4 - The NMJ and muscle contraction Flashcards
Define the neuromuscular junction
a specialised synapse between a motor neurone and a muscle fibre
What are the structures involved in a NMJ?
presynaptic nerve terminal
synaptic cleft
post endplate region of the muscle fibre
What is the neurotransmitter for voluntary striated muscle?
acetylcholine
What are the fibres coming out of the anterior and posterior roots of the spinal cord?
ventral root - motor fibres
dorsal root - sensory fibres
Where are the cell bodies of the sensory and motor neurones?
motor neurone cell bodies - in they grey matter of the spinal chord
sensory neurone cell bodies - in the dorsal root ganglia
What are the NMJ between specifically?
the axons terminal and the muscle fibres
Give the 7 stages of initiating muscle contraction at the NMJ
1 - Action potential open V-gated Ca2+ channels
2 - Ca2+ enters
3 - Ca2+ triggers exocytosis of vesicles
4 - Acetylcholine diffuses in cleft
5 - Acetylcholine binds to receptor-cation channel & opens channel
6 - Local currents flow from depolarized region and adjacent region; action potential triggered and spreads along surface membrane
7 - Acetylcholine broken down by acetylcholine esterase (enzyme). Muscle fibre response to that molecule of Acetylcholine ceases
What are MEPPs?
miniature end-plate potentials
individual vesicles releasing ACh at very low rates - they are cause by slight depolarisations
Give the layers of of skeletal muscle and what they are wrapped in
- muscle fibers (the muscle cells) are surrounded by endomysium
- muscle fibres are bundled together to form fasiculata
- fasiculata is bounded by perimysium
- the muscle is surrounded by epimysium
Give the microstructure of a muscle fibre
each muscle fibre is a cell containing myofibrils and many nuclei
- the myofibrils have capillaries running along their surface and indentations of the sarcolemma called t-tubules. The sarcoplasm has myoglobin and mitochondria present in it
- myofilaments are the single thread-like structures found in myofibrils. Comprised of actin and myosin. They are the structural unit of myofibrils. They extend along the entire length of myofibres. They have striated appearance.
What happens to the light and dark bands during contractions of skeletal muscles?
- I band becomes shorter
- A band remains the same length
- H zone narrows and disappears
Activation and relaxation process
1 - Action potential propagates along surface membrane and into T-tubules
2 - DHP (dihydropyridine) receptor in T-tubule membrane: senses V & changes shape of the protein link to Ryanodine receptor, opens the Ryanodine receptor Ca2+ channel in the SR membrane; Ca2+ released from SR into space around the filaments
3 - Ca2+ binds to Troponin & Tropomyosin moves allowing 4 - Crossbridges to attach to actin
5 - Ca2+ is actively transported into the SR continuously while action potentials continue. ATP- driven pump (uptake rate < or = release rate).
6 - Ca2+ dissociates from TN when free Ca2+ declines; TM block prevents new crossbridge attachment; Active force declines due to net crossbridge detachment
Give 3 disorders that arise as a result of pathological processes interfering with NMJ function and causing muscle weakness
- botulism
- myastenia gravis
- LEMS (Lambert-Eaton myastenic syndrome)
How does NMJ disfunction cause botulism?
the botulism toxin produces an irreversible disruption in stimulation-induced ACh release by presynaptic nerve terminal
prevents calcium influx into presynaptic nerve
What is myasthenia gravis?
an autoimmune disease
antibodies are directed against the acetylcholine receptor, block the receptors so ACh cannot bind
causes fatiguable weakness
affects muscle of the face and eyes: droopy eyelids, lack of facial muscle movement
