Introduction Structure of neuromuscular junction Presynaptic portion Synaptic cleft Post synaptic portion Mechanism of neuromuscular transmission Presynaptic events Synaptic events Events at end plate End plate potential Action potential Blockade of neuromuscular transmission Presynaptic blockade Postsynaptic blockade Neuromuscular dysfunctions Myasthenia gravis Lambert Eaton myasthenia syndrome Denervation hypersensitivity

1. Neuromuscular
Physiology
Dr Nameeda K S

2. Contents
1. Introduction
2. Structure of neuromuscular junction
1. Presynaptic portion
2. Synaptic cleft
3. Post synaptic portion
3. Mechanism of neuromuscular transmission
1. Presynaptic events
2. Synaptic events
3. Events at end plate
4. End plate potential
5. Action potential 2

3. Contents
1. Blockade of neuromuscular transmission
1. Presynaptic blockade
2. Postsynaptic blockade
2. Neuromuscular dysfunctions
1. Myasthenia gravis
2. Lambert Eaton myasthenia syndrome
3. Denervation hypersensitivity
3

4. o The junction between a motor neuron and muscle
fibre is known as neuromuscular junction.
Also called
▪ Myoneural junction
▪ Motor end plate
4
▪ Through which action potential from neuron is transmitted to the
muscle fibre.
▪ At this junction the neuronal membrane and the sarcolemma
remain opposed to each other, but do not touch.

5. 5
▪ Motor neurons have
their cell bodies in
anterior horn of
spinal cord or in
brain stem.
▪ Their axons are
myelinated and are
the largest diameter
axons in the body.

6. Presynaptic portion-
Axon terminal
• As the axon approaches the
muscle it loses its myelin
sheath and divides
extensively into fine branches
of 2µm diameter called axon
terminals.
• Terminals are covered by
schwann cells known as
teloglia.
6

7. Motor Unit
▪ Each terminal forms a junction
with the single skeletal muscle
fibre, midway along its length.
▪ Thus each muscle fibre is
supplied by one motor neuron
terminal.
▪ The motor neuron terminal,
including its axon and axon
terminals and the muscle fibres
supplied by it are called a motor
unit.
7

8. ▪ Each terminal is expanded at its
end to form knobby structure-
synaptic knob which contains
plenty of mitochondria and
neurotransmitter vesicles.
▪ The terminal button lies in the
synaptic trough on the muscle
cell membrane.
▪ Vesicles are clustered around
specific points called active
zones. The membrane at the
active zones is modified to form
dense bar that contains voltage
gated Ca++ channels.
8

9. 9
▪ 40- 100nm wide
▪ The basement
membrane in the cleft
consists of
acetylcholine esterase
– anchored to the
collagen fibrils.
Synaptic cleft

10. Postsynaptic portion –
end plate membrane
▪ Part of the sarcolemma
that lies directly under the
terminal button.
▪ End plate membrane-
increased area-
junctional folds.
▪ Ach receptors
▪ Voltage gated Na+
channels
10

11. 11

12. Ach Receptors at NMJ
▪ Nicotinic type.
▪ Around 15-40 million Ach receptors account for 10,000
AchR per square micrometre of end plate membrane.
▪ Chemicallly gated ion channel (2 Ach).
▪ Blocked by α- bungarotoxin.
▪ Large pore diameter- 0.8nm : permeable to both Na+
and K+ entry. (but electrochemical gradient favours NET
INFLUX OF Na+)
▪ AchR allow the passage of only cations (anions repelled
by the fixed negative charges present in it).
12

13. 13
▪ 5 subunits
□ 2 α
□ 1 β
□ 1 δ
□ 1 γ
Nicotinic receptors

14. Mechanism of
Neuromuscular
Transmission

15. Presynaptic events
15
Release
acetylcholine
into synaptic
cleft
PHASE 1
Action potential
depolrises the
membrane
PHASE 2
Opening of the voltage
gated Ca+ channels-
Ca influx .
PHASE 4
Vesicles fuse with the
membrane- exocytosis.
PHASE 3
Increased movement of
microtubules and
microfilaments
migration of
neurotransmitter
vesicles.

16. 16

17. Quantal Release
▪ Depolarization causes synchronous release of
about 60-200 vesicles from different parts of the
presynaptic membrane.
o Each vesicle contains 4000- 10,000 Ach molecules.
o Vesicle = quanta
o With each action potential the presynaptic terminal
discharges about the same amount of
neurotranmitter collectively known as quantal
content.
17

18. 18
Ach to AchR
allows Na+
influx- local
depolarization-
End Plate
Potential
EPP decreases
the membrane
potential to the
firing level and
opens the
voltage gated
Na+ channels –
Action Potential
Action potential
propagates
along the
sarcolemma
Ach- AchR
loose and
reversible-
rapidly
hydrolysed to
acetate and
choline by ActE
Closure of ion
channels-
cessation of
EPP – return to
resting state.

19. 19

20. End Plate Potential
▪ Is like a graded potential, the amplitude of
which is proportional to amount of
neurotransmitters released.
▪ Decrementel conduction.
▪ As Ach- gated ions are localized to end
plate membrane- confined to that region
only- the amplitude of EPP declines
progressively with increasing distance
from the end plate region. 20
First recorded in 1950
by Paul Fatt and
Bernard Katz.

21. EPSP
▪ In CNS most presynaptic neurons
generate excitatory post synaptic potential
(EPSP) less than 1 mV- several EPSP
required to attain threshold to generate
action potential in the post synaptic
neuron.
▪ Where as in NMJ the amplitude of EPP is
much larger than EPSP in CNS.
21

22. EPP vs EPSP
▪ Stimulation of single
motor neuron produces
an EPP of 70 mV
▪ Which is much bigger
than the required
(15mV) to bring
membrane potential to
firing level.
▪ Stimulation of a motor
nerve always produces
action potential in each
muscle fibre supplied by
it – EPP is always a
depolarizing potential.
▪ EPP rises quickly-
resulting in action
potential as soon as the
end plate membrane
attains threshold level.
22

23. 23

24. Miniature End Plate Potential
▪ When motor nerve is at rest spontaneous
occurring potentials of minute amplitude
(0.1- 4mV) are recorded from postsynaptic
membrane- MEPP.
▪ Properties like EPP
▪ Due to quantal event.
24

25. Resting Membrane Potential
▪ Neurons have a negative concentration gradient most of the time-
more positively charged ions outside than inside the cell. This regular
state of a negative concentration gradient is called resting membrane
potential.
▪ During the resting membrane potential there are:
□ more sodium ions outside than inside the neuron
□ more potassium ions inside than outside the neuron
▪ The exact value measured for the resting membrane potential varies
between cells, but -70 mV is most commonly used as this value.
▪ Leakage channels allow Na+ to slowly move into the cell or K+ to
slowly move out, and the Na+/K+ pump restores them. This may
appear to be a waste of energy, but each has a role in maintaining
the membrane potential.
25

26. Action Potential
▪ Action potentials are nothing more than a temporary shift (from
negative to positive) in the neuron’s membrane potential caused
by ions suddenly flowing in and out of the neuron.
26

27. ▪ This starts with a channel opening for
Na+ in the membrane. Because the
concentration of Na+ is higher outside the
cell than inside the cell by a factor of 10,
ions will rush into the cell that are driven
largely by the concentration gradient.
Because sodium is a positively charged
ion, it will change the relative voltage
immediately inside the cell relative to
immediately outside. The resting potential
is the state of the membrane at a voltage
of -70 mV, so the sodium cation entering
the cell will cause it to become less
negative. This is known as depolarization,
meaning the membrane potential moves
toward zero.
27

28. ▪ The concentration gradient for
Na+ is so strong that it will
continue to enter the cell even
after the membrane potential has
become zero, so that the voltage
immediately around the pore
begins to become positive. The
electrical gradient also plays a
role, as negative proteins below
the membrane attract the sodium
ion. The membrane potential will
reach +30 mV by the time
sodium has entered the cell.
28

29. ▪ As the membrane potential reaches +30 mV, other voltage-gated
channels are opening in the membrane. These channels are specific for
the potassium ion. A concentration gradient acts on K+, as well. As
K+ starts to leave the cell, taking a positive charge with it, the membrane
potential begins to move back toward its resting voltage. This is
called repolarization, meaning that the membrane voltage moves back
toward the -70 mV value of the resting membrane potential.
▪ Repolarization returns the membrane potential to the -70 mV value that
indicates the resting potential, but it actually overshoots that value.
Potassium ions reach equilibrium when the membrane voltage is below -
70 mV, so a period of hyperpolarization occurs while the K+ channels are
open. Those K+ channels are slightly delayed in closing, accounting for
this short overshoot.
29

30. 30

31. 31

32. 32

33. 33

34. Blockade of Neuromuscular
Transmission
▪ The neuromuscular transmission is disrupted
at different steps by drugs, chemicals, toxins
and trauma.
▪ Failure to produce EPP leads to paralysis of
the skeletal muscles.
▪ The drugs that cause muscle relaxation are
used in surgery and in some hyperactive
states.
▪ The blockade can occur presynaptic or
postsynaptic level. 34

35. Presynaptic Blockade
▪ Interruption of the events taking place at
the presynaptic axon terminal leads to
Impaired calcium influx causing decreased
vesicle release.
35

36. Botulinum toxins
▪ Bacteria Clostridium Botulinum releases a toxin that causes a
paralytic disease called botulism.
▪ Botulinum toxin is most potent natural toxin, a minute amount
can cause serious illness.
▪ Lethal dose- 2-3 μg
▪ Botulinum toxin B, D, F and G inactivate synaptobrevin- a vesicle
membrane protein that is required for the binding and fusion of
Ach vesicles with the plasma membrane of the axon terminal.
▪ Botulinum toxin A and B act on SNAP- 25 and toxic C breaks
down syntaxin.
▪ Thus botulinum toxin inhibits the release of ACh from the axon
terminals resulting in cessation of muscle contraction (flaccid
paralysis). 36

37. 37
Local injections of highly
diluted toxin produce
muscle relaxation at the
desired site- Botox
treatment
It is injected into the lower
oesophageal sphincter to
treat Achalasia cardia
Into extra ocular muscles to
decrease over activity-
used in the management of
strabismus and
blepharospam
Into facial muscles to
reduce aging related
skin wrinkle and used
in cases of cervical
dystonia
Misuse
Lethal bioterrorism agent
by contaminating food
supply
Disorders of secretion such
as Sialorrhea- under
research
Gooriah R, Ahmed F (2015) Therapeutic Uses of Botulinum Toxin. J Clin Toxicol 5:225. doi: 10.4172/2161-0495.1000225

38. Hemicholinium
▪ The drug inhibits
choline uptake by the
presynaptic terminal
resulting in depletion
of Ach.
▪ Consequently the
EPP decreases and
action potential will
not be formed. 38

39. 39
▪ Inhibits the transport of acetylcholine into the synaptic
vesicles in nerve terminals.
Vesamicol

40. Postsynaptic Blockade
▪ A variety of agents can act on the
postsynaptic membrane interfering with
the generation of EPP by different
mechanisms.
□ Competitive blockers
□ Depolarizing blockers
40

41. Competitive Blockers
▪ Block acetylcholine receptors.
▪ Curare and Gallamine.
▪ They diffuse to the end plate membrane and
compete with Ach for AChR.
▪ But their attachment with AChR does not lead to
opening of ionic channels as they do not have
biological activity as Ach.
▪ Moreover they do not get hydrolysed by
Acetylcholine Esterase.
▪ Binding site at receptor occupies permanently
▪ Hence no EPP and lack of muscle contraction. 41

42. Curare
▪ Plant product –
chondrodendron tomentosum.
▪ It is used by South America
tribals as arrow poison for
hunting.
▪ The animals got paralysed
but not killed by the arrow,
muscle paralyzing active
principles of curare are
alkaloids tubocurarine ,
toxiferine. 42

43. Gallamine
▪ Skeletal muscle relaxant before surgeries.
▪ Reduces the required dose of anaesthesia
as well as bleeding and other
consequences.
▪ As the respiratory muscles are also
relaxed, patients are supported with
artificial ventilation.
43

44. 44
Depolarizing Blockers
▪ Drugs like succinylcholine and carbamylcholine have the biological activity of Ach, but
they are not hydrolysed by AChE.
▪ Therefore their action is long lasting. When they bind to AchR , the ion channels remain
open in the end plate.
▪ The maintained depolarization keeps the voltage gated channels in an inactivated state
and blocks subsequent action potentials.
▪ There are other drugs that produce the blockade by inactivating AchE reversibly or
irreversibly.

45. Reversible AChE Inhibitors
▪ These agents compete with Ach to bind to
AchE, thereby prevent the hydrolysis of Ach
by AChE.
▪ The accumulated Ach at the synaptic cleft
binds to AChR, leading to depolarizing block.
▪ This is of use in case of diseases like
myasthenia gravis where excess Ach is
requirement in the case of decreased AChR.
▪ Neostigmine and physostigmine are
examples of reversible AChE inhibitors. Their
attachment with AChE decreases, when the
concentration of the drug falls and so the
block is removed after some time.
▪ This block can be overcome by application of
curare that attaches to AChR and reduces
the effect of excess ACh on the end plate
membrane.
45

46. Irreversible AChE
Inhibitors
▪ The organophosphorous compounds(pesticides
like parathion, malathion and baygon) and
nerve gases used in chemical warfare like
disopropylfluro- phosphate (DFP) and sarin,
bind to AChE very tightly. This binding is
irreversible.
▪ Lack of hydrolysis of Ach produces sustained
depolarization of the end plate resulting in no
further contractions in response to the
subsequent nerve stimulation.
▪ Poisoning with these agents lead to skeletal
muscle paralysis and death from asphyxiation.
▪ Nerve gases are also accumulation of Ach at
the cardiac pacemaker cells. Therefore,, in
cases of nerve gas exposure, Atropine (
muscarinic receptor antagonist) is given as
antidote. 46

47. 47

48. Local Anesthetics
▪ They prevent the conduction of nerve impulses by
blocking the voltage gated sodium channels.
▪ Local anaesthetics diffuse through the cell
membrane and bind to the voltage sensitive
sodium channel from the inner side of the cell
membrane. They prevent the increase in
permeability to Na+ and gradually rise the
threshold of excitation.
▪ With increase in concentration, impulse conduction
slows, rate of rise of action potential declines-
action potential amplitude decreases and finally
the ability to generate AP is abolished.
▪ Small nerve fibres are blocked first- autonomic
nerve fibres blocked first followed by sensory
fibres conducting pain, temperature, touch,
pressure and vibration. This is called differential
blockade. 48

49. 49
Neuron type Myelination Function Structures
supplied
A alpha Myelinated Motor- skeletal
muscles
Facial muscles
A beta Myelinated Sensory- touch,
pressure, vibration
Peripheral pulp and
inner dentin
A gamma Myelinated Motor- muscle
spindles
proprioception
Peripheral pulp and
inner dentin
A delta Myelinated Fast pain,
temperature
Mucosa and skin,
periodontal ligament
B Myelinated Autonomic- pre
ganglionic
sympathetic
Blood vessels,
salivary glands
C Unmyelinated Slow pain,
autonomic, post
ganglionic
sympathetic
polymodal
nocioceptors
Pulp of teeth,
mucosa

50. 50

51. Dantrolene
▪ Inhibits the muscle
contraction by
preventing calcium
release from the
sarcoplasmic reticulum.
▪ Binds to the ryanodine
receptor and blocks the
opening of ryanodine
channel. 51

52. Neuromuscular
Dysfunctions
52

53. Myasthenia Gravis
▪ Is a relatively rare
acquired, autoimmune disorder
caused by an antibody-mediated
blockade of neuromuscular
transmission resulting in skeletal
muscle weakness.
▪ Etiology- decrease in number of AChR
on motor end plate due to
autoantibodies against the receptors.
▪ Postsynaptic folds are flattened.
▪ Due to these changes, though ACh is
released normally magnitude of EPP
falls below threshold of initiating action
potential.
53

54. 54
Fatigue Women: male= 3:2
Muscle weakness
– proximal limb
muscles most
commonly
involved
Diplopia and
ptosis
Severe cases-
Respiratory
muscle paralysis
Features

55. Physiological basis
of treatment
▪ Administration of AChE
inhibitors- neostigmine-
increase the concentration of
Ach at NMJ.
▪ Thymectomy- blunts down
immune response and improves
the condition.
▪ Immunosuppression-
corticosteroids and azathiprine.
▪ Plasmapheresis- removes
AChR anti-bodies from plasma.
55

56. Lambert- Eaton Myasthenic Syndrome
▪ Presynaptic disorder of the neuromuscular junction due
to production of autoantibodies against voltage gated
Ca++ channels.
▪ The decreased Ca++ influx in the presynaptic terminal
leads to impaired Ach release from the nerve endings.
▪ The muscular weakness is primarily seen in the limbs
▪ Patients show incremental response to repetitive nerve
stimulation as Ca+ level raises wit each action potential.
56

57. Denervation
Hypersensitivity
▪ The phenomenon of
increased
responsiveness.
▪ Both is skeletal and
smooth muscles.
57

58. 58

59. References
▪ Comprehensive textbook of physiology GK
Pal
▪ Guyton and Hall textbook of physiology
▪ Essential medical pharmacology by KD
Tripathi.
▪ Essentials of Local Anesthesia by Stanley.
F MalamedS
59

60. 60
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you!
Have a beautiful day.