Properties Of Single Motor Units Research Articles

Consensus for experimental design in electromyography (CEDE) project: Single motor unit matrix.

The analysis of single motor unit (SMU) activity provides the foundation from which information about the neural strategies underlying the control of muscle force can be identified, due to the one-to-one association between the action potentials generated by an alpha motor neuron and those received by the innervated muscle fibers. Such a powerful assessment has been conventionally performed with invasive electrodes (i.e., intramuscular electromyography (EMG)), however, recent advances in signal processing techniques have enabled the identification of single motor unit (SMU) activity in high-density surface electromyography (HDsEMG) recordings. This matrix, developed by the Consensus for Experimental Design in Electromyography (CEDE) project, provides recommendations for the recording and analysis of SMU activity with both invasive (needle and fine-wire EMG) and non-invasive (HDsEMG) SMU identification methods, summarizing their advantages and disadvantages when used during different testing conditions. Recommendations for the analysis and reporting of discharge rate and peripheral (i.e., muscle fiber conduction velocity) SMU properties are also provided. The results of the Delphi process to reach consensus are contained in an appendix. This matrix is intended to help researchers to collect, report, and interpret SMU data in the context of both research and clinical applications.

Physical and electrophysiological motor unit characteristics are revealed with simultaneous high-density electromyography and ultrafast ultrasound imaging

Electromyography and ultrasonography provide complementary information about electrophysiological and physical (i.e. anatomical and mechanical) muscle properties. In this study, we propose a method to assess the electrical and physical properties of single motor units (MUs) by combining High-Density surface Electromyography (HDsEMG) and ultrafast ultrasonography (US). Individual MU firings extracted from HDsEMG were used to identify the corresponding region of muscle tissue displacement in US videos. The time evolution of the tissue velocity in the identified region was regarded as the MU tissue displacement velocity. The method was tested in simulated conditions and applied to experimental signals to study the local association between the amplitude distribution of single MU action potentials and the identified displacement area. We were able to identify the location of simulated MUs in the muscle cross-section within a 2 mm error and to reconstruct the simulated MU displacement velocity (cc > 0.85). Multiple regression analysis of 180 experimental MUs detected during isometric contractions of the biceps brachii revealed a significant association between the identified location of MU displacement areas and the centroid of the EMG amplitude distribution. The proposed approach has the potential to enable non-invasive assessment of the electrical, anatomical, and mechanical properties of single MUs in voluntary contractions.

Functional properties of single motor units in the human medial pterygoid muscle: Thresholds.

Little is known regarding the functional properties of single motor units (SMUs) in the medial pterygoid muscle (MPt) during jaw movements. The aims are (a) to report the thresholds of onset of MPt SMUs during 4 goal-directed jaw movement tasks, and (b) to determine whether the threshold of onset of SMU activation varies with the velocity of jaw movement and the location within the muscle. Intra-muscular electrodes were inserted in the right MPt of 18 participants performing ipsilateral (right), contralateral, protrusive and opening-closing jaw movements recorded at 2 velocities. Task phases were as follows: BEFORE, OUT, HOLDING, RETURN and AFTER. SMU onset thresholds were determined from the displacement (mm) of the lower mid-incisor point. Electrode location within 4 arbitrary muscle divisions was determined with computer tomography. Statistical tests: Spearman's correlations, Kruskal-Wallis tests; significance accepted at P<.05. A significant inverse relation occurred between velocity and threshold for the RETURN of the ipsilateral movement (n=62 SMU thresholds), while a significant positive relation occurred for the OUT of the contralateral movement (n=208); there were no significant associations for the protrusive (n=131) and opening-closing (n=58) tasks. Significant threshold differences occurred across the 4 muscle divisions only during the OUT of the contralateral and protrusive movements. Some evidence was provided for gender differences in MPt SMU properties. The absence of a significant inverse relation between velocity and SMU threshold for most recorded movements suggests the MPt acts as a stabilizer of the jaw in horizontal and opening-closing jaw movements.

Increased diaphragm motor unit discharge frequencies during quiet breathing in people with chronic tetraplegia.

Respiratory muscle strength is compromised in people with tetraplegia, which may be compensated for by an increase in neural drive to the diaphragm. We found that the discharge frequencies of diaphragm motor units are higher in people with chronic tetraplegia compared with able-bodied people during quiet breathing. Furthermore, we found that the area of single motor unit potentials was increased in people with tetraplegia. These results suggest an increased motoneurone output to the diaphragm and remodelling of diaphragm motor units to maintain ventilation in tetraplegia. People with tetraplegia have reduced inspiratory muscle strength, ∼40% of able-bodied individuals. Paralysed or partially paralysed respiratory muscles as a result of tetraplegia compromise lung function, increase the incidence of respiratory infections and can cause dyspnoea. We hypothesised that reduced inspiratory muscle strength in tetraplegia may increase neural drive to the inspiratory muscles to maintain ventilation. We recorded the discharge properties of single motor units from the diaphragm in participants with chronic tetraplegia (8 males, 42-78 years, C3-C6 injury, AIS A-C) and able-bodied control participants (6 males matched for age and body mass index). In each group, 117 and 166 single motor units, respectively, were discriminated from recordings in the costal diaphragm using a monopolar electrode. A linear mixed-effects model analysis showed higher peak discharge frequencies of motor units during quiet breathing in tetraplegia (17.8 ± 4.9Hz; mean ± SD) compared with controls (12.4 ± 2.2Hz) (P < 0.001). There were no differences in tidal volume, inspiratory time or mean air flow between groups. Motor unit potentials in tetraplegia, compared with controls, were larger in amplitude (1.1 ± 0.7mV and 0.5 ± 0.3mV, respectively, P = 0.007) and area (1.83 ± 1.49µV ms and 0.69 ± 0.52µV ms, respectively, P = 0.003). The findings indicate that diaphragm motor unit remodelling is likely to have occurred in people with chronic tetraplegia and that there is an increase in diaphragm motor unit discharge rates during quiet breathing. These neural changes ensure that ventilation is maintained in people with chronic tetraplegia.

18. Dysfunctional properties of single motor units in ALS

Background Abnormalities in the excitability of lower motor neurons in amyotrophic lateral sclerosis have been extensively examined using threshold tracking techniques. Several studies have found evidence for a down-regulation of K+, and an up-regulation of Na+ conductances. The results as a whole however have been somewhat mixed. Threshold-tracking studies test the excitability of a fixed fraction of the maximal compound action potential, and for most disease processes this accurately reflects the excitability of all axons of the same modality within a nerve. However, in ALS such a relationship is unreliable. The slow inexorable process of degeneration of individual motor units is likely to change their excitability, with hyperexcitability and reinnervation believed to impact on their viability. In ALS the disease process itself presents the opportunity to study single motor units identifiable in the recruitment curve, and in particular the large units which result from reinnervation of muscle fibres from motor units that have undergone degeneration. Such measurements provide an unequivocal snapshot of the excitability of an individual motor unit. Methods Measurements of the excitability of single motor units in the intrinsic muscles of the split hand were undertaken in 7 subjects, and mathematical modelling was performed to examine the nature of changes in individual motor units. Results As expected, the ALS single motor unit recordings were more varied in nature when compared to normal single motor units from a previous study. Significant differences were observed with a greater rheobasic current and lower resting I/V slope, relative refractory period and ‘fanned out’ threshold electrotonus in the ALS single motor units compared to the normal controls. Conclusions The excitability of individual motor units in ALS is heterogeneous in nature, with axonal abnormalities correlated with disease progression. The underlying biophysical mechanisms of these abnormal motor units were best modelled by an increase in the Na+/K+-ATPase pump current, and a near halving of slow K+ conductances. The relative proportions of changes to the Na+/K+-ATPase pump and slow K+ conductances are likely to vary between individual motor units, and with disease progression.

Changes in motor unit properties in SOD1 (G93A) rats

In amyotrophic lateral sclerosis (ALS), progressive death of motor neurons results in denervation and reinnervation of muscles. It is not clear how ALS affects the properties of motor units (MUs). Properties of single MUs in the medial gastrocnemius (MG) muscle of rats bearing the human mutated superoxide dismutase gene type 1 (SOD1) were determined at 3 stages: asymptomatic (ALS I); early symptomatic (ALS II); and terminal (ALS III). In ALS II, higher proportions of FF (fast fatigable) and S (slow) MUs were observed, whereas in ALS III higher percentages of S and lower percentages of FF MUs were noted compared with controls. S motor neurons reinnervated fast muscle fibers, and those MUs gained some properties of fast MUs, including lower fatigue resistance, greater force generation, and higher action potential amplitudes. Changes in MU properties of SOD1 rats have progressive and multidirectional character and speed depending on the MU type.

Threshold properties of single motor units in superior head of human lateral pterygoid muscle

The superior head of the human lateral pterygoid muscle (SHLP) may play a similar role in jaw movement as the inferior head (IHLP). Objective The aim was to determine whether threshold properties of single motor units (SMUs) within SHLP during jaw tasks were comparable to those identified for IHLP. Design In 24 human subjects, SMUs were recorded intramuscularly from computer-tomography verified sites within SHLP during standardised jaw tasks recorded by a jaw-tracking device. Results Of the 69 SMUs discriminated, 54 were active during contralateral, 52 during protrusive and 8 during ipsilateral jaw movements. The thresholds, at which SMUs commenced firing, decreased ( p < 0.05) as speed of contralateral or protrusive tasks increased. The data suggest an important role for SHLP in generation and control of contralateral and protrusive jaw movements. A number of lines of evidence were consistent with functional heterogeneity within SHLP. Conclusions The similarities in SHLP and IHLP functional properties support the proposal that both heads should be regarded as a system of fibers acting as one muscle.

Contractile properties of single motor units in elderly

Contractile properties of single motor units in elderly

Spike-triggered average torque and muscle fiber conduction velocity of low-threshold motor units following submaximal endurance contractions

The motor unit twitch torque is modified by sustained contraction, but the association to changes in muscle fiber electrophysiological properties is not fully known. Thus twitch torque, muscle fiber conduction velocity, and action potential properties of single motor units were assessed in 11 subjects following an isometric submaximal contraction of the tibialis anterior muscle until endurance. The volunteers activated a target motor unit at the minimum discharge rate in eight 3-min-long contractions, three before and five after an isometric contraction at 40% of the maximal torque, sustained until endurance. Multichannel surface electromyogram signals and joint torque were averaged with the target motor unit potential as trigger. Discharge rate (mean +/- SE, 6.6 +/- 0.2 pulses/s) and interpulse interval variability (33.3 +/- 7.0%) were not different in the eight contractions. Peak twitch torque and recruitment threshold increased significantly (93 +/- 29 and 12 +/- 5%, P <0.05) in the contraction immediately after the endurance task with respect to the preendurance values (0.94 +/- 0.26 mN.m and 3.7 +/- 0.5% of the maximal torque), whereas time to peak of the twitch torque did not change (74.4 +/- 10.1 ms). Muscle fiber conduction velocity decreased and action potential duration increased in the contraction after the endurance (6.3 +/- 1.8 and 9.8 +/- 1.8%, respectively, P <0.05; preendurance values, 3.9 +/- 0.2 m/s and 11.1 +/- 0.8 ms), whereas the surface potential peak-to-peak amplitude did not change (27.1 +/- 3.1 microV). There was no significant correlation between the relative changes in muscle fiber conduction velocity or surface potential duration and in peak twitch torque (R2= 0.04 and 0.10, respectively). In conclusion, modifications in peak twitch torque of low-threshold motor units with sustained contraction are mainly determined by mechanisms not related to changes in action potential shape and in its propagation velocity.

Commentary

Commentary

Recruitment of single human low-threshold motor units with increasing loads at different muscle lengths

We investigated the recruitment behaviour of low threshold motor units in flexor digitorum superficialis by altering two biomechanical constraints: the load against which the muscle worked and the initial muscle length. The load was increased using isotonic (low load), loaded dynamic (intermediate load) and isometric (high load) contractions in two studies. The initial muscle position reflected resting muscle length in series A, and a longer length with digit III fully extended in series B. Intramuscular EMG was recorded from 48 single motor units in 10 experiments on five healthy subjects, 21 units in series A and 27 in series B, while subjects performed ramp up, hold and ramp down contractions. Increasing the load on the muscle decreased the force, displacement and firing rate of single motor units at recruitment at shorter muscle lengths ( P<0.001, dependent t-test). At longer muscle lengths this recruitment pattern was observed between loaded dynamic and isotonic contractions, but not between isometric and loaded dynamic contractions. Thus, the recruitment properties of single motor units in human flexor digitorum superficialis are sensitive to changes in both imposed external loads and the initial length of the muscle.

Effects of experimental muscle pain on mechanical properties of single motor units in human masseter

Muscle pain is known to influence muscle activity but the details of its effects on the mechanical properties of single motor units (SMU) have not been described. We have recently reported a decreased firing rate of SMU in the human masseter muscle during painful contractions with a constant force output. Force output can be modulated by the SMU discharge rate in relation to the contractile properties of SMU. Therefore, the objective of the present study was to measure the mechanical properties of SMU in the masseter to clarify the mechanism which underlies the decrease in SMU firing rate during jaw-muscle pain. A spike-triggered averaging (STA) technique was used to determine the mechanical properties of low-threshold SMU in the masseter muscle recorded with fine wire electrodes during a voluntary isometric contraction. The twitch amplitude, contraction time, and half-relaxation time were determined from the averaged force records before and during experimental jaw-muscle pain induced by injection of 0.2 ml (100 microg/ml) capsaicin in 8 healthy subjects. Injections of 0.2 ml isotonic saline served as a non-painful control in 11 healthy subjects. The twitch amplitude was significantly increased during capsaicin-evoked muscle pain (P<0.001) without significant changes of half-relaxation time and contraction time. No significant changes in SMU twitch properties were observed during the control injections. Potentiation of twitch force could be a possible compensatory mechanism to maintain a constant force output during painful isometric contractions when SMU firing decreases. This finding therefore provides new information on the adaptation of motor function by muscle pain.

Extraocular motor unit and whole-muscle contractile properties in the squirrel monkey. Summation of forces and fiber morphology.

In order to understand the neural control of movement, many investigations have examined the contractile properties of single motor units contracting in isolation, and a great majority of those studies have been done in the cat. Fewer studies, again primarily in the cat, have examined motor units acting in concert in both hind-limb and extraocular muscles. It has been shown, in general, that when individual motor unit forces are added together they do not always add linearly, which makes our understanding of motor control somewhat more complicated. In addition, complex neuronal firing patterns can yield unexpected force outputs or muscle positions whether those patterns occur naturally or are induced through motoneuron stimulation. The current investigation extends these findings of nonlinearity to the primate extraocular system. In studies of the squirrel monkey lateral rectus muscle and its motor units, we show that individual units lose an average of 45% of their force output when they fire in concert with a small number of other motor units. Also, when individual motor units are stimulated at a constant rate of 100 Hz, the force output is most often dramatically different if that constant 100-Hz stimulation is preceded by brief (25 ms), high-frequency stimulation burst or pulse, as occurs during saccades. The force at 100 Hz is usually significantly higher than when no pulse is delivered. However, we now show that an identical stimulation pattern applied to a number of motor units simultaneously does not always yield these force differences. These "nonlinearities" are addressed in terms of the complex muscle architecture that we show in the squirrel monkey lateral rectus muscle. Muscle fibers do not always run in parallel from tendon to tendon. Instead, they may branch or attach to each other laterally or end to end, serially.

Functional properties of single motor units in the inferior head of human lateral pterygoid muscle: task firing rates.

The precise function of the inferior head of the human lateral pterygoid muscle (IHLP) is unclear. The aim of this study was to clarify the normal function of the IHLP. The hypothesis was that an important function of the IHLP is the generation and fine control of horizontal (i.e., anteroposterior and mediolateral) jaw movements. The activities of 50 single motor units (SMUs) were recorded from IHLP (14 subjects) during two- or three-step contralateral movement (n = 36) and/or protrusion (n = 33). Most recording sites were identified by computer tomography. There was a statistically significant overall increase in firing rate as the magnitude of jaw displacement increased between the holding phases (range of increments: 0.3-1.6 mm). The firing rates during the dynamic phases for each unit were significantly greater than those during the previous holding phases but less than those during the subsequent holding phases. For the contralateral step task at the intermediate rate, the cross-correlation coefficients between jaw displacement in the mediolateral axis and the mean firing rate of each unit ranged from r = 0.29 to 0.77; mean +/- SD; r = 0.49 +/- 0.13 (protrusive step task: r = 0.12-0.74, r = 0.44 +/- 0.14 for correlation with anterior-posterior axis). The correlation coefficients at the fast rate during the contralateral step task and the protrusive step task were significantly higher than those at the slow rate. The firing rate change of the SMUs per unit displacement between holding phases was significantly greater for the lower-threshold than for the higher-threshold units during contralateral movement and protrusion. After dividing IHLP into four regions, the SMUs recorded in the superior part exhibited significantly greater mean firing rate changes per unit displacement during protrusion than for the SMUs recorded in the inferior part. Significantly fewer units were related to the protrusive task in the superior-medial part. These data support previously proposed notions of functional heterogeneity within IHLP. The present findings provide further evidence for an involvement of the IHLP in the generation and fine control of horizontal jaw movements.

Effects of aging on force variability, single motor unit discharge patterns, and the structure of 10, 20, and 40 Hz EMG activity

The purpose of this investigation was to examine the discharge properties of single motor units and the structure of the rectified 10, 20, and 40 Hz electromyographic (EMG) activity to determine a physiological correlate for the greater force variability with aging. Young ( n=10; mean: 22±1 years), old ( n=10; mean: 67±2 years), and older-old ( n=10; mean: 82±5 years) adult humans produced isometric second finger abduction force in both constant and sine-wave tasks at 5, 10, 20, and 40% of their maximal voluntary contraction. Force and fine-wire intramuscular electromyography were recorded from the first dorsal interosseous muscle. The amount and time-dependent structure of the discharge rate variability of single motor units and Fourier analysis of the rectified intramuscular EMG was performed. Force output variability increased across the young, old, and older-old groups. The amount and time-dependent structure of the discharge rate variability of single motor units did not differ between the young and aging groups. There was a progressive decrease in the relative power of ∼40 Hz EMG activity from the young>old>older-old subjects across the 5, 10, 20, and 40% maximum voluntary contraction (MVC) force levels. There was also a progressive increase in the relative power of the ∼10 Hz EMG activity from young<old<older-old subjects at each target force level. The findings showed that a shift in the relative contribution of ∼40 Hz to ∼10 Hz neural activity is related to the reduced capacity of older adults to maintain optimal force control.

Functional properties of single motor units in inferior head of human lateral pterygoid muscle: task relations and thresholds.

The aim of this study was to clarify the normal function of the inferior head of the human lateral pterygoid muscle (IHLP). The hypothesis was that an important function of the IHLP is in the fine control of horizontal jaw movements. The activities of 99 single motor units (SMUs) were recorded from IHLP (22 recordings from 16 subjects). Most recording sites were identified by computer tomography (CT). All 99 SMUs were active during contralateral jaw movements with the teeth apart, and protrusive jaw movements with the teeth apart, and 81% (48 of 59 units studied during all 3 tasks) were active during submaximal jaw-opening movements. None were active on maximal ipsilateral or retrusive jaw movements with the teeth apart nor on jaw closing/clenching in intercuspal position; nor were they spontaneously active when the jaw was at the clinically determined postural jaw position. Thresholds of SMUs ranged from <0.2 mm of contralateral or protrusive horizontal displacements to 61-89% of the maximum contralateral or protrusive displacement, respectively. For the 35 units continuously active during the contralateral task, 23 (66%) were recruited within 2 mm of contralateral displacement [25 (63% of 40 units) for protrusion]. Recruitment thresholds (mm) of some of the units were rate dependent with thresholds significantly decreasing with increasing rate of horizontal jaw movement in protrusion and contralateral movements. At eight recording sites where up to six SMUs were able to be discriminated, the average thresholds of successively recruited SMUs were within a 1-mm increment of horizontal jaw displacement. After dividing IHLP into four regions, the SMUs recorded in the superior-medial zone exhibited significantly lower mean threshold values than for the SMUs recorded in the other zones (no units were recorded in the inferior-lateral zone). This provides suggestive evidence supporting previously proposed notions of functional heterogeneity within IHLP. Taken together, the data suggest that specific regions of the IHLP are capable of selective activation in a finely controlled manner to allow the application of the appropriate force vector (magnitude and direction) to effect the required condylar movement needed for the generation and control of horizontal jaw movements.

Mechanical properties of single motor units in the rabbit masseter muscle as a function of jaw position

Positions and contractile properties of rabbit masseter motor units were investigated at different jaw gapes. Twitch responses were measured at gapes ranging from dental occlusion (0 degree) to maximum opening (21 degrees), in steps of 3 degrees. The twitches were elicited by stimulating motoneurons extracellularly in the trigeminal motor nucleus. The units appeared to produce a large variety of force vectors. On average motor units in the deep parts of the masseter produced considerably less twitch force (average: 25-30 mN) than those in the superficial parts (average: 45-50 mN) and anteriorly located motor units were slower than posteriorly located units. With an increase of jaw angle, twitches became slower, reflected by an increase (30%) of the twitch contraction time. Most motor units had a parabolic-like active jaw angle-force relationship. A large variation in the shape of the curves was found. The average optimum jaw angle was reached at 12 degrees jaw opening. In general, force output was relatively low (20-60% of maximum force) at occlusion and relatively high (60-100% of maximum force) at maximal jaw opening. Anteriorly and posteriorly located motor units differed significantly in their angle-force curves. Anteriorly located motor units produced less relative force at occlusion, showed a steeper increase of force with an increase of jaw angle, reached maximum force at larger jaw angles and produced larger forces at maximum jaw opening. The larger force changes in the more anterior units are probably related to their longer distance from the axis of jaw rotation. The large variability of motor unit properties and angle-force curves suggests that a fine gradation of both force magnitude and direction is possible within the masseter and that the angle-force curve of the whole muscle or of whole muscle parts is broader than that of individual motor units. This broadening may be considered as a mechanism to sustain active muscle force throughout a large movement range.

EMG power spectrum and motor unit characteristics in the masseter muscle of the rabbit.

Masticatory muscles contain a large variety of motor units with different physiological and morphological properties. In this study, we tested the hypothesis that a relationship exists between the mechanical and myo-electric properties of single motor units in the masseter muscle of the rabbit. It was expected that faster-contracting motor units, which usually have a relatively large number of fibers with large diameters, should have faster action potentials with larger amplitudes than slower motor units. Single motor units were stimulated. A two-dimensional force transducer registered mechanical parameters of the units. EMG electrodes were used to determine amplitude and frequency parameters of the action potentials of the same units. The results showed that faster-contracting motor units indeed produced action potentials with higher conduction velocities. However, faster motor units had no significant larger amplitude of the action potential. Small but significant positive correlations were found between the tetanic peak force and the amplitude of the action potentials. Little difference was found among the various frequency and amplitude parameters, respectively, making them equally suitable to describe the action potential. Surprisingly, a negative correlation between the amplitude and frequency parameters of the action potential was found, which may result from variability in arrival times of action potentials at the electrode site. Regional differences in the frequency parameters were found between the anterior and posterior parts of the superficial masseter.

Response properties of hind limb single motor units in normal rats and after carrageenan-induced inflammation

The properties of single motor units from hind limb muscles and the changes in situations of hyperalgesia are not known in detail. We have therefore characterized the properties of single motor units in normal Wistar male rats and in rats with carrageenan-induced inflammation, under α-chloralose anaesthesia. Units were studied from three different muscles: peroneus longus, tibialis anterior and extensor digitorum longus. The properties of single motor units were not homogeneous in the three muscles studied in normal animals, showing different sizes of cutaneous receptive fields, thresholds for natural and electrical stimulation, and encoding of responses at different intensities of stimulation. Intraplantar injections of carrageenan induced a significant inflammation of the paw and a change in spontaneous behaviour observed in open field experiments. After inflammation, the responses to cutaneous stimulation of the single motor units became more homogeneous. The threshold for mechanical stimulation was lower for peroneus longus and tibialis anterior but not for extensor digitorum longus units when compared to normal animals. The receptive fields were larger when mapped with a 500 mN von Frey hair but not when mapped using a threshold intensity hair. The threshold for thermal stimulation was lower after inflammation than in normal conditions in all cases, whereas the threshold for electrical stimulation was lower in tibialis anterior and extensor digitorum longus units. An enhancement of responses related to the increase of stimulus intensity was seen in normal animals in all muscles for mechanical and electrical stimuli (but not for thermal). After inflammation, a relationship between firing rate and intensity of stimulation was seen in all cases studied. The firing of single motor units showed over 50% adaptation in the normal condition and over 75% after inflammation when stimulated for 10 s at mechanical threshold intensity. After inflammation, the rate of adaptation was significantly lower when suprathreshold intensity was used for mechanical stimulation. No differences were seen in the adaptation of units to thermal stimulation. We conclude that, in situations of hyperalgesia due to inflammation, the threshold, encoding of stimulus intensity and adaptation of single motor units from different muscles changed, resulting in a narrower range of responses and a more homogeneous population of units.

Cutaneous responsiveness of rat single motor units activated by natural stimulation

Recordings of withdrawal reflexes have been used extensively to study sensory-motor integration and processing of nociceptive information in the spinal cord. We describe here a new technique for the manufacture of improved EMG electrodes that permit the characterisation of the physiological properties of single motor units as well as the easy location of the muscles studied. Individual motor units from three rat hind-limb muscles: peroneus longus, tibialis cranialis and extensor digitorum longus, were activated by thermal and mechanical stimulation applied to their cutaneous receptive fields, which were located mainly on the 4th and 5th toes. Thresholds for thermal and mechanical (Von Frey hairs) stimulation were similar in the three muscles studied, with a value of 44±1°C and 100 mN (median), respectively. However, when a mechanical pincher with a stimulus area of 14 mm 2 was used, the values seen were similar for peroneus longus and tibialis cranialis (342±23 and 330±71 mN, respectively, mean±S.E.M.) but lower for extensor digitorum longus (220±37 mN, mean±S.E.M.). The firing rate of the single motor units was similar for all types of stimulation at threshold intensity, and showed a linear relationship with stimulus intensity, except for units of the tibialis cranialis, which showed a greater degree of adaptation.