Skeletal Muscle Part 2 Flashcards
What is the Sliding Filament Theory of Muscle Contraction?
The Sliding Filament Theory of Muscle Contraction explains muscle contraction by the sliding movement of thin (actin) and thick (myosin) filaments, causing the overlap of these filaments and shortening of sarcomeres.
What makes up the Thick Filament?
- Thick filaments made of myosin proteins
- Tail points towards center (M line) and globular heads point towards the Z disk
What is present on the Myosin head?
What are the 3 components of the Thin Filaments?
Actin
- Binding site for myosin
Tropomyosin
- Blocks myosin-binding site on actin (at rest)
Troponin
- Bound to tropomyosin
What are 3 Binding Sites of Troponin?
- Actin (I, inhibitory)
- Tropomyosin (T)
- Calcium (C)
What is the function of the Sacroplasmic reticulum?
store, release and uptake calcium
What are the Initial steps of Muscle Contraction?
- The Sacroplasmic Reticulum releases Calcium after encountering an AP
- The Ca binds to binding site on Troponin
- This causes a conformational change and Tropomyosin moves, exposing the active sites on actin.
- This allows Myosin to bind to actin and form a cross bridge
What is Cross Bridge Formation?
When Myosin had bound to Actin
(once tropomyosin has shifted)
What is Power Stroke?
Swiveling of Myosin heads, pulling the actin towards the M line of the sacromere. (USES ATP)
This cause the myofibrils to shorten, causing the whole muscle fiber to shorten and cause muscle contraction.
What is Cross-Bridge Cycling?
After one Power Stroke, the mysoin grabs on to another active site on actin and create another Cross-Bridge.
This results in another Power Stroke and Sacromere shortening.
What is defined as a one sacromere?
Z disc to Z disc
In what order to sacromeres shorten?
All sarcomeres shorten at the same time
What happens to the H zone as the muscle contracts?
H zone gets smaller and small, eventually not being visible.
What happens to the I band as the muscle contracts?
The I band gets smaller and smaller, eventually not being visible
What happens to the A band as the muscle contracts?
The A band remains the same size
What are Costameres?
What are there roles?
Proteins that link the z-disks of the outermost myofibrils to the sarcolemma/ECM (Extracellular Matrix)
- Maintains mechanical integrity of the sarcolemma
- Role in lateral force transmission
- Initiates intracellular signaling
What makes Costameres?
Dystrophin Proteins
What are Muscular Dystrophies?
- Can result from defects in any of the proteins that make up the costamere
- Causes Sacrolemma to lose its integrity
(For a structure to lose its integrity means that its overall strength, stability, or coherence has been compromised or diminished)
What is Duchenne Muscular Dystrophy?
Duchenne Muscular Dystrophy (DMD)
- Genetic condition that leads to non-functional dystrophin protein
- Myofibres are susceptible to structural deformation and/ or cell death
- Progressive Condition
What is Excitation-Contraction Coupling?
The sequence of events by which an action potential (AP) in the sarcolemma initiates the sliding of actin and myosin
What does an Action Potential in the Sacrolemma trigger?
The Action Potential Triggers release of Ca2+ from the sacroplasmic reticulum
Once an Action Potential reaches the Sacrolemma, how is is carried throughout the Muscle Fiber?
The T-Tubule carries the Action Potential deep into the Myofibers
What is a Motor Neuron?
A Neuron that innervates myofibres (can be a1 or a2)
- One motor neuron can innervate many myofibres
- Each myofibre is only innervated by one motor neuron
What is a Motor Unit?
What is the All or None Principal?
A motor neuron and all the myofibres it innervates
- When a motor neuron fires, all the myofibres in that motor unit will contract (the all or none principle)
What is the Significance of this Image?
Motor Neurons Contain either A1 or A2
A1 only cause A1 Myofibres to fully contract &
A2 only cause A2 Myofibres to fully contract
What is the Neuromuscular Junction?
The neuromuscular junction (NMJ) is the specialized synapse or connection between a motor neuron and a skeletal muscle fiber. It is a crucial point where nerve impulses are transmitted from the motor neuron to the muscle fiber, leading to muscle contraction.
Anatomy of the NMJ
What occurs during an Action Potential at a Motor Neuron?
- Once the AP reaches the Motor Neuron, Ca2+ enters voltage-gated channels in the Axon Terminal
- This cause a release of Acetylcholine from vesicles
- The Ach binds to receptors on the Sacrolemma and opens gated ion channels
- Na+ is released into the muscle cell, causing a change in membrane potential and the initiation of an action potential.
During an Action Potential, what happens once the Na+ is released and enters the muscle cell?
Once Na+ enters the muscle cell, they cause a rapid depolarization of the Sarcolemma
This depolarization propagates along the sarcolemma and into the T-tubules, triggering the release of Ca2+ from the sarcoplasmic reticulum.
