ANATOMY AND PHYSIOLOGY - Muscle

Question 1

What is the function of skeletal muscle?

Answer 1

Skeletal muscle is attached to the bones and supports the movement of the skeleton

Question 2

Is skeletal muscle under voluntary or involuntary control?

Answer 2

Skeletal muscle is under voluntary control

Question 3

Are skeletal muscle fibres syncytial?

Answer 3

No, skeletal muscle fibres and not syncytial and thus cannot have a rapid coordinated contraction

Question 4

(T/F) Skeletal muscle fibres (cells) run in parallel along the length of the muscle itself

Answer 4

TRUE

Question 5

Why are skeletal muscle fibres (cells) non-dividing cells?

Answer 5

Because skeletal muscle cells are multinucleated cells so cannot undergo normal cell division

Question 6

Why are the nuclei of skeletal muscle fibres found in the periphery of the cell?

Answer 6

Because the myofibrils within the cells take up most of the space, pushing the nuclei into the periphery

Question 7

Explain the process that leads to skeletal muscle cells (fibres) being multinucleated

Answer 7

In utero development, mononucleated myoblasts (embryonic muscle precursors) fuse together to form multi-nucleated skeletal muscle cells

Question 8

What is the structure that surrounds skeletal muscle fibres (i.e. the cell membrane)?

Answer 8

Salcolemma which is made up of an inner plasmalemma and outer basement membrane

Question 9

What is the most abundant organelle found in the skeletal muscle fibre which allows skeletal muscle contraction to occur?

Answer 9

Myofibrils

Question 10

Describe the structures that make up thick filaments in skeletal muscle

Answer 10

Thick filaments are made up of myosin molecules arranged in myosin tails and myosin heads. These myosin heads have an actin binding site and an ATPase binding site

Question 11

When muscle is at rest, which angle are the myosin heads to the myosin tails?

Answer 11

90 degrees to the longitudinal axis of the myosin tails

Question 12

Describe the structure of thin filaments in skeletal muscle?

Answer 12

Actin helix which has a tropomyosin molecule running along the helix, attached by troponin. The actin has a myosin binding site that in the relaxed state is covered by tropomyosin

Question 13

How are thick and thin filaments arranged within the myofibrils of skeletal muscle?

Answer 13

Thick and thin filaments are arranged in repeated sarcomeres along the longitudinal axis of the muscle fibres (cells)

Question 14

Which structures support the thick and thin filaments within the sarcomere arrangement?

Answer 14

M line: structural protein supporting the thick filaments within the sarcomere arrangement
Z line: structural protein supporting the thin filaments within the sarcomere arrangement

Question 15

Skeletal muscle contraction is a voluntary response mediated by ________

Answer 15

A signal from the brain transferred along somatic efferent (motor) neurones to each individual skeletal muscle cell via the neuromuscular junction

Question 16

What is the neuromuscular junction?

Answer 16

The neuromuscular junction is the synaptic connection between the terminal end of a somatic neuron and a skeletal muscle cell

Question 17

Describe how a signal from the brain is transferred from the terminal end of a somatic efferent neurone to a skeletal muscle cell via the neuromuscular junction

Answer 17

1. An action potential travels down the axon to the axon terminal of the somatic efferent neurone
2. This acton potential triggers the opening of voltage gated Ca2+ channels within the membrane of the somatic neurone, causing an influx of Ca2+
3. The influx of Ca2+ stimulates synaptic vesicles to release acetylcholine into the synaptic cleft
4. Acetylcholine binds to receptors located on the membrane of the skeletal muscle cell
5. This triggers the opening of ligand gates Na+ channels, triggering end plate potential which bring the membrane up to threshold to stimulate an action potential

Question 18

What is end plate potential?

Answer 18

A graded potential stimulated at skeletal muscle cells which ALWAYS stimulates an action potential

Question 19

What is a motor unit?

Answer 19

The branches of a single somatic efferent neurone and all of the skeletal muscle fibres it runs to

Question 20

What kind of communication system is a motor unit?

Answer 20

A motor unit is a divergent combination system between excitable cells

Question 21

Describe briefly the sliding filament theory of contraction

Answer 21

When muscle cells contract, the thick and thin filaments over lap, moving relative to each other to reducing the distance between the Z and M lines of the sarcomeres

Question 22

What is cross-bridge cycling?

Answer 22

The process which allows the thick and thin filaments to move relative to each other to allow muscle contraction to occur

Question 23

In which types of muscle does cross-bridge cycling occur?

Answer 23

All muscle types

Question 24

Describe briefly the process of cross-bridge cycling in skeletal muscle

Answer 24

1. ATP binds to the ATPase binding site of the myosin head, phosphorylating the myosin
2. When the muscle cell is stimulated by an action potential, actin binds to the phosphorylated myosin head to form a cross-bridge
3. The binding of the actin de-phosphorylates the myosin, causing the myosin head to move from a 90 degree to an acute angle
4. This movement of the myosin head slides the thick and thin filaments relative to each other, allowing for contraction
5. ATP can now bind to the myosin head again to break the cross bridge and return the myosin head to its original angle

Question 25

Why does cross-bridge cycling need to be regulated in all muscle types?

Answer 25

To prevent constant contraction of the muscle cells

Question 26

Describe how cross-bridge cycling is regulated in skeletal muscle cells

Answer 26

For cross-bridge cycling to occur, the phosphorylated myosin head need to bind to the myosin binding site present on the actin molecule, however this is covered by tropomyosin bound by troponin. Troponin has a Ca2+ binding site on its surface, which in the presence of elevated intracellular Ca2+, will stimulate the movement of the tropomyosin, exposing the myosin binding site to allow the formation of a cross-bridge

Question 27

What are the T-tubules?

Answer 27

T-tubules are extensions of the sarcolemma running transversely across the skeletal muscle cells

Question 28

What is the sarcoplasmic reticulum?

Answer 28

The sarcoplasmic reticulum is a membrane bound structure containing Ca2+ surrounding the myofibrils

Question 29

What are triads?

Answer 29

Triads are the junctions between the T-tubules and the sarcoplasmic reticulum

Question 30

Describe how intracellular calcium levels are elevated in skeletal muscle cells to allow for cross-bridge cycling to occur (excitation-contraction coupling)

Answer 30

Action potentials are propagated along the sarcolemma, the T-tubules and the sarcoplasmic reticulum via the triads. This action potential triggers the opening of voltage-gated Na+ channels on the membrane of the sarcoplasmic reticulum, allowing Ca2+ to move out of the sarcoplasmic reticulum and bind to the Ca2+ binding sites present on troponin, allowing cross-bridge cycling to occur

Question 31

How is Ca2+ taken back up into the sarcoplasmic reticulum after skeletal muscle contraction?

Answer 31

Ca2+ is taken back up into the sarcoplasmic reticulum by ATPase dependent Ca2+ channels

Question 32

What is a twitch?

Answer 32

A twitch is the mechanical response to a skeletal muscle contraction in response to a single action potential

Question 33

What is a tetanic contraction?

Answer 33

A tetanic contraction is the mechanical response to high frequency action potentials leading to the saturation of the troponin Ca2+ binding sites, causing the skeletal muscle to have no rest between contractions

Question 34

What is an unfused tetanic contraction?

Answer 34

An unfused tetanic contraction is the mechanical response to partial relaxation between tetanic contractions

Question 35

What is an isometric contraction?

Answer 35

An isometric contraction is when skeletal muscle is contracted and the length of the myofibril remains the same, but the tension changes

Question 36

What is an isotonic contraction?

Answer 36

An isotonic contraction is when skeletal muscle is contracted and the length of the myofibril shortens, but the tension remains the same

Question 37

What is the function of smooth muscle?

Answer 37

Smooth muscle surrounds visceral organs and vessels to regulate the flow of material

Question 38

Is smooth muscle under voluntary or involuntary control?

Answer 38

Smooth muscle is under involuntary control

Question 39

Are smooth muscle fibres syncytial?

Answer 39

Yes, smooth muscle fibres are syncytial and thus can have a rapid, coordinated contraction due to being connected by gap junctions

Question 40

Describe the structure of thick filaments in smooth muscle

Answer 40

Thick filaments are made up of myosin molecules arranged in myosin tails and myosin heads. These myosin heads have an actin binding site

Question 41

Describe the three structure of thin filaments in smooth muscle

Answer 41

Actin helix which has a tropomyosin molecule running along the helix. The actin has a myosin binding site

Question 42

Why is smooth muscle not striated?

Answer 42

Smooth muscle is not striated as the thick and thin filaments are not arranged in repeated sarcomeres

Question 43

How are thick and thin filaments arranged in smooth muscle fibres?

Answer 43

Smooth muscle contraction is multidirectional so thin filaments are arranged in multiple directions bound to dense bodies, surrounding thick filaments