5 The Neuromuscular Junction and Muscle Contraction Flashcards
Q: What is the neuromuscular junction? allows? Responsible for?
A: specialised synapse between a motor neuron and a muscle fibre
allows unidirectional communication before peripheral nerve and muscle
initiating muscle contraction
Q: What’s the structure of a standard chemical synapse? Role of specific vesicles?
A: presynaptic axon
presynaptic terminal (bouton) containing synaptic vesicles (propogate change in membrane potential -> carry AP)
synaptic cleft
postsynaptic membrane containing receptors
Q: What’s the structure of a neuromuscular junction? 3 main structures? Specific to striated muscle?
A: REFER
axon with myelin sheath and schwann cell right at end just before widening of terminal
flat presynaptic terminal contains mitochondria, achetylcholine containing vesicles
-> has voltage gated calcium channels
small synaptic cleft with highly folded post synaptic muscle membrane
membrane has ACh receptors at tops of folds and achetylcholinesterase in clefts
- presynaptic nerve terminal
- synaptic cleft
- postsynaptic endplate region on the muscle fibre
ACh serves as the NT for voluntary striated muscle
Q: Label a spinal cord cross section. (7)
A: REFER
dorsal/posterior
CS
anterior/ventral
- grey matter = within in butterfly shape (wider wings at ventral end)
- white matter around it
- posterior/dorsal root coming out of back= sensory fibres entering AND CONTAINS BULGE= ROOT GANGLIAN which contains body of sensory neuron
- anterior/ventral root coming out front = motor fibres (cell bodies reside in anterior horn of grey matter)
=> connect to make mixed nerve
Q: How do muscle fibres vary in terms of innnervation?
A: muscle fibres that are innervated by certain branches of the motor neuron are not fibres innervated by sensory neurons
under normal conditions, a neuron, can go to multiple muscle fibres but those muscle fibres will not be innervated by any other neurone
Q: What are the 7 steps in NMJ function? Result?
A: 1. Action potential reaches presynaptic terminal= opens V-gated Ca2+ channels
- Ca2+ enters
- Ca2+ triggers exocytosis of vesicles SNARE
- Acetylcholine diffuses in cleft
- Acetylcholine binds to receptor-cation channel & opens channel
- Local currents flow from depolarized region and adjacent region; action potential triggered and spreads along surface membrane
- Acetylcholine broken down by acetylcholine esterase (enzyme). Muscle fibre response to that molecule of Acetylcholine ceases
activate structures within skeletal muscle and cause contraction
Q: What’s the SNARE protein?
A: complex responsible for making membrane of vesicle fuse with membrane of nerve terminal-> contents of vesicle are emptied into synaptic cleft
Q: What occurs to NMJs at rest? causes? not? Variation?
A: release vesicles of ACh at v slow rate causing miniature end-plate potentials (MEPs)
don’t result in depolarisation of membrane = no contraction
vesicles contain specific (discrete) amounts of ACh
Q: What structure of muscle cell membranes aids conduction?
A: invaginations of membrane= produce T tubules that convey AP deep into muscle so that all muscle fibres can contract in muscle
Q: Describe cells of skeletal muscle? contain which essential organelle?
A: cells have merged to create multinucleated fibres
myofibrils
Q: What’s the structure of myofibres? (5) composed of?
A: -Covered by plasma membrane – sarcolemma
- T-tubules tunnel into centre
- Cytoplasm called sarcoplasm
- myoglobin and mitochondria present
- Network of fluid filled tubules – sarcoplasmic reticulum: Ca store -> released to allow contraction
Composed of myofibrils
Q: What are myofibrils? composed of? Diameter? Where are they?
A: any of the elongated contractile threads found in striated muscle cells
of two main types of protein – actin and myosin
1-2μm (10^-6) in diameter
Extend along entire length of myofibres
Q: What are microfilaments? Appearance? Where are they? Arrangement?
A: filaments of myofibrils
Light and dark bands give muscle striated appearance
Do not extend along length of myofibers
Overlap and are arranged in compartments called sarcomeres
Q: What is the structure of sarcomeres? (3)
A: dense protein Z discs separate sarcomeres
dark bands- A band (thick-myosin and mix)- stays same size
light bands- I band (thin-actin only)
Q: What are the sarcomere observations with the sliding filament theory? (3)
A: During contraction I band became shorter
A-band remained same length
H-zone narrowed or disappeared
