Lecture 31 - Muscular Dystrophies - Muscle Structure, Function and Neuromuscular Transmission Flashcards Preview

BIOM30002 - M2M > Lecture 31 - Muscular Dystrophies - Muscle Structure, Function and Neuromuscular Transmission > Flashcards

Flashcards in Lecture 31 - Muscular Dystrophies - Muscle Structure, Function and Neuromuscular Transmission Deck (47):

What are muscle disorders?

Affect one or more of the different muscle tissue type (smooth, cardiac, skeletal)

Muscles needed to move joints, thus these diseases result in motion disorder


Differentiate between myopathies and muscular dystrophies

• Genetic and acquired disorders of the muscle contractile apparatus (thick, thin filaments etc)
• Generally static pathology

Muscular dystrophies:
• Genetic disorders of the supporting structures (DAPC)
• Progressive degeneration
• Sacrolemmal proteins
• Proteins which anchor the contractile apparatus in place


Outline these features of skeletal muscle:
• Location
• Function
• Structure

Talk about the variation seen in these features

• Attached to bone

• Voluntary control
• Different contraction velocities depending on ability
• Variable metabolic processes used to generate energy

• Made up of muscle fibres (the cells)
• Striated
• Variable colour depending on myoglobin content


What are striations?

Muscle fibres (cells) containing alternating light and dark bands


What is myoglobin?

Oxygen storage protein for mitochondria

Has a higher affinity for oxygen that haemoglobin → oxygen movement into muscle cells from blood favoured


Discuss the differences in skeletal and smooth muscle

Skeletal muscle:
• Myofibres: parallel muscle fibres grouped into bundles
• Sacromeres

• Involuntary
• Not striated

NB both are:
• Multinucleated
• Contractile


Describe muscle fibres in detail

Muscle belly - Epimysium - Perimysium - Fasciculus - Endomysium - Muscle fibres (cells) - Myofibrils within cells

Fasciculi: bundles of muscle fibres

Muscle fibres: the cells
Consists of:
• Sarcolemma (membrane)
• Myofibrils: contractile component
• Sarcoplasm
• Multinucleated


What is the sarcolemma?

The membrane of muscle fibres


Compare the location of the following:
• Epimysium
• Perimysium
• Endomysium

These are all cocnective tissues surrounding various structures

• Ensheaths entire muscle

• Ensheaths fasciculi

• Ensheaths individual muscle fibres


What is a sarcomere?

Describe its structure in detail

Organised subunit repeated along the length of muscle fibres

Smallest contractile portion of a muscle

• Thick (myosin) filaments, w/ globular heads: 'Cross-bridges'
• Thin (actin) filaments
• M line: central anchor of sarcomere
• A band: length of myosin filaments, + actin (Dark band)
• H zone: centre of sarcomere, myosin filaments
• Z disc: attaches actin filaments in adjacent sarcomeres
• I band: isotropic, aligned actin filaments (Light band)
• Troponin
• Tropomyosin: weaved around the actin filaments


Describe the sliding filament model of muscle contraction

At rest:
• Tropomyosin covers myosin binding sites on actin

Muscle contraction:
1. Ca2+ influx into cytoplasm and binds to troponin
2. Tropomyosin moves, revealing myosin binding sites on actin
3. Myosin heads bind to actin forming cross bridges
4. Release of ADP and Pi
5. Conformational change of myosin head: powerstroke
6. Actin pulled into the centre of the sarcomere: Z discs pulled into centre
7. ATP binds myosin head, cross-bridge breaks, myosin returns to unattached position
8. ATP hydrolysis brings about cocking of myosin head


What does the sarcoplasm contain?

Fat particles


What are 'cross-bridges'?

The globular myosin heads protruding from myosin filaments


When do the following events occur:
• Cocking of myosin head
• Breakage of cross-bridges
• Powerstroke

Cocking of myosin head: ATP hydrolysis

Breakage of cross bridges: new ATP binding to myosin head

Powerstroke: ADP and Pi release from myosin head


Describe the events that lead to muscle contraction

1. Motor neuron ACh release onto motor end plate
2. ACh binds to nAChR
3. Change in nAChR conformation → influx of Na+ ions into muscle fibres
4. Initiation of post-synaptic action potential in muscle
5. AP travels along T (transverse) tubules until it reaches sarcoplasmic reticulum
6. AP changes permeability of SR → Ca2+ ions flow into sarcomere
7. Ca2+ binds troponin → tropomyosin pulled away to reveal myosin binding site on actin


List some of the supporting proteins for the myofibrils

• Dystrophin-associate glycoprotein complex

• Sarcoglycans

• Dystrophin

• Emerin, lamin A (proteins of the nuclear envelope)


Describe muscle metabolism

ATP as energy

Sources of ATP:
1. Within fibre
• Enough for muscle contraction for a few seconds

2. Creatine phosphate
• High energy molecule stored in muscle cells
• Transfers its high energy phosphate group to ADP to form ATP
• Enough ATP generated to maintain contraction for 15 seconds

3. Glycogen stored within cells
• Glycogenolysis → glucose
• ATP then generated from glucose

4. Glucose and fatty acids form blood stream
• Liver glycogen broken down into glucose
• Fatty acids from adipose cells and liver
• ATP generated from these by cellular respiration


What is CrPo4?

Creatine phosphate 4

High energy molecule

Transfers its high energy phosphate group to ADP to form ATP


Compare the various pathways of cellular respiration

1. Anaerobic (glycolysis)
• No O2 present, i.e. does not require oxygen
• Glucose → pyruvic acid → lactic acid
• Only 2 ATP generated
• Lactic acid diffuses into blood, and then to the liver, where it is converted back to pyruvic acid
(in the presence of O2 this can be oxidised into mitochondria)
• Occurs in cytosol

2. Aerobic (oxidative)
• Pyruvate & fatty acids → CO2 and H20
• Requires oxygen
• Occurs in mitochondria
• Produces 36 ATP
• Slower than anaerobic respiration


What happens when ATP from creatine phosphate is depleted?

Anaerobic respiration forced to begin


How long can anaerobic respiration provide ATP?

30 secs


Describe the timeline of cellular respiration in muscle

1. ATP/Cr system

2. Anaerobic respiration - 30 secs

3. Aerobic respiration

-- oxygen depleted --

4. Anaerobic respiration may still support further muscle contraction

5. Accumulation of lactic acid and depletion of ATP, O2 and glycogen lead to muscle fatigue → halted muscle contraction


What is the first thing to be depleted in the muscle?

ATP/Cr System


What energy production system takes the longest to kick in?

Aerobic respiration


Compare fibre types in skeletal muscle

Type I: red fibres
• Slow oxidative
• Slow twitch
• Fatigue resistant
• Large amount of myoglobin
• Many mitochondria
• Many blood capillaries (i.e. good blood supply)
• Slow rate of ATP splitting
• Aerobic respiration
• Slow contraction velocity
• Found in postural muscles

Type IIA: 'red' fibres
• Fast twitch 'A'
• Fatigue resistant (but not as much as Type I)
• Large amounts of myoglobin
• Many mitochondria
• Many capillaries
• High ATP generation capacity by oxidation
• Great rate of ATP splitting
• High contraction velocity

Type IIB: 'white' fibres
• Fast twitch B
• Fatigue-able
• Low myoglobin content
• Few mitochondria
• Few capillaries
• Large amount of glycogen
• Great rate of ATP splitting
• Anaerobic glycolysis
• Fatigue


Which types of muscle fibres contain large amounts of myoglobin?

Type I
Type IIA


Which are the fast twitch, fatigue resistant muscle fibres?

Type IIA


Compare the types of physical activity enabled by the various types of muscle fibres

Type I: endurance running

Type IIA: middle distance running, swimming

Type IIB: sprinting


Describe different muscle fibre composition of different muscles

What can change the proportions?

Individual muscles are a mixture of all 3 types of muscle fibre

Proportions vary depending on action of that muscle

Exercise can induce changes:
• Endurance athletes have more type I fibres
• Sprinters have more type IIB fibres


Describe the sex and age dependence of muscle fibre distribution

No sex or age differences

i.e. N° of mucles fibres constant throughout life


Describe muscle fibre participation in various movements

Weak contraction: only type I motor units activate

Stronger contraction: type IIA + type I

Maximal contractions: type IIB/X fibres


Heat production?

Muscle tissue also is a major organs giving rise to heat in the body


What does multinucleate cells indicate?

Multiple cells have fused to form the muscle fibres

These precursors are called myofibroblasts


What are the boundaries of the sarcomere?

From on Z disc to another


What are I bands?

Region containing only actin filaments
Spans ends of two sarcomeres

Decreases in length in muscle contraction


What are the A bands?

Length of the myosin filaments

Doesn't change size in muscle contraction


What is the H zone?

Only myosin

Disappears in muscle contraction


What are the:
• Lines
• Zones
• Bands
• Discs?

Zones: H

Bands: I and A

Discs: Z

Lines: M


Which filaments are attached to the Z discs?

Actin filaments


Which filament has a globular protein head?



Characterise the formation and breaking of the myosin heads to actin

Occur independently

If it happened synchronously, the actin fibres would slip out


What are T-tubules?

Pores that extend from the muscle fibre surface into the sarcoplasm and SR


Which fibre types are low in mitochondria, myoglobin and capillaries?

'White' fibres
Type IIB


Which fibre types are important in postural muscles?

Type I fibres


Which fibre types are fatigue-resistant?

Type I
Type IIA


What is the only thing that changes muscle fibre type distribution?


Age and sex do not affect composition


What is a sarcoplasm?

Similar to cytoplasm in regular cells