Skeletal Muscle Part 2

Question 1

What is the Sliding Filament Theory of Muscle Contraction?

Answer 1

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.

Question 2

What makes up the Thick Filament?

Answer 2

• Thick filaments made of myosin proteins
• Tail points towards center (M line) and globular heads point towards the Z disk

Question 3

What is present on the Myosin head?

Answer 3

Question 4

What are the 3 components of the Thin Filaments?

Answer 4

Actin

• Binding site for myosin

Tropomyosin

• Blocks myosin-binding site on actin (at rest)

Troponin

• Bound to tropomyosin

Question 5

What are 3 Binding Sites of Troponin?

Answer 5

• Actin (I, inhibitory)
• Tropomyosin (T)
• Calcium (C)

Question 6

What is the function of the Sacroplasmic reticulum?

Answer 6

store, release and uptake calcium

Question 7

What are the Initial steps of Muscle Contraction?

Answer 7

1. The Sacroplasmic Reticulum releases Calcium after encountering an AP
2. The Ca binds to binding site on Troponin
3. This causes a conformational change and Tropomyosin moves, exposing the active sites on actin.
4. This allows Myosin to bind to actin and form a cross bridge

Question 8

What is Cross Bridge Formation?

Answer 8

When Myosin had bound to Actin
(once tropomyosin has shifted)

Question 9

What is Power Stroke?

Answer 9

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.

Question 10

What is Cross-Bridge Cycling?

Answer 10

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.

Question 11

What is defined as a one sacromere?

Answer 11

Z disc to Z disc

Question 12

In what order to sacromeres shorten?

Answer 12

All sarcomeres shorten at the same time

Question 13

What happens to the H zone as the muscle contracts?

Answer 13

H zone gets smaller and small, eventually not being visible.

Question 14

What happens to the I band as the muscle contracts?

Answer 14

The I band gets smaller and smaller, eventually not being visible

Question 15

What happens to the A band as the muscle contracts?

Answer 15

The A band remains the same size

Question 16

What are Costameres?
What are there roles?

Answer 16

Proteins that link the z-disks of the outermost myofibrils to the sarcolemma/ECM (Extracellular Matrix)

1. Maintains mechanical integrity of the sarcolemma
2. Role in lateral force transmission
3. Initiates intracellular signaling

Question 17

What makes Costameres?

Answer 17

Dystrophin Proteins

Question 18

What are Muscular Dystrophies?

Answer 18

• 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)

Question 19

What is Duchenne Muscular Dystrophy?

Answer 19

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

Question 20

What is Excitation-Contraction Coupling?

Answer 20

The sequence of events by which an action potential (AP) in the sarcolemma initiates the sliding of actin and myosin

Question 21

What does an Action Potential in the Sacrolemma trigger?

Answer 21

The Action Potential Triggers release of Ca2+ from the sacroplasmic reticulum

Question 22

Once an Action Potential reaches the Sacrolemma, how is is carried throughout the Muscle Fiber?

Answer 22

The T-Tubule carries the Action Potential deep into the Myofibers

Question 23

What is a Motor Neuron?

Answer 23

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

Question 24

What is a Motor Unit?
What is the All or None Principal?

Answer 24

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)

Question 25

What is the Significance of this Image?

Answer 25

Motor Neurons Contain either A1 or A2
A1 only cause A1 Myofibres to fully contract &
A2 only cause A2 Myofibres to fully contract

Question 26

What is the Neuromuscular Junction?

Answer 26

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.

Question 27

Anatomy of the NMJ

Answer 27

Question 28

What occurs during an Action Potential at a Motor Neuron?

Answer 28

1. Once the AP reaches the Motor Neuron, Ca2+ enters voltage-gated channels in the Axon Terminal
2. This cause a release of Acetylcholine from vesicles
3. The Ach binds to receptors on the Sacrolemma and opens gated ion channels
4. Na+ is released into the muscle cell, causing a change in membrane potential and the initiation of an action potential.

Question 29

During an Action Potential, what happens once the Na+ is released and enters the muscle cell?

Answer 29

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.