Disturbances in the voluntary recruitment order of anterior tibial motor units in ataxia.

1. Recruitment order was studied in pairs of motor units of the medial gastrocnemius (MG) muscle of decerebrate cats with the use of dual microelectrode recording from intact ventral root filaments. Excitation was provided by stretch of MG, stretch of synergists [lateral gastrocnemius (LG), plantaris (PL), and soleus (SOL) muscles] or electrical stimulation of the caudal cutaneous sural (CCS) nerve. Motor units were characterized by axonal conduction velocity (CV), tetanic tension (Pmax), twitch contraction time (CT), and fatigue index (FI). 2. Consistent with the recruitment pattern described by others, most often in relation to either CV or Pmax, the first unit of a pair to be recruited by MG stretch was typically the one with the lower CV and Pmax, and the higher FI and CT. The proportion of pairs that agreed in rank order of each property and recruitment order was as follows: for CT, 94%; for CV, 87%; for Pmax, 84%; and for FI, 75%. With a single marginal exception (CT vs. FI), no motor-unit property proved to be significantly better than the others at predicting recruitment (G test; P greater than 0.05). 3. In all 11 tested pairs containing one slow (type S) and one fast (type F) unit, the S was more easily recruited by stretch. Type F units divided into groups with high (type FR), low (type FF), and intermediate (type FInt) values for FI were recruited in order from FR to FInt to FF in 8/11 pairs. Thus our findings were similar to earlier demonstrations that recruitment proceeds in order by type. 4. Stretch of MG synergists usually recruited units in the same order as MG stretch. In two S-S pairs, recruitment order was switched with synergist stretch. 5. Stimulation of the CCS nerve was generally excitatory to the MG units sampled. Most unit pairs were recruited by CCS stimulation in the same order as by MG stretch, but, for 6 of 39 pairs, CCS stimulation switched the order produced by stretch. Thus, whereas sural afferent input can preferentially excite some units over others as suggested by Kanda et al., that effect is not widespread or selective for unit type under these conditions. 6. Assuming that all MG motor units cooperate as a single functional pool in homonymous stretch reflexes, we support others in concluding that a motoneuron's recruitment threshold is not strictly determined by its size. However, our data do not distinguish other schemes that predict recruitment order more accurately than the size principle.(ABSTRACT TRUNCATED AT 400 WORDS)

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Cutaneous stimulation fails to alter motor unit recruitment in the decerebrate cat

Journal of Neurophysiology ◽

10.1152/jn.1993.70.4.1433 ◽

1993 ◽

Vol 70 (4) ◽

pp. 1433-1439 ◽

Cited By ~ 30

Author(s):

B. D. Clark ◽

S. M. Dacko ◽

T. C. Cope

Keyword(s):

Motor Unit ◽

Conduction Velocity ◽

Motor Units ◽

Firing Frequency ◽

Medial Gastrocnemius ◽

Electromyographic Activity ◽

Size Principle ◽

Decerebrate Cat ◽

Slow Twitch ◽

Recruitment Order

1. An attempt was made to repeat the observation that cutaneous input to the cat medial gastrocnemius (MG) muscle sometimes had the differential effect of inhibiting motoneurons with slow axonal conduction velocity while simultaneously exciting others with fast conduction velocity. Dual microelectrode recording from intact ventral root filaments was used to study the effects of cutaneous inputs on recruitment order and on firing frequency of physiologically characterized MG motor units in decerebrate cats. Motor responses to pinch of the skin over the lateral surface of the ankle as well as electrical stimulation of the caudal cutaneous sural (CCS) nerve were contrasted with the responses to static muscle stretch as well as muscle vibration. 2. In contrast to the prediction, recruitment order in pairwise tests was the same for skin pinch or CCS stimulation as it was for MG stretch or vibration in all 32 tested pairs of motor units. This sample included seven pairs comprising one slow-twitch (S) and one fast-twitch motor unit, where the predicted reversal of recruitment should have been most apparent. Regardless of the source of excitation, recruitment of motor units of the MG was consistent with Henneman's size principle in approximately 90% of trials. 3. Skin pinch increased the firing rate of 30 of 32 individual motor units previously activated by stretch or vibration, including 7 slow-twitch units. In the remaining two units, skin pinch transiently (100-400 ms) slowed the firing of an S unit in 11 of 13 vibration + pinch trials. The other unit (type unknown) showed one or two retarded spikes in each of four vibration + pinch trials. In three S units, including the lone inhibitable unit and two others that were only excited by skin pinch, there was a significant positive rank correlation between change in unit firing frequency and change in soleus integrated electromyographic activity.(ABSTRACT TRUNCATED AT 400 WORDS)

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Fatigue of motor units in the human anterior tibial muscle

Electroencephalography and Clinical Neurophysiology ◽

10.1016/0013-4694(85)90250-0 ◽

1985 ◽

Vol 61 (3) ◽

pp. S59 ◽

Cited By ~ 3

Author(s):

S. Andreassen ◽

L. Arendt-Nielsen

Keyword(s):

Motor Units ◽

Anterior Tibial Muscle ◽

Anterior Tibial

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Stability of Motor-Unit Force Thresholds in the Decerebrate Cat

Journal of Neurophysiology ◽

10.1152/jn.1997.78.6.3077 ◽

1997 ◽

Vol 78 (6) ◽

pp. 3077-3082 ◽

Cited By ~ 6

Author(s):

Timothy C. Cope ◽

Alan J. Sokoloff ◽

Stan M. Dacko ◽

Rebecca Huot ◽

Eleanor Feingold

Keyword(s):

Motor Unit ◽

Motor Units ◽

Relative Activity ◽

Rate Of Change ◽

Medial Gastrocnemius ◽

Unit Force ◽

Decerebrate Cat ◽

Axonal Conduction ◽

Physiological Properties ◽

Recruitment Order

Cope, Timothy C., Alan J. Sokoloff, Stan M. Dacko, Rebecca Huot, and Eleanor Feingold. Stability of motor-unit force thresholds in the decerebrate cat. J. Neurophysiol. 78: 3077–3082, 1997. To further test the hypothesis that some fixed property of motoneurons determines their recruitment order, we quantified the variation in force threshold (FT) for motoneurons recruited in muscle stretch reflexes in the decerebrate cat. Motor axons supplying the medial gastrocnemius (MG) muscle were penetrated with micropipettes and physiological properties of the motoneuron and its muscle fibers, i.e., the motor unit, were measured. FT, defined as the amount of MG force produced when the isolated motor unit was recruited, was measured from 20 to 93 consecutive stretch trials for 29 motor units. Trials were selected for limited variation in base force and rate of rise of force, which have been shown to covary with FT, and in peak stretch force, which gives some index of motor-pool excitability. Under these restricted conditions, large variation in FT would have been inconsistent with the hypothesis. Analysis of the variation in FT employed the coefficient of variation (CV), because of the tendency for FT variance and mean to increase together. We found that CV was distributed with a median value of 10% and with only 2 of 29 units exceeding 36%. Some of this variation was associated with measurement error and with intertrial fluctuations in base, peak, and the rate of change of muscle force. CV was not significantly correlated with motor-unit axonal conduction velocity, contraction time, or force. In three cases FT was measured simultaneously from two motor units in the same stretch trials. Changes in recruitment order were rarely observed (5 of 121 stretch trials), even when FT ranges for units in a pair overlapped. We suggest that the large variation in recruitment threshold observed in some earlier studies resulted not from wide variation in the recruitment ranking of motoneurons within one muscle, but rather from variation in the relative activity of different pools of motoneurons. Our findings are consistent with the hypothesis that recruitment order is determined by some fixed property of α-motoneurons and/or by some unvarying combination of presynaptic inputs that fluctuate in parallel.

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Change in recruitment order of motor units in human parasternal intercostal muscles with sleep state

Journal of Applied Physiology ◽

10.1152/jappl.1993.74.6.2718 ◽

1993 ◽

Vol 74 (6) ◽

pp. 2718-2723 ◽

Cited By ~ 1

Author(s):

W. A. Whitelaw ◽

K. P. Rimmer ◽

H. S. Sun

Keyword(s):

Sleep Stage ◽

Human Subjects ◽

Motor Units ◽

Excitatory Input ◽

Sleep State ◽

Intercostal Muscles ◽

Intramuscular Electrodes ◽

Normal Human ◽

Low Threshold ◽

Recruitment Order

Recruitment order of individual motor units in the early part of inspiration in parasternal intercostal muscles was observed in normal human subjects during wakefulness and non-rapid-eye-movement sleep. Electromyograms from bipolar fine wire intramuscular electrodes were recorded while the subjects lay supine in a sleep laboratory, and sleep stage was determined by polysomnography. From wakefulness to sleep there were numerous examples of shifts in order of recruitment among the low threshold units of early inspiration. There were corresponding shifts in the order of derecruitment of these units. Analysis of frequency of firing of units also suggested that the levels of excitatory input to one unit of a pair could be altered relative to the level of input of the other one. The data imply that there are at least minor differences in distribution of excitatory inputs from various sources among motoneurons of this muscle pool.

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Motor-unit recruitment in self-reinnervated muscle

Journal of Neurophysiology ◽

10.1152/jn.1993.70.5.1787 ◽

1993 ◽

Vol 70 (5) ◽

pp. 1787-1796 ◽

Cited By ~ 37

Author(s):

T. C. Cope ◽

B. D. Clark

Keyword(s):

Nerve Stimulation ◽

Sural Nerve ◽

Motor Units ◽

Cutaneous Reflexes ◽

Nerve Section ◽

Recruitment Pattern ◽

Nerve Crush ◽

Muscle Stretch ◽

Physiological Properties ◽

Recruitment Order

1. Recruitment order of motor units in self-reinnervated medial gastrocnemius (MG) muscles was studied in decerebrate cats 16 mo after surgical reunion of the cut MG nerve. Pairs of MG motor units were isolated by dual microelectrode penetration of ventral roots to measure their recruitment sequence during cutaneous reflexes in relation to their physiological properties. 2. Physiological properties of reconstituted motor units appeared normal, as expected. Also normal were the relationships among these properties: twitch and tetanic tension tended to increase with axonal conduction velocity and decrease with twitch contraction time. A small fraction of motor units (10/116) in reinnervated muscles produced either no measurable tension or unusually large amounts of tension compared with controls. This was the only distinct feature of the sample of reconstituted units. 3. In muscles reinnervated after nerve section, stretch was notably ineffective in eliciting reflex contraction of MG muscles or their constituent motor units (only 5/116 units). Incomplete recovery from nerve section was probably the cause of this impairment, because stretch reflexes were readily evoked in adjacent untreated muscles and in one reinnervated MG muscle that was studied 16 mo after nerve crush. In contrast with the ineffectiveness of muscle stretch, sural nerve stimulation succeeded in recruiting 49/116 units, a proportion fairly typical of normal MG muscles. 4. The contractions of the first unit recruited in cutaneous reflexes tended to be slower and less forceful than those of the other unit in a pair. By these measures, recruitment obeyed the size principle. This recruitment order with respect to unit contractile properties was not significantly different (P > 0.05) between untreated and reinnervated muscles but was significantly (P < 0.005) different from random order in both groups. The same recruitment pattern was observed for pairs of motor units sampled from the muscle reinnervated after nerve crush, whether units were recruited by muscle stretch or sural nerve stimulation. 5. The usual tendency for motor units with slower conduction velocity (CV) to be recruited in sural nerve reflexes before those with faster CV was not strong in reinnervated muscles. After nerve section the proportion of units exhibiting the usual recruitment pattern was not significantly different (P > 0.05) from a random pattern for CV. 6. The central finding is that the normal recruitment patterns recover from nerve injury in a muscle that is reinnervated by its original nerve. By contrast, stretch reflexes do not recover well from nerve section, and this deficiency may contribute to motor disability.

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