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.
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.
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 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.
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.
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.
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.
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.
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.
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
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 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.
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.
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