Structure of skeletal muscles

Question 1

What are muscles?

Answer 1

Muscles are effector organs that respond to nervous stimulation by contracting and so bring about movement

Question 2

What are the three types of muscle in the body?

Answer 2

Cardiac, smooth and skeletal

Question 3

What are individual muscles made up of?

Answer 3

Millions of tiny muscle fibres called myofibrils

Question 4

What are the two types of protein filament that make up myofibrils?

Answer 4

Myosin and Actin

Question 5

What is the difference between myosin and actin?

Answer 5

Actin is thin and consists of two strands twisted around one another whereas myosin is thick and consists of long rod-shaped fibres with bulbous heads that project to the sides

Question 6

Why do myofibrils appear striped?

Answer 6

They have alternating light and dark bands

Question 7

What are the light bands called and why are they light bands?

Answer 7

The light bands are called isotropic bands and they appear lighter because the actin and myosin filaments do not overlap in this region

Question 8

What are the dark bands called and why are they dark bands?

Answer 8

The dark bands are called anisotropic bands and they appear darker because the actin and myosin filaments overlap in this region

Question 9

What is at the centre of each anisotropic band?

Answer 9

A lighter-coloured region called the H-zone

Question 10

What is at the centre of each isotropic band?

Answer 10

A line called the Z-line

Question 11

What is the distance between the adjacent Z-lines?

Answer 11

A sarcomere

Question 12

When the muscle contracts, what happens to the sarcomeres?

Answer 12

They shorten and the pattern of light and dark bands changes

Question 13

What are two other important proteins that are found in muscle?

Answer 13

Tropomyosin and globular protein (troponin) involved in muscle contractions

Question 14

What are the two types of muscle fibre?

Answer 14

Slow-twitch and fast twitch

Question 15

What sort of contractions do slow-twitch muscle fibres produce?

Answer 15

They contract more slowly and provide less powerful contractions over a longer period

Question 16

What sort of work are slow-twitch muscle fibres adapted to?

Answer 16

Endurance work such as running marathon

Question 17

What type of respiration do slow-twitch muscle fibres carry out and why?

Answer 17

Aerobic respiration in order to avoid a build up of lactic acid which would cause them to function ineffectively

Question 18

How are slow-twitch muscle fibres adapted to their role?

Answer 18

They have a large store of myoglobin, a supply of glycogen to provide a source of metabolic activity, a rich supply of blood vessels to deliver oxygen and glucose and they have many mitochondria to produce ATP

Question 19

What sort of contractions do fast-twitch muscle fibres produce?

Answer 19

They contract more rapidly and produce powerful contractions but only for a short period of time

Question 20

What sort of work are fast-twitch muscles adapted to?

Answer 20

Intense exercise such as weightlifting and sprinting

Question 21

How are fast twitch muscles adapted to their role?

Answer 21

They have thicker, more numerous myosin filaments; A high concentration of enzymes involved in anaerobic respiration and a store of phosphocreatine that can rapidly generate ATP from ADP in anaerobic conditions

Question 22

What is a neuromuscular junction?

Answer 22

The point where a motor neurone meets a skeletal muscle fibre

Question 23

Why are many neuromuscular junctions present throughout a muscle?

Answer 23

Rapid muscle contractions are often essential for survival

Question 24

What is a motor neurone?

Answer 24

A neurone that transmits action potentials from the central nervous system to an effector

Question 25

What is a motor unit?

Answer 25

Where all muscle fibres are supplied by a single motor neurone and act together as a single functional unit

Question 26

How does a motor unit allow greater control over the force of contraction?

Answer 26

If only a slight force is required then only a few units are stimulated, but if a more powerful force is necessary then a larger number of units can be stimulated

Question 27

What happens when a nerve impulse is received at a neuromuscular junction?

Answer 27

The synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine which diffuses to the postsynaptic membrane, altering its permeability to sodium ions, which rapidly enter and depolarise the membrane

Question 28

What prevents the over-stimulation of the muscle?

Answer 28

The acetylcholine is broken down into choline and ethanoic can acid by acetylcholinesterase, which diffuse back into the neurone where they recombine using energy provided by mitochondria