Changes in contractile properties of motor units of the rat medial gastrocnemius muscle after spinal cord transection

Abstract

The effects of complete transection of the spinal cord at the level of Th9/10 on contractile properties of the motor units (MUs) in the rat medial gastrocnemius (MG) muscle were investigated. Our results indicate that 1 month after injury the contraction time (time-to-peak) and half-relaxation time were prolonged and the maximal tetanic force in most of the MUs in the MG muscle of spinal rats was reduced. The resistance to fatigue also decreased in most of the MUs in the MG of spinal animals. Moreover, the post-tetanic potentiation of twitches in MUs diminished after spinal cord transection. Criteria for the division of MUs into three types, namely slow (S), fast fatigue resistant (FR) and fast fatigable (FF), applied in intact animals, could not be directly used in spinal animals owing to changes in contractile properties of MUs. The 'sag' phenomenon observed in unfused tetani of fast units in intact animals essentially disappeared in spinal rats and it was only detected in few units, at low frequencies of stimulation only. Therefore, the MUs in spinal rats were classified as fast or slow on the basis of an adjusted borderline of 20 ms, instead of 18 ms as in intact animals, owing to a slightly longer contraction time of those fast motor units with the 'sag'. We conclude that all basic contractile properties of rat motor units in the medial gastrocnemius muscle are significantly changed 1 month after complete spinal cord transection, with the majority of motor units being more fatigable and slower than those of intact rats.

Time-related changes of motor unit properties in the rat medial gastrocnemius muscle after the spinal cord injury. II. Effects of a spinal cord hemisection

“…For each MU the same stimulation protocol and the same pattern of stimuli was applied as in our previous studies (Celichowski et al, 2006;Mrówczyń ski et al, 2009). The stimulation protocol was controlled by the computer program cooperating with the S88 Grass stimulator and the SIU5 isolation unit, and recorded MU activity had the following order: (1) the averaged twitch contraction (5 pulses at 1 Hz); (2) the unfused tetanus (500 ms train of stimuli at 40 Hz); (3) the fused tetanus (200 ms train of stimuli at 150 Hz); (4) series of tetani at progressively increasing stimulation frequencies (500 ms trains of stimuli at 10, 20, 30, 40, 50, 60, 75, 100 and 150 Hz in 10 s time intervals); (5) the averaged twitch contraction (5 pulses at 1 Hz); (6) the standard fatigue test (trains of 14 pulses at 40 Hz, repeated every 1 s for 3 min) (Burke et al, 1973).…”

“…In the fast fatigable motor units (FF) the contraction time and the half-relaxation time were prolonged, the twitch-to-tetanus ratio was increased whereas the ability to post-tetanic potentiation was diminished and maximal tetanus force was reduced, while the MUs of FR and S type became more fatigable. Moreover, the sag phenomenon in unfused tetanic contraction of fast MUs almost totally disappeared one month after the spinal cord injury (Celichowski et al, 2006). When MU contractile properties were analyzed 14, 30, 90 and 180 days after the spinal cord transection (Mrówczyń ski et al, 2009) all these changes in MU properties, mentioned above, developed progressively in time.…”

“…In our previous paper we described changes in contractile properties of MUs in the rat medial gastrocnemius (MG) muscle one month after the spinal cord total transection (Celichowski et al, 2006). The progressive development of those changes in the same parameters at various time points after the total spinal cord transection were the subject of an accompanying paper (Mrówczyń ski et al, 2009).…”

“…The experimental design of the evaluation of muscle morphology and function was different in those reports. They included investigations of contractile properties and morphometric measurements of whole muscles (Lieber et al, 1986a), histochemical studies of muscles and motor units (Mayer et al, 1984), electrophysiological studies on motoneurones and motor units (Cope et al, 1986;Munson et al, 1986) or on functionally isolated motor units (Celichowski et al, 2006). All these investigations demonstrated that the spinal cord transection leads to decreased muscle mass, muscle fiber atrophy and modification of several contractile properties such as force, contraction time, fatigue resistance (Mayer et al, 1984;Lieber et al, 1986b;Celichowski et al, 2006;Mrówczyń ski et al, 2009).…”

Isometric tetanic force measurement method of the tibialis anterior in the rat

“…6,12 Nevertheless, among other studies examining the maximum tetanic force of muscles other than the TA, there has been a wide and inconsistent variety of optimized stimulation parameters in their methods. 4,5,[13][14][15][16] From a review of the literature, four parameters necessary for testing each individual muscle were identified. These parameters include muscle length (preload force), stimulus intensity (voltage), stimulus pulse duration, 17 and stimulus pulse frequency.…”