Amplitude-related characteristics of motor unit and M-wave potentials during fatigue. A simulation study using literature data on intracellular potential changes found in vitro.

To realize possible reasons for changes in EMG amplitude characteristics with fatigue, we analyzed motor unit potentials (MUPs) and M-waves under simultaneous variations of the intracellular action potential (IAP) amplitude, duration, and shape as well as of the muscle fiber propagation velocity and desynchronization in activation of individual muscle fibers. Analysis was performed through computer simulation of MUPs and M-waves detected at different distances from active fibers in infinite anisotropic volume conductor. Changes in the IAP spike and negative after-potential were taken from in vitro experiments reported in the literature. It was shown that the amplitudes of MUP and M-wave detected simultaneously at different distances could decrease close to the active fibers, be almost unchanged at middle distances, and increase far from the fibers even under IAP amplitude decreasing. This reflected the distance-dependent effects of changes in the IAP profile along the fiber. Electrode position affected sensitivity of MUP and M-wave durations to changes in the IAP duration and propagation velocity. Thus, the signal area and RMS depended on electrode position and could change with fatigue in a way different from that of signal amplitude. The results can help to avoid misleading interpretation of EMG changes.     

Interpretation of EMG changes with fatigue: facts, pitfalls, and fallacies.

Failure to maintain the required or expected force, defined as muscle fatigue, is accompanied by changes in muscle electrical activity. Although studied for a long time, reasons for EMG changes in time and frequency domain have not been clear until now. Many authors considered that theory predicted linear relation between the characteristic frequencies and muscle fibre propagation velocity (MFPV), irrespective of the fact that spectral characteristics can drop even without any changes in MFPV, or in proportion exceeding the MFPV changes. The amplitude changes seem to be more complicated and contradictory since data on increased, almost unchanged, and decreased amplitude characteristics of the EMG, M-wave or motor unit potential (MUP) during fatigue can be found in literature. Moreover, simultaneous decrease and increase in amplitude of MUP and M-wave, detected with indwelling and surface electrodes, were referred to as paradoxical. In spite of this, EMG amplitude characteristics are predominantly used when causes for fatigue are analysed. We aimed to demonstrate theoretical grounds for pitfalls and fallacies in analysis of experimental results if changes in intracellular action potential (IAP), i.e. in peripheral factors of muscle fatigue, were not taken into consideration. We based on convolution model of potentials produced by a motor unit and detected by a point or rectangular plate electrode in a homogeneous anisotropic infinite volume conductor. Presentation of MUP in the convolution form gave us a chance to consider power spectrum (PS) of MUP as a product of two terms. The first one, PS of the input signal, represented PS of the first temporal derivative of intracellular action potential (IAP). The second term, PS of the impulse response, took into account MFPV, differences in instants of activation of each fibre, MU anatomy, and MU position in the volume conductor in respect to the detecting electrode. PS presentation through product means that not only changes in MFPV could be responsible for PS shift as is usually assumed. Changes in IAP duration and IAP after-potential magnitude, affecting the first term of the product, influence the product and thus MUP PS. Moreover, the interrelations between the two spectra and thus sensitivity of spectrum to different parameters change with MU-electrode distance because the second term depends on it. Thus, we have demonstrated that theory does not predict a linear relation between the characteristic frequencies (maximum, mean and median) and MFPV. IAP duration and after-potential magnitude are among parameters affecting MUP or M-wave PS and thus, EMG PS detected by monopolar and bipolar electrodes. Usage of single fibre action potential models instead of MUP ones can result in false dependencies of frequency characteristics. The MUP amplitude characteristics are determined not only by amplitude of IAP, but also by the length of the IAP profile and source-electrode distance. Due to the IAP profile lengthening and an increase in the negative after-potential, surface detected EMG amplitude characteristics can increase even when IAP amplitude decreases considerably during fatigue. Increase in surface detected MUP or M-wave amplitude should not be attributed to a weaker attenuation of the low-frequency components with distance. Simultaneous decrease and increase in amplitude of MUP and M-wave detected with indwelling and surface electrodes are regular, not paradoxical. Corner frequency of the high pass filter should be 0.5 or 1 Hz when muscle fatigue is analyzed. The area of MUP or M-wave normalized in respect of the amplitude of the terminal phase (that is produced during extinction of the depolarized zones at the ends of the fibres) could be useful as a fatigue index. Analysing literature data on IAP changes due to Ca(2+) increasing, we hypothesised that the ability of muscle fibres to uptake Ca(2+) back into the sarcoplasmic reticulum could be the limiting site for fatigue. If this hypothesis is valid, IAP changes are not a cause of fatigue; they are due to it.     

Simulation analysis of interference EMG during fatiguing voluntary contractions. Part II--changes in amplitude and spectral characteristics.

Capabilities of amplitude and spectral methods for information extraction from interference EMG signals were assessed through simulation and preliminary experiment. Muscle was composed of 4 types of motor units (MUs). Different hypotheses on changes in firing frequency of individual MUs, intracellular action potential (IAP) and muscle fibre propagation velocity (MFPV) during fatigue were analyzed. It was found that changes in amplitude characteristics of interference signals (root mean square, RMS, or integrated rectified value, IEMG) detected by intramuscular and surface electrodes differed. RMS and IEMG of surface detected interference signals could increase even under MU firing rate reduction and without MU synchronisation. IAP profile lengthening can affect amplitude characteristics more significantly than MU firing frequency. Thus, an increase of interference EMG amplitude is unreliable to reflect changes in the neural drive. The ratio between EMG amplitude and contraction response can hardly characterise the so-called 'neuromuscular efficiency'. The recently proposed spectral fatigue indices can be used for quantification of interference EMG signals. The indices are practically insensitive to MU firing frequency. IAP profile lengthening and decrease in MFPV enhanced the index value, while recruitment of fast fatigable MUs reduced it. Sensitivity of the indices was higher than that of indices traditionally used.     

Oxygen availability and motor unit activity in humans.

Six men were studied to determine the interrelationships among blood supply, motor unit (MU) activity and lactate concentrations during intermittent isometric contractions of the hand grip muscles. The subjects performed repeated contractions at 20% of maximal voluntary contraction (MVC) for 2 s followed by 2-s rest for 4 min with either unhindered blood circulation or arterial occlusion given between the 1st and 2nd min. The simultaneously recorded intramuscular MU spikes and surface electromyogram (EMG) data indicated that mean MU spike amplitude, firing frequency and the parameters of surface EMG power spectra (mean power frequency and root mean square amplitude) remained constant during the experiment with unhindered circulation, providing no electrophysiological signs of muscle fatigue. Significant increases in mean MU spike amplitude and frequency were, however, evident during the contractions with arterial occlusion. Similar patterns of significant changes in the surface EMG spectra parameters and venous lactate concentration were also observed, while the integrated force-time curves remained constant. These data would suggest that the metabolic state of the active muscles may have played an important role in the regulation of MU recruitment and rate coding patterns during exercise.     

Volume conduction models for surface EMG; confrontation with measurements

Volume conduction models are used to describe and explain recorded motor unit potentials (MUPs). So far it has remained unclear which factors have to be taken into account in a volume conduction model. In the present study, five different models are confronted with measured MUP distributions over the skin surface above the m. biceps brachii generated by MUs at different depths and recorded by small surface electrodes. All model simulations include fibres of finite length. The models differ in the size of the volume conductor (finite/infinite), the number of different layers (1, 2 or 3) and the conductivities of these layers (representing muscle, subcutaneous fat and skin). All measured and simulated MUPs contain a mainly negative propagating wave followed by a positive wave simultaneously present at all electrode positions. The magnitude of the different MUP components relative to each other and as a function of motor unit (MU) and electrode position differ between the models studied and the measurements. All simulated MUPs changed faster with observation distance than the measured MUPs. The three-layer model, in which muscle tissue was surrounded by a subcutaneous fat layer and by a layer of skin resulted in MUPs closest to the measured MUPs.     

Intramuscular and surface electromyogram changes during muscle fatigue

Twelve male subjects were tested to determine the effects of motor unit (MU) recruitment and firing frequency on the surface electromyogram (EMG) frequency power spectra during sustained maximal voluntary contraction (MVC) and 50% MVC of the biceps brachii muscle. Both the intramuscular MU spikes and surface EMG were recorded simultaneously and analyzed by means of a computer-aided intramuscular spike amplitude-frequency histogram and frequency power spectral analysis, respectively. Results indicated that both mean power frequency (MPF) and amplitude (rmsEMG) of the surface EMG fell significantly (P less than 0.001) together with a progressive reduction in MU spike amplitude and firing frequency during sustained MVC. During 50% MVC there was a significant decline in MPF (P less than 0.001), but this decline was accompanied by a significant increase in rmsEMG (P less than 0.001) and a progressive MU recruitment as evidenced by an increased number of MUs with relatively large spike amplitude. Our data suggest that the surface EMG amplitude could better represent the underlying MU activity during muscle fatigue and the frequency powers spectral shift may or may not reflect changes in MU recruitment and rate-coding patterns.     

Identification and characterization of functional genes encoding the mouse major urinary proteins.

Mouse Ltk- cells were stably transfected with cloned genes encoding the mouse major urinary proteins (MUPs). C57BL/6J MUP genomic clones encoding MUP 2 (BL6-25 and BL6-51), MUP 3 (BL6-11 and BL6-3), and MUP 4 (BL6-42) have been identified. In C57BL/6J mice, MUP 2 and MUP 4 are known to be synthesized in male, but not female, liver, and MUP 3 is known to be synthesized in both male and female liver and mammary gland. A BALB/c genomic clone (BJ-31) was shown to encode a MUP that is slightly more basic than MUP 2 and was previously shown to be synthesized in both male and female liver of BALB/c but not C57BL/6 mice. Comigration on two-dimensional polyacrylamide gels of the MUPs encoded by the transfecting gene provides a basis for tentative identification of the tissue specificity and mode of regulation of each gene. DNA sequence analysis of the 5' flanking region indicates that the different MUP genes are highly homologous (0.20 to 2.40% divergence) within the 879 base pairs analyzed. The most prominent differences in sequence occur within an A-rich region just 5' of the TATA box. This region (from -47 to -93) contains primarily A or C(A)N nucleotides and varies from 15 to 46 nucleotides in length in the different clones.     

Neither high-pass filtering nor mathematical differentiation of the EMG signals can considerably reduce cross-talk.

Using mathematical simulation of motor unit potentials (MUPs), detected by a point and rectangular plate electrode, we have shown that the muscle tissue does not act like a low-pass frequency filter on MUPs. Depending on the electrode type and its longitudinal position, the relative weight of the terminal phases (reflecting the excitation extinction) in MUPs and thus of high frequencies in the MUP power spectrum, increase with the MU depth. Therefore, high-pass filtering or differentiating signals detected neither monopolarly nor bipolarly could eliminate the cross-talk produced by high frequency components of MUPs from deep MUs. Such methods could be effective against the main components but not against the MUP leading edge and terminal phases. To reduce the cross-talk, position of the detecting electrodes should correspond to anatomy of muscles producing the cross-talk. Monopolar electrode should be located above the ends of the muscles. Cross-talk of the muscles located beyond the muscle of interest could be higher than that produced above the end-plate of deep muscles. On the contrary, under detection by a longitudinal bipolar electrode, the cross-talk is much smaller above the end-plate region or beyond deep muscles. The cross-talk is the greatest above the ends of the deep muscles.     

Variation in mouse major urinary protein (MUP) genes and the MUP gene products within and between inbred lines

The mouse major urinary proteins (MUPs) and the unprocessed in vitro translation products of MUP mRNA were each resolved by isoelectric focusing (IEF). The urinary MUPs showed about 15 distinct components, and the unprocessed MUPs about 20. In each case wide variation was observed in the relative intensities of individual bands. A comparison of three inbred lines (C57BL, BALB/c and JU) showed inter-line variation in the patterns both of the urinary MUPs and of the unprocessed MUPs. A series of experiments was carried out with a cloned MUP cDNA probe. All three inbred lines contain the same number (about 20) of MUP genes per haploid genome. In Southern blot analysis of genomic DNA the MUP genes displayed complex patterns which we interpret as showing variation on a common basic MUP gene sequence. For each combination of restriction enzymes tested, one size of fragment carried more than half of the total label, and this fragment was always the same in the three inbred lines. Inter-line differences were observed in the patterns of some of the less reactive fragments. MUP mRNA consists of at least two distinct species with sizes of 1 and 1.2 kb, which reacted with the probe in a label ratio of about 0.5 to 1. In the three inbred lines this ratio was essentially the same.     

Distribution of motor unit potential velocities in the biceps brachii muscle of sprinters and endurance athletes during prolonged dynamic exercises at low force levels

In surface electromyography (sEMG), the distribution of motor unit potential (MUP) velocities has been shown to reflect the proportion of faster and slower propagating MUPs. This study investigated whether the distribution of MUP velocities could distinguish between sprinters (n = 11) and endurance athletes (n = 12) in not-specifically trained muscle (biceps brachii) during prolonged dynamic exercises at low forces. sEMG was acquired during 4 min’ exercises: unloaded, 5%, 10% and 20% of maximal voluntary contraction (MVC). The features extracted from the sEMG were: the mean muscle conduction velocity – estimated using the inter-peak latency and cross-correlation methods, the within-subject skewness (expressing the proportions of faster and slower propagating MUPs) and the within-subject standard deviation of MUP velocities (SD-mup). Sprinters showed a greater proportion of faster propagating MUPs than endurance athletes. During fatigue, the SD-mup of sprinters broadened progressively, whereas that of endurance athletes did not. The findings suggest that sprinters conveyed a greater proportion of faster motor units than endurance athletes and that motor unit behavior during fatigue differed between groups. Thus, the distribution of MUP velocities enables distinction between a muscle of sprinters and endurance athletes during prolonged dynamic exercises at low forces.     

Simulation analysis of interference EMG during fatiguing voluntary contractions. Part I: What do the intramuscular spike amplitude–frequency histograms reflect?

Decline in amplitude of EMG signals and in the rate of counts of intramuscularly recorded spikes during fatigue is often attributed to a progressive reduction of the neural drive only. As a rule, alterations in intracellular action potential (IAP) are not taken into account. To test correctness of the hypothesis, the effect of various discharge frequency patterns as well as changes in IAP shape and muscle fibre propagation velocity (MFPV) on the spike amplitude-frequency histogram of intramuscular interference EMG signals were simulated and analyzed. It was assumed that muscle was composed of four types of motor units (MUs): slow-twitch fatigue resistant, fast-twitch fatigue resistant, fast intermediate, and fast fatigable. MFPV and IAP duration at initial stage before fatigue as well as their changes differed for individual MU types. Fatigability of individual MU types in normal conditions as well as in the case of ischaemic or low oxygen conditions due to restricted blood flow was also taken into account. It was found that spike amplitude-frequency histogram is poorly sensitive to MU firing frequency, while it is highly sensitive to IAP profile lengthening. It is concluded that spike amplitude-frequency analysis can hardly provide a correct measure of MU rate-coding pattern during fatigue.     

The muscle sound properties of different muscle fiber types during voluntary and electrically induced contractions.

Soundmyogram (SMG) and electromyogram signals were recorded simultaneously from the relatively fast medial gastrocnemius (MG) and slow soleus (SOL) during voluntary and electrically induced contractions. Using a spike-triggered averaging technique, the averaged elementary sound and corresponding MU spikes were also obtained from about 35 different MUs identified. The rms-SMG of MG increased as a function of force (P < 0.01). On the contrary, these values for SOL increased up to 60% MVC (P < 0.01), but decreased at 80% MVC. The relationship between the peak to peak amplitude of SMG and MU spike indicated significant positive correlations (r = 0.631 to approximately 0.657, P < 0.01). During electrical stimulation at 5 Hz, the SMG power spectral peak frequency (PF) was matched with stimulation frequency in both muscles. At higher stimulation frequencies, e.g., > 15 Hz, only in the MG was SMG-PF synchronized with stimulation frequency; the slow SOL did not show such synchronization. Our data suggest that the SMG frequency components might reflect active motor unit firing rates, and that the SMG amplitude depends upon mechanical properties of contraction, muscle fiber composition, and firing rate during voluntary and electrically induced contractions.     

Evaluating the effect of electrode location on surface EMG amplitude of the m. erector spinae p. longissimus dorsi

Variations in surface electromyography (SEMG) amplitude have been shown to be dependent on the dislocation of recording electrodes. Yet no literature is available about the effect of electrode dislocation on SEMG amplitude of the lower back muscles. In this project, the aim was to determine this effect by investigating changes in the SEMG root mean square (RMS), induced by a well-defined dislocation of the recording electrodes. Bipolar SEMG of the longissimus dorsi (LD) muscles was measured in 16 healthy subjects undertaking five functional tasks (standing, forward flexion, re-extension, unsupported sitting and arm/leg lifting), and for eight of those subjects the experiment was repeated within two weeks. Intra-class correlation coefficients (ICCs) were used to show the reliability of the RMS in relation to electrode dislocation, the repeatability of the tasks, and the test–retest reliability. Results showed that: (1) lateral dislocation causes a significant decrease (18%, p < 0.001) in RMS; (2) longitudinal dislocation does not change the RMS; and (3) the variability caused by electrode dislocation is comparable to the variability caused by repetitions of tasks or by electrode repositioning. Our conclusion is that positioning in the mediolateral direction should be exact to minimize changes in SEMG amplitude due to dislocation. However, precise longitudinal electrode positioning seems to be less critical in experimental setups which measure the SEMG of the lower back muscles.     

Nucleotide sequences of liver, lachrymal, and submaxillary gland mouse major urinary protein mRNAs: mosaic structure and construction of panels of gene-specific synthetic oligonucleotide probes.

The mouse major urinary proteins (MUPs) are encoded by a gene family of about 35 to 40 members. MUPs are synthesized in at least six secretory tissues under a variety of developmental and endocrine controls, but the identities of the individual genes expressed in each tissue have not previously been established. In this article, we present the nucleotide sequences of five MUP mRNAs which we designate MUP I through V. MUPs I, II, and III are the most abundant MUP mRNA species in the liver, and MUPs IV and V are the most abundant MUP mRNA species in the lachrymal gland and the submaxillary gland, respectively. The sequence data show that each of the five mRNAs is encoded by a distinct member of the gene family. The structures of the MUP mRNA consist of interspersed segments of variable and conserved sequences. On the basis of the sequences of the variable segments, gene-specific panels of synthetic oligonucleotide probes were prepared. The gene-specific panels were used to identify cloned genes and, as described in the accompanying paper (K. Shahan, M. Denaro, M. Gilmartin, Y. Shi, and E. Derman, Mol. Cell. Biol. 7:1947-1954, 1987), to characterize the expression of MUP genes I through V.     

The gene family for major urinary proteins: Expression in several secretory tissues of the mouse

The major urinary proteins (MUPs) of the mouse are encoded by a multigene family located at the Mup a locus on chromosome 4. Previous investigations have shown that the MUPs are synthesized in the liver, secreted and then excreted in the urine. We have found significant levels of MUP mRNA in several secretory tissues: the liver and the submaxillary, lachrymal and mammary glands. There are striking differences in hormonal and developmental regulation of MUP gene expression in these tissues. Furthermore, each tissue appears to express a characteristic pattern of MUP mRNAs. In particular, the lachrymal glands appear to express an entirely different set of MUP mRNAs. These results are discussed in relation to the organization of the MUP gene cluster and a possible function of the MUPs.     

Relation between electromyogram and force in fatigue

The relationship between the surface electromyogram (SEMG) and force was examined during maximal voluntary contraction (MVC). Isometric MVC of elbow flexors were studied in 18 subjects who performed 27 trials, each consisting of six MVCs lasting 45 s at intervals of 30 s. There was a decrease in the median frequency (Fm) of the SEMG and of the compound action potentials (CAP) during MVC. The CAPs demonstrated that the fall in Fm was associated with a proportional increase in signal power, whereas CAP amplitude did not decrease, indicating intact neuromuscular transmission. The SEMG root-mean-square amplitude remained fairly constant, progressively deviating from force with time of contraction (r = 0.40). When SEMG amplitude was corrected for the Fm change, it tracked force more closely (r = 0.68), indicating a fall in motoneuron drive during MVC. The corrected SEMG was used to calculate the change in the generalized firing rate of motoneurons. The firing rate decreased 60% in the first and sixth contractions, tracked force closely, and corresponded to the firing rate fall seen in late adaptation of motoneurons (r = 0.90, P less than 0.001).     

Motor unit activity and surface electromyogram power spectrum during increasing force of contraction

Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71 +/- 31.3 to 505 +/- 188 microV, p less than 0.01) and mean power frequency (89 +/- 13.3 to 123 +/- 23.5 Hz, p less than 0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike amplitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412 +/- 79 to 972 +/- 117 microV, p less than 0.01) and mean firing frequency (17.8 +/- 5.4 to 24.7 +/- 4.1 Hz, p less than 0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.     

Spatial distribution of surface action potentials generated by individual motor units in the human biceps brachii muscle

This study analyses the spatial distribution of individual motor unit potentials (MUPs) over the skin surface and the influence of motor unit depth and recording configuration on this distribution. Multichannel surface (13 × 5 electrode grid) and intramuscular (wire electrodes inserted with needles of lengths 15 and 25 mm) electromyographic (EMG) signals were concurrently recorded with monopolar derivations from the biceps brachii muscle of 10 healthy subjects during 60-s isometric contractions at 20% of the maximum torque. Multichannel monopolar MUPs of the target motor unit were obtained by spike-triggered averaging of the surface EMG. Amplitude and frequency characteristics of monopolar and bipolar MUPs were calculated for locations along the fibers’ direction (longitudinal), and along the direction perpendicular (transverse) to the fibers. In the longitudinal direction, monopolar and bipolar MUPs exhibited marked amplitude changes that extended for 16–32 mm and 16–24 mm over the innervation and tendon zones, respectively. The variation of monopolar and bipolar MUP characteristics was not symmetrical about the innervation zone. Motor unit depth had a considerable influence on the relative longitudinal variation of amplitude for monopolar MUPs, but not for bipolar MUPs. The transverse extension of bipolar MUPs ranged between 24 and 32 mm, whereas that of monopolar MUPs ranged between 72 and 96 mm. The mean power spectral frequency of surface MUPs was highly dependent on the transverse electrode location but not on depth. This study provides a basis for the interpretation of the contribution of individual motor units to the interference surface EMG signal.     

Pheromone binding by polymorphic mouse major urinary proteins

Mouse major urinary proteins (MUPs) have been proposed to play a role in regulating the release and capture of pheromones. Here, we report affinity measurements of five recombinant urinary MUP isoforms (MUPs-I, II, VII, VIII, and IX) and one recombinant nasal isoform (MUP-IV) for each of three pheromonal ligands, (+/-)-2-sec-butyl-4,5-dihydrothiazole (SBT), 6-hydroxy-6-methyl-3-heptanone (HMH), and (+/-)dehydro-exo-brevicomin (DHB). Dissociation constants for all MUP-pheromone pairs were determined by isothermal titration calorimetry, and data for SBT were corroborated by measurements of intrinsic protein fluorescence. We also report the isolation of MUP-IV protein from mouse nasal extracts, in which MUP-IV mRNA has been observed previously. The affinity of each MUP isoform for SBT (K(d) approximately 0.04 to 0.9 micro M) is higher than that for DHB (K(d) approximately 26 to 58 micro M), which in turn is higher than that for HMH (K(d) approximately 50 to 200 micro M). Isoforms I, II, VIII, and IX show very similar affinities for each of the ligands. MUP-VII has approximately twofold higher affinity for SBT but approximately twofold lower affinity for the other pheromones, whereas MUP-IV has approximately 23-fold higher affinity for SBT and approximately fourfold lower affinity for the other pheromones. The variations in ligand affinities of the MUP isoforms are consistent with structural differences in the binding cavities of the isoforms. The data indicate that the concentrations of available pheromones in urine may be influenced by changes in the expression levels of urinary MUPs or the excretion levels of other MUP ligands. The variation in pheromone affinities of the urinary MUP isoforms provides only limited support for the proposal that MUP heterogeneity plays a role in regulating profiles of available pheromones. However, the binding data support the proposed role of nasal MUPs in sequestering pheromones and possibly transporting them to their receptors.