filament proteins) is in principle always ‘ready to go’, but is prevented from force generation at low myoplasmic Ca2+ concentration.
role of tropomyosin at rest
At rest, the tropomyosin molecules, Tm and filament proteins prevents interaction between actin and myosin by inhibiting the X formation.
what happens when cytosolic Ca2+ concentration increases?
An increase in cytosolic [Ca2+] initiates cross-bridge cycling, and the binding of an energized myosin cross-bridge to actin. When activated, myosin with ADP● Pi bound has a high affinity for its actin binding site.
When cytosolic Ca2+ concentration increases, four Ca2+ ions bind cooperatively to troponin C
causing a conformational change in the troponin complex.
The tropomyosin molecule then shifts with respect to the actin filament, removing the steric interference to cross-bridge interaction.
Crossbridge cycling generates what and what the 2 rules
Crossbridge cycling generates force (for as long as Ca2+ remains high and ATP is present)
what proteins are involved in Intracellular Calcium Regulation at LOW conc and where does Ca go for release
Myoplasmic Ca2+ concentration maintained at very low levels by Ca2+ pump (ATPase) in the longitudinal SR. SR Ca2+ pump (SERCA) has very high affinity for calcium.
Ca2+ transported to release sites in terminal cisternae where it binds to the protein calsequestrin.
Excitation-Contraction (EC) Coupling:
what receptors are present, what one is ESSENTIAL
- general process of EC coupling
dihydropyridine receptors (DHPR, members of the voltage gated L type Ca2+ channel family) in the T-tubular (TT) membrane. They are essential for EC coupling
AND
ryanodine receptors (RyRs) (Ca release channels) in the SR terminal cisternae.
Depolarisation of the TT membrane ‘flips’ the DHPR which induces a conformational change
in the RyR. Protein-protein interaction between DHPR & the large cytoplasmic domain of RyR1 (foot) occurs on depolarisation to bring about SR Ca2+ release.
The mechanism of skeletal muscle Ca2+ release is termed
“voltage dependent calcium release”
Benefits of voltage dependent EC coupling (4)
Rapid kinetics (consistent with requirements of skeletal muscle),
No dependence on current flow per se (ie, a steep F-Em relation),
No reliance on diffusion of any substance from the sarcolemma, and
Activation can therefore occur in the absence of extracellular Ca2+, even (experimentally) in the presence of Ca2+chelators in isolated muscle preparations.
Skeletal muscle roles
1.Provide support
2. produce heat
3.regaulate glucose homeostasis
Examples of striated muscle and non striated
Cardiac and skeletal (striated)
Smooth muscle (non striated
Function classification of muscle and example (cardiac, smooth and skeletal)
Volunatary (skeletal) and involunatary (cardiac and smooth muscle)
Skeletal muscle facts:
innervated or nerverated
Branched or unbranched
Shape?
Diameter?
Innervated
unbranched
long and cylinderal
10-100um
Are muscle cells (myofibres) connected to other myofibres?
No, they lack connections to other myofibres
Where are T tubules located
at the junction of overlap between A and I bands.
Assosicated with 2 terminal cisternae of SR (triad)
draw the thin filaments
Lecture slide
Tropomysoin, G actin molecule, tropoin, nebulin
Thin filaments: Actin
composition/structure
what band does it make
Composed of globular- (G)-actin molecules linked into two strands, twisted into a helix to form F-actin
Makes up I band
Thick filaments: Myosin
forms what band
A band
Thin filaments:
Troponin complex:
3 proteins and their roles
Draw the complete complex
TnT (troponin tropomyosin)
Positions the complex on the tropomyosin (Tm) molecule
TnC (troponin calcium) Contains the Ca2+ binding sites
TnI (troponin inhibitor)
Binds actin & ‘inhibits’ the myosin head from binding to the
actin binding site when cytosolic [Ca2+] is low.
increased plasma levels of Tnl means?
Muscle injury
Tn Complex + Tropomyosin =
Ca sensitive switch
Muscle force depends on
the interaction of the contractile proteins, actin and myosin.
Cross bridge process
- Ca2+ levels increase in cystol
- Ca binds to tropion
- Tropoin-Ca complex pulls tropomysoin away from the actins myosin binding site
- Myosin binds strongly to actin and completes the power stroke
- Actin filament moves
2 roles of ATP
- ATP binding to myosin breaks the link formed between actin & myosin, allowing the cycle to repeat.
- ATP hydrolysis ultimately provides the energy for cross-bridge movement & force development.
What causes the rise in
intracellular Ca2+ that initiates
cross-bridge cycling ?
When Ca2+ is released from the SR
Trop C and calcium binding sites
binds up to 4 ca ions
has 2 high affinity binding sites (always occupied by Ca or Mg) and low affinity Ca sites
Ryanodine Receptors (RyR1) inhibitors
Cytosolic [Mg2+] inhibits skeletal RyR1 activation.
Voltage sensor (DHPR) activation overcomes the Mg2+ inhibition.
