Facility of motor unit control during tasks defined directly in terms of unit behaviors

Distribution of activity within the cat's peroneus longus muscle when activated in different ways via the central nervous system

We have studied whether a mechanically homogenous muscle (all units giving joint-torques in the same direction) would display different distributions of activity in response to different types of activation via the central nervous system. In adult cats anaesthetized with pentobarbitone, long-lasting contractions were evoked in m. peroneus longus (PerL), a mixed ankle dorsiflexor muscle. The contractions were elicited by continuous repetitive stimulation of: (1) the superficial peroneal nerve (NP, flexion reflex); and (2) the contralateral motor cortex (MC). During these contractions, isometric force was monitored and fine-wire electromyographic recordings (EMG) were simultaneously obtained from anterior and posterior portions of the PerL. The relative degree of activation in anterior vs posterior muscle portions was quantified by measuring, at various force levels: (1) the average spike amplitude (Spike); and (2) the total number of spikes per unit time (Count; 0.5-s measurement periods). For both types of stimulation (MC and NP), the results indicated that activation was more effective for posterior than for anterior muscle portions: the Spike-measurements were typically higher in posterior than in anterior PerL and, for MC-elicited contractions, this was true for the Count-measurements as well. With respect to both types of EMG-quantification (Spike and Count), the 'posterior bias' of activity was significantly more pronounced for the cortically evoked contractions than for those elicited via stimulation of the peroneal nerve. As it is known that anterior and posterior PerL portions tend to be innervated by rostral and caudal motoneurones, respectively, these findings indicate that different inputs to the PerL motoneurone pool may differ significantly with respect to the intraspinal spatial distribution of synaptic effects among the motoneurones. The results were discussed in relation to the organization of task-related recruitment schemes.

The recruitment order of stapedius motoneurons in the acoustic reflex varies with sound laterality

In many muscles, motor units are recruited in a fixed order with increasing strength of muscular contraction. We show that for the stapedius muscle of the cat, vastly different recruitment orders can be obtained, depending on which ear is acoustically stimulated. The data support the idea that the distribution of inputs to a motoneuron pool can be inhomogeneous and a significant factor in determining recruitment order.

Relation between location of a motor unit in the human biceps brachii and its critical firing levels for different tasks

Critical firing levels (CFLs) of single motor units (MUs) in the long head of the human biceps brachii muscle were determined in combinations of two isometric tasks: flexion of the elbow, supination of the lower arm, and exorotation of the humerus, as well as the corresponding antagonistic tasks. The MU activity was recorded by 25-micron bipolar wire electrodes. Four main patterns of MU recruitment, related to the recording location in the muscle, were found: (i) MUs active only when flexing the elbow were located mostly laterally. (ii) MUs active only when supinating were all located medially. (iii) MUs whose CFL depended on a linear combination of flexion and supination forces were all located medially. Some of these MUs could not be recruited during pronation. (iv) Nonlinearly behaving MUs, located centrally. The relative weights of flexion and supination input were constant for all units, whose CFL depended on a linear sum of flexion and supination forces, as well as for the nonlinearly behaving units. Supination and exorotation showed equivalent CFL changes when they were combined with the flexion task. Extension did not change the CFL for supination- or exorotation tasks. No clear difference was found between the ratios of the peak twitch forces in flexion and supination direction for laterally and medially located small muscle areas or single MUs. A simple model of the motoneuron pool organization is proposed to explain our findings.

The effect of digital nerve stimulation on recruitment order of motor units in the first deep lumbrical muscle of the cat

The normal recruitment order of EMG spikes of the first deep lumbrical muscle of the cat's hindpaw, usually seen during cortical stimulation, pad pinch and weak plantar nerve stimulation, was temporarily reversed after stimulation of the medial digital nerve of the foot at 50 Hz for 2 min, and normal order was recovered in 2 to 10 min. The longer the period of stimulation of the medial digital nerve was, the longer the time for recovery. Alteration of order is repeatable and reversible after an interval of more than 15 min. After prolonged medial digital nerve stimulation, EMG responses to pad pinch and plantar nerve stimulation were facilitated. Combination of some stimuli (e.g. cortical stimulation and plantar nerve stimulation, pad pinch and plantar nerve stimulation, plantar nerve and medial digital nerve stimulation) also produced reversal of recruitment order during the period of stimulation. A functionally single motor nerve fiber to the first deep lumbrical muscle was isolated from the motor nerve axons in L7 and S1 of the spinal cord of cats, and physiological properties of pairs of motor units whose recruitment order was temporarily altered by plantar nerve stimulation after prolonged stimulation of the medial digital nerve of a foot were examined. Motor units with large action potentials were more facilitated than motor units with small action potentials after prolonged stimulation of the medial digital nerve. The former motor units showed fast contraction time, large twitch tension, low resistance to fatigue and presence of sag-behavior. The latter motor units showed slow contraction time, small twitch tension and high resistance to fatigue. High threshold motor units with middle-sized surface EMG records were recruited after motor units with large potentials had been recruited; motor units with middle-size action potentials were beta motor axons branching to both intrafusal and extrafusal muscle fibers. The beta motor axons also showed fast conduction velocity and the presence of sag-behavior.

The influence of recruitment order and temperature on muscle contraction with special reference to motor unit fatigue

Although recruitment in man often proceeds from the slowest to fastest motor units, a variety of stimuli can alter this order. To study the effect of recruitment order on isometric performance, the tetanic tension of individual fast and slow twitch motor units was measured during fatiguing isometric contractions of the medial gastrocnemius muscle of the cat with recruitment proceeding from either the slowest to fastest or fastest to slowest motor units throughout the contractions. The muscles were stimulated electrically through the ventral roots of the spinal cord. Fatiguing isometric tensions were examined at 10, 20, 40, 70, or 100% of the initial strength of the muscles at 28 and 38 degrees C. The results of these experiments showed that for contractions at low isometric tensions (20 and 40% of the muscles' strength), especially at the lower temperature, the endurance was longer when recruitment proceeded from the slowest to fastest units. The reason for this appeared to be linked to the more rapid rate of fall of tension over time in fast as opposed to slow twitch motor units.