Nerve - Muscle Physiology

1. Neuromuscular
Transmission and its
Pharmacology

2. Neuromuscular Junction
 Synapses are the junctions where the
axon or some other portion of one cell
(the presynaptic cell) terminates on the
dendrites, soma, or axon of another
neuron or in some cases a muscle or
gland cell (the postsynaptic cell)
 Synapses between axons of motor
neurons and skeletal muscle fibers are
called NEUROMUSCULAR JUNCTION or
MYONEURAL JUNCTIONS

3. Motor Unit
 Motor unit
 One neuron
 Muscle cells
stimulated by
that neuron
 Motor neuron &
all the muscle
fibers it supplies

4. Motor Unit

5. Physiological Anatomy of NMJ
1. PRESYNAPTIC MEMBRANE (TERMINAL
BOUTONS/END FEET) : small, clear vesicles
containing acetylcholine
2. SYNAPTIC CLEFT/SPACE : space between pre
and post-synaptic membrane, 20-40 nm, also
contain enzyme ACETYLCHOLINESTERASE
3. POSTSYNAPTIC MEMBRANE (MOTOR END
PLATE): thickened portion of muscle
membrane at the junction

7. Physiological Anatomy of NMJ
 Axon makes a single point of synaptic
contact with a skeletal muscle fiber,
midway along the length of the fiber
 As axon approaches its termination, it
loses its myelin sheath and divides into a
number of terminal buttons, or endfeet
 The endfeet contain many small, clear
vesicles that contain acetylcholine (Ach),
the transmitter at these junctions

8. Physiological Anatomy of NMJ
 Vesicles – formed in the cell body
and are transported by fast axonal
transport via the microtubules
 ACh is synthesized in the nerve
terminal – outside the vesicle –
from choline and acetyl coenzyme
A by the enzyme choline
acetyltransferase
 The ACh moves into the synaptic
vesicles via a specific ACh-H+
exchanger

10. Release of Acetylcholine
 Nerve impulse reaching axon terminal
increases permeability of pre-synaptic
membrane to Ca2+
 Ca2+ enters the nerve terminal through
voltage-gated channels
 Ca2+ causes fusion of synaptic vesicles
with the pre-synaptic membrane
 Amount of neurotransmitter released is directly
proportional to Ca2+ influx
 Mg2+ decreases this process

11. Physiological Anatomy of NMJ
 Synaptic vesicles fuse at differentiated
regions of the presynaptic membrane
called Active Zones and releases ACh
 Endings fit into junctional folds, which
are depressions in the motor end plate
 Synaptic basal lamina contains high
concentration of enzyme AChE
(Acetylcholinesterase) - terminates
transmission by rapidly hydrolyzing free
ACh to choline and acetate

14. Effect of ACh on Postsynaptic
Membrane
 ACh binds to ACh Receptors which are
ligand-gated ion channels in postsynaptic
membrane
 ACh receptor is a heteropentamer with a
subunit composition of α2βγδ
 Subunits are homologous to one another
 Each α subunit has 4 membrane spanning
segments

15. Acetylcholine Receptor

16. Acetylcholine Receptor

17. Acetylcholine Receptor

18. Effect of ACh on Postsynaptic
Membrane
 Binding of acetylcholine to these receptors
increases the Na+ and K+ conductance
 Na+ influx, creates a local positive potential
change inside the muscle fiber membrane called
the END PLATE POTENTIAL (EPP)
 ACh released into synaptic cleft is removed
rapidly by enzyme Acetylcholinesterase,
present in synaptic cleft. Removal is rapid to
prevent re-excitation of the receptors after
first action potential

19. Entry of sodium
ions
(Depolarization)
Depolarization causes opening
of voltage gated calcium
channels.
Calcium entry
Opening of potassium
channels
(hyperpolarization)
Closure of calcium channels
Entry of Calcium after the arrival of
impulse (action potential)
Events at the end of the nerve fiber

20. Release of
vesicles
Entry of Calcium
Movement of vesicle
towards membrane
Release of Ach
One impulse = 125 vesicles
released
At rest = 1 – 2 Hz release
Events at the end of the nerve fiber

21. Binding to AChR
Opening of Ligand gated channel
Events on the muscle fiber

23.  Binding of ACh to receptor increases the Na+ and
K+ conductance of the membrane
 Resultant influx of Na+ produces a depolarizing
potential - end plate potential
 Current sink created by this local potential
depolarizes the adjacent muscle membrane to its
firing level
 Normally not recordable as action potential is
almost always generated
End Plate Potential (EPP)

24. End Plate Potential (EPP)
 Average human end plate contains about
15–40 million ACh receptors
 Each nerve impulse releases about 60 ACh
vesicles, and each vesicle contains about
10,000 molecules of the neurotransmitter
 Can activate 10 times the number of
acetylcholine receptors
 Therefore, a propagated response in the
muscle is regularly produced

25. End Plate Potential (EPP)
 Only 6 vesicles are reqd for activation
from -90 to -65 mv
 Each nerve impulse releases
60 ACh vesicles
 Every vesicle has 10,000 molecules of ACh
10-fold safety factor
 Action potential always generated

26. Depolarization due to net entry of cations
threshold
Time (ms)
Membranevoltage
(mv)
End plate potential (graded potential)
Action potential
Events on the Muscle Fiber

27. End Plate Potential (EPP)
 EPP can be seen if the tenfold safety
factor is overcome
 Administration of small doses of curare
(competitive inhibitor of ACh) is used for
studying EPP
 The response is then recorded only at the
end plate region and decreases
exponentially away from it
 EPP undergoes temporal summation

28. Miniature End Plate Potential (MEPP)
 At rest - Random release of small packets /
quanta of ACh - Quantal Release of
Transmitter
 Small depolarising spike
 Amplitude = 0.5mv
 Amount released ∞ Ca2+ concentration
1/∞ Mg++ concentration
 When a nerve impulse comes, no. of quanta
released increases resulting in large EPP
that exceeds the firing level of the muscle
fiber producing AP

29. Muscle Nerve
RMP -90mv -70mv
Action
Potential
Duration
2-4msec varies
Velocity 5 m/sec
Varies with
fiber
Absolute
Refractory
period
1-3 msec 2-4 msec
Ionic distribution is similar to that in nerve

31. Transmission of Nerve Impulse to
Muscle
 Sodium rushes into the cell generates an
action potential (AP)
 The action potential travels along the T-
tubules to the SR to stimulate release of
Calcium ions
 The Ca2+ ions travels to the muscle tissue
and bind to the ACTIN regulatory proteins
(Troponin C)

32. Transmission of Nerve Impulse to
Muscle
 This UNCOVERS Myosin Head BINDING Sites on
ACTIN so as to allow CROSS BRIDGING ( once
myosin is powered by ATP)
 Activation by nerve causes myosin heads
(crossbridges) to attach to binding sites on the
thin filament
 Myosin heads then bind to the next site of the
thin filament - Sliding Filament Theory of
Muscle Contraction

33. Transmission of Nerve Impulse to
Muscle

34. End of Neuromuscular
Transmission
 Acetylcholine, the neurotransmitter is broken
down by the enzyme acetylcholinesterase
 SO the stimulus to muscle ceases! – prevents
continued muscle reexcitation
 Calcium ions are actively transported back to the
SR by SERCA (calcium ATPase)
 The actin and myosin cross bridges break
 RELAXES------S T R E T C H of the sarcomere

35. Drugs that Enhance or Block
Transmission at N-M Junction
1. STIMULATE THE MUSCLE FIBER BY
ACh LIKE ACTION:
methacholine, carbachol and nicotine:
o these drugs are not destroyed or are
destroyed very slowly by cholinesterase,
action persists for many minutes to
several hours
o can cause muscle spasm

36. 2.STIMULATE THE NEURO-MUSCULAR
JUNCTION BY INACTIVATING ACETYL-
CHOLINESTERASE
• neostigmine, physostigmine : inactivate
acetylcholinesterase for upto several hours
• diisopropyl flourophosphate : inactivate
acetylcholinesterase for weeks, “nerve” gas
poison
Drugs that Enhance or Block
Transmission at N-M Junction

37. 3. BLOCK TRANSMISSION AT NEURO-
MUSCULAR JUNCTION
 Curariform drugs
 d-Tubocurarine blocks the action of ACh
on the muscle fiber Acetylcholine
receptors, thus preventing sufficient
increase in permeability of the muscle
membrane channels to initiate an action
potential
Drugs that Enhance or Block
Transmission at N-M Junction

38. Neuromuscular blocking agents:
 Used in surgery because they relax muscle
and abolish reflexes
 Reduces the dose of anesthetic agent
necessary
 Patient who receive neuromuscular
blockers, need artificial respiration,
because respiratory muscles
(S.K.muscles/diaphragm) being paralysed,
may lead to death within minutes

39. Drugs that Enhance or Block
Transmission at N-M Junction

40. Botulinum and Tetanus toxin
Decrease
Ach release
Tetanus toxin – markedly
increases activity of motor neurons
Causes spastic paralysis - Lockjaw
Botulinum toxin( A-G)
Causes Flaccid paralysis

41. Myasthenia Gravis
Eaton Lambert Syndrome
Clinical disorders of NMJ

42. Myasthenia Gravis
 Afflicts 25-125 of every million people
 Can occur at any age but has a bimodal
distribution with peak occurrences in 20s (mainly
women) and 60s (mainly men)
 Antibodies against ACh receptors are present in
blood of most patients with this disease
 Autoimmune disease
 Antibody detected in
 50% of pts with pure ocular MG
 90-95% of pts with generalized MG

43. Myasthenia Gravis
 Antibodies destroy some of the receptors
and bind others to neighboring receptors,
triggering their removal by endocytosis
 Normally, number of quanta released
declines with successive repetitive stimuli
 Neuromuscular transmission fails at these
low levels of quantal release
 Leads to the major clinical feature of the
disease–muscle fatigue with sustained or
repeated activity

44. Myasthenia Gravis

45. Myasthenia Gravis
 Two major forms of the disease –
 Involves weakness of only the extraocular
muscles
 Results in generalized weakness of all skeletal
muscles
 In severe cases, paralysis of respiratory
muscles can lead to death

46. Clinical Manifestation of MG
 Symptoms worsen with exercise, end of day (Fatigue)
 Ocular
 Droopy eyelids (ptosis)
 Double vision (diplopia)
 Extremity weakness
 Arms > legs
 Dysarthria & Dysphagia
 Respiratory
 Shortness of breath

47. Muscle Weakness of MG
EASY
FATIGABILITY

48. Myasthenia Gravis
 Treatment: neostigmine or some other
anticholinesterase drug – alone or
combined with thymectomy or
immunosuppression
 Cholinesterase inhibitors prevent metabolism
of ACh compensating for the normal decline in
released neurotransmitters during repeated
stimulation
 Removal of Thymoma leads to clinical
improvement in 75% of cases

49. Clinical Problem
A 18-year-old college woman comes to the
student health service complaining of
progressive weakness.
 She reports that occasionally her eyelids
“droop”
 she tires easily, even when completing
ordinary daily tasks such as brushing her
hair.
 She has fallen several times while climbing
a flight of stairs.
 These symptoms improve with rest.

50. Lambert-eaton Myasthenic Syndrome
(LEMS)
 Antibodies against Ca2+ channels in the
motor nerve terminals
 no. of Ca2+ channels decrease less
calcium enters the nerve terminal and less
neurotransmitter is released
 Symptoms - muscular weakness &
diminished stretch reflexes
 Muscle strength increases with prolonged
contraction as more Ca becomes available

51. Lambert-eaton Myasthenic Syndrome
(LEMS)
 The major clinical finding is progressive weakness
that does not usually involve the respiratory
muscles and the muscles of face
 In contrast to MG, symptoms of LEMS tend to be
worse in the morning and improve with exercise
 The proximal parts of the legs and arms are
predominantly affected
 Many patients have autonomic symptoms like dry
mouth or impotence. Reflexes are usually
reduced or absent

52. Differences between
MG LEMS
 Antibodies are formed
against the ACh
Receptors on the Post
synaptic membrane
 Primarily attacks the
ocular and bulbar
muscles
 Repeated muscle
stimulation leads to
decrease in
contractile strength
 Antibodies are formed
against the
presynaptic Calcium
channels
 Primarily attacks the
limb muscles
 Repeated muscle
stimulation leads to
increasing contractile
strength

53. N-M Junction in Smooth & Cardiac
Muscle

54.  Autonomic nerve fibers that innervate
smooth muscle branch diffusely on top of
sheet of muscle fibers
 These fibers do not make direct contact
with smooth muscle fiber cell membranes
but form diffuse junctions
 Vesicles may contain ACh in some & NE in
other autonomic nerve fiber endings
N-M Junction in Smooth & Cardiac
Muscle

55. N-M Junction in Smooth & Cardiac
Muscle
 No typical end feet as seen in skeletal muscle,
axons have multiple varicosities distributed along
their axes
 Varicosities are about 5 μm apart, with up to
20,000 varicosities per neuron
 Transmitter is liberated at each varicosity, ie, at
many locations along each axon
 This arrangement permits one neuron to innervate
many effector cells
 The type of contact in which a neuron forms a
synapse on the surface of another neuron or a
smooth muscle cell and then passes on to make
similar contacts with other cells is called a
synapse en passant

56. NMJ: Smooth muscles

57. Denervation Hypersensitivity
 When the motor nerve to skeletal muscle
is cut and allowed to degenerate
 muscle gradually becomes extremely sensitive
to acetylcholine -denervation
hypersensitivity or supersensitivity due to
an upregulation of its receptors
 Muscle atrophies
 Also seen in smooth muscle
 Does not atrophy
 hyperresponsive to the chemical mediator that
normally activates it

58. Summary

59. Thank you
References:
Guyton- Textbook of Medical Physiology
Ganong’s- Review of Medical Physiology
Boron-Medical Physiology
Kandel-Principles of Neural Science
Silbernagl-Color atlas of Physiology
Ira Fox- Medical Physiology