Muscle structure and function

Question 1

What is a muscle?

Answer 1

A composition of cell types that provide the contractual force required to move body parts or maintain their position against an opposing force

Question 2

What is skeletal muscle?

Answer 2

Large striated fibres that appear nucleated under a microscope

Question 3

What is cardiac muscle?

Answer 3

Has striated fibres but are smaller than skeletal muscle and are branched and uninucleate
Cells are joined in series by junctions called intercalated disks

Question 4

What is functional syncytium?

Answer 4

Each cardiomyocyte contracts in coordination with its neighbouring cells

Question 5

What is smooth muscle?

Answer 5

Small unstriated fibres

Question 6

Describe the composition of skeletal muscle

Answer 6

A skeletal muscle is made up of bundles of muscle fibres
Each muscle fibre is a multinucleate cell containing numerous microfibrils which are highly ordered assemblages of thick myosin and thin actin

Question 7

What is a sarcomere?

Answer 7

A unit of contraction

Question 8

Describe the structure of a sarcomere

Answer 8

Each thin filament is anchored by the Z disk (protein capz) and capped at the end by tropomodulin
Thick filaments are held in position by the elastic molecule Titin

Question 9

Describe the sliding filament theory

Answer 9

The myosin heads pull the actin towards the centre of the myosin filament
During the cross-bridge cycle, the myosin heads interact with actin but this is prevented as tropomyosin covers the myosin-binding sites
The position of tropomyosin is controlled by troponin
When Ca2+ binds to troponin the position of the tropomyosin changes thus exposing the myosin-binding sites

Question 10

Where is the Ca2+ store in skeletal muscle?

Answer 10

In the sarcoplasmic reticulum

Question 11

During contraction does the A band change?

Answer 11

No

Question 12

During contraction does the I band change?

Answer 12

Yes becomes narrower

Question 13

What is the sarcoplasmic reticulum?

Answer 13

A tubular network within the muscle fibre

Question 14

Describe excitation-contraction coupling

Answer 14

A vertebrate muscle fibre is innervated by one motorneuron
Muscle cells can generate action potentials
The action potentials travel along T-tubules and depolarisation leads to a structural change in the DHP receptor which is linked to the RyR receptor
The RyR receptor opens and releases Ca2+ from the sarcoplasmic reticulum

Question 15

What is recruitment?

Answer 15

The more motor neuron units are active, the stronger the tension of the muscle

Question 16

What is 1 motor neuron unit?

Answer 16

1 unit=1 motor neuron+fibres innervated by the motor neuron

Question 17

How is strength controlled in the muscles?

Answer 17

By variations in the number of active motor units

Question 18

What is tetanus?

Answer 18

At high frequencies, the muscles are maximally contracted

Question 19

What are the 2 types of muscle fibres?

Answer 19

Fast and slow-twitch

Question 20

What is the difference between fast and slow twitch genes?

Answer 20

They express different myosin genes with different rates of ATPase activity

Question 21

Describe slow-twitch fibres

Answer 21

Alos called oxidative or red muscle

Contain large amounts of myoglobin, mitochondria and are well supplied with blood vessels

Question 22

Describe fast-twitch fibers

Answer 22

Also called glycolytic or white muscle
Few mitochondria
Little to no myoglobin
Few blood vessels
Major energy source is glycolysis
Can develop max. tension faster and fatigues rapidly

Question 23

What is isometric contraction?

Answer 23

A muscle can build up tension without changing its length

Question 24

What is isotonic contraction?

Answer 24

Where the muscle shortens

Question 25

How are heart muscle cells controlled?

Answer 25

Not controlled by motor neurons

Contractions originate within the muscle and are myogenic

Question 26

How do heart muscle cells have auto-rhythmicity?

Answer 26

A small percentage of muscle cells are specialised to generate and conduct action potentials and are located within the SA node

Question 27

How are the heart muscle cells connected?

Answer 27

Electrically connected by gap junctions within intercalated disks

Question 28

What is the function of smooth muscle?

Answer 28

Causes slow contractions of many internal organs

Question 29

Why is smooth muscle called so?

Answer 29

The actin and myosin are not as regularly arranged as in skeletal or cardiac muscle

Question 30

Describe the Ca2+ mediated change in smooth muscle

Answer 30

The change is on myosin instead of the actin-tropomyosin filament

Question 31

Describe depolarisation in smooth muscle

Answer 31

When as smooth muscle cell is depolarised by a neurotransmitter, Ca2+ enters the cytoplasm and binds to calmodulin which then activated an enzyme that phosphorylates the myosin heads causing them to bind to actin

Question 32

How is smooth muscle organised in muscles?

Answer 32

In sheets with individual cells in electrical contact with one another through gap junctions thus the contractions are coordinated

Question 33

Describe peristasis

Answer 33

In the digestive tract, a coordinated spreading wave of smooth muscle contraction pushes the contents through the central lumen