Muscle

Question 1

T/F - Skeletal muscle accounts for about 40% of total body mass

Answer 1

TRUE

Question 2

T/F - Simultaneous binding of two acetylcholine molecules to the two alpha subunits of the nicotinic ACh receptor initiates a conformational change that opens a channel through the centre of the receptor

Answer 2

TRUE

Question 3

T/F - Ryanodine receptor (RyR1) is found in the sarcoplasmic reticulum

Answer 3

TRUE

It is linked to the dihydropyridine receptor on the T tubules, and when activated opens its linked Calcium channel to permit Ca2+ efflux from the SR to the sarcoplasm

Question 4

T/F - Contraction of skeletal muscle is initiated by attachment of calcium ions to tropomyosin

Answer 4

FALSE

Calcium ions bind to troponin C subunit, which causes a conformational change and makes the troponin-tropomyosin complex change shape and move into the groove between the two actin helices, allowing the myosin heads to bind to the myosin-binding site on the actin and thus contraction of skeletal muscle

Question 5

T/F - Foetal type nicotinic ACh receptors are resistant to non-depolarising NMBDs

Answer 5

TRUE

Foetal nicotinic ACh receptors are more sensitive to succinylcholine but more resistant to non-depolarising NMBDs

Question 6

T/F - Motor nerve terminals innervate several muscle cells

Answer 6

TRUE

Motor unit = 1 motor neuron innervating a number of muscle fibres (myocytes)

Question 7

T/F - The motor nerve terminals end inside the sarcolemma

Answer 7

FALSE

The motor nerve terminals end at the neuromuscular junction and depolarise the sarcolemma, creating a muscle action potential which then results in muscle contraction

Question 8

T/F - Skeletal and cardiac muscle act as a functional syncytium during depolarisation

Answer 8

FALSE

Skeletal muscle does not act as a functional syncytium, as each myocyte is innervated by a motor neuron and there is no communication between the myocytes to facilitate synchronised depolarisation.

Cardiac muscle does as a functional syncytium, with spread of action potentials via gap junctions that allow current to pass from cell to cell.

Question 9

T/F - Cardiac and fast skeletal muscle are very rich in mitochondria

Answer 9

TRUE

Cardiac and fast skeletal muscle both rely primarily on oxidative metabolism for energy production, which requires large amounts of mitochondria

Question 10

T/F - Troponin is a complex of protein molecules attached to tropomyosin

Answer 10

TRUE

Troponin is a protein consisting of 3 subunits:

• Troponin C - binds to calcium
• Troponin T - binds to tropomyosin
• Troponin I - binds to the myosin binding site on the actin to prevent myosin from binding

Question 11

T/F - ATP can only supply energy for skeletal muscle contraction for 5-8 seconds

Answer 11

FALSE

ATP already present within the myocyte can only sustain skeletal muscle contraction for 1-2 seconds. Thereafter, phosphocreatine is used to yield more ATP for a further 5-8 seconds of muscle contraction. After that, the glycolytic pathway and oxidative metabolism pathways take over to produce more ATP for further skeletal muscle contraction

Question 12

T/F - The first source of energy to regenerate ATP in skeletal muscle contraction comes from glycolysis

Answer 12

TRUE

Glycolysis is used as the first source of energy to regenerate ATP and phosphocreatine in skeletal muscle

Question 13

T/F - The major supply of energy for long term skeletal muscle contraction comes from fats

Answer 13

TRUE

More than 95% of all energy used by the muscles for sustained long-term contraction is derived via oxidative metabolism. The primary energy substrates are fats, and up to 1/2 of the energy can come from stored carbohydrates for periods of 2-4 hours.

Question 14

T/F - After a long period of rest, initial contraction strength of a muscle is lower than after usage

Answer 14

TRUE

Known as the Staircase Effect or Treppe. The mechanism behind this is unknown

Question 15

T/F - Skeletal muscles receive nerve impulses at rest to maintain muscle tone

Answer 15

TRUE

Skeletal muscle tone results from a low rate of nerve impulses from the spinal cord. They are controlled partly by the brain and also by muscle spindles.

Question 16

T/F - Skeletal muscle fatigue increases in almost direct proportion to muscle glycogen depletion

Answer 16

TRUE

Muscle fatigue occurs primarilyfrom inability of the contractile and metabolic process of the muscle fibres to continue supplying the same work output as the glycogen stores are depleted. Muscle fatigue also occurs due to:

• Diminished transmission of the nerve signal through the NMJ
• Muscle contraction impairing blood supply and thus supply of oxygen and nutrients
• Build up of metabolic end products such as lactic acid

Question 17

T/F - Loss of nerve supply causes immediate atrophy, and then skeletal muscle fibre degeneration after 2 months

Answer 17

TRUE

Atrophy begins almost immediately after denervation, as the contractile signals to maintain normal muscle size are lost. After 2 months, degenerative changes start to appear in the muscle fibres. As time passes, the potential for return of full muscular function becomes less and less, and recovery is not possible after 6-12 months

Question 18

T/F - In the final stage of denervation atrophy, muscle cells have little capability to regenerate myofibrils.

Answer 18

TRUE

Most of the muscle fibres are destroyed and replaced with fibrous tissue. Those that remain are composed of a long cell membrane with a lineup of muscle cell nuclei but few or no contractile properties and no capacity for regeneration of myofibrils.

Question 19

T/F - Most muscle fibres have only one neuromuscular junction

Answer 19

TRUE

The neuromuscular junction is located around the middle of the length of the myocyte. Only 2% of myocytes have more than one NMJ

Question 20

T/F - Acetylcholine release at the skeletal muscle junction is triggered by calcium influx through voltage gated calcium channels

Answer 20

TRUE

The calcium influx through VG Ca2+ channels causes an increased concentration of intracellular calcium ions, which activates Ca2+-calmodulin dependent protein kinase, which phosphorylates synapsin proteins to facilitate exocytosis of acetylcholine molecules from the storage vesicles

Question 21

T/F - Activation of skeletal muscle acetylcholine receptors caused depolarisation by efflux of K+ ions

Answer 21

FALSE

Although the channel is permeable to K+, Na+ is the only ion that moves into the cell. K+ is attracted to the large negative charge inside the cell (-90mV) and therefore is close to balanced at its electrochemical gradient based on its Nernst potential (-94mV)

Question 22

T/F - A normal impulse at the neuromuscular junction causes three times as much end plate potential as that required to stimulate the muscle fibre

Answer 22

TRUE

The sudden Na+ influx into the muscle fibre causes the local area of the end plate to increase to +50 - +75mV, when all that is requires is +20-+30mV to stimulate Na+ channel opening and action potential initiation.