Anatomy & physiology of neuromuscular junction & monitoringPresentation Transcript
Presented by Dr.Rashmi.G Chairperson Dr.M.Jajoor MD,DA PROFESSOR & HOD
Red Indians used arrow poison to hunt their food, using Curare as the poison. Unknown to them they laid the foundation of blocking the NMJ.
Through subsequent ages new methods have been studied and technique refined to produce a blockade where we control the patient.
Curare was first used clinically in 1942 by Griffith and Johnstone.
The NMJ is a synapse at which an electrical impulse travelling down a motor nerve, releases chemical transmitter which cause the muscle to contract.
PHYSIOLOGY OF NEUROMUSCULAR TRANSMISSION
NMJ is specialised on the nerve side and on the muscle side to transmit and recieve chemical messages.
Each motor neuron runs without interruption from the ventral horn of spinal cord to NMJ as a large myelinated axon.
As it approaches muscle it branches to contact many muscle cells together into functional group known as Motor unit.
PARTS OF NMJ
The anatomy of NMJ consist of following parts:
The nerve is separated from the surface of the muscle by a gap of about 20nm called junctional cleft.
Presynaptic membrane contains prejunctional acetylcholine receptors and active zone.
Synaptic cleft: Lies Between the muscle endplate and nerve terminal which are held in tight alignment by basal lamina.
Post synaptic membrane – acetylcholine receptors: At the post synaptic membrane the area overlying the nerve terminal is called muscle end plate. The membrane here is thrown into primary and secondary clefts.
At the shoulder of these clefts numerous acetylcholine receptors are present.
The acetylcholine receptors are nicotinic and are of following types
Present in the post junctional membrane of the motor end plate & are of nicotinic type. These receptors exist in pairs.
It consists of protein made up of 1000 amino acids, made up of 5 protein subunits designated as alpha, beta, delta and epsilon joined to form a channel that penetrates through and projects on each side of the membrane.
In the fetus gama replaces epsilon subunit.
Each receptor has central funnel shaped core which is an ion channel, 4 nm in diameter at entrance narrowing to less than 0.7nm within the membrane.
The receptor is 11 nm in length and extends 2nm into the cytoplasm of the muscle cell.
The receptor has 2 gates, an Upper voltage- dependent and a Lower time-dependent.
When acetylcholine receptors bind to the pentameric complex, they induce a conformational change in the proteins of the alpha subunits which opens the channel and occurs only if it binds to both the alpha binding sites.
For ions to pass through the channel both the gates should be open.
Cations flow through the open channel, sodium and calcium in and potassium out, thus generating end plate potential.
Na ions are attracted to the inside of the cell which induces depolarisation.
These are nicotinic receptors that control ion channel specific for calcium which is essential for synthesis and mobilization of acetylcholine.
They contain protein subunits that are blocked by non depolarising muscle relaxants resulting in fade and exhaustion.
They are also blocked by aminoglycosides and polymyxin antibiotics
These tend to be concentrated around the end plate, where they mix with post junctional receptors but may be found anywhere on the muscle membrane. In them, the adult epsilon subunit is replaced by the fetal gamma subunit.
They are not found in normal active muscle, but appear very rapidly after injury or whenever muscle activity has ended.
They can appear within 18hrs of injury and an altered response to neuromuscular blocking drugs can be detected in 24hrs of the insult.
When a large number of extrajunctional receptors are present, resistance to non-depolarising muscle relaxants develops, yet there is an increased sensitivity to depolarising muscle relaxants.
In most extreme cases, increased sensitivity to succinylcholine results in lethal hyperkalemic receptors with an exaggerated efflux of intracellular potassium.
The longer opening time of the ion channel on the extrajunctional receptor also results in larger efflux.
It is formed by thin actin , thick myosin filaments tropomyosin & troponin.
The shortening of this apparatus causes the contraction of the muscle
Synthesized in the Presynaptic terminal from substrate Choline and Acetyl CoA.
CHOLINE + ACETYL CoA ACETYL CHOLINE
50% Carrier Facilitated Transport Release
CHOLINE + ACETYL CoA ACETYL CHOLINE
Different pools of acetylcholine in the nerve terminal have variable availability for release
The immediately releasable stores, VP2: Responsible for the maintainance of transmitter release under conditions of low nerve activity. 1% of vesicles
The reserve pool, VP1: Released in response to nerve impulses. 80% of vesicles
The stationary store: The remainder of the vesicles.
Each vesicle contains approx 12,000 molecules of acetylcholine, which are loaded into the vesicles by an active transport process in the vesicle membrane involving a magnesium dependent H+ pump ATPase.
Contents of a single vesicle constitute a quantum of acetylcholine.
Release of acetylcholine may be
In response to a nerve impulse.
When a nerve impulse invades the nerve terminal, calcium channels in the nerve terminal membrane are opened up.
Calcium enters the nerve terminal and there is calcium dependant synchronous release of the contents from 50-100 vesicles.
The number of quanta released by each nerve impulse is very sensitive to extracellular ionized calcium concentrations. Increased calcium concentration results in increased quanta released.
To enable this, vesicle must be docked at special release sites (active zones) in that part of the terminal where the axonal membrane faces the postjunctional acetylcholine receptors.
These are vesicle from the immediately releasable stores
Once the contents have been discharged, they are rapidly refilled from the reserve stores.
The reserve vesicles are anchored to actin fibrils in the cytoskeleton, by vesicular proteins called synapsins
Some calcium that enters the axoplasm, on the arrival of the nerve impulse binds to calmodulin, which activates protein kinase-2 which phosphorylates synapsins, which, in turn dissociates the vesicle from the actin fibrils allowing it to move forward to the release site.
Docking of the vesicle and subsequent discharge of acetylcholine by exocytosis, involves several other proteins.
Membrane protein called SNAREs ( Soluble N-ethylmatrimide sensitive attachment proteins) are involved in fusion, docking, and release of acetylcholine at the active zone.
SNARE includes – synaptic vesicle protein synaptobrevin, synataxin and SNAP-25.
The released acetylcholine diffuses to the muscle type nicotinic acetylcholine receptors which are concentrated at the tops of junctional folds of membrane of the motor end plate.
Binding of acetylcholine to these receptors increases Na and K conductance of membrane and resultant influx of Na produces a depolarising potential, end plate potential.
The current created by the local potential depolarise the adjacent muscle membrane to firing level.
Acetylcholine is then removed by acetylcholinesterase from synaptic cleft, which is present in high concentration at NMJ.
Action potential generated on either side of end plate and are conducted away from end plate in both directions along muscle fiber.
The muscle action potential in turn initiates muscle contraction
STRUCTURE OF NA CHANNEL
This Na channel is cylindrical
Has membrane protein
Its two ends act as gates
Both should be open to allow passage of ions.
Voltage dependent gate is closed in resting state and opens only on application of a depolarising voltage, remains open as long as the voltage persists
The time dependent gate is normally open at rest closing a few milliseconds after the voltage gate opens and remains closed as long as the voltage gate is open
It reopens after the voltage gate closes.
The channel is patent, allowing sodium ions only when the gates are open.
POSSIBLE CONFIGURATION OF Na CHANNELS
Resting state : Voltage gate closed
Time gate open
Depolarization: Voltage gate open
Time gate open
With in a few milliseconds: Voltage gate open
Time gate closed
End of depolarization: Voltage gate closed
Time gate open
ROLE OF CALCIUM
The concentration of calcium and the length of time during which it flows into the nerve ending, determines the number of quanta release.
Calcium current is normally stopped by the out flow of potassium.
Calcium channels are specialized proteins, which are opened by voltage change accompanying action potentials
Part of calcium is captured by proteins in the endoplasmic reticulum & are sequestrated.
Remaining part is removed out of the nerve by the Na/Ca antiport system
The sodium is eventually removed from the cell by ATPase
This protein enzyme is secreted from the muscle, but remain attached to it by thin stalks of collagen, attached to the basement membrane.
Acetylcholine molecules that don’t interact with receptors are released from the binding site & are destroyed almost immediately by acetylcholinesterase, in <1 ms, after its release into the junctional cleft.
PHYSICAL CHANNEL BLOCKADE
Various drugs can block the neuromuscular junction and prevent depolarisation.
Blockade can occur in two modes
Blocked when open
Blocked when closed
OPEN CHANNEL BLOCK
In this, the drug molecule enters a channel which has been opened by acetylcholine.
This is use dependent block
Physical blockade by a molecule of an open channel relies on the open configuration of the channel and the development of this is proportional to the frequency of channel opening.
This mechanism may explain the synergy that occurs with certain drugs such as local anaesthetic, antibiotics and muscle relaxants.
In addition, the difficulty in antagonizing profound neuromuscular blockade may be due to open channel block by the muscle relaxants
CLOSED CHANNEL BLOCK
The drugs occupy the mouth of the channel and prevents ions from passing through the channel to depolarise the end plate.
Tricyclic drugs and naloxone may cause physical blockade of a closed by impending interaction of acetylcholine with the receptor.
For drugs interfering with the function of the acetylcholine receptor, without acting as an agonist or antagonist, the receptor lipid membrane interface may also be another site of action.
Eg: Volatile agents, Local anaesthetic and Ketamine
Neuromuscular monitoring is based on two important issues:
1. on the variable response to muscle relaxants and
2 because of the narrow therapeutic window.
There is no detectable block until 75 to 85% of receptors are occupied and paralysis is complete at 90 to 95% receptor occupancy.
Neuromuscular monitoring permit optimal surgical relaxation and reverses the block spontaneously or revesed quickly with antagonists.
Residual neuromuscular block is a major risk factor for many critical events in the immediate postoperative period such as ventilatory insufficiency, hypoxemia and pulmonary infections.
The most satisfactory method is by peripheral nerve stimulator and observation of evoked response in the muscle supplied.
FEATURES OF NEUROSTIMULATION
Key features of exogenous nerve stimulation are:
Nerve stimulator: A battery powered device that delivers depolarizing current via the electrodes.
Pulse width: Is the duration of the individual impulse delivered by the nerve stimulator.
Each impulse should be <0.5msec and 0.1sec in duration to elicit nerve firing at a readily attainable current. Pulse width >0.5msec extends beyond the refractory period of the nerve resulting in repetitive firing.
Current intensity: Is the amperage(mA) of the current delivered by the nerve stimulator. The current output of most stimulators can range from 0-80mA.
Supramaximal current: Is approximately 10-20% higher intensity than the current required to depolarize all fibres in a particular nerve bundles.
Submaximal current: A current intensity that induces firing of only a fraction fibres in a given nerve bundle and is less painful
Stimulus frequency: The rate (Hz) at which each impulse is repeated in cycles per second (Hz).
Single twitch is commonly repeated at 10 second intervals i.e. 0.1 Hz and
Tetanic stimulation commonly consists of 50 impulses/ sec i.e. 50 Hz.
Surface Electrodes: They contain gel conducting surfaces for transmission of impulses to the nerves through the skin. With careful skin preparation the threshold for which response is generally <15mA.
Needle Electrodes: Subcutaneous needles deliver the impulse in the immediate vicinity of the nerve. These are highly effective because they bypass the tissue impedance so that the tissue impedance is typically <2000 Ohms.
Single twitch: This is the simplest form 0f neurostimulation entailing a single twitch at 0.1 to 0.12 msec.
Single twitch is delivered at a supramaximal current, it induces a single nerve action potential in each fiber of the nerve bundle.
During nondepolarizing block the response to single twitch stimulation is not reduced until atleast 75 to 80% of receptors are occupied and therefore does not detect block of less than 70%.
Train of four(TOF) :
This is a popular mode of stimulation for clinical monitoring of neuromuscular junction first described by ali et al.
Four successive stimuli are delivered at 2 Hz (every 0.5sec). In the presence of non depolarizing relaxants, the margin of safety is decreased such that some end plates in train of four progressively fade.
In the absence of non depolarizing block, the T4/T1 ratio is approximately one.
For complete recovery T4/T1 ratio should be more than 0.9
ADVANTAGES OF TOF STIMULATION
This pattern of stimulation can be applied at anytime during the neuromuscular block and can provide quantification of depth of block without the need for control measurement before relaxant administration.
It is more sensitive to lesser degree of receptor occupancy than single twitch.
The relatively low frequency allows response to be evaluated manually or visibly.
There is no post tetanic facilitation therefore can be repeated every 10 to 12 sec.
It may be delivered at sub maximal current which is less painful and is associated with same degree of fade.
High frequency stimulation (50Hz or more) results in sustained or tetanic contraction of the muscle during normal neuromuscular transmission despite decrement in acetylcholine release.
During tetanus, progressive depletion of acetylcholine output is balanced by increased synthesis and transfer of transmitter from its mobilization stores.
The presence of nondepolarizing muscle relaxants reduces the margin of safety by reducing the number of free cholinergic receptors and also by impairing the mobilization of acetylcholine within the nerve terminal there by contributing to the fade in the response to tetanic and TOF stimulation.
A frequency of 50Hz is physiological as it is similar to that generated during maximal voluntary effort. Fade is first noted at 70% receptor occupancy.
It has been shown that tetanic response to 50 Hz for five sec is sustained when TOF ratio is greater than 0.7.
Is post tetanic facilitation which depends on frequency and duration of block
It is very painful and therefore not suitable for unanaesthetised patients.
DOUBLE BURST STIMULATION
TOF ratio of less than 0.2 to 0.3 is difficult to detect even by trained observers.
To improve the detection rate, a new mode of stimulation which consist of two short tetani, separated by a interval long enough to allow relaxation, evaluating the ratio of second to first response has been proposed.
Many patterns have been suggested but the most promising one consists of two train of three impulse of 50 Hz separated by 750msec.
POST- TETANIC COUNT (PTC)
Tetanus at 50 Hz for five seconds is applied followed 3 sec later by single twitch stimulation at 1 Hz.
The number of evoked post-tetanic twitches detected is called the post-tetanic count (PTC).
PTC is a prejunctional event, the response can vary with the nondepolarising muscle relaxant used.
A PTC of 8 to 9 indicated imminent return of TOF.
APPLICATION OF PTC
Evaluating the degree of neuromuscular blockade when there is no reaction to single twich or TOF as after administration of large dose of nondepolarizing muscle relaxant.
PTC can also be used whenever sudden movement must be eliminated (Ophthalmic Surgery). Elimination of responses to tracheobronchial stimulation requires intense neuromuscular blockade of zero PTC.
PTC can be used during continuous infusion of intermediate nondepolarizing muscle relaxant as a guidance to intensity of neuromuscular blockade.
PTC predicts time to reappearance of first response to TOF stimulation
The specific nerve-muscle site utilized for monitoring has drawn interest in the recent years because of the variability among muscle groups in sensitivity and onset time
Relative sensitivities of muscle groups to nondepolarizing muscle relaxants MUSCLES SENSITIVITY Vocal cord Most Resistant Diaphragm Orbicularis oculi Abdominal rectus Adductor pollicis Masseter Pharyngeal Extraocular Most sensitive
The nerve is most commonly used for neuromuscular monitoring in the perioperative period.
The ulnar nerve innervates the adductor pollicis, abductor digiti mimimi, abductor pollicis brevis and dorsal interosseous muscles.
One stimulating electrode is typically placed more than 2cm proximally on the volar forearm or over the olecranon groove.
The recording electrodes are placed over the appropriated muscle.
The response to the stimulation is monitored commonly at the orbicularis oculi (contraction of eyebrow) and orbicularis oris(contraction of the lip)
NERVES OF THE FOOT
The posterior tibial nerve may be stimulated as it comes behind the medial malleolus, caused plantar flexion of the great toe and foot.
The peroneal nerve and lateral popliteal nerve elicit dorsi flexion of the foot
ASSESSMENT OF EVOKED RESPONSE TO NEUROMUSCULAR STIMULATION
Visual or tactile assessment: Visual or tactile methods of evaluation of the evoked response to stimulation is the simplest means of assessment
During recovery of neuromuscular function all responses of TOF can be felt.
An estimation of TOF ratio may be attempted but the method is not sensitive enough to exclude possibility of residual neuromuscular blockade.
Fade is usually undetected until TOF ratio values are less than 0.5
Greater sensitivity for fade detection is achieved with DBS
RECORDING DEVICES FOR MEASURING NEUROMUSCULAR FUNCTION
Compound muscle action potential: It is the cumulative electrical signal generated by the individual action potentials of the individual muscle fibres.
Electromyogram(EMG): It records the compound MAP via recording electrodes place near the mid portion or motor point of the muscle.
The latency of the compound MAP is the interval between stimulus artifact and evolved muscle response
The amplitude of the compound MAP is proportional to the number of muscle units that generate an MAP within the designated time interval (epoch) and this correlates with the evoked mechanical responses.
This method is used mostly for experimental studies
This technique used a miniature piezoelectric transducer to determine the rate of angular acceleration.
This is a nonisometric measurement and there are less stringent requirements for immobilization of arm, fingers and thumb and also no preload is necessary.
However recording of tetanic responses is not possible as the movement is an essential component of accelerography.
It is a simple method useful in operating room and in the intensive care unit.
To differentiate depolarising block and Non-depolarising block
To see efficacy of depolarising block after administration of drug and to judge whether the patient is fully relaxed
To see whether the pt is out of effect of block of depolarising muscle relaxed
As a guide to administer first dose of non-depolarising muscle relaxant
As a guide to see whether completeness of non-depolarising neuromuscular block
As a guide to for starting of reversal of non-depolarising block
To see a completeness of recovery
As a guide for incremental doses administration of non-depolarising muscle relaxant
To differentiate respiratory paralysis i.e. central or peripheral due to nueromuscular block
To diagnose overdose of sedatives cerebral depressants or muscle relaxants
To diagnose phase II block after suxamethonium
To diagnose various neuromuscular disorders
To diagnose site of nerve injury
To diagnose electrolyte imbalance or disturbances affecting NM Transmission
As a guide in diagnosis of prolonged apnea or recovery after balanced anaesthesia
WHICH PATIENT SHOULD BE MONITORED
By the foregoing discussion, it would seem prudent to monitor NMJ in all pts receiving NMBs.
Such monitoring is advisible particularly in conditions where the pharmacokinetics and pharmacodynamics of NMBs are altered significantly as listed below:
Several renal, liver disease
Neuromuscular disorders such as myasthenia gravis, myopathies , and upper and lower motor neuron lesions
Pts with severe pulmonary disease or marked obesity to ensure adequate recovery of skeletal muscle function.
Neuromuscular blockade achieved with continuous infusion of NMBs
Pts receiving long-acting NMBs
Pts undergoing lengthy surgical procedures
LIMITATIONS OF NEUROMUSCULAR MONITORING
Despite the important role of NMJ monitoring in anaesthesia practice, it is necessary to use a multifactorial approach for the following reasons:
Neuromuscular responses may appear normal despite persistence of receptor occupancy y NMBs. T4:T1 ratio is 1 even when 40-50% of the receptors are occupied.
Because of wide individual variability in evoked responses some pts may exhibit weakness at TOF ratio as high as high as 0.8 to 0.9
The established cut-off values for adequate recovery do not guarantee adequate ventilatory function or airway protection
Increased skin impedence resulting from perioperative hypothermia limits the appropriate interpretation of evoked responses