Lecture 29- Myofibrils And Neuromuscular Junctions Flashcards
Myofibrils
Contain contractile element of muscle cell
Account for 80% of cellular volume
A bands
Dark striation of myosin
I bands
Light striations of actin
Sarcoplasmic reticulum
smooth endoplasmic reticulum that regulates availability of calcium ions, surrounds each myofibril, and forms terminal cisterns at the A band–I band junction.
Dark A band (part of sacromere)
Middle of sacromere
Both a thick(myosin) and thin (actin) myofilaments
Light i band (part of sacromere)
on either side of the A band, has only thin (actin) myofilaments.
H zone (parts or sacromere)
slightly lighter region in the centre of the A band, has only thick (myosin) myofilaments
M line (parts of sacromere)
middle of the H zone, is where fine protein strands hold adjacent thick (myosin) myofilaments together.
Thick filaments
Composed of bundles of myosin
Thin filaments
Composed of strands of actin
Titian
Anchors myosin filaments
Regulatory proteins (tropomyosin)
wraps around actin filaments, stabilizing them and blocking the myosin binding sites.
Regulatory protein (troponin)
binds to both actin and tropomyosin,
and binds calcium ions if they are present.
What do actin filaments consist of?
polymerized globular actin (G actin) subunits that have active sites that bind myosin heads during contraction.
What does it take for a muscle to contract?
fibre must be stimulated by a nerve ending so there is a change in membrane potential
muscle fibre will generate an action potential, in its sarcolemma.
The action potential propagates along the sarcolemma
Intracellular calcium levels rise briefly, providing the final trigger for contraction.
Neuromuscular junction
connection between an axon terminal of a somatic motor neuron and a muscle fibre
is the route of electrical stimulation of the muscle cell.
Gap that separates axon terminal and muscle fibre
Synaptic cleft
Synaptic vesicles
Within axon terminal
Contain acetylcholine
Motor neuron
stimulates a skeletal muscle fibre when a nerve impulse causes the release of ACh to the synaptic cleft, which diffuses across the cleft and binds to Ach receptors on the sarcolemma, creating a graded end plate generator potential that leads to an action potential.
After acetylcholine binds to ACh receptors, an enzyme in the synaptic cleft, acetylcholine, breaks down acetylcholine to acetic acid and choline, to prevent continued contraction in the absence of stimulation.
Motor neuron stimulate muscle order
Action potential arrives at axon terminal at the neuromuscular junction
Acetylcholine is released and binds to chemically-gated channels on the sarcolemma, producing a graded response.
The graded potential exceeds the action potential threshold and opens membrane voltage-gated channels.
Acetylcholinesterase breaks down acetylcholine in the synapse.
