Recruitment Order Among Motoneurons From Different Motor Nuclei

Alan J. Sokoloff, Sondra G. Siegel, and Timothy C. Cope

Department of Physiology, Emory University, Atlanta, Georgia 30322


    ABSTRACT
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Sokoloff, Alan J., Sondra G. Siegel, and Timothy C. Cope. Recruitment order among motoneurons from different motor nuclei. The principles by which motoneurons (MNs) innervating different multiple muscles are organized into activity are not known. Here we test the hypothesis that coactivated MNs belonging to different muscles in the decerebrate cat are recruited in accordance with the size principle, i.e., that MNs with slow conduction velocity (CV) are recruited before MNs with higher CV. We studied MN recruitment in two muscle pairs, the lateral gastrocnemius (LG) and medial gastrocnemius (MG) muscles, and the MG and posterior biceps femoris (PBF) muscles because these pairs are coactivated reliably in stretch and cutaneous reflexes, respectively. For 29/34 MG-LG pairs of MNs, the MN with lower CV was recruited first either in all trials (548/548 trials for 22 pairs) or in most trials (225/246 trials for 7 pairs), whether the MG or the LG MN in a pair was recruited first. Intertrial variability in the force thresholds of MG and LG MNs recruited by stretch was relatively low (coefficient of variation = 18% on average). Finally, punctate stimulation of the skin over the heel recruited 4/4 pairs of MG-LG MNs in order by CV. By all of these measures, recruitment order is as consistent among MNs from these two ankle muscles as it is for MNs supplying the MG muscle alone. For MG-PBF pairings, the MN with lower CV was recruited first in the majority of trials for 13/24 pairs and in reverse order for 9/24 pairs. The recruitment sequence of coactive MNs supplying the MG and PBF muscles was, therefore, random with respect to axonal conduction velocity and not organized as predicted by the size principle. Taken together, these findings demonstrate for the first time, that the size principle can extend beyond the boundaries of a single muscle but does not coordinate all coactive muscles in a limb.


    INTRODUCTION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The explanation of how the CNS controls posture and movement requires an understanding of how the activation of motoneurons (MNs) is organized during these tasks. Within muscles, MNs are activated in an orderly sequence: the last MNs recruited into activity are preceded by MNs that have slower conduction velocities and that connect with muscle fibers that contract with lower force, slower contraction time, and lower susceptibility to fatigue (for review, see Calancie and Bawa 1990; Henneman and Mendell 1981; see also Cope and Clark 1991). This orderly recruitment sequence of MNs and their muscle fibers, i.e., motor units, commonly is referred to as the size principle (Calancie and Bawa 1990; Cope and Pinter 1995; Henneman and Mendell 1981).

Recruitment by the size principle has been documented extensively within skeletal muscles (see reviews by Calancie and Bawa 1990; Henneman and Mendell 1981). In contrast, order among MNs supplying different muscles has not been examined directly to our knowledge. Indirect support for operation of the size principle across muscles comes from the seminal experiments of Elwood Henneman. In 1965, Henneman and coworkers observed a strong tendency for motor axons in a single ventral root filament in the decerebrate cat to be recruited in order according to spike amplitude (Henneman et al. 1965a,b). Because the recruitment stimuli used in these studies typically activate multiple muscles, it is possible that the sample of motor axons exhibiting order supplied different muscles. Definitive conclusion about order across muscles cannot be reached, however, because the identities of motor axons were not established in these studies.

Wyman, Waldron, and Wachtel (1974) recognized that the experimental findings just described were consistent with orderly recruitment among different possible groups of MNs. They used the term "motor pool" to define a functionally coordinated group of MNs recruited in accordance with the size principle, and suggested that the motor pool might consist of "all the motoneurons innervating a single muscle, all the motoneurons in a given ventral root (probably subdivided into flexor and extensor groups), or all the motoneurons activated by a given stimulus." Either of the latter two schemes might result in orderly recruitment across motor nuclei. Wyman et al. (1974), however, did not test these possibilities, and in the ensuing 25 yr, this issue has not been investigated further.

Expansion of the size principle across muscles could have some important benefits that can be appreciated by recognizing that tens to hundreds of MNs are selected into activity from multiple muscles in virtually all movements and postures. Because muscles and their motor units have diverse mechanical actions, the smooth and predictable trajectory of limb movement would seem to require coordination of the sequence in which MNs are recruited. Recruitment by the size principle could meet this requirement. Additionally, this scheme would simplify the task of coordinating large numbers of MNs by relying on differences in the intrinsic excitability of MNs to rank order MN firing sequence. In this way, the group of MNs selected into activity would self-organize.

Here we directly test the hypothesis that MNs selected into activity from different muscles can be recruited in accordance with the size principle. Also tested is the hypothesis that all MNs brought into activity from different motor nuclei in a discrete movement or reflex necessarily are recruited in order by the size principle (cf. Bawa and Calancie 1989; Riek and Bawa 1992). These tests require that the MNs under study supply muscles that can be coactivated. This condition was met for pairs of muscles activated reflexively in decerebrate cats. The medial and lateral gastrocnemius muscles (MG and LG, respectively) are coactivated by muscle stretch applied through the Achilles tendon (e.g., Nichols 1989), and in a putative nociceptive reflex (see Sokoloff and Cope 1996). The posterior biceps femoris muscle (PBF) is activated together with MG in the cutaneous reflex elicited by stimulation of the caudal cutaneous sural nerve (see Siegel et al. 1999). Studies reported here of MNs recruited from these muscles demonstrate that recruitment order by the size principle can cross muscle boundaries but that this order does not necessarily encompass the entire set of MNs recruited in a discrete motor reflex.

Preliminary results of this work were reported in abstract form (Sokoloff and Cope 1995).


    METHODS
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INTRODUCTION
METHODS
RESULTS
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Experimental preparation

Thirteen cats of either sex ranging in weight from 2.5-3.5 kg were studied in acute experiments similar to those detailed in earlier reports from this laboratory (Cope and Clark 1991; Cope et al. 1997). Anesthesia was induced and maintained with a gaseous mixture of 1.5-3% halothane in O2 and NO2 (1:1). Surgery proceeded when withdrawal and eye-blink reflexes were suppressed completely. A cannula was inserted into the trachea and connected to a respirator for delivery of gases throughout the surgery described in the following text. The rate and volume of artificial respiration were adjusted to maintain end-tidal CO2 between 3.5 and 4%. Cannulae were secured in the right common carotid artery to measure blood pressure and in the right external jugular vein for infusion of dextran (10%) in Ringer solution and, on occasion, a sympathomimetic amine (Aramine) as needed to support mean blood pressure >60 mmHg. The left common carotid was ligated in preparation for decerebration (see below). Body temperature was monitored continuously and held at 36-38°C by radiant heat. At the end of the experiment, cats were killed by barbiturate overdose [pentobarbital sodium (Nembutal): 150 mg/kg].

Spinal roots L5-S1 were exposed by laminectomy and by opening the overlying dura mater. An incision was made in the left posterior hindlimb. In one set of experiments (n = 7), the MG and LG muscles were freed from surrounding tissues but the continuity between these muscles was not disturbed. The common MG/LG tendon was severed at the insertion on the calcaneus and tied with Dacron cord for later attachment to a force transducer. The nerves supplying the MG, LG, and soleus were dissected for later placement, in continuity, on bipolar electrodes. In a second set of experiments (n = 6), the nerves to the MG and the PBF were dissected for later placement, in continuity, on bipolar electrodes. The cat was secured in a rigid recording frame with the left hindlimb clamped at the distal ends of the femur and the tibia. Exposed tissues were bathed in warm-oil pools (36-38°C). Cats were decerebrated by removal of brain structures rostral to intercollicular transection of the brain stem after which anesthetic was discontinued.

Data acquisition

The axons of two individual MNs, either one LG and one MG MN, one MG and one PBF MN, or two PBF MNs, were penetrated simultaneously in S1 or L7 ventral rootlets each axon by a glass micropipette (2 M K-acetate; 10-25 MOmega ). MNs were identified by antidromic action potentials produced when stimulating their respective peripheral nerves. Because the nerve to the LG contains axons projecting to the soleus, in the LG/MG experiments the soleus muscle nerve also was stimulated to allow conclusive discrimination of LG and soleus MNs. The delay and distance traveled by the antidromic action potentials from stimulus to recording sites were used to calculate axonal conduction velocity (CV) for each MN (see Cope and Clark 1991). Because of the variability in repeated measures of conduction delay (<= 25 µs), we adopted the criterion that the difference in CV must exceed 2 m/s for MNs in a pair to be reliably ranked by CV. Motoneuron pairs not meeting this criterion are excluded from this report. We relied on axonal conduction velocity to test recruitment order because under the conditions of this study, i.e., MN recruitment in stretch and cutaneous reflexes in decerebrate cats, CV is as good as other parameters for testing the size principle (Cope and Clark 1991; see also Bawa et al. 1984). MN action potentials were digitized and collected on videotape and computer for later analysis.

LG-MG MN pairs

To study LG and MG behavior, MNs were recruited primarily during the reflex evoked by the combined stretch of the LG and MG muscles. The combined tendon of insertion of these muscles (the Achilles tendon with the soleus tendon removed) was attached to a strain-gauge transducer (a semiconductor strain gauge attached to a U-shaped aluminum beam) connected in-series with a feedback-controlled servomotor. This arrangement allowed us to apply constant ramp-hold-release stretches (100 ms each for the ramp and release phases and 300 ms for the hold phase) of the LG and MG muscles while simultaneously recording muscle force (bandwidth DC-5 kHz; ADC sampling at 2.5 kHz). We are confident that this sampling rate did not introduce aliasing because the upper frequency band in the force power spectra (250 Hz) was well below the Nyquist limit of 1.25 kHz (Lynn 1989). The initial muscle length and the ramp-stretch amplitude (4, 6, or 8 mm) were set at levels that consistently recruited action potentials in the penetrated motor axons during the ramp phase of muscle stretch. The speeds and amplitudes of triceps surae stretch were adopted from those used by Nichols (1989) to mimic length changes experienced by the triceps surae during locomotion in the cat. Motoneuron recruitment was scored when at least one action potential in each MN was elicited during the ramp phase of muscle stretch. The reproducibility of recruitment order was checked for each MN pair in replicate trials of muscle stretch repeated once every 4 s. Some MNs also were recruited in reflex contractions elicited in the LG and MG muscles by penetrating the skin over the heel pad with the fine points of jeweler's forceps (see Sokoloff and Cope 1996).

Recruitment order among pairs of LG-MG motor axons was studied in two ways. The primary method was to determine which MN in a pair was the first to fire an action potential during the ramp phase of muscle stretch (differences in firing onset were reliably distinguishable within 250 µs). Recruitment by the size principle was supported when the MN with the lower CV fired before the MN with the higher CV. Order was measured only when both motoneurons in the pair were recruited because order can be determined unequivocally only when this criterion is met (see Cope and Pinter 1995). The advantage of this pair-wise analysis is that recruitment sequence is determined unequivocally and with no underlying assumptions. A disadvantage is that each MN's recruitment sequence is specified in relation to only one other MN. For this reason, we also measured MN force threshold, i.e., the muscle force recorded at the time when the MN is recruited into activity. Force threshold has been used to specify one MN's recruitment in relation to the population of all previously recruited MNs (e.g., see Henneman et al. 1965a). Recognizing that muscle force represents the population of recruited MNs only indirectly and probably inaccurately to some extent (see Cope et al. 1997), we used force threshold merely to obtain a rough estimate of the stability in recruitment order of one MN (LG or MG) in relation to all other active LG and MG MNs. For each of nine LG and nine MG MNs, we measured the coefficient of variation in force threshold expressed as a percentage [(SD/mean force threshold) × 100] over repeated recruitment trials (>20). To minimize contributions to force threshold variation from sources other than the recruitment process itself, trials were selected for low variation (<20% coefficient of variation in base and peak forces) in the muscle responses to stretch (see Cope et al. 1997).

MG-PBF and PBF-PBF MN pairs

These pairs were recruited during reflex contraction elicited by stimulating the ipsilateral caudal cutaneous sural nerve (100 pulses/second for 2 s). Stimulus strength was adjusted to levels that consistently recruited both motor axons. The stimulus strength was not the same for different axon pairs but was similar for all recruitment trials for a given pair. Intermittent tests using widely different stimulus strengths (up to 10-fold range) revealed no dependency of recruitment order on stimulus strength. Moreover, when tested by pinching the skin overlying the ipsilateral lateral malleolus (dermatome of the caudal cutaneous sural nerve), recruitment order was invariably the same as that produced by electrical stimulation of the nerve. To allow for stable MN recordings during sural stimulation, Pancuronium (0.1 mg/ml iv) was administered as needed to maintain paralysis. Because of the paralysis, muscle force, and thus MN force threshold, were not determined.

Six or more stimulation trials, with intertrial intervals of >= 1 min were conducted for each muscle pair. MG and PBF MNs typically fired repetitively in response to sural stimulation. Recruitment order was judged in PBF-PBF MN pairs and in MG-PBF MN pairs only when the activity of the two MNs of a pair overlapped. In two pairs, sural nerve stimulation produced alternating activity between the MG MN and the PBF MN. Because recruitment order by the size principle can be determined only when both MNs are active together, these two cases were not included in our analysis. Alternating activity was never observed among PBF-PBF pairs.


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Recruitment order among MG-LG motoneurons

Thirty-four MN pairs, each composed of one LG and one MG MN, were studied in replicate trials of stretch of the combined LG and MG muscles. Stretch recruited both of the MNs in >= 4 and <= 66 stretch trials. Axonal conduction velocity for MNs included in these 34 pairings (each MN was used in <= 3 pairings) ranged from 68 to 131 m/s for LG MNs [n = 32; 101 ± (SD) 18 m/s] and from 71 to 130 m/s for MG MNs (n = 29; 105 ± 15 m/s), ranges similar to those reported for MG MNs in previous studies (see Emonet-Denand et al. 1988; McDonagh et al. 1980).

In 22 of the 34 pairs, the order of recruitment was unchanged in all (548/548) stretch trials. Figure 1 shows that these MNs always were recruited in order by increasing conduction velocity, whether the MN recruited first was LG (14 pairs, open circle ) or MG (8 pairs, ). Thus recruitment by muscle stretch was as orderly among MG and LG motor axons as it is among MG motor axons (Table 1).



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Fig. 1. Axonal conduction velocity (CV) of later vs. earlier recruited unit from coactive pairs of 1 lateral gastrocnemius (LG) and 1 medial gastrocnemius (MG) motoneuron (MN). Data shown for 22 pairs in which both MNs were active and order was the same for all (>4) trials. MN with the slower CV recruited before the MN with the faster CV as predicted by the size principle for all 22 pairs, whether the first recruited MN was LG (open circle ) or MG (). ---, line of identity; - - -, limits of measurement uncertainty.


                              
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Table 1. Recruitment ordering for different motoneuron pairs

Some intertrial variation in recruitment order was observed for the remaining 12/34 pairs (Fig. 2). In 7 of these 12 pairs, recruitment was ordered predominantly from the MN with the slowest to fastest conduction velocity (225/246 stretch trials). In four pairs, the MN with the fastest conduction velocity was the first recruited in most trials (95/111 stretch trials). In one pair, recruitment order was equivocal (slowest CV recruited first in 19/37 stretch trials). Overall, recruitment order was predominantly from low to high CV for 29/34 pairs of motor axons or 85% of the sample.



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Fig. 2. Recruitment order switched from trial to trial for 12/34 pairs of coactive LG/MG MNs. MN pairs are ranked from the pair with the lowest (MN Pair 1) to the pair with the highest (MN Pair 12) number of trials in which the predominant recruitment order was opposite to that predicted by the size principle, i.e., MN with high CV recruited before MN with low CV. All but pair 9 exhibit a clear tendency in recruitment order, either low before high CV in most trials (pairs 1-7; ) or high before low CV in most trials (pairs 8, 10-12; ). Numbers next to each bar (e.g., *) list the CVs of MNs in the order in which the MNs were most commonly recruited.

Motoneurons recruited by mechanical stimulation of heel

Mechanical stimulation of the skin over the heel produces reflex excitation of LG and MG muscles (see Sokoloff and Cope 1996). The strong extensor response to this stimulus often displaced the microelectrodes from the axon(s) under study, thereby precluding reliable determination of recruitment order by this stimulus. However, order was determined for four pairs of MNs recruited in four or more stimulation trials. These pairs are reported to allow evaluation of recruitment order across the LG and MG during an additional natural stimulus. In all trials of these pairs (27/27), the MN with the slowest CV was recruited first. This was true regardless of which MN was the first recruited of the pair (MG MN first in 3 of the 4 pairs).

Motoneuron force threshold

We measured intertrial variability in force thresholds for nine LG MNs and nine MG MNs. At least 20 stretch trials were collected for each MN (see Cope et al. 1997). The samples of LG and MG MNs were similar in measures of CV (for LG, range 71-110 m/s, mean = 92 m/s; MG range 68-111 m/s, mean = 91 m/s) and included 13 units used in the pair-wise analysis. Values for the coefficient of variation in force threshold ranged from 1.3 to 37.3%. For all 18 MNs, the mean and median values were 18 ± 9 and 19%, respectively. In comparison, the coefficients of variation obtained under similar conditions for 29 MG MNs in relation to the force of contraction produced by the MG muscle alone (Cope et al. 1997) were distributed with range = 2.3-82%, mean = 18 ± 18% (SD), and median = 10%. Thus force threshold variation was comparable whether measured from one MN in relation to its parent muscle or in relation to multiple muscles.

Recruitment order among MG-PBF motoneurons

Twenty-four pairs of MG-PBF MNs were each studied in six or more trials of sural nerve stimulation. Conduction velocity measures for MG and PBF MNs were similar, ranging from 78 to 117 m/s for MG MNs (n = 24; 92 ± 10 m/s) and from 78 to 108 m/s for PBF MNs (n = 24; 92 ± 8 m/s). In 14/24 pairs, the order of recruitment was unchanged in 108/108 trials. Figure 3 shows that, in contrast to the results for LG-MG pairs, only half of the 14 MG-PBF pairs were recruited in accordance with the size principle. For seven pairs (55/55 stretch trials) MNs always were recruited in order by increasing CV, whether the MN recruited first was MG (3 pairs,  in Fig. 3) or PBF (4 pairs, open circle ). For seven pairs (53/53 stretch trials), the MN with the highest CV always was recruited first, whether the MN recruited first was MG (3 pairs,  in Fig. 3) or PBF (4 pairs, open circle ). The proportion of pairs exhibiting recruitment order from low to high CV was not statistically distinguishable from random (Table 1).



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Fig. 3. Axonal CV of later vs. earlier recruited unit from coactive pairs of 1 MG and 1 posterior biceps femoris (PBF) MN. Data shown for 14 MG-PBF pairs in which both MNs were active and order was the same for all (>6) trials. MN with the slower CV recruited before the MN with the faster CV for 7 pairs, as predicted by the size principle. MN with the faster CV recruited before the MN with the slower CV for 7 pairs, i.e., order reversed to that predicted by the size principle. , 1st recruited MN in pair is MG. open circle , 1st recruited MN in pair is PBF. ---, line of identity; - - -, limits of measurement uncertainty.

For the remaining 10/24 pairs, recruitment order varied across trials in proportions illustrated in Fig. 4. In 6 of these 10 pairs, recruitment was ordered predominantly from the MN with the slowest to fastest CV (slowest CV recruited first in 46/56 trials). In two pairs, the MN with the fastest CV was the first recruited in most trials (fastest CV recruited 1st in 11/16 trials). And in two pairs, recruitment was equivocal (slowest CV recruited 1st in 8/16 trials). For the sample as a whole, recruitment order from low to high CV in all or most stretch trials was observed in only 13/24 pairs of MG/PBF MNs, i.e., 54% of the sample.



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Fig. 4. Recruitment order switched from trial to trial for 10/24 pairs of coactive MG-PBF MNs. MN pairs are ranked from the pair with the lowest (MN Pair 1) to the pair with the highest (MN Pair 10) number of trials in which the predominant recruitment order was opposite to that predicted by the size principle, i.e., MN with high CV recruited before MN with low CV. All but pairs 7 and 8 exhibit a clear tendency in recruitment order, either low before high CV in most trials (pairs 1-6; ) or high before low CV in most trials (pairs 9, 10; ). Numbers next to each bar (e.g., *) list the CVs of MNs in the order in which the MNs were most commonly recruited.

Recruitment order among PBF-PBF motoneuron pairs

Table 1 presents data taken from earlier studies demonstrating that the size principle holds for MG MNs recruited in sural reflexes. The lack of recruitment according to the size principle among MG-PBF pairs might have resulted if recruitment is not ordered among MG or PBF MNs. To test this possibility among PBF MNs, 27 MN pairs were each studied in six or more trials of sural nerve stimulation. Conduction velocity measures of PBF MNs ranged from 72 to 115 m/s (93 ± 11m/s). In 22/27 pairs, the order of recruitment was unchanged across trials (179/179 stretch trials). For 19 of these pairs, the first recruited MN had the lowest CV; in 3 of these pairs the first recruited unit had the highest CV (Table 1 and Fig. 5). For the remaining five pairs, the lowest CV MN was recruited first in 8/9, 8/9, 7/8, 4/12, and 1/7 trials. In sum, recruitment order from low to high CV was observed in all or in a majority of trials for 22/27 (82%) of this sample of PBF MN pairs.



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Fig. 5. Axonal CV of later vs. earlier recruited unit from coactive pairs of PBF MNs. Data shown for 22 PBF-PBF pairs in which both MNs were active and order was the same for all (>6) trials. MN with the slower CV recruited before the MN with the faster CV as predicted by the size principle for 19 of 22 pairs. ---, line of identity; - - -, limits of measurement uncertainty.


    DISCUSSION
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INTRODUCTION
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DISCUSSION
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In this study, we tested the hypothesis that motoneurons supplying different coactive muscles can be recruited collectively in order by the size principle. To this end, hindlimb muscles studied in pairs were coactivated reflexively in either muscle stretch or cutaneous reflexes in decerebrate cats. Our findings provide the first direct support of the hypothesis. However, we also found that the extent of this order is limited and not expressed in the complete set of all motoneurons concurrently recruited in a discrete motor reflex. Thus recruitment order by the size principle can but need not cross muscle boundaries.

Recruitment order by the size principle across muscles

LG and MG muscles can be coactivated in muscle stretch reflexes elicited by stretch of their common Achilles tendon (see Nichols 1989). The reflex response of each muscle is driven by stretch activation of muscle spindle afferents from both muscles, i.e., by both homonymous and heteronymous afferents (Eccles et al. 1962; Nichols 1989; Scott and Mendell 1976). In this combined stretch reflex, we found that MG and LG MNs sampled in pairs were most often recruited in order from low to high CV regardless of whether the MG or the LG MN in a pair was recruited first. Recruitment order by the size principle also was observed for a small subset of this sample of LG-MG MN pairs recruited by a putative nociceptive reflex.

This orderly behavior found among MNs recruited from the LG and MG motor nuclei is similar to that found among MNs recruited from one of these motor nuclei, the MG. In the cat MG, MN recruitment in order of CV was observed in 83% of MN pairs during MG muscle stretch (Cope and Clark 1991) and 76% of MG MNs recruited by brain stem stimulation (Tansey and Botterman 1996). Bawa et al. (1984) found 97% of MNs recruited in order by CV during triceps surae stretch after excluding cases of equivocal order. We are unaware of any demonstration that LG MNs are recruited in order by the size principle, but this can be inferred from the order that we find among MG MNs and among MG/LG MNs.

The simplest mechanistic explanation for the recruitment order observed across motor nuclei is that MG and LG MNs are collectively rank-ordered by their intrinsic excitability in relation to axonal CV and that they share similar amounts of stretch-evoked synaptic input. Supporting this notion is first, the observation that the ranges in rheobase current, a measure of intrinsic excitability, overlap for MG and LG MNs (cf. Fleshman et al. 1981 with Vanden Noven et al. 1986), as do the ranges in CV (see RESULTS). Second, the strength of synaptic input from Ia afferents of LG and MG combined (Eccles et al. 1957a) is about the same onto MG motoneurons (7.3 mV) as it is onto LG motoneurons (6.0 mV). Taken together, the intermingling of MG and LG MNs in rank order by excitability and their receipt of common synaptic input, from muscle stretch, nociceptive, and possibly other inputs, lead us to expect orderly recruitment across these motor nuclei. This expectation is strengthened even further by recent computer simulation which shows that the distribution of Ia synaptic current, at least from homonymous Ia afferents onto MG motoneurons, promotes orderly recruitment beyond that generated by intrinsic motoneuron properties alone (Heckman and Binder 1993). These considerations suggest that shared Ia synaptic input may be important for orderly recruitment by the size principle among motoneurons from different motor nuclei. If this is the case, then demonstrations of shared Ia synaptic input among a variety of motor nuclei supplying the cat hindlimb (Eccles et al. 1962; see also Nichols et al. 1999) lead us to predict additional instances of orderly recruitment across motor nuclei, e.g., between peronei longus and brevis. We also would anticipate our finding that MG and PBF motoneurons are not recruited in order (see following text) because these motor nuclei do not share Ia synaptic feedback (Eccles et al. 1962).

Functional benefits of orderly recruitment across motor nuclei

The direction of joint movement or torque depends on the relative activity of musculature (whole muscles, portions of muscles, and motor units) with different mechanical actions. For example, the triceps surae of cat produces different torque directions at the ankle in response to the differential activation of these muscles in ipsilateral versus contralateral cutaneous reflexes (Nichols et al. 1993). The recruitment order of LG and MG MNs in relation to one another (as well as with MNs supplying other active muscles) becomes an important factor, therefore, in determining the moment-to-moment direction of ankle movement (see Cope and Sokoloff 1998). One way of coordinating these MNs would be through the orderly recruitment of the set of all active MNs combined, as shown in this report for LG and MG MNs. This recruitment scheme would ensure reproducibility and, therefore, predictability in the direction of ankle torque. In addition, ordering the recruitment of motor units from muscles producing disparate torque directions would have the effect of smoothing limb trajectory. It remains to be determined whether the set of active triceps surae motor units are recruited in order by the size principle under all conditions, irrespective of joint torque direction. Sequencing the recruitment of different sets of active motor units by the size principle would not constrain joint direction, which still could be changed dependent on the set of motor units selected into activity from different muscles.

The goal of coordinating the recruitment sequence of MNs supplying different muscles might be achieved in a variety of ways. In one case, MNs supplying each muscle could be recruited in order by the size principle, and some additional neural mechanism could set the time at which the recruitment sequence begins for each muscle. This additional mechanism would not be required if, instead, recruitment among MNs from different muscles was determined predominantly by intrinsic properties of the MNs themselves. Differences in the intrinsic properties of MNs act to distribute varying levels of excitability among MNs (see Pinter 1990), such that uniform synaptic input to these MNs results, in computer simulation (Heckman and Binder 1993), in an orderly sequencing of recruitment, with the most excitable MNs firing first. On this backdrop of distributed MN excitability, MNs selected into activity from various motor nuclei by a common source of synaptic excitation would be expected to fire in a sequence established automatically by MN excitability. This selection across nuclei might be achieved by any of a variety of sources of divergent synaptic drive, whether descending (e.g., cortico-spinal and rubro-spinal projections) (see for example, Araki et al. 1976; Hongo et al. 1969) or segmental (e.g., muscle and cutaneous afferents) (see for example, Eccles et al. 1957b; LaBella et al. 1989).

Recruitment disorder among coactive pairs of motoneurons

To test whether coactivity is a condition sufficient to ensure orderly recruitment, as hypothesized first by Bawa and coworkers (Bawa and Calancie 1989; Riek and Bawa 1992) and later by Cope and Sokoloff (1998), we examined a second pair of muscles. MG and PBF were selected for study following our observation (Siegel et al. 1999) that these muscles are strongly coactivated by sural nerve stimulation in decerebrate cats. In contrast to LG-MG MN pairs, we found that MG-PBF MN pairs were not recruited in accordance with the size principle: the prediction of recruitment order by axonal CV was no better than by random selection (Table 1).

The recruitment disorder observed among MG/PBF MN pairs was not explainable by differences in the peripheral stimulus because each MN in a pair was recruited by the same stimulus and within the same stimulus trials. Neither did disorder across muscles result from disorder within these muscles (Table 1) because sural stimulation recruited pairs of MG MNs in order by the size principle (Cope and Clark 1991; see also Clark et al. 1993; Haftel et al. 1997) just as it recruited pairs of PBF MNs in order by the size principle (Fig. 5). Other possible explanations are more likely. One possibility is that the MNs themselves differ. Recruitment disorder across nuclei could arise if the threshold differences, i.e., threshold spacing (Bakels and Kernell 1994), between MNs is not the same for the MG and PBF motor nuclei. Disorder also would result for MG and PBF MNs combined if the relationship between intrinsic excitability and axonal CV is not the same for MNs in each motor nucleus.

Other explanations for disorderly recruitment could be accountable to differences in synaptic input to MG and PBF MNs originating either from sural nerve or other sources. Studies of synaptic potentials indicate that input to MG and PBF MNs can arrive via different polysynaptic pathways. For example, low-frequency sural nerve stimulation produces primarily excitatory postsynaptic potentials (EPSPs) in PBF-semitendinosus MNs (Schomburg and Steffens 1986) but EPSPs, inhibitory postsynaptic potentials (IPSPs), or mixed EPSPs/IPSPs in MG MNs (Burke et al. 1970; Pinter et al. 1982; Powers and Binder 1985). However, other studies indicate that these differences in synaptic potentials may depend on either the method of anesthesia (see LaBella et al. 1989) or the frequency of presynaptic stimulation (Heckman et al. 1992). Thus although we can conclude from these studies that different sural pathways to MG and PBF MNs exist, we cannot be sure that they were in operation during our experiments. It remains a formal possibility, however, that different synaptic pathways from sural primary afferents to these two motor nuclei were in operation in our experiments and introduced differences in synaptic delay or recruitment gain (Kernell and Hultborn 1990) that perturbed recruitment order across these nuclei. Another factor to consider is the synaptic input to MG and PBF MNs from sources other than the sural nerve. Gossard et al. (1994) suggest that synaptic noise produces random fluctuations in MN excitability, and if differentially distributed to motor nuclei, this noise could disrupt recruitment order among the combined set of MNs as we observed here.

Proposed organizational scheme for motor unit recruitment

Together with earlier descriptions of MN recruitment, the findings of this study lead us to propose the following scheme for the functional organization of a group of MNs engaged in a discrete motor activity. We refer to this group as an ensemble, in which members act together to produce a specific motor effect. The principles involved in selecting members of the ensemble in the control of movement and posture are not yet elucidated, although they undoubtedly involve consideration of biomechanical actions and probably energy use (see Windhorst 1988). These selection principles, though still unclear, are separate from the size principle, which in our proposal acts as a sequencing principle applied to all MNs selected into the ensemble. Degrees of freedom in motor unit combinations thus are achieved in the selection process and not in the recruitment process. Support for this notion comes from the recent work of Riek and Bawa (1992). This study demonstrates recruitment order by the size principle within each of two independently activated subpopulations of MNs as well as within the group formed when these subpopulations are recruited in combination. Similarly, recruitment in accordance with the size principle occurs among MG MNs during recruitment of the MG muscle alone (Cope and Clark 1991) and among MG and LG MNs during recruitment of the parent muscles together as shown in the present study. In this light, the lack of orderly recruitment among MG-PBF pairs is seen to represent the participation of MNs from each muscle in a different ensemble during sural reflex excitation.


    ACKNOWLEDGMENTS

This work was supported by National Institutes of Health Grant NS-20123.


    FOOTNOTES

Address for reprint requests: A. J. Sokoloff, Dept. of Physiology, Emory University, 1648 Pierce Dr., Atlanta, GA 30322.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 2 December 1998; accepted in final form 11 January 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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0022-3077/99 $5.00 Copyright © 1999 The American Physiological Society