15.7 Structure of Skeletal Muscle

Question 1

What are muscles

Answer 1

Effector organs that respond to nervous stimulation by contracting and so bringing about movement

Question 2

What are the three types of muscle in the body and where are they found

Answer 2

1. Cardiac muscle - heart only
2. Smooth muscle - walls of blood vessels and the gut
3. Skeletal muscle - makes up the bulk of body muscle in verterbrates. It is attached to bone and acts under concious control

Question 3

What are individual muscles made up of

Answer 3

Millions of tiny muscle fibres called myofibrils

Alone, myofibrils are weak - but collectively they can be very strong and powerful

Question 4

If muscle was made up of individual cells joined end to end it wouldnt be able to contract effectively. Why is this>

Answer 4

Because the junction between adjacent cells would be a point of weakness that would reduce the overall strength

Question 5

How has the structure of muscles overcome having points of fundamental weakness

Answer 5

The separate cells have become fused together into muscle fibres.

These muscle fibres share nuceli and also a cytoplasm, called a sarcoplasm.

Question 6

Where is the sarcoplasm found

Answer 6

Around the circumference of the fibre

Question 7

What is found in large concentrations within the sarcoplasm

Answer 7

Mitochondria and endoplasmic reticulum

Question 8

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

Answer 8

1. Actin

2. Myosin

Question 9

Describe the structure of actin

Answer 9

Thin, two strands wrapped around eachother

Question 10

Describe the structure of myosin

Answer 10

Thicker and consists of long rod-shaped tails with bulbous heads that project to the side

Question 11

Why do myofibrils appear striped

Answer 11

Because of their alternating light-coloured and dark-coloured bands

Question 12

What are light bands

Answer 12

Isotropic bands

Question 13

What are dark bands

Answer 13

Anisotropic bands

Question 14

Why do isotropic bands appear light

Answer 14

Because the thicker and thin filaments dont overlap (actin and myosin)

Question 15

Why do anisotropic bands appear dark

Answer 15

Because the thicker and thin filaments overlap (actin and myosin)

Question 16

What are the thin filaments

Answer 16

Actin

Question 17

What are the thick filaments

Answer 17

Myosin

Question 18

What is the centre of the anisotropic (A band) band called

Answer 18

A lighter coloured region called the H-zone

Question 19

What is the centre of the Isotropic (I Band) band called

Answer 19

A line called the Z-Line

Question 20

What is the distance between Z-lines called

Answer 20

The sarcomere

Question 21

When muscles contract, describe what happens to sarcomeres

Answer 21

The sarcomeres shortern and the pattern of light and dark bands change

Question 22

What is tropomyosin

Answer 22

Fibrous strand around the actin filament

Question 23

What are the two different types of muscle fibres

Answer 23

1. Fast-twitch fibres

2. Slow-twitch fibres

Question 24

Describe the nature of slow-twitch fibres

Answer 24

• They contract slower than fast-twitch fibres

- They provide less powerful contractions over a longer period of time. They are therefore adapted for endurance work

Question 25

Where are slow-twitch fibres most commonly found

Answer 25

The calf muscle, to maintain the body upright

Question 26

What adaptations do slow-twitch fibres have

Answer 26

1. Large myoglobin store which stores oxygen
2. Rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration
3. Numerous mitochondria for ATP production

Question 27

Describe the nature of fast-twitch fibres

Answer 27

• They contract much more rapidly than slow-twitch fibres

- They provide powerful contractions for a short period. They are therefore adapted for weight lifting

Question 28

Where are fast-twitch fibres most commonly found

Answer 28

Biceps

Question 29

What adaptations do fast-twitch fibres have

Answer 29

• Thicker and more myosin filaments
• High glycogen concentration
• High concentration of enzymes required for anaerobic respiration
• A store of phosphocreatine which is a molecule that can rapidly produce ATP from ADP in anaerobic conditions and so provide energy for contraction

Question 30

What is a neuromuscular junction

Answer 30

The point where a motor neurone meets a skeletal muscle fibre

Question 31

What would happen if there were only one junction, rather than many along the muscle

Answer 31

It would take time for a wave of contraction to travel across the muscle, in which case not all the fibres would contract simultaneously and the movement would be slow

Question 32

Why are there many neuromuscular junctions along the muscle

Answer 32

Because rapid and coordinated muscle contraction is essential for survival.

Question 33

What do multiple muscle fibres supplied by a single motor neurone act together as

Answer 33

A function unit known as a motor unit.

Question 34

What does acting lime a motor neurone allow muscle fibres to do

Answer 34

Give control over the force exerted. If only a slight force is needed, few units are stimulated. If much force is needed, more units are stimulated

Question 35

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

Answer 35

The synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine

The acetylcholine diffuses to the postsynaptic membrane (muscle fibres membrane), altering its permeability to Na+ ions, which enter rapidly, depolarising the membrane

The acetylcholine is broken down by acetylchloinesterase to ensure the muscle isnt over-stimulated

The resulting choline and ethanoic acid diffuse back into the neurone, where they are recombined to form acetylcholine using energy from the mitochondria

Question 36

What are the similarities between a neuromuscular junction and a synapse

Answer 36

1. Both have neurotransmitters that are transported via diffusion
2. Have receptors, that on binding with the neurotransmitters, cause an influx of sodium ions
3. Use a Na/K pump to repolarise the axon
4. Use enzymes to breakdown the neurotransmitter