Muscle 1

Question 1

What is skeletal muscle responsible for?

Answer 1

-voluntary movement of bones
-control of inspiration
-skeletal muscle pump

Question 2

Describe the appearance of skeletal muscle

Answer 2

Striated

Question 3

Describe the structure of skeletal muscle

Answer 3

-muscle cell/fibre surrounded by endomysium
-muscle cells come together to make fascicles surrounded by perimysium
-muscle units made up of many fascicles surrounded by the epimysium
-myofibrils run through muscle cells + are made up of sarcomeres

Question 4

What are sarcomeres made up of?

Answer 4

-thick filaments (myosin)
-thin filaments (actin)

Question 5

How do the I and A bands appear and why?

Answer 5

-I band appears light - predominantly actin
-A band appears dark - overlap of actin and myosin

Question 6

Where is the Z line/disc in the sarcomere?

Answer 6

At the ends

Question 7

What are the 2 other proteins in the sarcomere and what do they do?

Answer 7

-nebulin protein is linked around actin
-titin protein anchors myosin to z disc

Question 8

When the muscle contracts, what changes in the sarcomere?

Answer 8

Actin is pulled inward, so the I band becomes smaller and the A band stays the same

Question 9

How is muscle contraction initiated?

Answer 9

-ACh released at neuromuscular junction causing action potential in plasma membrane of muscle fibre
-depolarisation passes along sarcolemma + through T-tubule network to cell interior
-T tubule runs near 2 areas of sarcoplasmic reticulum, forming a triad (skeletal muscle)
-depolarisation causes increases in intracellular calcium

Question 10

How many actin filaments are around myosin?

Answer 10

6

Question 11

How are cross-bridges formed?

Answer 11

-myosin head is tightly bound with actin
-ATP binds to myosin head, weakening its binding to actin, so it dissociates
-ATP is hydrolysed, causing myosin head to return to resting configuration
-myosin head binds loosely to new actin molecule, forming a cross-bridge

Question 12

How does the sarcomere contract?

Answer 12

-after cross-bridge is formed, phosphate is released, causing a strong association between actin + myosin
-myosin head changes from open to closed position, pulling actin towards middle of sarcomere
-ADP released, returning head to original position

Question 13

How can the myosin head attach to a new actin molecule further down the filament?

Answer 13

When the myosin head returns to resting configuration, it’s in an open position, lengthening it

Question 14

What happens to calcium ions after contraction?

Answer 14

Taken out of cytoplasm, so muscle goes into relaxation period

Question 15

What happens to skeletal muscle at an increased frequency (10 Hz)?

Answer 15

-muscle doesn’t fully relax before net contraction
-stronger contraction
-temporal summation

Question 16

What happens to skeletal muscle at 25 Hz?

Answer 16

-unfused tetanus
-contraction force plateaus

Question 17

What happens to skeletal muscle at 50 Hz?

Answer 17

-fused tetanus
-not time for calcium to be taken out of cytoplasm
-constant generation of force

Question 18

What are the 3 classes of muscle fibres?

Answer 18

-type I - slow oxidative
-type IIa - fast oxidative
-type IIb - fast glycolytic

Question 19

Are the classes of muscle fibres fast or slow?

Answer 19

-type I - slow
-type II - fast

Question 20

Are slow fibres larger or smaller, and do they take longer to contract?

Answer 20

-slow fibres are half the diameter of fast fibres
-take longer to contract

Question 21

Describe the speed and force of the 3 classes of muscle fibre

Answer 21

-type I - react slowly + generate a low level of force
-type IIb - react quickly + high level of force
-type IIa - in between type I and IIb

Question 22

Describe the unfused tetanus force + fatigability of the 3 muscle fibre classes

Answer 22

-type I - muscle doesn’t tire, continually generates the same force
-type IIa - generates a large force, sustains for a while but drops off quickly
-type IIb - generates a large force, drops off quickly

Question 23

What is the difference between isometric and isotonic muscle contraction

Answer 23

-isometric - muscle remains at fixed length
-isotonic - muscle stimulation causes a change in length

Question 24

Describe how transmission occurs at the neuromuscular junction

Answer 24

-action potential enters the neuron
-sodium channels open + sodium moves in, causing depolarisation
-activates calcium channels + calcium moves in
-causes fusion of vesicles containing ACh with membrane
-ACh released unto synaptic cleft + stimulates AChR receptors
-depolarisation in muscle, so muscle sodium channels open + wave of depolarisation can continue

Question 25

What does acetylcholinesterase do?

Answer 25

Breaks down ACh to prevent overstimulation of muscle

Question 26

Why do potassium ion channels open in the neuron at the neuromuscular junction?

Answer 26

Potassium ions move out, repolarising neuron, so new wave of depolarisation can occur

Question 27

What are the V snares and T snares and where are they?

Answer 27

-V snare in vesicle - synaptobrevin
-T snares in membrane - SNAP-25 + syntaxin

Question 28

What is the function of T snares and v snares?

Answer 28

Complex forms between them, causing the vesicle containing ACh to fuse with the membrane and release ACh into cleft

Question 29

How do botulinum toxins act as inhibitors?

Answer 29

Botulinum toxins cleave V and T snares, so vesicle doesn’t fuse with the membrane, so ACh not released

Question 30

Which types of botulinum toxin cleave which snares?

Answer 30

-A,C,E - SNAP-25
-B,D,F,G - synaptobrevin
-C - syntaxin