Neuromuscular Junction Flashcards
How is skeletal muscle innervated
By motor neurones with myelinated axons and cell bodies in spinal cord
How does the motor neurone innervate the muscle
Axon divides into unmyelinated branches near to the muscle
Each branch innervates an individaul skeletal muscle fibre
What is a motor unit
The neurone and the number of fibres that it innervates
What are some key features of neuromuscular junction
Terminal bouton
Synaptic vesicles
Synaptic cleft
End plate region
What are active zones
Synaptic vesicles containing ACh awaiting release cluster at sites
How is choline ttansported into the terminal
By choline transporter
Where is ACh syn and form what
Syn in cytosol from choline and acetyl coenzyme A by enzyme
Choline acetyl transferase
What does the arrival of the AP do to the terminal
Causes depo and opening of voltage activated calcium channels
Allowing calcium entry to terminal
Calcium causes vesicles at active sites to fuse with presyn mem release ACh to cleft
Activate post syn nicotinic ACh receps in end plate region
What does ACh activate
Nicotinic ACh receps located at muscle end plate
Where are the nicotinic ACh receps
Assembled as pentamers of glycoprotein subunits that surround central cation selective pore which has a gate which is closed without ACh
Coz driving force for sodium > pottasium =
Influx of sodium greater than efflux pottasium which is depo known as end late potentail
What causes a miniature end plate potential
Electrical repsonse of one quantum of transmitter due to activation of nicotinic ACh receptors at the end plate
What do multiple mepp sum mate to produce
The end plate potential a graded electrical response
What does an End plate potentail do
If it exceeds the threshold it exhibits an all or none response initiate contraction
Why is the muscle action potential required for contraction
The voltage activated channel propagates the end plate over the length of the muscle fibre
Whilst the AP propagates over the surface mem of skeletal ,sucked what does it affect
T tubules which close to sarcoplasmic reticulum
Which triggers release of calcium by interacting with troponin associated with myofibrils
How can a neuromuscular transmission be rapidly terminated
By hydrolysis ACh by using acetylcholinesterase anytime associated with end plate
Hydrolyses to: choline and acetate
What is neuromyotonia
Isaacs syndrome
Symptoms - multiple disorders of skeletal muscle including cramps, stiffness, slow relaxation
Autoimmune origin - antibodies against voltage activated pottasium channels in motor neurone disrupt function resulting in hyper excitability
What is lambert-eaton myasthenia syndrome
Characterised by muscle weakness in the limbs, very rare and associated with small cell carcinoma
Autoimmune origin
Drugs used to help increase the conc of ACh in synaptic cleft
Myasthenia gravis
Increasing muscle weakness during activty
Autoimmune origin - antibodies against ACh receps in end plate result in reduction in the num of functional channels and hence amplitude o the epp
Drug treat - anticholinesterasss ehci increase ACh in syn cleft
What is botulinum toxin
Potent exotoxin toxin that acts at motor neurone terminals to reversible inhibit ACh release
Enter presyn nerve terminals to enzymatic modify protiens involved in vesicles contains ACh
BOTOX anti wrinkle and treat over active muscles
What are curare-like compounds
Interfere with postsyn action fo ACh by acting as competitive antagonists of nicotinic ACh receps
Reduce amplitude of endplate potentail to below threshold for muscle fibre AP gen
Clinically used to induce reversible muscle paralysis
What is skeletal muscle
Voluntary muscle
