NMJ, muscle contraction and EMG Flashcards
What is a neuromuscular junction?
A specialised synapse between a motor neuron and a muscle fibre.
A specialized structure incorporating the distal axon terminal and the muscle membrane that allows for the unidirectional chemical communication between peripheral nerve and muscle
What another word for a presynaptic terminal?
Bouton
What does a synapse allow contact between?
Allows for contact from neuron to muscle or from neuron to neuron.
What are the main structures constituing the Neuromuscular Junction?
–presynaptic nerve terminal
–synaptic cleft
–postsynaptic endplate region on the muscle fibre
What serves as the neurotransmitter for voluntary striated muscle?
Acetylcholine serves as the neurotransmitter for voluntary striated muscle
List 7 steps that occur at a NMJ?
•The NMJ is responsible for initiating muscle contraction.
An action potential arrives at the synaptic knob
The voltage-gated calcium ion channels open
Calcium ions diffuse into the synaptic knob
The calcium ions cause the synaptic vesicles to move to and fuse with the presynaptic membrane
Acetylcholine is released by exocytosis
Acetylcholine molecules bind to the receptor sites on the sodium ion channels in thee postsynaptic membrane
The sodium ion channels open
Sodium ions diffuse across the postynaptic membrane into the postsynaptic neurone
A generator potential is or excitory postsynaptic potential (EPSP) is created
If sufficient generator potentials combine then the potential across the postsynaptic membrane reaches the threshold potential
A new action potential is generated in the postsynaptic neurone
What happens at rest at a NMJ?
At rest, individual vesicles release ACh at a very low rate causing miniature end-plate potentials (MEPPs)
lable the diagram below
What are myofibres composed of?
•Composed of myofibrils
What are myofibres covered with?
•Covered by plasma membrane – sarcolemma
What are the diameters of myofibrils?
•1-2μm in diameter
What types of protein are myofibrils composed of?
•Composed of two main types of protein – actin and myosin
What gives muscles a striated appearence?
•Light and dark bands give muscle striated appearance
Sketck a myofilament?
Myofilaments do not extend along length of myofibres?
True or False
True
Do not extend along length of myofibers
Myofilaments overalp and are arranged in compartments called …………………
Myofilaments overlap and are arranged in compartments called sarcomeres
What is the role of dense protein Z-discs in myofilaments?
Dense protein Z-discs separate sarcomeres
What are the dark bands (A bands) in myofillaments composed of
•Dark bands – A band (thick - myosin)
What are light bands ( I band composed of )
•Light bands – I band (thin - actin)
Lable the image below
What happens to the I band, A band and H-zone in contraction?
- During contraction I band became shorter
- A-band remained same length
- H-zone narrowed or disappeared
Lable the diagram below
Recall the activation and relaxation process
What is a motor end plate?
The large and complex terminal formation by which the axon of a motorneuron establishes synaptic contact with a striated muscle fiber. Also called motor plate .
T-Tubles tunnel unto the centre
To cause calcium release in myofilaments in the centre
Name 3 disorders of NMJs and define neruomuscular junction?
- Pathological processes interfering with NMJ function can cause muscle weakness
- Examples:
–Botulism
–Myastenia gravis (MG)
–Lambert-Eaton myastenic syndrome (LEMS)
What is botulism
Botulinum toxin produces an irreversible disruption in stimulation-induced acetylcholine release by the presynaptic nerve terminal
Botulism is a rare and potentially fatal illness caused by a toxin produced by the bacterium Clostridium botulinum.
What is Myasthenia Gravis?
- An autoimmune disorder where antibodies are directed against the acetylcholine receptor.
- There may be a personal or family history of other autoimmune diseases. It cause fatigable weakness (i.e. becomes more pronounced with repetitive use) and may affect the ocular, bulbar, respiratory or limb muscles.
- Antibodies are detected in nearly 90% of cases and EMG examination will confirm the diagnosis.
- In severe cases the antibodies in the blood can be removed via plasma exchange which allows rapid improvement to occur.
What is Lambert-Eaton myastenic syndrome?
•an autoimmune disease caused by antibodies directed against the voltage-gated calcium channel (VGCC) - associated with lung cancer
In normal neuromuscular function, a nerve impulse is carried down the axon (the long projection of a nerve cell) from the spinal cord. At the nerve ending in the neuromuscular junction, where the impulse is transferred to the muscle cell, the nerve impulse leads to the opening of voltage-gated calcium channels (VGCC), the influx of calcium ions into the nerve terminal, and the calcium-dependent triggering of synaptic vesicle fusion with plasma membrane. These synaptic vesicles contain acetylcholine, which is released into the synaptic cleft and stimulates the acetylcholine receptors on the muscle. The muscle then contracts.[1]
In LEMS, antibodies against VGCC, particularly the P/Q-type VGCC, decrease the amount of calcium that can enter the nerve ending, hence less acetylcholine can be released from the neuromuscular junction.
.
