3.6.3 skeletal muscles are stimulated Flashcards
what are the three muscle types?
- cardiac - in wall of heart, involuntary, myogenic
- skeletal - attached to skeleton, voluntary
- smooth muscle - walls of guts, blood vessels, bladder, involuntary
what is the structure of skeletal muscle?
each muscle is made up of many structures called myofibrils, which is grouped into muscle fibres
fibres between 0.1mm and 0.01mm
myofibrils is arranged in order to provide maximum strength
what is the structure of striated muscle?
each muscle fibre is made from several cells on which the plasma membrane separating them is broken down
cytoplasm of fibre contains many nuclei = large multinucleated structure (syncytium)
each contains a bundle of myofibrils (protein)
each syncytium shares nuclei and cytoplasm = sarcoplasm
each fibre can be seen to be filled with numerous parallel striations
made up of light and dark staining bands of adjacent myofibrils
what is the structure of myofibril?
each myofibril is made up of thick filaments (myosin) and thin filaments (actin), aligned with sarcomeres
arrangements of thick and thin filaments result in light and dark bands when viewed with TEM
what is the structure of a sarcomere?
the sarcomere runs from Z-line to Z-line
Z lines are lined up to adjacent myofibrils, which project down towards centre of sarcomere
A band corresponds to length of thick filaments
H zone contains only thick filaments
I band contains thin filaments
connections among the thick filaments form form M line - H zone
describe myosin filaments
myosin form thick filaments
each filament is made up of many staggered myosin molecules
each molecule has a fibrous tail region and globular head
arranged in bundles
half heads are towards one side of the sacromere
tails meeting form the M line
describe actin filaments
forms thin filaments, actin molecules link together to form long chains
2 CHAINS FORM A THIN FILAMENT
contains two proteins: tropomyosin and troponin
tropomyosin: forms a long fibrous strand that lies in the groove between the two chains of actin molecules
troponin: globular protein that binds to the actin molecules at regular intervals - anchored into Z lines
what is a neuromuscular junction?
point where a motor neurone meets a skeletal muscle fibre
many of them along the length of the fibre ensure muscle contraction is rapid and powerful when it is simultaneously simulated by APs
one motor neurone and all the muscle fibres it stimulates = motor unit
describe what occurs at a neuromuscular junction
- motor unit gives control over the force that the muscle exerts
- small force is stimulated by a few units, greater force is stimulated by a large number of units
FEWER THE NUMBER OF FIBRES PER NEURON»_space;» THE FINER THE MOVEMENT
- end of the motor neurone does not come in contact with muscle fibre
- small gap between the membrane of the axon + sarcolemma
compare a neuromuscular synapse and a synapse
synapse:
- may be excitatory or inhibitory
- links neurones to neurones or neurones to other effector organs
- motor, sensory and intermediate neurones are involved
- a new AP may be produced along another neurone
- Ach binds to receptors on membranes of post-synaptic neurones
neuromuscular synapse:
- only excitatory
- only links neurones to muscles
- only motor neurones are involved
- AP ends here
- Ach binds to the receptors on membrane (sarcolemma) of muscle fibre
what is the sliding filament theory?
process involving actin and myosin filaments sliding past one another
occurs in a relaxed muscle fibre
each sarcomere is about 2.5um long
what evidence is available for the sliding filament mechanism?
thin filaments in each sarcomere slide in between the thick filament
I bands become narrower, Z lines become closer together and H zones becomes narrower
shortens sarcomere, this force is generated in heads of myosin molecule
A band remains the same width
how does the shape of proteins suggest the sliding filament theory?
The bulbous heads of the myosin filaments form cross-bridges with the actin filaments. They do this by attaching themselves to binding sites on the actin filaments, and then flexing in unison, pulling the actin filaments along the myosin filaments. They then become detached and, using ATP as a source of energy, return to their original angle and re-attach themselves further along the actin filaments. This process is repeated up to 100 times per second.
describe muscle stimulation
- AP arrives at presynaptic membrane of neuromuscular junction
- causes calcium ion channels to open and calcium ions to flood into synaptic knob
- calcium ions cause synaptic vesicles to fuse with the presynaptic membrane + Ach is emptied into synaptic cleft
- Ach diffuses across the cleft and binds with receptors on the postsynaptic membrane
depolarisation spreads along T-tubules and is carried right into the centre of the muscle cell
- where the T tubules contact the sarcoplasmic reticulum, the sarcoplasmic reticulum releases calcium ions
DIFFUSES DOWN CONCENTRATION GRADIENT
describe muscle contraction
- calcium ions bind with the troponin molecules on actin filaments
- troponin changes shape, causes troponin and tropomyosin to move away from specific binding sites for myosin
- myosin head can bind ATP + hydrolyse it to ADP + Pi = causes the head to change shape to a configuration which can bind to exposed binding sites on an actin, following an AP
- AP and myosin bind to newly exposed sites on actin, forming cross bridges = myosin head bend’s in on its self, pulling the filament towards the centre of the sarcomere + releasing ADP
- cross-bridge is broken when a new molecule of ADP binds to myosin head = ATP-ase hydrolyses the ATP
- cycle repeats itself and myosin reattach’s further along the actin filament = each myosin head forms and reforms many cross bridges per second, driving filaments past each other and muscles contract
- when nervous stimulation ceases, calcium ions are pumped back into the sarcoplasm reticulum = allows tropomyosin and troponin to block myosin binding sites on actin so muscle relaxes
