NMJ and Muscle contraction Flashcards
(26 cards)
What is a neuromuscular junction?
Specialised structure incorporating the distal exon terminal and muscle membrane that allows for unidirectional chemical communication between peripheral nerve and muscle
What is a synapse?
Allows unidirectional flow
Contact ratio varies from 1:1 in muscle and 10^3:1 in the CNS
What enzyme breaks down acetylcholine?
Acetylcholinesterase
What are 3 points of failure of the neuromuscular junction?
Demyelination
Loss of ACHesterase
Faulty VGCCs
What happens in demyelination?
Reduced conduction velocity
Weak/uncoordinated muscle movement
Where does the upper motor neurone go?
in the brain
Where does the lower motor neurone do?
in brainstem if to face
in spinal chord if to elsewhere
What do motor neurones to effectors look like
Soma in butterfly shaped grey matter
Surrounded by white matter and tracts
With axons that leave the anterior ventral horn
How many neurone branches does each muscle fibre receive?
One
What happens if a muscle is reinnervated by a damaged nerve?
Then the muscle can have branches form multiple neurones and be innervated by branches from previously unconnected nerves
What happens in the synapse in normal physiology?
AP opens VGCC causing Ca2+ influx
Ca2+ influx triggers vesicles exocytosis so ACh diffuses into the cleft and binds to receptor cation channels, opening these on the post synaptic neurone
Local currents flow from depolarised region and adjacent region, triggering an AP
ACh broken down by acetylcholinesterase and muscle fibre response ceases
What happens in synapses at rest?
Individual vesicles release ACh at low rate causing minature end plate potentials (MEPPs)
Voluntary or involuntary control?
Smooth muscle
Skeletal muscle
Cardiac muscle
Smooth muscle: involuntary - around round organs eg GI tract and vessels
Skeletal muscle: voluntary - allow for limb/body movement
Cardiac muscle: involuntary - cause myocardial contraction
What is the structure of myofibre?
Covered by sarcolemma plasma membrane with t-tubules tunnelling to centre, and a sarcoplasm (cytoplasm) with myoglobulin and mitochondria present.
What is the sarcoplasmic reticulum?
Network of fluid filled tubules that store Ca2+ ions
What do myofibrils look like?
1-2um diameter extending along entire length of myofibres, composed of actin and myosin (and other proteins)
What do myofilaments look like?
Straited due to light and dark bands
Do not extend along whole length of myofibres
Overlap and arranged into compartments called sarcomeres (Z-Z)
What happens in eccentric muscle contraction?
Muscle gets longer during contraction
Painful
Can lead to shearing and ripping out tendons/muscles
What happens in concentric muscle contraction
Muscle shortens during contraction
What happens in relaxation of muscle?
Ca2+ dissociates from troponin when free Ca2+ declines
Preventing new crossbridges forming
Ca2+ actively transported back to SR during AP so for contraction release => return
What are examples of NMJ disorders?
Botulism
Myasthenia Gravis
Lambert Eaton myasthenic syndrome
What happens in botulism disorder?
Botulism toxin leads to irrevocable disruption in stimulation induced ACh release by presynaptic nerve terminal
What happens in myasthenia gravis disorder?
Autoimmune disorder leading to Ab production against ACh receptors causing fatigable weakness -
affect ocular, bulbar (brainstem), respiratory or limb muscles - confirmed by EMG; may need plasma exchange/AChesterase inhibitors
What happens in Lambert-Eaton myasthenic syndrome? (LEMS)
Autoimmune Ab production against VGCCs associated with lung cancer, may need ACh analogue
