Muscle: molecular and cellular basis of contraction

Question 1

What do the Z lines define?

Answer 1

determine where the sarcomere is

Question 2

How do actin and myosin generate force?

Answer 2

Move relative to one another

Question 3

How to identify the A band?

Answer 3

It appears dark and light cannot penetrate through it easily, head of myosin sticking out there is no clear passageway for light
- anisotropic band

Question 4

How to identify the I band?

Answer 4

Z line in the middle, just contains actin which are a thin filament arranged in a very regular spatial pattern
- isotropic as light can pass through it

Question 5

What is the M line?

Answer 5

Middle of the myosin area

Question 6

What is nebulin?

Answer 6

Essentially a molecular ruler that sets the length of the actin filaments, starts at Z disc

Question 7

What is titin?

Answer 7

Starts as Z disc but goes as far as the M line, forms a measurement for the length of a sarcomere

Question 8

How to define fibre type of muscle?

Answer 8

Particular isoform of myosin that it contains

Question 9

What is the sliding filament theory of muscle contraction?

Answer 9

Z discs get closer to each other when the muscle contracts, when the muscle is stimulated and contracts
- achieved by myosin heads moving actin filaments towards the centre towards the M line
In relaxed state Z discs are fairly far apart

Question 10

Why is force generation dependant on filament overlap?

Answer 10

In the optimal case, we have substantial overlap, but not too much of the actin in the myosin and that produces OPTIMAL RESTING LENGTH, can produce the most force
As the actin is pulled away from the myosin, the amount of force that can be generated falls until the point where the actin and myosin don’t overlap at all, zero ability to produce force
If the actin is driven too far into the myosin, we also get a drop off in force, particularly if the actin overlaps, force production is essentially nil because the structure cannot function as it should

Question 11

What is the excitation-contraction coupling?

Answer 11

1. Stimulus produces action potential moves along to motor end plate. Where the motor neurone branches and the branches connect with muscle fibre, specialised membrane formation forming the motor end plate
2. Terminal of motor neurone contains lots of mitochondria and vesicles with neurotransmitter (Ach)
3. When a stimulus reaches the end of the motor neurone causes an opening of the Ca2+ channels, allowing Ca2+ to fuse with the edge of the terminal releases Ach into synaptic cleft and binds to Ach receptors on muscle membrane
4. Binding causes depolarisation stimulates action potential which travels along the sarcolemma through a system of tubules in the endoplasmic reticulum pockets

Question 12

How does calcium released from the sarcoplasmic reticulum aid muscle contraction?

Answer 12

As action potential spreads across sarcolema it goes down T tubules and triggers the opening of voltage gated Ca2+ channel (dihydroperidine receptor DHP), allows Ca2+ to enter the sarcoplasm of muscle fibres
- triggers opening in hydnodinc receptors that are present within the temrinal cisternae of sarcoplasmic reticulum
Ca2+ ATPases in sacroplasmic reticulum that withdraw Ca2+ from sacroplasm and allow muscle to go back to its resting state

Question 13

What proteins are involved in muscle contraction?

Answer 13

Actin filamet = 𝛂 - helical arrangement of actin monomers

• Tropomyosin
• Tropnin complex:
• • TnT - binds tropomyosin
• • TnC - binds calcium
• • TnI - inhibits tropomyosin

Question 14

How are myosin binding sites exposed?

Answer 14

Ca2+ binds to troponin C, tropomyosin moves exposing myosin binding sites

Question 15

What is the cross bridge cycle?

Answer 15

1. Without ATP, crossbridge is bound strongly to actin
2. ATP binds to myosin causing crossbridge to detach and moves the position of the head
3. ATP is hydrolysed into ADP and Pi which remain bound
4. In the presence of actin, ADP and Pi are displayed and the power stroke occurs

Question 16

What are the muscle fibre types?

Answer 16

Type I
Type IIa
Type IIx(b)

Question 17

What are features of Type I muscle fibres?

Answer 17

Slow oxidative (SO)
High myoglobin 
Small diameter 
Many capillaries 
Used when tension is needed for a prolonged period 
Soleus muscle

Question 18

What are features of Type IIa muscle fibres?

Answer 18

Fast oxidative glycolytic (FOG)
Intermediate
Quadriceps are a fast muscle

Question 19

What are features of Type IIx (b) muscle fibres?

Answer 19

Fast Glycolytic (FG)
Low myoglobin
Large diameter
Few capillaries

Question 20

What are four major cellular functional compartments?

Answer 20

1. Membrane excitation
2. Excitation - contraction coupling
3. Contraction
4. Energy supply

Question 21

How to identify fibre type? (Classically)

Answer 21

Reveal ATPase of the myosin head, incubate with appropriate solutions @ different pH, could identify type I or IIa

Question 22

How to identify fibre type? (Now)

Answer 22

Use antibodies that recognise specific myosin isoforms

- fast v slow myosin

Question 23

What are advantages of myosin immuno-staining?

Answer 23

Co-expression
Labelling lasts
Post mortem

Question 24

What is the triple labelling system for multiple monoclonal antibodies?

Answer 24

By labelling each antibody with a different flurophore/colour reagent, can identify variety of different myosin isoforms
- fast = red
- slow - green
- neonatal = pink
Some fibres express more than one type of myosin, seen in disease or where there has been a training effect that might have altered the balance of myosin isoforms

Question 25

How is force regulated?

Answer 25

Contraction of single muscle fibre is all or none
Regulation of force comes from:
- rapidity of nerve impulses
- number of muscle fibres that contract at once

Question 26

What is a motor unit?

Answer 26

Single motor neuron and fibre it innervates
- eye muscles = 1:1 muscle:nerve
- hamstrings = 300:1 muscle:nerve
Gradual increase in motor unit involvement to increase force
Smaller motor units stimulated first, followed by larger units