Anaesthesia, 2009, 64 (Suppl. 1), pages 1–9
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Basic principles of neuromuscular transmission
J. A. J. Martyn,1,2 M. Jonsson Fagerlund3 and L. I. Eriksson4
1 Professor, Harvard Medical School, Director Clinical & Biological Pharmacology Laboratory, Department of
Anesthesiology, Massachusetts General Hospital, 2 Anaesthetist-in-Chief, Shriners Hospital for Children, Boston, MA,
USA
3 Resident in Anesthesiology and Intensive Care Medicine, 4 Professor and Academic Chairman, Department of
Anesthesiology, Karolinska Institutet and Karolinska University Hospital, Stockholm, Sweden
Summary
Neuromuscular transmission at the skeletal muscle occurs when a quantum of acetylcholine from
the nerve ending is released and binds to the nicotinic acetylcholine receptors on the postjunctional
muscle membrane. The nicotinic acetylcholine receptors on the endplate respond by opening
channels for the influx of sodium ions and subsequent endplate depolarisation leads to muscle
contraction. The acetylcholine immediately detaches from the receptor and is hydrolysed by
acetylcholinesterase enzyme. Suxamethonium is a cholinergic agonist stimulating the muscle nicotinic acetylcholine receptors prior to causing neuromuscular block. Non-depolarising neuromuscular blocking drugs bind to the nicotinic acetylcholine receptors preventing the binding of
acetylcholine. Non-depolarising neuromuscular blocking drugs also inhibit prejunctional a3b2
nicotinic acetylcholine autoreceptors, which can be seen in the clinical setting as train-of-four fade.
In some pathological states such as denervation, burns, immobilisation, inflammation and sepsis,
there is expression of other subtypes of nicotinic acetylcholine receptors with upregulation of these
receptors throughout the muscle membrane. The responses of these receptors to suxamethonium
and non-depolarising neuromuscular blocking drugs are different and explain some of the aberrant
responses to neuromuscular blocking drugs.
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Correspondence to: J. A. Jeevendra Martyn
E-mail: [email protected]
Accepted: 15 December 2008
Introduction
The mammalian neuromuscular junction is the prototypical and most extensively studied synapse. From a
broad perspective, neuromuscular transmission occurs by
a fairly simple and straightforward mechanism. The nerve
synthesises acetylcholine and stores it in small, uniformly
sized packages called vesicles. Arrival of an action
potential at the distal motor nerve ending leads to an
instant opening of voltage gated Ca2+-channels with a
subsequent abrupt increase in intracellular calcium concentration [1]. This increased calcium concentration
triggers a cascade of intracellular signalling events leading
neurotransmitter-containing vesicles to migrate to the
surface of the nerve, rupture and discharge acetylcholine
into the cleft separating nerve from muscle [1]. Nicotinic
acetylcholine receptors (nAChRs) in the endplate of the
muscle, being activated by the released acetylcholine,
respond by opening their channels for influx of sodium
2009 The Authors
Journal compilation 2009 The Association of Anaesthetists of Great Britain and Ireland
ions into the muscle to depolarise the muscle. The
endplate potential created is propagated along the muscle
membrane by the opening of the sodium channels present
throughout the muscle membrane, leading to muscle
contraction [1]. Acetylcholine immediately detaches from
the receptor and is destroyed by the nearby acetylcholinesterase located in the synaptic cleft. Non-depolarising
neuromuscular blocking drugs (NMBs) act on the
nAChRs, by preventing acetylcholine from binding to
the receptor, thereby inhibiting depolarisation of the
receptor. Although NMBs are known to have effects on
the presynaptic and postsynaptic nAChRs of the neuromuscular junction, recent evidence also demonstrates that
this class of agents used in anaesthesia and intensive care
can react with nicotinic and muscarinic acetylcholine
receptors other than those at the neuromuscular junction,
including those within the carotid body, vagus
innervations of the heart, and in bronchial smooth muscle
[2, 3].
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Organisation of the neuromuscular junction
The neuromuscular junction is specialised on the nerve
side and on the muscle side to transmit and receive
chemical messages [4]. Each motor neuron runs without
interruption from the ventral horn of the spinal cord or
medulla to the neuromuscular junction as a large,
myelinated axon. As it approaches the muscle, it branches
repeatedly to contact many muscle cells and to gather
them into a functional group known as a motor unit. The
architecture of the nerve terminal is quite different from
that of the rest of the axon. As the terminal reaches the
muscle fibre, it loses its myelin to form a spray of terminal
branches against the muscle surface and is covered by
Schwann cells (Fig. 1). The nerve is separated from the
surface of the muscle by a gap called the junctional or
synaptic cleft. The muscle surface is heavily corrugated,
with deep invaginations of the junctional cleft, the
primary and secondary clefts, between the folds in the
muscle membrane; thus the endplate’s total surface area is
very large. The shoulders of the folds are densely
populated with nAChRs, about 5 million of them in
each junction. These receptors are sparse in the depths
between the folds. Instead, these deep areas contain
sodium channels (Fig. 1).
The peri-junctional zone is the area of muscle immediately beyond the junctional area, and it is critical to the
function of the neuromuscular junction. The perijunctional zone contains a mixture of the receptors,
which include a smaller density of nAChRs and highdensity sodium channels (Fig. 1). The admixture
enhances the capacity of the peri-junctional zone to
respond to the depolarisation (i.e. endplate potential)
produced by nAChRs and to transduce it into the wave
of depolarisation that travels along the muscle to initiate
muscle contraction. The density of sodium channels in
the peri-junctional area is richer than in more distal parts
of the muscle membrane [1, 5]. The peri-junctional zone
is close enough to the nerve ending to be influenced by
transmitter released from it. Moreover, special variants,
i.e. isoforms, of nAChRs and sodium channels can appear
in this area at different stages of life and in response to
abnormal decreases in nerve activity. Congenital or
acquired abnormalities in the nAChRs or the sodium
Figure 1 Structure of the adult neuromuscular junction with the three cells that constitute the synapse: the motor neuron (i.e. nerve
terminal), muscle fibre and Schwann cell. As the nerve approaches its muscle fibres, and before attaching itself to the surface of the
muscle fibre, the nerve divides into branches that innervate many individual muscle fibres. The motor nerve loses its myelin and
further subdivides into many presynaptic boutons to terminate on the surface of the muscle fibre. The nerve terminal, covered by a
Schwann cell, has vesicles clustered about the membrane thickenings, which are the active zones, toward its synaptic side and
mitochondria and microtubules toward its other side. A synaptic gutter or cleft, made up of a primary and many secondary clefts,
separates the nerve from the muscle. The muscle surface is corrugated, and dense areas on the shoulders of each fold contain
acetylcholine receptors. The sodium channels are present at the bottom of the clefts and throughout muscle membrane. The
acetylcholinesterase, proteins and proteoglycams which stabilise the neuromuscular junction are present in the synaptic clefts.
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Neuromuscular transmission
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and calcium channels, i.e. mutations, are also known,
such as Eaton Lambert syndrome [6, 7]. These variabilities
seem to contribute to the differences in response to
NMBs that are seen in patients with different pathologic
conditions [8, 9].
Quantal theory
The contents of the nerve ending are not homogeneous.
As shown in Fig. 1, the vesicles are congregated in the
area towards the junctional surface, whereas the microtubules, mitochondria, and other support structures, are
located towards the opposite side. The vesicles containing
transmitter are arranged in repeating clusters alongside
small, thickened, electron-dense patches of membrane,
referred to as active zones or release sites. This thickened
area is a cross section of a band running across the width
of the synaptic surface of the nerve ending that is believed
to be the structure to which vesicles attach (active zones)
before they rupture into the junctional cleft. Highresolution scanning electron micrographs reveal small
protein particles arranged alongside the active zone
between vesicles. These particles are believed to be
voltage-gated calcium channels, that allow calcium to
enter the nerve and initiate a series of events that
ultimately lead to release of vesicles [1]. The rapidity with
which the neurotransmitter is released (200 ls) suggests
that the voltage-gated calcium channels are close to the
release sites. Proteomic studies suggest that at least 26
genes encode presynaptic proteins, and 12 of them cause
defects in presynaptic structure that can lead to decreased
acetylcholine release and muscle weakness [10]. These
defects can be related to exocytosis, endocytosis, formation of active and peri-active zones, vesicle transport, and
neuropeptide modulation [10].
When observing the electrophysiologic activity of a
skeletal muscle, small, spontaneous, depolarising potentials at the postsynaptic muscle membrane can be seen.
These small-amplitude potentials are called miniature
endplate potentials (MEPPs). MEPPs have only onehundredth the amplitude of the evoked endplate potential
produced when the motor nerve is stimulated, which
leads to muscle contraction. Except for amplitude, these
potentials resemble the endplate potential in the time
course and the manner in which they are affected by
drugs. The stimulus-evoked endplate potential is the
additive depolarisation produced by the synchronous
discharge of quanta from several hundred vesicles. The
action potential that is propagated to the nerve ending
allows entry of calcium into the nerve through voltagegated calcium channels, and this causes vesicles to migrate
to the active zone, fuse with the neural membrane, and
discharge their acetylcholine into the junctional cleft.
2009 The Authors
Journal compilation 2009 The Association of Anaesthetists of Great Britain and Ireland
Postsynaptically, the ions (mostly Na+ and some Ca++)
that flow through the channels of the activated nAChRs
cause a maximum depolarisation of the endplate, which
causes an endplate potential that is greater than the
threshold for stimulation of the muscle. The signal is
carried by more molecules of transmitter than are needed,
and they evoke a response that is greater than needed. At
the same time, only a small fraction of the available
vesicles and receptors are used to send each signal.
Consequently, transmission has a substantial margin of
safety, and at the same time, the system has substantial
capacity in reserve [11].
The number of quanta released by a stimulated nerve is
greatly influenced by the concentration of ionized
calcium in the extracellular fluid. If calcium is not present,
depolarisation of the nerve, even by electrical stimulation,
will not produce release of transmitter. Doubling the
extracellular calcium results in a 16-fold increase in the
quantal content of an endplate potential [12]. The calcium
current persists until the membrane potential is returned
to normal by outward fluxes of potassium from inside the
nerve cell. An effect of increasing the calcium in the nerve
ending is also seen clinically as the so-called post-tetanic
potentiation, which occurs after a nerve of a patient
paralysed with a non-depolarising NMB is stimulated at
high, tetanic frequencies. Calcium enters the nerve with
every stimulus, but because it cannot be excreted as
quickly as the nerve is stimulated, it accumulates during
the tetanic period. Because the nerve ending contains
more than the normal amount of calcium for some time
after the tetanus, a stimulus applied to the nerve during
this time causes the release of more than the normal
amount of acetylcholine. The abnormally large amount of
acetylcholine antagonises non-depolarising NMBs and
causes the characteristic increase in the size of the twitch
after tetanic stimulation. Higher than normal concentrations of bivalent inorganic cations, e.g. magnesium,
cadmium and manganese, can also inhibit calcium entry
into nerve and profoundly impair neuromuscular transmission. This is the mechanism for muscle weakness in the
mother and fetus when magnesium sulphate is given to
treat pre-eclampsia.
Synaptic vesicles and recycling
There seem to be two pools of vesicles that release
acetylcholine: a readily releasable pool and a reserve pool
[13, 14]. The vesicles in the former are a bit smaller and
are limited to an area very close to the nerve membrane,
where they are bound to the active zones. These vesicles
are the ones that normally release transmitter. Electron
microscopic studies have demonstrated that the majority
of the synaptic vesicles are sequestered in the reserve pool
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Anaesthesia, 2009, 64 (Suppl. 1), pages 1–9
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tethered to the cytoskeleton in a filamentous network
composed mainly of actin, synapsin (an actin-binding
protein), synaptotagmin and spectrin [13, 14].
Studies have shed some light on the inner workings by
which the vesicle releases its contents. The whole process
is called exocytosis. The soluble N-ethylmaleimidesensitive-factor attachment receptor (SNARE) protein
plays a pivotal role in the release of acetylcholine [14].
The SNAREs include the synaptic-vesicle protein synaptobrevin and the plasmalemma-associated proteins
syntaxin and synaptosome-associated protein of 25 kDa
(SNAP-25) [14]. The current model for protein-mediated
membrane fusions in exocytosis is as follows: syntaxin and
SNAP-25 are complexes attached to plasma membrane.
After initial contact, the synaptobrevin on the vesicle
forms a ternary complex with syntaxin ⁄ and SNAP-25.
Synaptotagmin is the protein on the vesicular membrane
that acts as a calcium sensor and localises the synaptic
vesicles to synaptic zones rich in calcium channels,
stabilising the vesicles in the docked state. The assembly
of ternary complex forces the vesicle close to the
underlying nerve terminal membrane, i.e. the active
zone, and the vesicle is then ready for release. An action
potential in the nerve terminal allows the entry of calcium. Hence, the close proximity of release sites, calcium
channels, and synaptic vesicles, and the use of a calcium
sensor lead to the burst of new transmitter release
synchronous with the stimulus [14, 15].
Clostridial toxins, including botulinum toxin and
tetanus neurotoxins, which selectively digest one or all
of these SNARE proteins, block exocytosis of the vesicles
[16]. The result is muscle weakness or paralysis. These
toxins in effect produce a partial or complete chemical
denervation. Botulinum toxin is used therapeutically to
treat spasticity or spasm in several neurological and
surgical diseases (blepharospasm, cerebral palsy, torticollis,
anal sphincter spasm, etc), to reduce excessive sweating,
and cosmetically to correct wrinkles [17, 18]. Recent
reports indicate increased incidence of clostridial infections, both in Canada and the United States with
clostridium botulinum infection particularly common
after traumatic injuries among drug abusers and after
muscular skeletal allografts [19]. While systemic paralysis
can occur after systemic clostridial infections, local
injection for therapeutic purposes will usually result in
localised paresis, although systemic effects have been
reported [20].
Acetylcholinesterase enzyme
Acetylcholine transmitter molecules that do not react
immediately with a receptor or those released after
binding to the nAChRs are destroyed almost instantly by
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acetylcholinesterase, which is a type-B carboxylesterase
enzyme located in the synaptic cleft with a smaller
concentration in the extra-junctional area. The enzyme is
secreted from the muscle but remains attached to it by
thin stalks of collagen fastened to the basement membrane
[1]. Acetylcholine is a potent messenger, but its actions
are very short lived because it is destroyed in less than
1 ms after its release. Notably, approximately 50% of the
released acetylcholine is immediately hydrolysed before
reaching the receptor. Acetylcholine molecules released
from the nerve initially pass between the enzymes to
reach the postsynaptic receptors, but as they are released
from the receptors, they invariably encounter acetylcholinesterase and are destroyed. Under normal circumstances, a molecule of acetylcholine reacts with only one
receptor before it is hydrolysed into choline and acetate.
Choline is taken up by the nerve terminal and re-used for
synthesis of acetylcholine.
Nicotinic acetylcholine receptors (nAChRs)
At present, three subtypes of nAChRs found in the
neuromuscular junction are of clinical importance: a3b2
presynaptically; a1b1d ⁄ c and a7 postsynaptically.
The nAChRs belong to the superfamily of Cys-loop
ligand gated ion channels, which have a common
architecture of four transmembrane domains building up
each subunit, and also include glycine, 5-HT3 and
GABAA receptors. The nAChRs are synthesised in
muscle cells and are anchored to the endplate membrane
by a special 43-kDa protein known as rapsyn [1]. This
cytoplasmic protein is associated with the nAChR in a
1 : 1 ratio. The nAChRs, formed of five subunit proteins,
are arranged like the staves of a barrel into a cylindrical
receptor with a central pore for ion channelling. The key
features of postsynaptic nAChRs are sketched in Fig. 2.
Every receptor subunits consists of approximately 400–
500 amino acids. The receptor protein complex passes
entirely through the membrane and protrudes beyond the
extracellular surface of the membrane and into the
cytoplasm.
To date, 17 nicotinic subunits have been cloned in
vertebrates: the muscle a1, b1, d, c and subunits, and
the neuronal a2–10 and b2–4 subunits [21, 22].
The fetal, immature or extra-junctional muscle
nAChR consists of two a1, and one of each b1, d and
c subunit (a1b1dc) and is present during decreased
activity in muscle, as seen in the fetus before innervation
or after chemically or physically-induced immobilisation;
after lower or upper motor neuron injury, burns or
sepsis – or after other events that cause increased muscle
protein catabolism including sepsis or generalised inflammation [8, 9]. Notably, some indirect evidence suggests
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and one low affinity binding site at the interface between
a1 and or c. Heteromeric neuronal nAChRs have two
binding sites situated at the interface between an a- and a
b-subunit, and both subunits contributes to the pharmacological specificity within each receptor subtype [29].
The neuronal homomeric subtypes have five potential
binding sites. However, the number of agonists needed
for receptor activation is not known [22].
Figure 2 Sketch of postsynaptic nicotinic acetylcholine receptor
channels. The mature, or junctional, receptor consists of two
a1-subunits and one each of b1-, d-, and -subunits. The
immature, extra-junctional or fetal form consists of two a1- and
one each of b1, d, and c-subunits. The latter is thus called
c-subunit receptor. Recently, a neuronal receptor consisting of
five subunits of a7 has been described in muscle. All subunits are
arranged around the central cation channel. The immature
isoform containing the c-subunit shows long open times and
low-amplitude channel currents (not shown). The mature isoform containing the -subunit shows shorter open times and
high-amplitude channel currents during depolarisation. Substitution of the -subunit for the c-subunit gives rise to the
fast-gated, channel with prolonged open time. As expected,
acetylcholine application to the a7 nAChR also results in a fast,
rapidly decaying inward current (not shown). All of these
depolarising events are insensitive to the treatment with atropine
but sensitive to treatment with a-bungarotoxin or non-depolarising NMBs, blocking current flow.
that the immature isoform is not seen in muscle protein
catabolism and wasting that occurs with malnutrition
[23].
After innervation, the c-subunit is replaced by , which
creates the adult, mature or junctional muscle nAChR
(a1b1d) present throughout a healthy life.
The neuronal nAChRs include both homomeric and
heteromeric receptors, with the a7–9 subunits forming
homomeric nAChRs. The heteromeric receptors are
formed by a combination of a2–6 and b2–4, where most
of these receptors are formed by a single a and a single b
subunit, with a stoichiometry of 2 a and 3 b. Although
there are many potential combinations of neuronal
nAChRs, only a few have been found to be of biological
importance. In the neuromuscular junction, the neuronal
a3b2 subtype is found at the presynaptic nerve ending
[24] and a7 is found postsynaptically at the muscle
membrane during development and denervation [25].
Expression of neuronal nAChRs in in vitro systems has
confirmed that non-depolarising NMBs and their metabolites can inhibit these neuronal nAChRs [26, 27].
Interestingly, non-depolarising NMBs decrease hypoxic
ventilatory response in partially paralysed humans [28],
and the mechanism behind the depression might be
related inhibition of neuronal nAChRs within the carotid
body [2].
The muscle nAChRs have two distinct agonist binding
sites, one high affinity binding site between the a1 and d,
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Journal compilation 2009 The Association of Anaesthetists of Great Britain and Ireland
Postsynaptic nAChRs
As previously mentioned, three different subtypes of
nAChRs have been found at the postsynaptic muscle
membrane: the adult muscle a1b1d nAChR and, during
development and denervation, the fetal muscle a1b1dc
and neuronal a7 subtype of nAChRs.
Shortly after birth, the motor nerve axons grow into
the developing muscle, and these axons bring in nervederived signals (i.e. growth factors), including agrin and
neuregulins (NRb-1 and NRb-2) that are key to
maturation of the myotubes to muscle [1–4]. Agrin is a
protein from the nerve that stimulates postsynaptic
differentiation by activating muscle-specific kinase
(MuSK), a tyrosine kinase expressed selectively in muscle.
Agrin, together with other growth factors (neuregulins,
etc), induce the clustering of nAChRs and other critical
muscle-derived proteins, including MuSK, rapsyn, and
Erbb proteins, all of which are necessary for maturation
and stabilisation of the nAChRs at the junction [30, 31].
Clustering brings all of these proteins from the extrajunctional to the junctional region. Just before and shortly
after birth, the immature, c-subunit containing nAChRs
are converted to the mature, -subunit-containing receptors. Although the mechanism of this change is unclear, a
neuregulin, NRb-1, also called ARIA, which binds to
one of the ErbB receptors, seems to play a role [30, 31].
Conversion of all of the c-subunit- to -subunitcontaining nAChRs in the peri-junctional area continues
to take place after birth. In the rodent, it takes about
2 weeks [4]. In humans, this process takes longer. As to
when a7 nAChRs disappear in the fetus or newborn is
also unknown.
Immature (fetal) receptors have a smaller single-channel
conductance and a 2- to 10-fold longer mean channel
open time than mature receptors. The changes in subunit
composition may also alter the sensitivity or affinity, or
both, of the receptor for specific ligands. Depolarising or
agonist drugs such as suxamethonium and acetylcholine
depolarise immature receptors more easily, resulting in
cation fluxes: one-tenth to one-hundredth of doses necessary for mature receptors, can effect depolarisation in
immature receptors [9]. Once depolarised, the immature
channels also stay open for a longer time. Potency of nondepolarising NMBs is also decreased which is demon5
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strated by a resistance to non-depolarising NMBs, as
documented in burns, denervation and immobilisation
[8, 9]. This resistance may be related to decreased affinity
of the immature muscle a1b1dc and a7 nAChRs to nondepolarising NMBs and to the upregulation of receptors
in the peri-junctional area.
Although the names junctional and extra-junctional
imply that each is located in the junctional and extrajunctional areas, this is not strictly correct. Junctional
receptors are always confined to the endplate (perijunctional) region of the muscle membrane. The immature, or extra-junctional receptor, may be expressed
anywhere in the muscle membrane. During development
and in certain pathologic states, the junctional and extrajunctional receptors can coexist in the peri-junctional area
of the muscle membrane. The synthesis of immature
receptors is initiated within hours of inactivity, but it takes
several days for the whole muscle membrane to be fully
covered with receptors. This upregulation of receptors
has implications for the use of depolarising and nondepolarising NMBs. The changes in a7 nAChRs seem to
be parallel the expression of immature receptors, although
this has not been well studied. The differences in the
protein structure of these two new subtypes (i.e. a1b1dc
and a7) cause significant qualitative variations among the
responses of individual patients to NMBs and seem to be
responsible for some of the anomalous results that are
observed when giving NMBs to particular individuals
who express these subtypes under pathologic conditions
[8, 9, 32].
In pathological states where nAChRs are scattered
over a large surface of the muscle, the nAChR channels
opened by the agonist (suxamethonium) allow potassium
to escape from the muscle and enter the blood. If a large
part of the muscle surface consists of upregulated
(immature) receptor channels, each of which stays open
for a longer time, the amount of potassium that moves
from muscle to blood can be very large. The resulting
hyperkalaemia can cause dangerous disturbances in
cardiac rhythm, including ventricular fibrillation [9].
Moreover, it is difficult to prevent the hyperkalaemia by
the prior administration of non-depolarising NMBs
because extra-junctional receptors are not very sensitive
to block by non-depolarising NMBs in the usual doses
[8]. Larger than normal doses of non-depolarising NMBs
may attenuate the increase in blood potassium but
cannot completely prevent it. However, hyperkalaemia
and cardiac arrest can occur after suxamethonium
administration, even in the absence of denervation
states. This is seen in certain congenital muscle dystrophies, in which the muscle membrane is prone to
damage by suxamethonium releasing potassium into the
circulation [33].
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Presynaptic nAChRs
Presynaptic- or nerve terminal-associated cholinergic
receptors have been demonstrated pharmacologically and
by molecular biology techniques, but their form and
functions are not completely understood when compared
with those in the postsynaptic area [34]. A clue to
differences between presynaptic and postsynaptic nAChRs
was the finding that presynaptic nAChRs can bind bbungarotoxin only, while postsynaptic receptors bind to
a-bungoratoxin. Additional clues to differences between
pre- and postsynaptic nAChRs is the response of these
receptors to different nAChR agonists and antagonists [21].
The nicotinic receptor on the presynaptic surface of the
nerve senses transmitter in the cleft and, by means of a
positive- and negative-feedback system, causes the release
of more transmitter. This positive feedback is also
complemented by a negative-feedback system that senses
when the concentration of transmitter in the synaptic cleft
has increased appropriately and shuts down the release
system. It is believed that tetanic fade and train-of-four
fade during neuromuscular block due to non-depolarising
NMBs arise from inhibition of presynaptic cholinergic
autoreceptors at the motor nerve ending [35, 36]. The
neuronal nAChR subtype on the nerve terminal that
causes fade has been identified as a3b2 [36]. Quite in
contrast, suxamethonium does not inhibit the presynaptic
a3b2 nicotinic autoreceptor at clinically relevant concentrations [37]. This may be reason for the typical lack of
train-of-four fade during suxamethonium-induced neuromuscular block.
Depolarising neuromuscular blocking drugs
Depolarising NMBs simulate the effect of acetylcholine
and therefore can be considered agonists despite the fact
that they block neurotransmission after initial stimulation.
Structurally, suxamethonium is two molecules of acetylcholine bound together. It is therefore not surprising that
it can mimic the effects of acetylcholine at the muscle
nAChR. Suxamethonium binds to the muscle type
nAChR, opens the channel and the current passes,
depolarising the endplate. The response to acetylcholine
is over in milliseconds because of its rapid degradation by
acetylcholinesterase, and the endplate reverts to its resting
state long before another nerve impulse arrives. In
contrast, the depolarising NMBs characteristically have a
biphasic action on muscle: an initial contraction followed
by relaxation lasting minutes to hours. The depolarising
NMBs, because they are not susceptible to hydrolysis by
acetylcholinesterase, are not eliminated from the synaptic
cleft until after they are eliminated from the plasma. The
time required to clear the drug from the body is the
principal determinant of the duration of block. The quick
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shift from excitation and muscle contraction to block of
transmission by depolarising NMBs occurs because the
endplate is continuously depolarised. Notably, although
suxamethonium is built upon two acetylcholine molecules, it does not interact with the presynaptic a3b2
nicotinic autoreceptors [37], and therefore does not cause
a fade phenomenon in the clinical dose range. Similarly,
suxamethonium does not interact with neuronal a3b4
nAChRs found in autonomic ganglia [37]. This suggests
that the autonomic side effects of suxamethonium (tachyarrhythmias) are not mediated by stimulation of a3b4
nAChRs in the autonomic ganglia. However, in concentrations higher than normal, suxamethonium can produce
block of a3b4 and a3b2 nAChRs [37].
Non-depolarising neuromuscular blocking drugs
All non-depolarising NMBs impair or block neurotransmission by competitively preventing the binding of
acetylcholine to its receptor. The final outcome (i.e.
block or transmission) depends on the relative concentrations of the chemicals and their comparative affinities
for the receptor. Normally, acetylcholinesterase enzyme
destroys acetylcholine and removes it from competition
for a receptor, so that the non-depolarising NMB has a
better chance of inhibiting transmission. If, however, an
inhibitor of the acetylcholinesterase such as neostigmine is
added, the cholinesterase cannot destroy acetylcholine.
The concentration of agonist in the cleft remains high,
and this high concentration shifts the competition
between acetylcholine and the non-depolarising NMB
in favour of the former, improving the chance that two
acetylcholine molecules will bind to a receptor even
though non-depolarising NMBs are still in the environment. Cholinesterase inhibitors overcome the neuromuscular paralysis produced by non-depolarising NMBs by
this mechanism. The channel opens only when acetylcholine attaches to recognition sites on the two a-subunits. Thus, a single molecule of non-depolarising
NMB, binding to one a-subunit of the nAChR, is
adequate to prevent the depolarisation of that receptor.
This modifies the competition by biasing it strongly in
favour of the antagonist (non-depolarising NMBs).
Antagonism of neuromuscular block
The non-depolarising NMBs block neuromuscular transmission predominantly by competitive antagonism of
acetylcholine at the postjunctional receptor. The most
straightforward way to overcome their effects is to
increase the competitive position of acetylcholine. Two
factors are important, the first of which is the concentration of acetylcholine. Increasing the number of mol 2009 The Authors
Journal compilation 2009 The Association of Anaesthetists of Great Britain and Ireland
ecules of acetylcholine in the junctional cleft changes the
agonist-to-antagonist ratio and increases the probability
that agonist molecules will occupy the recognition sites of
the receptor. It also increases the probability that an
unoccupied receptor will become occupied. Normally,
only about 500 000 of the 5 million available receptors
are activated by a single nerve impulse, and a large
number of receptors is in ‘reserve’ and could be occupied
by an agonist. The second factor important to the
competitive position of acetylcholine is the length of time
acetylcholine is in the cleft. Acetylcholine must wait for
the antagonist (non-depolarising NMB) to dissociate
spontaneously before it can compete for the freed site.
The non-depolarising NMBs bind to the receptor for
slightly less than 1 ms, which is longer than the normal
lifetime of acetylcholine. The destruction of acetylcholine
normally takes place so quickly that most of it is destroyed
before any significant number of antagonist molecules
have dissociated from the receptor. Prolonging the time
during which acetylcholine is in the junction allows time
for the available acetylcholine to bind to receptor when
the antagonist dissociates from the receptors. A recently
developed c-cyclodextrin compound, sugammadex
(Bridion, Schering Plough, Kenilworth, New Jersey,
USA), can directly bind steroidal NMBs, reversing
their action on the neuromuscular junction [38]. Details
of this reversal drug are discussed elsewhere in this
supplement.
Summary
The neuromuscular junction provides a rich array of
receptors and substrates for drug action. Most drugs used
clinically have multiple sites of action. The NMBs are not
exceptions to the rule in that most drugs have more than
one site or mechanism of action. The major site of action
of non-depolarising NMBs is the postjunctional receptors. Neuromuscular transmission is impeded by nondepolarsing NMBs because they prevent the access of
acetylcholine to its recognition site on the postjunctional
receptor. The paralysis is also potentiated by the prejunctional actions of the NMB, preventing the release of
acetylcholine. The latter can be documented as fade
occurs with increased frequency of stimulation. Nondepolarising NMBs modify neurotransmission by interacting with nAChRs which include a3b2, a1b1dc,
a1b1d, and the neuronal homomeric a7 nAChR.
Inhibition of the postjunctional acetylcholinesterase by
anticholinesterases increases concentration of acetylcholine, which can compete and displace the NMB-reversing
paralysis. The actions of certain NMBs such as rocuronium can be reversed by binding of the NMB by
exogenous compounds (c-cyclodextrin). Depolarising
7
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J. A. J. Martyn et al.
Neuromuscular transmission
Anaesthesia, 2009, 64 (Suppl. 1), pages 1–9
. ....................................................................................................................................................................................................................
compounds initially react with the acetylcholine recognition site and, like acetylcholine, open ion channels and
depolarise the endplate membrane. Unlike the transmitter, they are not subject to hydrolysis by acetylcholinesterase and so remain in the junction. Soon after
administration of the drug, some receptors are desensitised and, although occupied by an agonist, do not open
to allow current to flow to depolarise the area.
Conflicts of interest
Work described was supported in part by Grants
GM31569, GM21500-Project IV, GM55082 from the
National Institute of Health, Bethesda, MD (to JAJM),
Swedish Research Council Medicine (K2008-53X13405-09-3, to LIE), Karolinska Funds, Stockholm City
Council, Stockholm, Sweden (to MJF and LIE) and from
Shriners Hospitals Philanthropy, Tampa, FL (to JAJM).
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