Muscle Flashcards
T/F - Skeletal muscle accounts for about 40% of total body mass
TRUE
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
TRUE
T/F - Ryanodine receptor (RyR1) is found in the sarcoplasmic reticulum
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
T/F - Contraction of skeletal muscle is initiated by attachment of calcium ions to tropomyosin
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
T/F - Foetal type nicotinic ACh receptors are resistant to non-depolarising NMBDs
TRUE
Foetal nicotinic ACh receptors are more sensitive to succinylcholine but more resistant to non-depolarising NMBDs
T/F - Motor nerve terminals innervate several muscle cells
TRUE
Motor unit = 1 motor neuron innervating a number of muscle fibres (myocytes)
T/F - The motor nerve terminals end inside the sarcolemma
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
T/F - Skeletal and cardiac muscle act as a functional syncytium during depolarisation
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.
T/F - Cardiac and fast skeletal muscle are very rich in mitochondria
TRUE
Cardiac and fast skeletal muscle both rely primarily on oxidative metabolism for energy production, which requires large amounts of mitochondria
T/F - Troponin is a complex of protein molecules attached to tropomyosin
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
T/F - ATP can only supply energy for skeletal muscle contraction for 5-8 seconds
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
T/F - The first source of energy to regenerate ATP in skeletal muscle contraction comes from glycolysis
TRUE
Glycolysis is used as the first source of energy to regenerate ATP and phosphocreatine in skeletal muscle
T/F - The major supply of energy for long term skeletal muscle contraction comes from fats
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.
T/F - After a long period of rest, initial contraction strength of a muscle is lower than after usage
TRUE
Known as the Staircase Effect or Treppe. The mechanism behind this is unknown
T/F - Skeletal muscles receive nerve impulses at rest to maintain muscle tone
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.
T/F - Skeletal muscle fatigue increases in almost direct proportion to muscle glycogen depletion
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
T/F - Loss of nerve supply causes immediate atrophy, and then skeletal muscle fibre degeneration after 2 months
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
T/F - In the final stage of denervation atrophy, muscle cells have little capability to regenerate myofibrils.
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.
T/F - Most muscle fibres have only one neuromuscular junction
TRUE
The neuromuscular junction is located around the middle of the length of the myocyte. Only 2% of myocytes have more than one NMJ
T/F - Acetylcholine release at the skeletal muscle junction is triggered by calcium influx through voltage gated calcium channels
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
T/F - Activation of skeletal muscle acetylcholine receptors caused depolarisation by efflux of K+ ions
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)
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
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.
