Lecture 9 - Skeletal Mysclr Structure And Function Part 2 Flashcards
Where is the NMJ normally located and why?
In the middle third of the fibres length, so the wave of depolarization (action potential, AP) spreads over the sacrolemma away from the NMJ in both directions
What is the sacrolemma?
The plasma membrane of the muscle fibre
What cation does excitation heavily rely on?
Calcium
What is excitation contraction coupling?
The term used to describe the steps from plasma membrane excitation to calcium release to muscle contraction
What is the first step of excitation contraction coupling?
The action potential from the nerve causes the synaptic transmission at the NMJ to trigger an AP in the muscle fibre
What is the second step of excitation contraction coupling?
The muscle AP spreads over the surface of the sacrolemma and invades the T-tubular system
What is the third step of excitation contraction coupling?
Depolarization within the T-tubular system triggers release of the Ca2+ from the nearby terminals of the SR
Mechanical activation of the sacroplasmic reticulum (SR) Ca2+ release channel by the VS
What is step four of excitation contraction coupling?
Ca2+ release into the sacroplasm promotes binding of Ca2+ to the contractile apparatus
Myosin then binds actin and the filaments slide
Force is generated (contraction!)
What is step five of excitation contraction coupling?
Calcium is pump3 from the intracellular space and back to the SR via SERCA
Calcium unbinds from contractile apparatus
What is step seven of excitation contraction coupling?
Relaxation!
What is cross bridge sliding?
The sliding filament theory explains the mechanism of muscle contraction based on muscle proteins that slide past each other to generate movement
What is the first state of the cross bridge cycle and what does it do?
First state = attached state
We start at the cycle with a myosin cross bridge attached to actin. At “rest” (Ca2+ levels vary low in fibre cytoplasm)
What is the second state of the cross bridge cycle and what does it do?
Second state = released state
ATP binding causes the dissociation of myosin from actin.
What is the third state of the cross bridge cycle and what does it do?
Third state = cocked state
ATP hydrolysis causes a shape change so that the myosin head is “cocked”. The products of hydrolysis ADO and the inorganic phosphate remain bound
What is the fourth state of the cross bridge cycle and what does it do?
Fourth state = cross-bridge state
A cross bridge forms and myosin head binds to a new position on actin.
Cocking of the myosin head puts it in line with a new binding site on the actin filament
What is the fifth state of the cross bridge cycle and what does it do?
State = power-stroke state
P is released. Myosin heads change conformation, resulting in the power stroke. The filaments slide past each other.
Myosin binds to actin and the power stroke occurs. It is this power stroke they generates force, pulling the thin filament toward the Centre of the sacromere
What is the last step of the cross bridge cycle and what does it do?
State = attached state
ADP is released. Binding of another ATP molecule causes dissociation of myosin from actin and the cycle repeats itself
Muscle tension (or force) depends on what two things?
- The rate at which the muscle is stimulated
2. The number of muscle fibres recruited
What are two things that determine the amount of force delivered when a muscle is activated?
- The amount of force produced by each fibre
2. The number of fibres activated
What determines the number of cross bridges that can form?
The sacromere length
What is a sacromere?
A sacromere is the basic contractile unit of muscle fibre. Each sacromere is composed of two main protein filaments - actin and myosin - which are the active structure responsible for muscular contraction
What does the myosin head determine?
How strong the force or tension will be in the muscle
What is a recruitment?
The process of activating more motor units to make more force
