Firing rate of the lower motoneuron and contractile properties of its muscle fibers after upper motoneuron lesion in man.

We studied motor unit (MU) firing rate and contractile properties and myosin isoform composition of single muscle fibers after upper motoneuron lesion. Single-MUs and surface electromyogram (EMG) were recorded during voluntary contractions and locomotion in the paretic (P) and nonparetic (NP) tibialis anterior (TA) of 15 hemiparetics. P TA low-threshold MUs fired within the lower end of their normal range. High-threshold MUs fired below their normal range or were not recruited. Surface EMG was abnormally low and high in the P TA and NP TA, respectively. On muscle cross sections stained with histochemical methods, type I fibers represented 99.4%, 74.3% and 66.6% of NP, P, and control TA, respectively. P TA fibers expressing type I myosin heavy chain (MyHC) were smaller, weaker, and slower. In conclusion, low MU firing rate and activity in the P TA was associated with slower type I MyHC fibers, while increased activity in NP TA resulted in homogenous expression of type I MyHC.

Fatigue of chronically overused motor units in prior polio patients.

This study was undertaken to investigate the mechanisms underlying fatigue of chronically overused motor units (MUs). The force of the tibialis anterior muscle (TA) and the firing properties of single MUs were studied during prolonged maximum voluntary effort in 10 prior polio patients selected such that daily living required all residual TA power. Almost all TA fibers were hypertrophic type I. Activities of intermyofibrillar succinate dehydrogenase (SDH) and calcium-stimulated myofibrillar adenosine triphosphatase (ATPase) were measured in single TA fibers from a representative patient. Neither insufficient motoneuron activation nor peripheral blocking of the electrical impulse played a major role in the loss of force during prolonged contraction or for slow recovery after contraction. The ratio of SDH to calcium-stimulated ATPase, representing the relation between energy resynthesis and energy utilization, was significantly (P < 0.001) lower in prior polio patients (0.230 +/- 0.096) compared to control (0.515 +/- 0.097) type I fibers.

Contractile properties in single muscle fibres from chronically overused motor units in relation to motoneuron firing properties in prior polio patients.

The relation between motoneuron firing rate in vivo and maximum velocity of unloaded shortening (Vmax) and myosin isoform composition in single chemically skinned muscle fibres was investigated in chronically overused motor units. Ten patients with loss of a large proportion of the motoneuron pool due to a prior polio lesion and compensatory overuse of residual neurones were studied and compared with normal individuals. The tibialis anterior muscle (TA) was chosen and prior polio patients who used all residual TA motor units at high rates during the normal step cycle were selected. In prior polio patients, all motor units fired at approximately 40 Hz when maximum voluntary force was reached. A common firing rate of 30 Hz yielded 70-90% maximum force. In normal subjects, on the other hand, maximum TA force was reached when low threshold units fired at 25-30 Hz and high threshold units at 50 Hz. Myosin heavy chain (MHC) and light chain (MLC) isoforms were resolved by 6% and 12% sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), respectively, and quantified densitometrically. In the whole biopsy cross-sections, types I, IIA and IIB MHC proportions were 97, 3 and 0% in a typical prior polio patient and 65, 25 and 10% in an age- and sex-matched control subject. Vmax differed significantly (p < 0.001) between type I fibres from the patient (0.54 +/- 0.12 ML/s) and the control subject (0.29 +/- 0.08 ML/s). The composition and relative contents of essential and regulatory MLC isoforms differed in single type I MHC fibres from the control subject and prior polio patient. 65% of the fibres co-expressed the fast and slow isoform of the regulatory light chain (MLC2) in the patient, while this combination was only observed in one of the control type I fibres. All prior polio fibres with a Vmax higher than 0.45 ML/s, except one, co-expressed MLC2s and MLC2f and the only control fibre co-expressing the slow and fast MLC2 isoform had the highest Vmax (0.50 ML/s) among control fibres. On the other hand, a high relative content of MLC3 was not associated with a high Vmax in type I MHC fibres. It is suggested that the composition of fast and slow isoforms of MLC2 has a significant modulatory influence on Vmax within type I MHC fibres. This combination of MLCs and high Vmax in type I MHC fibres is probably induced by chronic motor unit overuse and an altered motoneuron firing pattern.

Motoneuron firing and isomyosin type of muscle fibres in prior polio.

In patients with prior polio there was an excessive use of remaining motor units and an absence of type II muscle fibres in the tibialis anterior (TA). In the present study, eight subjects with prior polio with more than 90% type I fibres in the TA were examined. The aim was to elucidate whether the lack of type II muscle fibres was due to a selective loss of motoneurons with high threshold and high axonal conduction velocity or due to a muscle fibre transition from type II to type I. There was no decrease of the proportion of motoneurons with high threshold and high axonal conduction velocity. Monoclonal antibodies against fast and slow myosin heavy chains (MHC) were used as histochemical markers and many muscle fibres of type I according to ATPase stainability showed a binding of both anti-fast and anti-slow MHC. It is suggested that the type I muscle fibre dominance in prior polio subjects with excessive use of TA during walking is due to a muscle fibre transition from type II to type I and not to a loss of one class of motor units.

Effect of exercise on the motor unit.

The motoneuron part of this review deals with the changes in recruitment and firing rates of the motor unit types upon changes from a physically inactive life to endurance or strength training. The muscle fibers react to prolonged exercise by adaptation to a higher level of performance. A matter of discussion is the prerequisites for a transformation between the basic muscle fiber types, slow twitch and fast twitch, during voluntary (transsynaptic) activity, which is demonstrated after artificial nerve stimulation. The review includes current knowledge of muscle fiber transformation as an adaptive response to increased usage either by electrical stimulation or by transsynaptic neuronal activity. The metabolic adaptation related to increased endurance is reviewed with special reference to effects on muscle fibers. The increase in strength as a result of high resistance training is mainly the result of increased muscle cross-section. Whether this is solely the result of an increase in size of individual fibers or an increased fiber number is a controversial matter.

Motor neuron firing range, axonal conduction velocity, and muscle fiber histochemistry in neuromuscular diseases.

The voluntary discharge properties and axonal conduction velocity of single motor units were studied in patients with neuromuscular diseases with retained differentiation of the muscle fibers into type 1 and type 2, and in patients with late-onset hereditary distal myopathy in which muscle fibers have only intermediate histochemical properties. In the patients with muscle fiber differentiation, the findings were similar to those in normal subjects; that is, there was a continuum between motor units which fired tonically at low rates and had a low axonal conduction velocity, and motor units which fired phasically at high rates and had a high axonal conduction velocity. In the patients without muscle fiber differentiation, all motor units had intermediate firing properties and a low axonal conduction velocity. It is suggested that in chronic pathologic states, the differentiation of the muscle fiber histochemistry remains only as long as the differentiation of the motor neurons remains.