neuromuscular junctions Flashcards
What is a neuromuscular junction?
Specialised synapse between motor neuron and muscle fibre
What main characteritics of a synaspe allows transmission of the signal?
Presynaptic axon-into presynaptic bouton
Bouton filled with snyaptic vesicles
Post synaptic membrane is just after the synaptic clef, and has receptors for neurotransmitters
How large is the synaptic cleft?
10-50um
What receptors are used for NMJ?
Acetylcholine
Are motor neurons myelinated
Yes
What are junctional folds in NMJ?
Evnagination of the post synaptic membranes in which acetylcholine esterase destroy remamant neurotransmitter
How would the strcucture of the NMJ best described as?
A specilaises tructure incrpotporating the distal axon terminal and muscle membrane, allowing unidriectional communication
Main constituants-Presynaptic nerve terminal, synaptic clef, post synaptic endplate on muscle fibre
How do axons oraganise themselves on a muscle?
Axons of motor units each arrive and spead over a short region of a muscle fiber-each fibre only has 1 axon activating it, but one neuron can activate several fibres
Describes the events leading to sygnal transmission across the NMJ-and its clearance
Action potential triggers VGCC (in synaptic termnial), entrance of Ca2+ and exocytosis of acetycholine in cleft
ACh brinds receptor cations channel (Na+) and allows flow in
Local current flow (depolarised)-action potential triggered and speads down to soma
ACh broken down by ACh esterase in junctional folds-causing it to cease
What are miniature end plate potentials?
At rest, individual vescicules release ACh causing small potentials
Describe the overall stucture of a muscle
Muscles are made of collections of Muscle fibres;
Muscle fibers in themselves are made from bundles of myofibril-large mutinuceleated cells, fused, which contain myfibrils bunch (myofibril made off functional sarcomeres bundles (myofilaments)
Large Sarcoplasmic reticulums in cell, with T-tubules in membrane (envagination)
Cappilaries also run alongside
Cytoplasm is called sarcoplasm, which has myoglonin
What are Z discs, A bands and I bdans in myofilaments
z-lines are dark and seperate each sarcomere
A line are dark BANDS-thick myosin
I line are light bands, actin
Describe the sucession of events leading to contraction of muscle
Action potential goes through t-tubule, opening DHP (dihydropyridine) receptors. In skeletal msucle, DHPR physycally change the shape of SR RyR receptor-allows Ca to exit the SR
Ca binds troponin and tropomyosin, allowing binding of actin to myosin
Ca is activelly transported into SR as AP in on (ATP pump)
Ca dissociates from troponin as free Ca declines, TM blocks new cross bridge-active force decrease
What are three major exemples of NMJ disorders?
Botulism, Myastenia gravis and lamber-eaton myastenic syndrome
What is botulism?
Botulinum is a toxin which caused irreversible disruption of stimulation induced ACh release by NMJ-msucle weakness
What is Myastenia gravis?
Autoimmune disorder where AB target ACh receptor
Might have family ties
causes fatiguable weakness (increase as use), also ocular, bulbar, resp or limb
EMG examinations to confirm
In severe cases, remove AB from blood for rapid improvement
What is Lambert eaton myastenic syndrome?
Autoimmunde dosrder caused by AB against VGCC-associated with lung cancer
What is twitch in relation to muscle?
Small, local involuntary movement of a muscle, caused by spontaneous depolarisation of a lower motor neuron-so not many myofibrils affected
A twitch is a single contraction and relaxation cycle produced by an action potential within the muscle fiber itself
The time between a stimulus to the motor nerve and the subsequent contraction of the innervated muscle is called the latent period, which usually takes about 10 ms and is caused by the time taken for nerve action potential to propagate, the time for chemical transmission at the neuromuscular junction, then the subsequent steps in excitation-contraction couplin
What is tetanus in relation to muscle?
Happens when stimuli are delivered to fast-normally just makes a twicth but if their frequency overtakes the duration of the twitch, makes a continuous cramp
What is summation in relation to muscle?
The fact that even at maxiumum effort, not all of the fibres are contracting at the same time. To protect tendon, about 1/3 fire at once and they alternate
What is EMG?
A measure of the action potentials produced by muscle cells in response to nerve action of electrical stimulation
Extracellular recording
What is the difference between intra and extracellular measurments of AP
in Intracellular, one electrode is in the cell, outiside being a reference. There you just measure how the cells differ from outside (normal -70mV) in relation to an action
Extracellular, both electrodes are outside. Cells have negative RMP, and skin is positive
As an AP propagates, muscle cells have positive intracellular, which causes the skin to have negative charges-measured by electrode.
With two electrodes (one reference peaks up, the other the opposite once the AP arrives), you can measure when an AP is at one position, then having the other electrode downstram, can measure it there-check speed, strength and more
What is the principle for Ulnar nerve stimulation EMG?
Group of muscle of the thumb are innervated by the median nerve-measure the AP at the arm part, then again at the thumb muscle part
How does force of contraction respond to an increase in amplitude?
As the implitude increases, the force increases and happens over a longer period of time
How does force of contraction respond to an increase of frequency?
at slow stimuli, the peaks are all individual-no summation, just happening-not very strong
WHen reach a certain speed, peak start to merge and add on top of one another-start making a logarythmic looking curve. At sufficuent frequency, makes a nice looking addition of force that peaks and stays constant until stimuli stopped-tetanic proper contraction
What are the main difference between Voluntary and electical stimuli in terms of EMG and force?
EMG once makes a frequency-peaks of activating electronics. Voluntary has a much smoother, all together mix of AP
results are about the same, and force adds up the same-but activally a bits higher, just less stable
