15.7 Structure of Skeletal Muscle Flashcards

1
Q

What are muscles

A

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

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2
Q

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

A
  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
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3
Q

What are individual muscles made up of

A

Millions of tiny muscle fibres called myofibrils

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

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4
Q

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

A

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

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5
Q

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

A

The separate cells have become fused together into muscle fibres.

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

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6
Q

Where is the sarcoplasm found

A

Around the circumference of the fibre

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7
Q

What is found in large concentrations within the sarcoplasm

A

Mitochondria and endoplasmic reticulum

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8
Q

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

A
  1. Actin

2. Myosin

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9
Q

Describe the structure of actin

A

Thin, two strands wrapped around eachother

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10
Q

Describe the structure of myosin

A

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

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11
Q

Why do myofibrils appear striped

A

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

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12
Q

What are light bands

A

Isotropic bands

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13
Q

What are dark bands

A

Anisotropic bands

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14
Q

Why do isotropic bands appear light

A

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

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15
Q

Why do anisotropic bands appear dark

A

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

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16
Q

What are the thin filaments

17
Q

What are the thick filaments

18
Q

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

A

A lighter coloured region called the H-zone

19
Q

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

A

A line called the Z-Line

20
Q

What is the distance between Z-lines called

A

The sarcomere

21
Q

When muscles contract, describe what happens to sarcomeres

A

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

22
Q

What is tropomyosin

A

Fibrous strand around the actin filament

23
Q

What are the two different types of muscle fibres

A
  1. Fast-twitch fibres

2. Slow-twitch fibres

24
Q

Describe the nature of slow-twitch fibres

A
  • 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

25
Where are slow-twitch fibres most commonly found
The calf muscle, to maintain the body upright
26
What adaptations do slow-twitch fibres have
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
27
Describe the nature of fast-twitch fibres
- They contract much more rapidly than slow-twitch fibres | - They provide powerful contractions for a short period. They are therefore adapted for weight lifting
28
Where are fast-twitch fibres most commonly found
Biceps
29
What adaptations do fast-twitch fibres have
- 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
30
What is a neuromuscular junction
The point where a motor neurone meets a skeletal muscle fibre
31
What would happen if there were only one junction, rather than many along the muscle
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
32
Why are there many neuromuscular junctions along the muscle
Because rapid and coordinated muscle contraction is essential for survival.
33
What do multiple muscle fibres supplied by a single motor neurone act together as
A function unit known as a motor unit.
34
What does acting lime a motor neurone allow muscle fibres to do
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
35
What happens when a nerve impulse is received at the neuromuscular junction
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
36
What are the similarities between a neuromuscular junction and a synapse
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