Nervous control of skeletal muscle Flashcards
Describe the structure of muscle tissue (from gross structure to microstructute)
Muscle (organ) = many fascicles run together, surrounded by the epimysium
Fascicle (a portion of the muscle) = a bundle of muscle fibres surrounded by the perimysium
Muscle fibre (cell) = a long, multinucleate cell with a striated appearance. cellular fluid inside the muscle fibre is called the sarcoplasm and each is covered by the endomysium (sarcolemma). it contains many straight organelles called myofibrils.
Myofibrils (organelles) = rod-like contractile elements that occupy most of the muscle cell volume. comprised of sarcomeres arranged end to end.
Sarcomeres (segments of myofibrils) = the contractile unit of muscle. composed of myofilaments made of contractile proteins.
Myofilaments (units of a sarcomere) = two types of myofilament (thick and thin). thick filaments are bundled myosin molecules with many heads. thin filaments contain actin and some other molecules. there are also elastic filaments which provide muscle with the ability of elastic recoil. the sliding of thin filaments past thick filaments is what causes muscle shortening/contraction.
Define sarcomere
sarcomere = space between two Z discs, the smallest and functional unit of a muscle. averages 2 micrometers in length.
List and describe the features of a sarcomere
I bands = the space in between the ends of thick filaments. it contains thin filaments and elastic fibres. it also includes the z discs. it is the light colour in muscle striation
A bands = the area of the sarcomere containing thick filaments, including some thin filament and the M band. it is the dark colour in muscle striation
H zone = area around the M line containing only thick filaments
M line = middle line of the sarcomere formed by proteins called myomesin
Z line = end points of the sarcolemma. contains thin fibres and elastic fibres and is the midpoint of the I band.
What is the molecular composition of the fibres in a sarcomere?
thick filaments = composed primarily of myosin molecules which have a long body and a bulbous head
thin filaments = composed primarily of actin but also contains tropomyosin and troponin molecules
elastic filament = composed primarily of titin
What happens to a sarcomere during muscle contraction?
- I bands shorten
- Distance between Z discs shortens as they pull towards the midline
- H zones disappear
- Adjacent A bands move closer together (though their individual length does not change)
Describe the organisation of sarcoplasmic reticulum and T tubules on a myofibril
sarcoplasmic reticulum = an elaborate smooth endoplasmic reticulum that stores and releases calcium ions. it covers the surface of a myofibril. terminal cisterns are large channels of of sarcoplasmic reticulum found at the junction of the I band and A band. also at this junction there is a T tubule. the T tubule is a large projection of sarcolemma into the inside of the muscle cell.
A T tubule flanked by two terminal cisterns is called a triad.
provide a general outline of the process of a neuron causing a muscle to contract
- action potential travels down a motor neuron
- motor neurons axon releases ACh into the synaptic cleft
- ACh diffuses across the synapse and binds to specific receptors in folds of the sarcolemma on the muscle cell.
- ACh binding causes local depolarisation called an end plate potential in the muscle sarcolemma
- end plate potential triggers an action potential in the sarcolemma
- the action potential in the sarcolemma travels down the T tubules in the triads
- depolarisation of the T tubule membrane causes proteins to shift in terminal cisterns and sarcoplasmic reticulum releases Ca2+
- Ca2+ binds to troponin which shifts tropomyosin out of the way of myosin-binding sites on actin molecules. This allows myosin heads to bind to actin.
- contraction occurs via cross bridge cycling
describe what happens at the neuromuscular junction to cause a muscle to contract
- action potential arrives at the axon terminal
- voltage gated ion channels open and allow Ca2+ into the cell
- Ca2+ causes ACh to be released into the synapse via exocytosis
- ACh diffuses across the synaptic cleft and binds to specific ACh receptors in the sarcolemma
- ACh binding causes chemically gated ion channels to allow Na+ in to the cell and K+ out of the cell which causes a localised depolarisation (end plate potential)
- This wave of depolarisation continues across the sarcolemma (action potential)
Describe the process of excitation contraction coupling
- action potential propagates along the sarcolemma and down the T tubules
- calcium ions are released: the action potential causes voltage sensitive tubule proteins to change shape, causing Ca2+ channels in the sarcoplasmic reticulum to open. Ca2+ flows into the cytosol of the muscle fibre.
- Ca2+ binds to troponin, which removes tropomyosin from the myosin-binding sites on actin filaments. (by changing its shape)
- myosin can now bind to actin. cross bridge cycling can then begin and excitation contraction coupling is over.
define excitation contraction coupling
the process by which transmission of an action potential along the sarcolemma leads to the sliding of myofilaments
describe the cross bridge cycle
- cross bridge formation: energised myosin head binds to actin
- power/working stroke: ADP and P are released. myosin head bends, pulling actin towards the M line
- cross bridge detachment: ATP attaches to myosin, myosin head then detaches from actin
- myosin hydrolises ATP to ADP and P (providing it with energy) and re-prepares to form the cross-bridge
- repeat
under what conditions does cross bridge cycling go wrong?
- the cycle will only continue as long as Ca2+ is bound to troponin so the myosin binding sites are exposed. if no Ca2+ is present then this cannot occur.
- if there is no supply of ATP, for example if respiration stops, myosin heads cannot detach from actin and rigor mortis sets in. it takes 2-3 hours to develop but lasts up to 80 hours as only enzymatic protein breakdown leads to myosin-actin decoupling.
what are the effects of muscle ageing (Sarcopenia)?
- myosin production decreases
- mitochondrial malfunction becomes more common (less ATP available)
- motor axon atrophy occurs
- muscle fibres begin to atrophy (due to lack of motor neuron signal input)
what is a motor unit?
one motor neuron and all the muscle fibres that it innervates. for example, there are 700 motor units in the biceps.
Motor units allow force to be generated incrementally and smoothly through recruitment of individual motor units rather than the whole muscle at once. during sustained effort, motor units may switch on and off, preventing fatigue.
how can muscle movement be made smooth?
- recruiting of different size muscle fibres: low effort only recruits small muscle fibres, increasing effort leads to increasingly large muscle fibres being recruited.
- recruitment of individual motor units
- frequency coding = as the frequency of action potentials increase, the muscle fibres don’t have time to relax in between stimuli and therefore are constantly contracted (which stops twitching movements during a sustained movement or activity)
