Spontaneous and evoked ectopic discharges recorded from single human axons

Electromyography of the muscle spindle

In needle electromyography, there are two spontaneous waveforms, miniature end plate potentials and “end plate spikes”, appearing usually together. Miniature end plate potentials are local, non-propagating postsynaptic waves, caused by spontaneous exocytosis of acetylcholine in the neuromuscular junction. The prevailing hypothesis states that “end plate spikes” are propagated postsynaptic action potentials of muscle fibers, caused by presynaptic irritation of the motor nerve or nerve terminal. Using several small concentric needle electrodes in parallel with the muscle fibers, most “end plate spikes” are strictly local or propagating for 2–4 mm. At the end plate zone, there are miniature end plate potentials without “end plate spikes”. Local “end plate spikes” are junctional potentials of intrafusal gamma neuromuscular junctions of the nuclear bag fibers, and propagated “end plate spikes” are potentials of nuclear chain muscle fibers of muscle spindles. Miniature end plate potentials without “end plate spikes” at the end plate zone derive from alpha neuromuscular junctions. These findings contrast with the prevailing hypothesis. The history of observations and different hypotheses of the origin of end plate spikes are described.

Long-term changes in trigeminal ganglionic and thalamic neuronal activities following inferior alveolar nerve transection in behaving rats

The transection of the inferior alveolar nerve (IANx) produces allodynia in the whisker pad (V2 division) of rats. Ectopic discharges from injured trigeminal ganglion (TG) neurons and thalamocortical reorganization are possible contributors to the sensitization of uninjured V2 primary and CNS neurons. To test which factor is more important, TG and ventroposterior medial nucleus (VPM) neurons were longitudinally followed before, during, and after IANx for up to 80 d. Spontaneous discharges and mechanical stimulation-evoked responses were recorded in conscious and in anesthetized states. Results show (1) a sequential increase in spontaneous activities, first in the injured TG neurons of the IAN (2-30 d), followed by uninjured V2 ganglion neurons (6-30 d), and then VPM V2 neurons (7-30 d) after IANx; (2) ectopic discharges included burst and regular firing patterns in the IAN and V2 branches of the TG neurons; and (3) the receptive field expanded, the modality shifted, and long-lasting after-discharges occurred only in VPM V2 neurons. All of these changes appeared in the late or maintenance phase (7-30 d) and disappeared during the recovery phase (40-60 d). These observations suggest that ectopic barrages in the injured IAN contribute more to the development of sensitization, whereas the modality shift and evoked after-discharges in the VPM thalamic neurons contribute more to the maintenance phase of allodynia by redirecting tactile information to the cortex as nociceptive.

Mechanisms of cramp contractions: peripheral or central generation?

We analysed the cramp threshold (i.e. the minimum frequency of electrical stimulation capable of inducing a cramp) and the behaviour of individual motor units during cramps electrically elicited in the absence (intact condition) and presence (blocked condition) of a peripheral nerve block in eight healthy subjects. The cramp threshold was significantly greater in the blocked than in the intact condition (18 ± 3 Hz vs. 13 ± 3 Hz; P = 0.01). Cramp duration and peak EMG amplitude in the intact condition (55.6 ± 19.2 s and 47.5 ± 24.8 μV, respectively) were significantly greater compared to the blocked condition (2.6 ± 1.3 s and 13.9 ± 8.8 μV; P < 0.01). All motor units identified in the blocked condition (n = 38) had a shorter interval of activity and a greater discharge rate compared to the intact condition (n = 37) (respectively, 1.1 ± 1.0 s vs. 29.5 ± 21.8 s, P < 0.0001; 25.7 ± 11.6 pulses s(-1) vs. 20.0 ± 5.9 pulses s(-1); P < 0.05). The motor unit activity detected during the blocked condition corresponded to spontaneous discharges of the motor nerves, while in the intact condition the motor unit discharge patterns presented the typical characteristics of motor neuron discharges. These results indicate a spinal involvement at the origin of cramps and during their development.

Myofascial syndrome and pain: A neurophysiological approach

It has been debated whether muscle spindles have a role in myofascial pain or not. We present a number of arguments for the former hypothesis. It was hypothesized that firing of intrafusal muscle fibres, i.e. fusimotor activity can be observed as "end plate spikes" (EPSs) in electromyography (EMG). The EPSs may be found in local active spots of muscle, often associated with miniature end plate potentials (MEPPs). Insertion of EMG needle electrodes into an active spot is painful, indicating nociception in the muscle spindle. Myofascial syndrome patients have taut bands with active trigger points (TrPs) in painful muscles. End plate activity (EPSs and MEPPs) is a significantly more common finding in TrPs of myofascial pain than in control points of the muscle, indicating the presence of muscle spindles. However, some control sites may show EPSs of normal muscle spindles. Increased amount of inflammatory metabolites have been observed in active TrPs. Muscle spindle is a capsulated gel-filled container, where inflammatory and contraction metabolites may be heavily concentrated during sustained fusimotor activation. Thus the intrafusal chemosensitive pain mediating III- and IV-afferents are sensitized and activated. Intrafusal inflammation causes further reflex activation of the fusimotor and skeletofusimotor systems via sensitized III- and IV-afferents. The taut band itself may be a contracture (rigor) of local skeletofusimotor (beta) units caused by sustained reflex drive by the given muscle spindles. In EMG this may be seen as complex repetitive discharges. We conclude that TrPs of myofascial pain are related to painful muscle spindles in taut bands.

Pathophysiology of peripheral nerve injury during regional anesthesia

BACKGROUND AND OBJECTIVES

Despite attention to technical details in performance of regional anesthetics, damage to nerves continues to be a concern. Understanding of pathophysiological mechanisms may aid in decreasing the incidence and severity of such injuries.

METHODS

Studies from both clinical and basic science perspective are reviewed.

RESULTS

Exposure of peripheral nerves to local anesthetics may result in axonal damage, particularly if the solution is injected intrafascicularly, if the concentration is high, and if duration of exposure is prolonged. Disruption of numerous cellular functions may contribute to neuronal damage by local anesthetics, but elevated intracellular calcium levels may play a central role. Needle penetration of a nerve results in minimal lasting damage unless this is combined with local anesthetic administration within the nerve fascicle. Direct compression by a pronged tourniquet application may damage axons particularly of large myelinated fibers. Ischemia may also contribute to neuronal injury in proportion to the duration of blood flow interruption.

CONCLUSIONS

The relative importance of these pathogenic factors in cases of nerve injury after regional anesthesia is not resolved.

Spontaneous bursting neuronal discharges recorded from peripheral nerve in human: injury discharges or not?

This paper deals with a spontaneous, bursting neuronal activity which can not be altered by any stimulation in the periphery or voluntary actions or by cognitive tasks. An initial description of such units led to the conclusion that this activity was generated ectopically at the site of a previous or present impalement of a nerve fibre. The aim of the current study was to record a larger number of these units by using microneurography, in order to characterise their firing properties and particularly, see if any subtypes of units could be identified. In conclusion, this paper suggests that some of these discharges could be related to an injury of the nerve fibre, however most of them could not. Some hypothesis regarding the nature of these bursting activities are suggested.

Modulation of ongoing EMG by different classes of low-threshold mechanoreceptors in the human hand

1. We have previously demonstrated that the input from single FA I and SA II cutaneous mechanoreceptors in the glabrous skin of the human hand is sufficiently strong to modulate ongoing EMG of muscles acting on the digits. Some unresolved issues have now been addressed. 2. Single cutaneous (n = 60), joint (n = 2) and muscle spindle (n = 34) afferents were recorded via tungsten microelectrodes inserted into the median and ulnar nerves at the wrist. Spike-triggered averaging was used to investigate synaptic coupling between these afferents and muscles acting on the digits. The activity of 37 % of FA I (7/19), 20 % of FA II (1/5) and 52 % of SA II afferents (11/21) evoked a reflex response. The discharge from muscle spindles, 15 SA I and two joint afferents did not modulate EMG activity. 3. Two types of reflex responses were encountered: a single excitatory response produced by irregularly firing afferents, or a cyclic modulation evoked by regularly discharging afferents. Rhythmic stimulation of one FA I afferent generated regularly occurring bursts which corresponded to the associated cyclic EMG response. 4. Selectively triggering from the first or last spike of each burst of one FA I afferent altered the averaged EMG profile, suggesting that afferent input modulates the associated EMG and not vice versa. 5. The discharge from single FA I, FA II and SA II afferents can modify ongoing voluntary EMG in muscles of the human hand, presumably via a spinally mediated oligosynaptic pathway. Conversely, we saw no evidence of such modulation by SA I, muscle spindle or joint afferents.

Mechanisms of paresthesias arising from healthy axons

Paresthesias are common manifestations of central and peripheral pathological processes and are due to ectopic impulse activity in cutaneous afferents or their central projections. Cutaneous afferents are more excitable than motor axons, due to differences in their biophysical properties. These differences probably include more persistent Na(+) conductance and inward rectification on cutaneous afferents, properties which probably confer greater protection from impulse-dependent conduction failure but create a greater tendency to ectopic activity. Ectopic discharges can be induced in normal afferents by four maneuvers: hyperventilation, ischemia, release of ischemia, and prolonged tetanization. The alkaline shift produced by hyperventilation selectively increases the persistent Na(+) conductance, while the membrane depolarization produced by ischemia affects both transient and persistent Na(+) channels. Postischemic and posttetanic paresthesias occur when hyperpolarization by the Na(+)/K(+) pump is transiently prevented by raised extracellular K(+). The electrochemical gradient for K(+) is reversed, and inward K(+) currents trigger regenerative depolarization. These mechanisms of paresthesia generation can account for paresthesias in normal subjects and may be relevant in some peripheral nerve disorders.

Evidence for strong synaptic coupling between single tactile afferents and motoneurones supplying the human hand

1. Electrical stimulation of digital nerves elicits short-latency excitatory and inhibitory spinal reflex responses in ongoing EMG in muscles acting on the fingers and thumb. Similar responses are elicited by stimulating a population of muscle spindles but not when a single muscle spindle is activated. The current study investigated whether short-latency EMG responses could be evoked from the discharge of a single cutaneous afferent. 2. Thirty-three tactile afferents were recorded via tungsten microelectrodes in the median nerve of awake humans. Spike-triggered averaging revealed EMG events time-locked to the afferent discharge. The afferents were activated by an external probe and the EMG was elicited by a weak voluntary contraction. 3. Eleven cutaneous afferents (33 %) showed a short-latency response in the ongoing EMG. Overt increases or decreases in EMG were observed for seven afferents (onset latency 20.0-41.1 ms). For four slowly adapting (SA) type II afferents, EMG showed a periodicity that was correlated to the afferent interspike interval (r = 0.99). 4. The EMG associated with two rapidly adapting (FA) type I afferents (29 %) showed a short-latency excitation while five showed neither excitation nor inhibition. Seven SA II afferents (39 %) showed excitation and 11 no response; and none of the six SA I afferents showed any response. 5. We conclude that, unlike muscle spindle afferents, the input from a single cutaneous afferent is strong enough to drive, via interneurones, motoneurones supplying muscles acting on the digits. The potent short-latency response we found supports the important role of cutaneous mechanoreceptors in fine motor control of the human hand.