NMJ and EMG Flashcards
What is the neuromuscular junction?
A specialised structure incorporating the distal axon terminal and the muscle membrane that allows for the unidirectional chemical communication between peripheral nerve and muscle
What are the main structures constituting the NMJ?
- Presynaptic nerve terminal
- Synaptic cleft
- Postsynaptic endplate region on the muscle fibre
what are each muscle fibre innervated by?
Although mixture of muscle fibres are being innervated by a mixture of neurotransmitters, but it only receives innervation from one neuron.
Lots of nerves innervate lots of different muscle fibres, but during selective development the muscle fibre will only be innervated by one neuron.
What is the neurotransmitter for voluntary striated muscle?
Acetylcholine
What is the organisation of the motor neuron?
- Organised into upper and lower Upper - in the brain/brain stem Lower - Brain stem if going to the face - Spinal cord if going to arms and legs
- Highly branched nerves
- However, a single muscle fibre only receives a branch from one neuron (see the picture in the slide - only receive innovation from one colour of neurons and not the other colour)
What are myofibers?
- Covered by plasma membrane (sarcolemma)
- T-tubules tunnel into centre
- Cytoplasm called sarcoplasm (includes myoglobin and mitochondria present)
- Network of fluid filled tubules (sarcoplasmic reticulum)
- Composed of myofibrils
What are myofibrils?
myofibrils?
- 1-2 micrometers in diameter
- Extend along entire length of myofibers
- Composed of two main type of proteins - actin and myosin
What elements are the skeletal muscle made of?
- Dense protein Z-discs separate sarcomeres
- Dark bands - A bands (thick - myosin)
- Light bands - I bands (thin - actin)
- Myosin and actin filaments overlap
What are the observations during the sliding filament theory?
- During contraction, the I band become shorter
- A-band remains the same length
- H-zone narrowed or removed
What is the process of activation and relaxation in skeletal muscles?
- Action potential propagates along surface membrane and into T-tubules
- DHP (dihydropyridine) receptor in T-tubule membrane: senses change in voltage & 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
- Ca2+ binds to Troponin & Tropomyosin moves allowing
Crossbridges to attach to actin - Ca2+ is actively transported into the SR continuously while action potentials continue.
ATP- driven pump (uptake rate < or = release rate). - Ca2+ dissociates from TN when free Ca2+ declines; TM block prevents new crossbridge attachment; Active force declines due to net crossbridge detachment
What are the 3 NMJ disorders?
- Botulism
- Myasthenia Gravis (MG)
- Lambert-Eaton Myasthenic Syndrome (LEMS)
All result in the prevention of muscle contraction
What is Botulism?
Prevents the release of acetylcholine from the pre-synaptic neurone
- Botulinum toxin produces an irreversible disruption in stimulation-induced acetylcholine release by the presynaptic nerve terminal
- When an action potential arrives at the presynaptic terminal causes set of events that cause muscle contraction.
- However, the release of acetylcholine is prevented
What is Myasthenia Gravis?
Prevents the acetylcholine that has been released from having an effect by causing defective receptor to the acetylcholine, which is caused by antibodies
- An autoimmune disorder where antibodies are directed against the acetylcholine receptor
- Cause fatigable weakness (i.e. becomes more pronounced with repetitive use)
- May affect the ocular, bulbar (motor tracts carrying signals to the muscles of face and eyes), respiratory or limb muscles.
- In severe cases, the antibodies in the blood can be removed via plasma exchange which allows rapid improvement to occur.
What is Lambert-Eaton Myasthenic Syndrome (LEMS)?
- An autoimmune disease caused by antibodies directed against the voltage-gated calcium channels (VGCC)
- Associated with lung cancer
Do not allow calcium inside in the pre-synaptic.
What is an Electromyogram (EMG)?
- Recording of the action potentials occurring in skeletal muscle fibres
