Lecture 9 - Skeletal Mysclr Structure And Function Part 2

Question 1

Where is the NMJ normally located and why?

Answer 1

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

Question 2

What is the sacrolemma?

Answer 2

The plasma membrane of the muscle fibre

Question 3

What cation does excitation heavily rely on?

Answer 3

Calcium

Question 4

What is excitation contraction coupling?

Answer 4

The term used to describe the steps from plasma membrane excitation to calcium release to muscle contraction

Question 5

What is the first step of excitation contraction coupling?

Answer 5

The action potential from the nerve causes the synaptic transmission at the NMJ to trigger an AP in the muscle fibre

Question 6

What is the second step of excitation contraction coupling?

Answer 6

The muscle AP spreads over the surface of the sacrolemma and invades the T-tubular system

Question 7

What is the third step of excitation contraction coupling?

Answer 7

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

Question 8

What is step four of excitation contraction coupling?

Answer 8

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!)

Question 9

What is step five of excitation contraction coupling?

Answer 9

Calcium is pump3 from the intracellular space and back to the SR via SERCA

Calcium unbinds from contractile apparatus

Question 10

What is step seven of excitation contraction coupling?

Answer 10

Relaxation!

Question 11

What is cross bridge sliding?

Answer 11

The sliding filament theory explains the mechanism of muscle contraction based on muscle proteins that slide past each other to generate movement

Question 12

What is the first state of the cross bridge cycle and what does it do?

Answer 12

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)

Question 13

What is the second state of the cross bridge cycle and what does it do?

Answer 13

Second state = released state

ATP binding causes the dissociation of myosin from actin.

Question 14

What is the third state of the cross bridge cycle and what does it do?

Answer 14

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

Question 15

What is the fourth state of the cross bridge cycle and what does it do?

Answer 15

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

Question 16

What is the fifth state of the cross bridge cycle and what does it do?

Answer 16

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

Question 17

What is the last step of the cross bridge cycle and what does it do?

Answer 17

State = attached state

ADP is released. Binding of another ATP molecule causes dissociation of myosin from actin and the cycle repeats itself

Question 18

Muscle tension (or force) depends on what two things?

Answer 18

1. The rate at which the muscle is stimulated

2. The number of muscle fibres recruited

Question 19

What are two things that determine the amount of force delivered when a muscle is activated?

Answer 19

1. The amount of force produced by each fibre

2. The number of fibres activated

Question 20

What determines the number of cross bridges that can form?

Answer 20

The sacromere length

Question 21

What is a sacromere?

Answer 21

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

Question 22

What does the myosin head determine?

Answer 22

How strong the force or tension will be in the muscle

Question 23

What is a recruitment?

Answer 23

The process of activating more motor units to make more force