Muscle Function and Contraction

Question 1

What is a muscle fibre/cell?

Answer 1

an elongated multinucleate cell which has a striated appearance and is surrounded by endomysium

Question 2

What is a fascicle?

Answer 2

This is a discrete bundle of muscle cells which are segregated from the rest of the muscle by a connective tissue sheath called perimysium

Question 3

What is a muscle?

Answer 3

it is an organ which consists of hundreds to thousands of muscle cells as well as connective tissue wrappings, blood vessels and nerve fibres and this whole bundle is wrapped externally by epimysium

Question 4

What is a myofibril?

Answer 4

a contractile rod-like element which is composed of sarcomeres arranged end-to-end and appear banded

Question 5

What is a sarcomere?

Answer 5

the segment of a myofibril which is the contractile unit composed of myofilaments made up of contractile proteins

Question 6

What is a myofilament?

Answer 6

extended macromolecular structure of the filament where there is actin and myosin

Question 7

What is a myotendinous junction?

Answer 7

where the collagen fibres of tendons are continuous with the connective tissue of the muscle

Question 8

Outline the four zones which classify the attachment of tendon to bone histologically

Answer 8

1) tendon/ligament has aligned collagen fibres and embedded fibroblasts throughout
2) uncalcified fibrocartilage has less parallel collagen bundles and has ovoid-shaped aligned cells and then a wavy line (tidemark) to mark calcification
3) calcified fibrocartilage is mineralised tissue where there hypertrophy of chondrocytes which become more circular
4) then there is bone

Question 9

Describe the histology of a muscle cell/fibre

Answer 9

sarcoplasm contains myoglobin to store oxygen, there are many proteins including actin and myosin (main) and accessory proteins such as troponin and tropomyosin. The SR is in the SER of the muscle cells and consists of interconnecting tubules which surround each myofibril which have terminal cisternae which store Ca2+. There is also a T-tubule network which are invaginations of the sarcolemma which starts at actin-myosin overlap

Question 10

What is the M-line?

Answer 10

where the myosin is thickest (centre of sarcomere)

Question 11

What is the Z-line?

Answer 11

start/end of sarcomere

Question 12

What is the I-band?

Answer 12

the presence of only actin

Question 13

What is the H-zone?

Answer 13

where only myosin is present

Question 14

What is the A-band?

Answer 14

all of the myosin present, including the overlap with actin

Question 15

Describe how a sarcomere changes upon muscular contraction?

Answer 15

H-zone shortens (more actin-myosin overlap), I-band shortens (due to more overlap) and Z-lines move closer together

Question 16

Describe the physiology of muscular contraction at the neuromuscular junction

Answer 16

Action potential in terminal bouton at NMJ causes voltage-gated Ca2+ channels to open, calcium influx causes ACh vesicles to leave the cell by exocytosis. ACh binds to receptors on sarcolemma which causes influx of sodium, which causes a depolarisation of the sarcolemma which travels down the T-tubules and stimulates the SR to release calcium which binds to troponin, moving myosin away from the myosin head attachment receptor, allowing the myosin head to attach and pull, causing the contraction of the muscle fibre.

Question 17

What is a ‘motor unit’?

Answer 17

A motor neuron and al of the muscle fibres supplied by it

Question 18

What is the ‘muscle spindle’?

Answer 18

Spindle are mechanosensitive proprioceptors which are specialised intrafusal muscle fibres inbetween ordinary muscle fibres which detect changes in muscle length. Therefore, when the muscle is stretched, the spindle fibres are stretched and the sensory nerves are stimulated and cause contraction to be perceived.