MD Surg PG lecture

1. Neuromuscular
Physiology
Prof. Vajira Weerasinghe
Senior Professor of Physiology
Faculty of Medicine
University of Peradeniya

2. Nerve conduction
• Electrochemical basis
• concentration gradient, membrane permeability
ionic channels
• Resting membrane potential (RMP)
• K+ efflux, Na/K pump
• Leaky channels
• Action potential (AP)
• depolarisation, repolarisation
• Voltage-gated channels
• Propagation of AP
• Local current flow

3. • Excitability of a tissue depends on its
membrane potential
– Excitable tissues have more negative RMP
( - 70 to - 90 mV)
– Non-excitable tissues have less negative RMP
( - 40 mV)
excitable Non-excitable
Red cell
GIT
neuron
muscle

4. Factors contributing to RMP
• One of the main factors is K+ efflux
• (Nernst Equilibrium Potential: -94mV)
• Contribution of Na influx is little
• (Nernst Equilibrium Potential: +61mV)
• Na/K pump causes more negativity inside the
membrane
•
Negatively charged protein remaining inside due to
impermeability contributes to the negativity
• Net result: -70 mV inside

5. Ionic channels
• Leaky channels (K-Na leak channel)
– More permeable to K
– Allows free flow of ions
K+
Na+

6. Na/K pump
• Active transport system for Na-K exchange using
energy
• It is an electrogenic pump since 3 Na efflux coupled
with 2 K influx
• Net effect of causing negative charge inside the
membrane
3 Na+
2 K+
ATP ADP

7. -70
+35
RMP
Hyperpolarisation

8. Physiological basis of AP
• When the threshold level is reached
– Voltage-gated Na channels open up
– Since Na conc outside is more than the inside
– Na influx will occur
– Positive ion coming inside increases the positivity of the membrane potential and
causes depolarisation
– When it reaches +35, Na channels closes
– Then Voltage-gated K channels open up
– K efflux occurs
– Positive ion leaving the inside causes more negativity inside the membrane
– Repolarisation occurs
• Na/K pump restores Na and K conc slowly
– By pumping 3 Na ions outward and 2 K ions inward

9. • At rest: the activation gate is closed
• At threshold level: activation gate opens
– Na influx will occur
– Na permeability increases to 500 fold
• when reaching +35, inactivation gate closes
– Na influx stops
• Inactivation gate will not reopen until resting
membrane potential is reached
outside
inside
outside
inside
-70 Threshold level +35
Na+ Na+
outside
inside
Na+m gate
h gate

10. – At rest: K channel is closed
– At +35
• K channel open up slowly
• This slow activation causes K efflux
– After reaching the resting still slow K channels may
remain open: causing further hyperpolarisation
outside
inside
outside
inside
-70 At +35
K+ K+
n gate

11. Propagation of AP
• When one area is depolarised
• A potential difference exists between that site
and the adjacent membrane
• A local current flow is initiated
• Local circuit is completed by extra cellular fluid

12. Propagation of AP
• This local current flow will cause opening of
voltage-gated Na channel in the adjacent
membrane
• Na influx will occur
• Membrane is deloparised

13. Propagation of AP
• Then the previous area become repolarised
• This process continue to work
• Resulting in propagation of AP

14. Propagation of AP

15. Propagation of AP

16. Propagation of AP

17. Propagation of AP

18. AP propagation along myelinated
nerves
• Na channels are conc around nodes
• Therefore depolarisation mainly occurs at
nodes

19. AP propagation along myelinated
nerves
• Local current will flow one node to another
• Thus propagation of A.P. is faster. Conduction
through myelinated fibres also faster.
• Known as Saltatory Conduction

23. Membrane stabilisers
• Membrane stabilisers (these decrease excitability)
• Increased serum Ca++
– Hypocalcaemia causes membrane instability and spontaneous activation
of nerve membrane
– Reduced Ca level facilitates Na entry
– Spontaneous activation
• Decreased serum K+
• Local anaesthetics
• Acidosis
• Hypoxia
• Membrane destabilisers (these increase excitability)
• Decreased serum Ca++
• Increased serum K+
• Alkalosis
• Caffeine
• strychnine

24. NMJ function
• Pre-synaptic membrane
• Ca channels
• Acetycholine release
• Postsynaptic membrane
• Acetylcholine receptors
• Ligand-gated channels
• Synaptic cleft
• cholinesterase

25. Synapse
• A gap between two neurons
• More commonly chemical
• Rarely they could be electrical (with gap junctions)
• Although an axon conducts bothways, conduction
through synapse is oneway

26. Presynaptic terminal (terminal knob,
boutons, end-feet or synaptic knobs)
 Terminal has synaptic vesicles and mitochondria
 Mitochondria (ATP) are present inside the presynaptic
terminal
Vesicles containing neurotransmitter (Ach)

27. Presynaptic terminal (terminal knob,
boutons, end-feet or synaptic knobs)
 Presynaptic membrane contain voltage-gated Ca2+ channels
 The quantity of neurotransmitter released is proportional to
the number of Ca2+ entering the terminal
 Ca2+ ions binds to the protein molecules on the inner surface
of the synaptic membrane called release sites
 Neurotransmitter binds to these sites and exocytosis occur

28. Ca2+ Ca2+

29. Synthesis of Ach
• Synthesised from choline and acetyl-coenzyme A (acetyl-coA) in the
terminal axoplasm of motor neurons
• Acetyl-coA is synthesised from pyruvate in the mitochondria in the axon
terminals
• Approximately 50% of the choline is extracted from extracellular fluid by a
sodium dependent active transport system
• Other 50% is from acetylcholine breakdown at the neuromuscular junction.
• Majority of the choline originates from the diet with hepatic synthesis only
accounting for a small proportion

30. • Postsynaptic membrane contain nicotinic
acetylcholine receptor
Ach
Na+
• This receptor contains several
sub units (2 alpha, beta, delta &
epsilon)
• Ach binds to alpha subunit
• Na+ channel opens up
• Na+ influx occurs
• End Plate Potential (EPP)
•This is a graded potential
•Once this reaches the threshold
level
•AP is generated at the
postsynaptic membrane

31. Ach vesicle docking
• With the help of Ca entering the presynaptic terminal
• Docking of Ach vesicles occur
• Docking:
– Vesicles move toward & interact with membrane of
presynaptic terminal
• There are many proteins necessary for this purpose
• These are called SNARE proteins
• eg. Syntaxin, synaptobrevin etc

32. NMJ blocking
• Useful in general anaesthesia to facilitate inserting tubes
• Muscle paralysis is useful in performing surgery
• Commonly used to paralyze patients requiring intubation
whether in an emergency as a life-saving intervention or for a
scheduled surgery and procedure
• Indications for intubation during an emergency
– failure to maintain or protect the airway
– failure to adequately ventilate or oxygenate
– anticipation of a decline in clinical status

33. Earliest known NMJ blocker - Curare
• Curare has long been used in South America as
an extremely potent arrow poison
• Darts were tipped with curare and then
accurately fired through blowguns made of
bamboo
• Death for birds would take one to two minutes,
small mammals up to ten minutes, and large
mammals up to 20 minutes
• NMJ blocker used in patients is tubocurarine
• Atracurium is now used

34. Non-depolarising blocking agents
– Competitive
– Act by competing with Ach for the Ach receptors
– Binds to Ach receptors and blocks
– Prevent Ach from attaching to its receptors
– No depolarisation
– Late onset, prolonged action
– 70–80% of receptors should be occupied to produce an effect
– To produce complete block, at least 92% of receptors must be occupied
– Ach can compete & the effect overcomes by an excess Ach
– Anticholinesterases can reverse the action
– eg.
• Curare
• Atracurium
• Rocuronium
• Vencuronium

35. Depolarising blocking agents
– non-competitive, chemically act like Ach
– Bind to motor end plate and once depolarises
– Persistent depolarisation leads to a block
• Due to inactivation of Na channels
– Phase I block
• After a depolarizing agent binds to the motor end plate receptor, the agent remains
bound and thus the end plate cannot repolarize
• during this depolarizing phase the transient muscle fasciculation occur
• Absence of fade to tetanic stimulation
– Phase II block
• After adequate depolarization has occurred, phase II (desensitizing phase) sets in and
the muscles are no longer receptive to acetylcholine released by the motor neurons
• It is at this point that the depolarizing agent has fully achieved paralysis
• Fade after tetanic stimulation
– Ach cannot compete
– Quick action start within 30 sec, recover within 3 min and is complete within 12–
15 min
– Hydrolysed by plasma cholinesterase (also called pseudocholinesterase)
produced in the liver
– Prolonged blockade is seen in liver disease or pregnancy
– Inhibition of plasma cholinesterase occurs with OP compounds
– eg. Succinylcholine
– Side effect: hyperkalaemia
– Bind to nicotinic and muscarinic Ach, causes bradycardia
– Contraindicated in burns

36. Na+
Acetylcholine
Depolarization
Na+
- - - -+ + + +
- - - -
+ + + +
+ + + +
+ + + +- - - - - - - -

37. Na+
AcetylcholineTubocurarine
Na+
+ + + +
- - - -- - - -
+ + + +
Competitive neuromuscular blocking drugs

38. Na+
Depolarized
Na+
PHASE I
Membrane depolarizes
resulting in an initial
discharge which
produces transient
fasciculations followed
by flaccid paralysis
- - - -
+ + + ++ + + +
- - - - - - - -+ + + + + + + +
- - - -- - - -
Depolarizing Neuromuscular blocking drugs

39. Repolarized
PHASE II
Membrane repolarizes
but the receptor is
desensitized to effect
of acetylcholine
+ + + +- - - -+ + + +- - - -
- - - -+ + + +
- - - -+ + + +
Depolarizing Neuromuscular blocking drugs

40. Anticholinesterases
• AchE inhibitors
– Inhibit AchE so that Ach accumulates and causes
depolarising block
• Reversible
– Competitive inhibitors of AChE
– Block can be overcome by curare
• physostigmine, neostigmine, edrophonium
• Irreversible
– Binds to AChE irreversibly
• , insecticides, nerve gases

41. NMJ disorders
• Myasthenia gravis
– Antibodies to Ach receptors
– Post synaptic disorder
• Lambert Eaton Syndrome (myasthenic syndrome)
– Presynaptic disorder (antibodies against Ca channels)
• Botulism
– Presynaptic disorder
– Binds to the presynatic region and prevent release of Ach

42. Botulinum toxin
• Most potent neurotoxin known
• Produced by bacterium Clostridium botulinum
• Causes severe diarrhoeal disease called botulism
• Action:
– enters into the presynaptic terminal
– cleaves proteins (syntaxin, synaptobrevin) necessary for Ach vesicle
release with Ca2+
• Chemical extract is useful for reducing muscle spasms, muscle
spasticity and even removing wrinkles (in plastic surgery)

44. Organophosphates
• Phosphates used as insecticides
• Action
– AchE inhibitors
– Therefore there is an excess Ach
accumulation
– Depolarising type of postsynaptic
block
• Used as a suicidal poison
• Causes muscle paralysis and death
• Nerve gas (sarin)

45. Snake venom
• Common Krait (bungarus
caeruleus)
– Produces neurotoxin known as
bungarotoxin
– Very potent
• Causes muscle paralysis and death
if not treated
• Cobra
– venom contain neurotoxin

46. Myasthenia gravis
• Serious neuromuscular disease
• Antibodies form against acetylcholine nicotinic
postsynaptic receptors at the NMJ
• Characteristic pattern of progressively reduced
muscle strength with repeated use of the muscle and
recovery of muscle strength following a period of rest
• Present with ptosis, fatiguability, speech difficulty,
respiratory difficulty
• Treated with cholinesterase inhibitors