MSK 2: Muscle

Question 1

What are the three types of muscle?

Answer 1

Smooth muscle
Cardiac muscle
Skeletal muscle

Question 2

Which type of muscle is under voluntary control?

Answer 2

Skeletal muscle

Question 3

What influences the contraction of cardiac muscle?

Answer 3

Autonomic nervous system and circulating chemicals.

Question 4

Which division of the nervous system causes skeletal muscle to contract?

Answer 4

Somatic nervous system

Question 5

What is the function of skeletal muscle?

Answer 5

To bring about movement

Question 6

What is a fasicle?

Answer 6

A bundle of myofibres

Question 7

Order from macroscopic to microscopic: Myofibril, myofilament, myofibre.

Answer 7

Myofibre, myofibril, myofilament.

Question 8

Which covers fasicles, perimysium or endomysium?

Answer 8

Perimysium

Question 9

What is the name of the muscle cell plasma membrane?

Answer 9

Sarcolemma

Question 10

What is a T-tubule?

Answer 10

A tunnel into the centre of the centre of the myofibre.

Question 11

What types of protein is muscle composed of?

Answer 11

Actin and myosin

Question 12

What gives the muscle a striated appearance?

Answer 12

The light and dark bands, arranged in sarcomeres.

Question 13

Label (c)

Answer 13

Actin, myosin, H zone, M line, Z disc, A band, M line,

Question 14

What is the structure of myosin?

Answer 14

A protein with two globular heads and a tail formed from two alpha helices.

Question 15

What is the structure of actin?

Answer 15

A protein twisted into helix, each molecule with a myosin binding site. Filaments also contain troponin and tropomyosin.

Question 16

What is the sliding filament theory?

Answer 16

During contraction the I bands become shorter, the A band remains the same length and the H zone narrows or disappears, due to the filaments sliding over one another.

Question 17

How are muscle contractions initiated?

Answer 17

• AP opens voltage gated calcium channels.
• Calcium ions enter pre-synaptic terminal and triggers exocytosis of vesicles containing acetylcholine.
• ACh binds to receptors and induces an AP in muscle
• Local currents flow from depolarised region to adjacent region, so AP spreads along surface.
• ACh broken down by acetylcholine esterase.

Question 18

When the AP is on the surface of the muscle, how is a contraction then activated?

Answer 18

• AP propagates along surface membrane and into t tubules.
• Dihydropyridine (DHP) receptor in T-tubule membrane senses increased voltage and changes shape of protein linked to ryanodine receptor, opening to allow calcium out of the sarcoplasmic reticulum into space around filaments.
• Ca2+ binds to troponin
• Tropomyosin moves allowing crossbridges to attach to actin.
• Calcium is actively transported into the sarcoplasmic reticulum continuously while APs continue.

Question 19

What is excitation contraction coupling?

Answer 19

• In the presence of calcium ions, troponin moves from the tropomyosin chain.
• Movement exposes myosin binding site on surface of actin chain
• ‘Charged’ myosin heads bind to exposed site
• Binding and discharge of ADP causes myosin head to picot (power stroke), pulling actin towards centre of sarcomere.
• ATP binding releases myosin head from chain
• Hydrolysis to ADP recharges myosin head.

Question 20

Where is voluntary neural control from?

Answer 20

Upper motor neurons in brain and lower motor neurons in brainstem or spinal cord.

Question 21

What is a motor unit?

Answer 21

A single motor neuron and all the muscle fibres it innervates.

Question 22

How many muscle fibres does each motor neuron supply on average?

Answer 22

Around 600

Question 23

What the three types of motor unit?

Answer 23

Slow (type I)
Fast, fatigue-resistant (type IIa)
Fast, fatiguable (type IIb)

Question 24

What are the differences between slow and fast types of motor units?

Answer 24

Slow type motor units have smaller diameter cell bodies, smaller dendritic trees, thinner axons and higher aerobic capacity but lower anaerobic capacity.

Question 25

Is the myoglobin content in type IIb muscle fibres high or low compared to type I and IIa?

Answer 25

Low

Question 26

How are motor units classified?

Answer 26

Amount of tension generated
Speed of contraction
Fatiguability of the motor unit

Question 27

What are the mechanisms by which the brain regulates the force that a single muscle can produce?

Answer 27

Recruitment and rate coding.

Question 28

What is recruitment?

Answer 28

Motor units are not randomly recruited, they are governed by the size principle. Smaller units are recruited first and as more force is required, more units are recruited.

Question 29

What is rate coding?

Answer 29

A motor unit can fire at a range of frequencies (slower units fire at a lower frequencies), as the firing rate increases, the force produced by the unit increases. Summation occurs when units fire at a frequency too fast to allow the muscle to relax between arriving action potentials.

Question 30

What are neurotrophic factors?

Answer 30

A type of growth factorthat prevent neuronal death and promote growth of neurons after injury.

Question 31

What are motor unit and fibre characteristics dependent on?

Answer 31

The nerve that innervates them.

Question 32

What are the three types of muscle contraction?

Answer 32

Concentric (shorten)
Eccentric (lengthen)
Isometric (no change in length)

Question 33

What is the most common example of plasticity of motor neurons?

Answer 33

Type IIb to type IIa following training.

Question 34

What is an example of type I to II?

Answer 34

Severe deconditioning or spinal cord injury.

Question 35

What change in motor neurons is ageing associated with?

Answer 35

Loss of type I and II, but also preferential loss of type II fibres. leading to slower contraction times.