Muscle Contraction Flashcards
myofibrils
== specialised contractile elements
found in muscle fibres
Myofibrils are composed of individual contractile proteins or myofilaments (thick and thin filaments)
Each myofibril consists of regular arrangement of thick and thin filaments:
- Thick filaments – myosin
- Thin filaments – actin (+ regulatory proteins)
=titin connects thick filaments to the Z line
sarcomere structure
I band : (isotropic) only actin, no overlap
A band : (aisotropic, not isotropic SO NOT FULLY BLACK) entirety of myosin, so there is overlap
H zone : only myosin, no overlap
myofibril : thick filament
thick filament : myosin molecule
titin anchors the myosin molecules on the Z line
has a hinge region which joins myosin heads to the myosin tails
hinge region : undegoes conformational change
–> initaited by hydrolysis of ATP
–> allow power stroke
–> myosin and actin molecules move together to achieve contraction and relaxation
- myosin heads have actin binding site:
binds to the G-actin molecules on the thin filament (actin) - also myosin ATPase site
= for hydrolysis of ATP to result in the conformational change into the cocked position
myofibrils: thin filaments
thin filament : actin molecule
also consists of tropomyosin, troponin
actin molecules anchored on the z line
G-actin molecules are on the nebulin and bind to the binding site on the myosin heads
troponin : regulatory protein
== sits on the G-actin molecules to block the binding
tropomyosin : regulatory protein
== exposes the G-actin molecules when Ca2+ binds for muscle contraction
Sliding filament mechanism
= contraction
during cross bridge cycling and filaments slide past each other
==> = shortening of the sarcomere as there is greater overlap between the thick and thin filaments
==> H zone shortens (H zone where there is no overlap and just thick filament) but A band doesn’t change (sarcomere length shortens not the filaments itself)
Contraction
- Troponin stabilises tropomyosin
- SR releases Ca2+
- Myosin heads are cocked due to ATP hydrolysis (ADP bound), but not in its excited position until the binding sites of G-actin molecules to the myosin heads are exposed
- When Ca2+ binds to troponin, causes tropomyosin to move and be displaced to expose myosin binding sites
- Myosin cross-bridges (thick filaments) can bind with actin molecules (thin filaments)
excited state causes power stroke, myofilaments glide past each other, pulls thin filament towards the M line of the sarcomere
==> heads release the ADP molecules during the contraction of the sarcomere
ATP molecules bind to the myosin heads, breaking the bonds between actin and myosin and stopping this pulling
ATP ==> ADP + Pi and heads are cocked, back to initial positions to begin muscle contraction
Triggering a contraction: AP to motor endplate depolarization
- stimulus at the dendrites of the alpha motor neuron that exceeds threshold
- sufficient depolarisation to engage voltage gated sodium channels and initiate AP that will propagte down entire length of axon to terminal
- high conc of voltage gated Ca2+ channels at the terminal respond to depolarisation by allowing the flow of Ca2+ ions down its electrochemical gradient into presynaptic cell/synaptic terminal
- increase in increase in intracellular calcium will result in exocytosis of your vesicles containing your neurotransmitter acetylcholine
- That acetylcholine can then rapidly diffuse across the synaptic cleft to your nACh receptors on the postsynaptic cell membrane.
- binding of ACh on these receptors allow the influx of Na+ ions and depolarises the sarcolemma
- voltage gated sodium channels initiate AP within cell along length of sarolemma
- AP travels down T-tubule
Triggering a contraction: Motor endplate – Ca2+release
in skeletal
DHP recepors (basically voltage gated calcium channels) and 2 RyR (ryanodine recptors) are directly coupled
==> make Ca2+ flow out of SR and into cytosol
in cardiac muscle:
Ca2+ from the ECF in the T-tubule directly bind to the RyR
RyR allow release of calcium from SR
motor unit
motor neuron and muscle fibres
muscle fibres are only innervated by one single neurons, by one motor neurone can innervate mor than one muscle fibre
And so when you stimulate one motor neuron that will stimulate that will cause contraction in the entirety of that muscle of that motor unit.
Force production by skeletal muscle
- Varying the number of fibres contracting in a muscle
–> more motor units, increasing number of fibres contractin = MORE FORCE - Varying the amount of force developed by each contracting fibre
–> frequency/rate of AP firing
The length-tension relationship
*optimal length (l o ) at which maximum force can be achieved
*If muscle is longer or shorter than lo at onset of contraction, less tension can be achieved
*The maximum tension achieved at the optimal muscle length is greater than the maximum tension produced when the muscle length is less than or greater than the optimal length
overcontraction : beyong optimal force
–> stretched too far, myosin heads fo too far and alignment of mysoin heads and actin molecules
–> less direct overlap and so less capacity to initiate cross bridges and perform power strokes.
undercontraction : less than optimal length
–> insufficient overlap between the actin and myosin filaments, resulting in fewer cross-bridges and decreased force production
muscle contraction differences
skeletal :
* DHP directly coupled to RyR
cardiac :
* Ca2+ enters through DHP = increase in intracellular calcium
* calcium travels to then directly bind to RyR itself opening receptor opening to allow muscle contraction
* calcium dependent calcium release
smooth :
* no sacromeres, still got thick and thin filaments, myosin heads pointing in opposite directions on actin molecules for contraction
* no troponin or tropomyosin : not about having increased intracellular calcium to actually expose those binding sites.
* instead, calmodulin (CaM) converts inactive MLCK to active MLCK (kinase)
* phosphorylates the myosin heads, to cleave ATP more rapidly for faster power strokes and cross bridge cycling
relaxtion
* myosin phosphotase removes Pi groups to reduce their activity and relax the muscles
