Neuroscience 4- Neuromusclular Junction Flashcards
Define neuromuscular junction
A specialised synapse between a motor neurone and a muscle fibre
Describe the basic structure of a neuromusclular junction
- Acetylcholine receptors are on the muscle membrane
- Acetylcholineesterase is also present in the muscle membrane, which breaks down acetylcholine.
- Neurotransmitters released into synaptic cleft
What is the neurotransmitter for voluntary striated muscle?
Acetylcholine
What are miniature end plate potentials, and what causes them?
- Vesicles containing acetylcholine collide spontaneously with the nerve terminal and release acetylcholine without a signal from the axon.
- These small depolarizations are not enough to reach threshold and so an action potential in the postsynaptic membrane does not occur.
Describe the structure of myofibres
- Covered by plasma membrane – sarcolemma
- T-tubules tunnel into centre
- Cytoplasm called sarcoplasm – myoglobin and mitochondria present
- Network of fluid filled tubules – sarcoplasmic reticulum
- Composed of myofibrils
Describe the structure of myofibrils
- 1-2μm in diameter
- Extend along entire length of myofibres
- Composed of two main types of protein – actin and myosin
Describe the structure of myofilaments
- Z discs separate sarcomeres
- A band is myosin
- L band is actin (light band)
- H zone is just myosin, with no overlap of actin and myosin
- M line is the middle of the myosin fibres
What happens during contraction to the sarcomere?
- I band becomes shorter
- A band remains the same length
- H zone is narrowed
What disorders of NMJ function cause muscle weakness?
- 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.
- Used in botox
What is myathenia gravis?
- An autoimmune disorder where antibodies are directed against the acetylcholine receptor, therefore it no longer works.
What is a bulbar muscle?
Muscle in the brainstem.
What is Lambert-Eaton myastenic syndrome?
An autoimmune disease caused by antibodies directed against the voltage-gated calcium channel (VGCC) - associated with lung cancer.
What is the difference between concentric and eccentric muscle contraction?
- Concentric is were the muscle contracts simultaneously and shortens
- Eccentric is where the muscle contracts as it lengthens, for example when picking up something that is too heavy, or walking downhill (braking contraction).
