Large motor neurons originates from the anterior horn cells of spinal cord
They are myelinated nerve fibers
They innervates skeletal muscles
Each nerve fiber after entering the muscle belly, branches and stimulates 3- several hundreds of skeletal muscle fibers
Each nerve ending makes a junction – Neuromuscular Junction
NMJ is present at midpoint of the muscle
AP initiated in the muscle fiber by the nerve impulse, travels in both directions towards the muscle fiber ends
2. Learning objectives
Draw and label a typical neuromuscular junction
Describe the sequence of events occurring at NMJ
Classify drugs acting on NMJ
Describe the basis and features of myasthenia gravis
3. Introduction
Large motor neurons originates from the anterior horn cells of spinal cord
They are myelinated nerve fibers
They innervates skeletal muscles
Each nerve fiber after entering the muscle belly, branches and stimulates 3-
several hundreds of skeletal muscle fibers
Each nerve ending makes a junction – Neuromuscular Junction
4. Introduction
NMJ is present at midpoint of the muscle
AP initiated in the muscle fiber by the nerve impulse, travels in both
directions towards the muscle fiber ends
6. Physiological anatomy
The nerve terminals invaginate the surface of the skeletal muscle fiber
But lie outside the muscle fiber plasma membrane
The entire structure is called motor end plate
The invaginated membrane – synaptic gutter / synaptic trough
The space between the nerve terminal and muscle membrane – synaptic
space /synaptic cleft
7. Physiological anatomy
Synaptic space is 20-30 nm wide
At the bottom of gutter, numerous small folds of muscle membrane – sub
neural cleft
Sub neural cleft – greatly increase the surface area where N.T act
The axon terminal has numerous mitochondria – to supply ATP
ATP is energy source for synthesis of the acetylcholine
8. Physiological anatomy
Acetylcholine intern excites muscle fibers
Ach is synthesized in the cytoplasm of the terminal
Then rapidly absorbed into many small vesicles
Around 300,000 vesicles in one end plate
In synaptic space large quantities of acetylcholine esterase is present
It destroys Ach after a few mili sec after Ach has been released from vesicles
9.
10. Secretion of Ach by the nerve terminals
Nerve impulse reaches NMJ
About 125 vesicles of Ach is released from nerve terminals
Ach is released into the synaptic space
On inside the surface of the neural membrane are linear dense bars
On each side of dense bars there are proteins that penetrate the membrane
They are voltage gated calcium channels
11.
12. Secretion of Ach by the nerve terminals
Spreading of AP in the nerve terminal
Opening of voltage gated calcium channels
Diffusion of calcium to the interior of nerve terminal
Activation of calcium calmodulin protein kinase
Phosphorylates synapsin proteins
Synapsin anchors the vesicles to cytoskeleton of presynaptic terminal
13. Secretion of Ach by the nerve terminals
Docking of the vesicles at the releasing sites
Fuse with neural membrane
Empty the Ach into synaptic space by exocytosis
14. Events at post synaptic membrane
Muscle fiber membrane has many small acetylcholine receptors
They are acetylcholine gated ion channels
Lies immediately below the dense bars
The channel is closed
When two Ach molecules attaches to the receptors
Conformational change occurs and channel opens
15.
16. Events at post synaptic membrane
The channel has diameter about 0.65nm
Large enough to allow important positive ions
Sodium, potassium, calcium
They can transmit 15,000-30,000 sodium ions in 1 milli sec
Negative ions do not pass through due to strong negative charges in the
mouth of the channel that repel these negative ions
17. Events at post synaptic membrane
More sodium ions pass through these channels than any other ions
Sodium is large amounts in ECF
Potassium is large amounts in ICF
Negative potential on the inside of the muscle membrane (-80 to -90 mv)
Pulls the positively charged sodium ions inside the fiber
Prevents efflux of potassium ions
18. Events at post synaptic membrane
Influx of large amounts of sodium ions
Creates local positive potential change inside the muscle fiber membrane
Endplate potential
This endplate potential initiates action potential
AP spreads along the muscle membrane
Causes muscle contraction
19. Destruction of acetylcholine
Ach once released into synaptic space
Continues activating Ach receptors
As long as Ach persists in the synaptic space
However it is removed rapidly by two means
Most of Ach is destroyed by enzyme acetylcholine esterase
Small amount is diffused out of the synaptic space and no longer available
20. Destruction of acetylcholine
Ach remains in synaptic space only few milli sec
But that time is sufficient to excite muscle fiber
Rapid removal of Ach prevents continuous muscle reexcitation
21. Events at post synaptic membrane
Opening of Ach gated channels
Sudden insurgence of sodium ions into the muscle fiber
Electrical potentials at the local area of end plate
Potential increases in positive direction
Local potential – end plate potential
If EPP is stronger, it causes enough sodium channels to open - AP
22.
23. Molecular biology of Ach formation
Small vesicles about 40 nm in size
Formed in the Golgi apparatus in the cell body of motor neuron
These vesicles are then transported by axoplasm
All the way to the NMJ at the tips of the peripheral nerve fibers
About 300,000 of these small vesicles present at the nerve terminal
24. Molecular biology of Ach formation
Ach is synthesized in the cytosol of nerve fiber terminal
Immediately transported through the membranes of vesicles
To their interior
It is stored in the vesicles
10,000 molecules of Ach in each vesicle
25. Molecular biology of Ach formation
Arrival of Action potential at nerve terminal
Opens many calcium channels in the membrane of nerve terminal
Increase in the calcium influx
Calcium concentration increases 100 fold
Rate of fusion of ach vesicles with membrane increases 10,000 fold
Rupture of many of vesicles
26. Molecular biology of Ach formation
Exocytosis of Ach into synaptic space
About 125 vesicles rupture with each AP
After few milli sec, Ach is split by Ach esterase
Acetate and choline
Choline is reabsorbed to neural terminal
Reused in Ach synthesis
27. Molecular biology of Ach formation
This sequence of events occurs within a period of 5-10 millisec.
28. Drugs that acts on NMJ
Drugs that prevent release of Ach
Botulinum toxin
Product of bacteria clostridium botulinum
Prevents Ach vesicles from fusing with pre-synaptic membrane
Prevents release of NT into synaptic cleft
29. Drugs that acts on NMJ
Drugs that stimulate the muscle fiber by ach like action and block
transmission
Methacholine, carbachol, nicotine
Acts like Ach
Binds to Ach receptors
EPP and AP and muscle contraction initially
30. Drugs that acts on NMJ
These drugs can not be destroyed by Ach esterase
Their actions persist for many minutes to several hours
Persistent depolarization
Muscle paralysis
31. Drugs that acts on NMJ
Drugs that stimulate the NMJ by inactivating the Ach esterase
Well known drugs
Neostigmine, Physostigmine, diisopropyl flurophosphate
Inactivate ach esterase
Ach can not be hydrolyzed
Stimulation of muscle fiber repeatedly
32. Drugs that acts on NMJ
Muscle spasm
May also cause death of the person by laryngeal spasm
Neostigmine and physostigmine inactivate Ach esterase for several hours
After that these drugs are displaced from the ach esterase
Esterase once again become active
Di isopropyl fluorophosphate –powerful nerve gas poison
33. Drugs that acts on NMJ
Di isopropyl fluorophosphate –powerful nerve gas poison
Inactivates Ach esterase for weeks
Lethal poison
34. Drugs that acts on NMJ
Drugs that block transmission at NMJ
Curariform drugs
Prevent passage of impulses from nerve ending into the muscle
D-tubocurarine blocks the action of Ach on Ach receptors
Prevent opening of sodium channels
No EPP and No AP and no muscle contraction
35. Myasthenia Gravis
Autoimmune disease
Occurs in 1 in every 20,000 people
Cause muscle weakness
Inability of NMJ to transmit signals from nerve fiber to muscle fiber
Pathologically antibodies that attack the Ach receptors are present in the
blood of the patients
36. Pathological changes at NMJ
EPP are too weak to open enough sodium channels
Depolarization do not occurs
No action potential
No muscle contraction
Muscle paralysis
37. Pathological changes at NMJ
If disease is intense
Patient may die
Respiratory muscle weakness
Respiratory failure
Initial stages – drooping of eyelid, difficult in speech and swallowing
41. Treatment
Administration of Neostigmine
Or some other anticholine esterases
Increase Ach concentration
People can begin to function almost normally within minutes
New dose required few hours later
42. Nerve Ap vs Muscle AP
Nerve Ap- RMP is -90 mv
Muscle AP- RMP is -80 to -90 mv