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

1. Neuromuscular
Transmission
Dr. Sai Sailesh Kumar G

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

5. Definition
Structural and functional junction between the motor nerve fiber and the
skeletal muscle fiber is called neuromuscular junction.

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

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

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

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

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

39. Pathological changes at NMJ
Loss of post junctional folds
Reduced Ach receptors

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