Structure of skeletal muscle

Question 1

What is tropomyosin?

Answer 1

-forms a fibrous strand around the actin filament blocking the myosin binding site

Question 2

What are the two types of protein filament in skeletal muscle?

Answer 2

• actin: thinner than myosin with two strands twisted round each other
• myosin: thicker than actin and consists of long rod-shaped tails with bulbous heads

Question 3

What are myofibrils?

Answer 3

• myofibrils make up muscle fibres
• myofibrils appear striped due to their alternating light-coloured and dark-coloured bands
• the light bands are called I bands
• they appear lighter because the thick and thin filaments do not overlap
• the dark bands are called A bands
• they appear darker because the thick and thin filaments overlap
• at the centre of each A band is a light region called the H-zone
• at the centre of each I band is a line called the Z-line
• the distance between adjacent Z-lines is called a sarcomere which shortens when the muscle contracts I

Question 4

What are the types of muscle fibres?

Answer 4

• slow-twitch fibres

- fast-twitch fibres

Question 5

What are slow-twitch muscle fibres?

Answer 5

• these contract more slowly than fast-twitch fibres and provide less powerful contractions but over a longer period
• they are therefore adapted to endurance work, such as running a marathon
• in humans they are more common in muscles like the calf muscle, which must contract constantly to maintain the body in an upright position
• they are suited to this role by being adapted for aerobic respiration in order to avoid a build-up of lactic acid, which would cause them to function less effectively and present long-duration contraction

Question 6

What are fast-twitch fibres?

Answer 6

• these contract more rapidly and produce powerful contractions but only for a short period
• they are therefore adapted to intense exercise, such as weight-lifting
• as a result they are more common in muscles which need to do short bursts of intense activity, like the biceps muscle of the upper arm

Question 7

How are slow-twitch muscles adapted to fit their role?

Answer 7

• they have a large store of myoglobin (a bright red molecule that stores oxygen, which accounts for the red colour of slow-twitch fibres)
• a rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration
• numerous mitochondria to produce ATP

Question 8

How are fast-twitch muscles adapted to fit their role?

Answer 8

• thicker and more numerous myosin filaments
• a high concentration of glycogen
• a high concentration of enzymes involved in anaerobic respiration which provides ATP rapidly
• a store of phosphocreatine, a molecule that can rapidly generate ATP from ADP in anaerobic conditions and so provide energy for muscle contraction

Question 9

What are neuromuscular junctions?

Answer 9

• a neuromuscular junction is the point where a motor neurone meets a skeletal muscle fibre
• there are many such junctions along the muscle
• If there were only one junction of this type 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
• as rapid and coordinated muscle contraction is frequently essential for survival there are many neuromuscular junctions spread throughout the muscle
• this ensures that contraction of a muscle is rapid and powerful when it is simultaneously stimulated by action potentials
• all muscle fibres supplied by a single motor neurone act together as a single functional unit and are known as a motor unit
• this arrangement gives control over the force that the muscle exerts
• if only slight force is needed, only a few unite are stimulated
• if a greater force is required, a larger number of units are stimulated
• when a nerve impulse is received at the neuromuscular junction, the synaptic vehicles fuse with the presynaptic membrane and release their acetylcholine
• the acetylcholine diffuses to the postsynaptic membrane which is the membrane of the muscle fibre, altering its permeability to sodium ions (Na+), which enter rapidly, depolarising the membrane
• the acetylcholine is broken down by acetylcholinesterase to ensure that the muscle is not over-stimulated
• the resulting collie and ethanoic acid (acetyl) diffuse back into the neurone, where they are recombined to form acetylcholine using energy provided by the mitochondria found there

Question 10

what are the similarities between the neuromuscular junction and a synapse?

Answer 10

• have neurotransmitters that are transported by diffusion
• have receptors, that on binding with the neurotransmitter, cause an influx of sodium ions
• use a sodium-potassium pump to repolarise the axon
• use enzymes to breakdown the neurotransmitter

Question 11

What are the differences between a neuromuscular junction and cholinergic synapse?

Answer 11

• the neuromuscular junction is only excitatory where as the cholinergic synapse may be excitatory or inhibitory
• the neuromuscular junction only links neurones to muscles where as the cholinergic synapse links neurones to neurones, or neurones to other effector organs
• neuromuscular junction is the end of a neural pathway where as the cholinergic synapse is where a new action potential may be produced along another neurone which is the postsynaptic neurone
• in the neuromuscular junction only motor neurones are involved where as in the cholinergic synapse the motor, sensory and intermediate neurones may be involved

Question 12

What are the three types of muscle in the body?

Answer 12

• cardiac muscle
• smooth muscle
• skeletal muscle

Question 13

What is the smooth muscle?

Answer 13

-the smooth muscle is found in the walls of blood vessels and the gut

Question 14

What is the structure of skeletal muscles?

Answer 14

• individual muscles are made up of millions of tiny muscle fibres called myofibrils
• in themselves they produce almost no force while collectively they can be extremely powerful
• just as the threads in a rope are lined up parallel to each other in order to maximise its strength so the myofibrils are arranged in order to give maximum force
• just as the threads of a rope are grouped into strings, the strings are grouped into small ropes and small ropes are grouped into bigger ropes, so muscle is composed of smaller units bundled into progressively larger ones
• if muscle was made up of individual cells joined end to end it would not be able to perform the function of contraction very efficiently
• this is partly because the junction between adjacent cells would be a point of weakness that would reduce the overall strength of the muscle
• to overcome this, muscles have a different structure
• the separate cells have become fused together into muscle fibres
• these muscle fibres share nuclei and also cytoplasm, called sarcoplasm, which is mostly found around the circumference of the fibre
• within the sarcoplasm is a large concentration of mitochondria and endoplasmic reticulum