15.7 Structure of Skeletal Muscle Flashcards
What are muscles
Effector organs that respond to nervous stimulation by contracting and so bringing about movement
What are the three types of muscle in the body and where are they found
- Cardiac muscle - heart only
- Smooth muscle - walls of blood vessels and the gut
- Skeletal muscle - makes up the bulk of body muscle in verterbrates. It is attached to bone and acts under concious control
What are individual muscles made up of
Millions of tiny muscle fibres called myofibrils
Alone, myofibrils are weak - but collectively they can be very strong and powerful
If muscle was made up of individual cells joined end to end it wouldnt be able to contract effectively. Why is this>
Because the junction between adjacent cells would be a point of weakness that would reduce the overall strength
How has the structure of muscles overcome having points of fundamental weakness
The separate cells have become fused together into muscle fibres.
These muscle fibres share nuceli and also a cytoplasm, called a sarcoplasm.
Where is the sarcoplasm found
Around the circumference of the fibre
What is found in large concentrations within the sarcoplasm
Mitochondria and endoplasmic reticulum
What are the two types of protein filament that make up myofibrils
- Actin
2. Myosin
Describe the structure of actin
Thin, two strands wrapped around eachother
Describe the structure of myosin
Thicker and consists of long rod-shaped tails with bulbous heads that project to the side
Why do myofibrils appear striped
Because of their alternating light-coloured and dark-coloured bands
What are light bands
Isotropic bands
What are dark bands
Anisotropic bands
Why do isotropic bands appear light
Because the thicker and thin filaments dont overlap (actin and myosin)
Why do anisotropic bands appear dark
Because the thicker and thin filaments overlap (actin and myosin)
What are the thin filaments
Actin
What are the thick filaments
Myosin
What is the centre of the anisotropic (A band) band called
A lighter coloured region called the H-zone
What is the centre of the Isotropic (I Band) band called
A line called the Z-Line
What is the distance between Z-lines called
The sarcomere
When muscles contract, describe what happens to sarcomeres
The sarcomeres shortern and the pattern of light and dark bands change
What is tropomyosin
Fibrous strand around the actin filament
What are the two different types of muscle fibres
- Fast-twitch fibres
2. Slow-twitch fibres
Describe the nature of slow-twitch fibres
- 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
Where are slow-twitch fibres most commonly found
The calf muscle, to maintain the body upright
What adaptations do slow-twitch fibres have
- Large myoglobin store which stores oxygen
- Rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration
- Numerous mitochondria for ATP production
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
Where are fast-twitch fibres most commonly found
Biceps
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
What is a neuromuscular junction
The point where a motor neurone meets a skeletal muscle fibre
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
Why are there many neuromuscular junctions along the muscle
Because rapid and coordinated muscle contraction is essential for survival.
What do multiple muscle fibres supplied by a single motor neurone act together as
A function unit known as a motor unit.
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
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
What are the similarities between a neuromuscular junction and a synapse
- Both have neurotransmitters that are transported via diffusion
- Have receptors, that on binding with the neurotransmitters, cause an influx of sodium ions
- Use a Na/K pump to repolarise the axon
- Use enzymes to breakdown the neurotransmitter
