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Experimental Physiology 91.5 pp 887-895
DOI: 10.1113/expphysiol.2005.033076
© The Physiological Society 2006
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Changes in contractile properties of motor units of the rat medial gastrocnemius muscle after spinal cord transection

Jan Celichowski1, Wlodzimierz Mrówczynski1, Piotr Krutki1, Teresa Górska2, Henryk Majczynski2 and Urszula Slawinska2

1 Department of Neurobiology, University School of Physical Education, Poznañ, Poland 2 Nencki Institute of Experimental Biology, Warsaw, Poland


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
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.

(Received 15 December 2005; accepted after revision 18 May 2006; first published online 25 May 2006)
Corresponding author J. Celichowski: Department of Neurobiology, University School of Physical Education, 55 Grunwaldzka Street, 60-352 Poznan, Poland. Email: celichowski{at}awf.poznan.pl


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
Many different experimental models have been used to study the effects of inactivity on skeletal muscles: denervation, immobilization or tenotomy (Vrbová, 1963; Pette et al. 1973). One approach to produce reduced activity in hindlimb muscles is spinal cord transection (Mayer et al. 1984; Munson et al. 1986). The effect of this procedure have been investigated in rats mainly in two antagonistic muscles: soleus (Sol) and extensor digitorum longus (EDL; Davey et al. 1981; Lieber et al. 1986; Hutchinson et al. 2001). These two muscles have different physiological functions, i.e. dorsi flexion (EDL) or extension (Sol) of the ankle joint. Moreover, the Sol is a slow muscle composed mainly of slow motor units (MUs) while EDL is a fast muscle composed mainly of fast MUs (Ariano et al. 1973; Chamberlain & Lewis, 1981).

After spinal cord transections at the thoracic level in rats, the hindlimb movements are severely impaired, thus they are dragged most of time behind the hindquarters over the ground and the dorsiflexors of the ankle joint such as EDL are kept in a lengthened position, whereas the extensors of the ankle joint (Sol, gastrocnemius medialis and lateralis) are in a shortened position. This change in the length of the muscle may by itself cause changes in functional properties of the motor units and muscle fibres. It is known that slow muscles kept in a shortened position become usually fast contracting and atrophic, whereas the same manipulation with fast muscles has no dramatic effects on their contractile properties (Frischknecht et al. 1990).

In order to study the effect of reduced activity and shortened position of different types of muscle fibres within the same muscle, the properties of MUs in the medial gastrocnemius (MG) after spinal cord transection were examined. The medial gastrocnemius muscle in the rat is composed of three different types of motor units in following proportions: 35.4% fast fatigable and fast intermediate (FF + FI); 47.9% fast fatigue resistant (FR); and 16.7% slow (S; Kanda & Hashizume, 1989). This composition is evidently different from that reported in the cat: (68% FF, 18% FR and 14% S; Krutki et al. 2006). It was found that in the cat after spinal cord transection (Mayer et al. 1984; Munson et al. 1986), slow motor units in the MG muscle became faster contracting, whereas there was no dramatic change in the speed of contraction of fast motor units. In addition, all motor units became more fatigable. It is not known whether these consequences of spinal cord transection represent a general rule among mammals or depend on the composition of muscles. Therefore, in the present paper we investgated whether spinal cord transection in the rat would produce similar changes in motor units of MG muscle to those described in the cat (Mayer et al. 1984; Munson et al. 1986).


   Methods
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 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
Experiments were performed on 13 adult female Wistar rats, weighing 260–310 g. All experimental procedures used in this study were approved by the local Ethics Committee and followed EU guidelines on animal care. Adequate care was taken to minimize pain and discomfort in the experimental animals.

Spinal cord transection

Spinal cord transection at a low thoracic level (Th9/10) was performed in nine 3-month-old Wistar rats under deep Equithesin (9.7 mg pentobarbitone, 7.6 mg ethyl alcohol, 42.5 mg chloral hydrate, 428 mg propylene glycol, 21 mg MgSO4 per mililitre sterilized H2O) anaesthesia (3 ml kg–1, I.P.). The surgical procedures were described in detail in our previous papers (Slawiñska et al. 2000; Majczyñski et al. 2005). To prevent possible axonal regrowth through the cavity of the lesion, 2–3 mm of the spinal cord tissue was cut by fine scissors and then gently aspirated using a glass pipette. Subsequently, the layers of paravertebral muscles on the back of animal and the overlying fascia were closed using sterile sutures (Mersilk 0.22 mm). The skin was closed with stainless-steel surgical clips. An adequate level of anaesthesia during surgery was ensured by regular testing for the lack of cutaneous withdrawal reflexes of the forelimbs and an additional dose of anaesthetic (1 ml kg–1, I.P.) was given when needed. After surgery, the animals received a non-steroidal anti-inflammatory and analgesic treatment (Tolfedine (tolfedine acid, Vétoquinol, France) 0.4 mg (100 g)–1, I.P.). All animals recovered from anaesthesia within 2–3 h after surgery. During the following 10 days, the animals were routinely (daily)given antibiotics (gentamicin (LEK, Poland) 0.2 mg (100 g)–1 and Baytril (Enrofloxacin, Bayer Healthcare) 0.5 mg (100 g)–1, I.P.). During this postoperative period, the bladder was emptied manually twice a day until the voiding reflex was re-established (i.e. after about 10–12 days).

Contractile properties of MUs: final acute experiment

The properties of the medial gastrocnemius MUs were electrophysiologically investigated in nine animals: four control intact rats and five rats 1 month after complete spinal cord transection. The experimental procedures were described in detail in our previous papers (Grottel & Celichowski, 1990; Celichowski, 1992).

Briefly, rats were anaesthetized with pentobarbitone, with an initial dose of 60 mg kg–1 I.P., supplemented after 3 h with additional doses of 10 mg kg–1 h–1 I.P. The depth of anaesthesia was verified throughout the experiment by assessing the pinna reflexes.

The spinal cord was exposed by laminectomy at the level L2–S1 segments. Ventral roots were cut proximally to the spinal cord and covered with warm paraffin oil that filled a pool formed by skin flaps. The MG muscle was isolated and dissected free from surrounding tissues. The blood vessels and the respective nerve branch to MG were left intact, while other nerve branches of the sciatic nerve innervating other muscles were cut. The Achilles tendon was dissected free and connected to a force transducer. The hindlimb was immobilized by two steel clamps attached to the tibia and the sacral bone and was immersed in warm paraffin oil in a special pool. The temperatures of both the animal and the paraffin oil were automatically maintained at 36–37°C. The MG was stretched to a passive tension of 100 mN, to generate the maximal twitch forces of motor units (Celichowski & Grottel, 1992).

To isolate individual motor units, the L5 ventral root was split into thin filaments which were stimulated with rectangular electrical pulses of 0.1 ms duration and amplitude up to 0.5 V. The evoked twitches were accepted as the single motor unit activity when their tensions as well as action potentials were of the ‘all or none’ type. Action potentials were recorded by two thin silver electrodes inserted into the muscle. The force and action potentials were stored on a computer disc using an analog-to-digital converter (RTI-800 Utilities, Analog Devices, Inc., USA). A sampling rate of 1 kHz for force and 10 kHz for action potentials was applied for recordings.

Each experiment was performed under the same physiological conditions, with the same stimulation protocol. The pattern of stimuli was generated by a computer program controlling an S88 Grass stimulator and an SIU 5 isolation unit (Grass Instrument Company, USA). Responses of each isolated motor unit were recorded in 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 (500 ms trains of stimuli at 10, 20, 30, 40, 50, 60, 75, 100 and 150 Hz, separated by 10 s breaks); (5) the averaged twitch contraction (5 pulses at 1 Hz); and (6) the fatigue test (responses to trains of 14 pulses at 40 Hz, repeated every 1 s for 3 min; Burke et al. 1973). The rest time between each stimulation pattern was 10 s. At the end of the experiment, each animal was killed with an overdose of pentobarbitone (180 mg kg–1, I.P.), and the investigated medial gastrocnemius muscle was removed and weighed.

Data analysis

Three physiological properties were determined from a single twitch: the contraction time (measured from the onset of the contraction to the force peak), the half-relaxation time (measured from the force peak to half of its maximum) and the twitch force (measured from the baseline to the peak). The post-tetanic potentiation (PTP) was calculated as a ratio of the force of the potentiated twitch (see 5th point of the stimulation protocol) to the initial one (see 1st point of stimulation protocol). The maximal tetanic force was measured in the fused tetanic contraction at 150 Hz. In order to control the presence of the ‘sag’ phenomenon, the profiles of unfused tetani at 20, 30 and 40 Hz were analysed. In intact animals, the motor units with ‘sag’ and with a contraction time under 18 ms were classified as fast (F), while those without ‘sag’ and with a contraction time over 18 ms were classified as slow (S; Burke et al. 1973; Grottel & Celichowski, 1990). However, in spinal rats this criterion appeared to be inapplicable owing to a lack of ‘sag’ in most of investigated motor units. Therefore, in spinal rats the motor units were classified as slow and fast exclusively on the basis of the contraction time, with a shifted border value from 18 to 20 ms (see Results). This change of classification criterion was made because in spinal animals the contraction time of a few motor units with ‘sag’ was longer than in control intact rats (for detailed description see Results). During the fatigue test, the fatigue index was calculated as the ratio of force after 2 min of the test to the initial maximum force. Fast units with a fatigue index greater than 0.5 were classified as fast resistant to fatigue (FR) type, while those with a fatigue index less than 0.5 were classified as fast fatigable (FF; Kernell et al. 1983; Grottel & Celichowski, 1990).

The properties of motor units are presented as mean values ± S.D. Statistical comparisons between control and spinal rats were made with Student's unpaired t test.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
Behavioural observation

One month after complete spinal cord transection, before the acute experiments, the rats were observed during natural exploratory behaviour in the home cage to check the degree of hindlimb impairment. In agreement previous findings (Lieber et al. 1986; Slawiñska et al. 2000; Majczyñski et al. 2005), none of the rats with complete transection of the spinal cord was able to support the body weight with the hindlimbs during spontaneous locomotor movements 1 month after injury. The spinal rats moved around using their forelimbs, while the hindlimbs and hindquarters were dragged behind. When sitting motionless, spinal rats kept their hindlimbs passively extended behind the body.

Contractile properties of MUs

In acute experiments, a total of 86 motor units were examined in intact rats, while 78 motor units were investigated in spinal rats 1 month after injury. In control (intact) rats, there were clear-cut differences in the contraction time and half-relaxation time between fast and slow motor units (for fast units the contraction time was shorter than 18 ms, see Fig. 1A). Moreover, the division into fast and slow motor units was supported by the presence of ‘sag’, i.e. a decline in force after its initial increase during unfused tetanic contraction of fast units (Grottel & Celichowski, 1990; Celichowski et al. 2005). In MG muscle of intact rats, ‘sag’ was consistently detected in all fast motor units (twitch contraction time < 18 ms). This phenomenon was rarely observed in motor units of spinal rats and could not be used for their fast or slow classification. The distribution of the contraction time in motor units of spinal rats is presented on Fig. 1B. With respect to the contraction time below and above 20 ms, two distinct groups of motor units could be distinguished. The first group consisted of units, classified as fast (n = 60), with a contraction time ranging from 12 to 20 ms and a fatigue index ranging between 0.01 and 0.97. The ‘sag’ was visible only in four units and only at lower frequencies of stimulation (20 Hz). These four units had a contraction time of 20 ms and two of them had low values of fatigue index (characteristic for FF units; 0.20 and 0.21), lower in comparison to units with contraction times longer than 20 ms. Therefore they were accepted as fast units. The second group consisted of units with a contraction time of 20–35 ms, high fatigue index (0.79–1.00) and no ‘sag’ in unfused tetani at all applied frequencies of stimulation. They were therefore classified as slow (n = 18). The reduced number of MUs with unfused tetani with ‘sag’ phenomenon in spinal rats is a significant difference compared to intact animals. Figure 2 shows examples of records of unfused tetani evoked by 20, 30 and 40 Hz stimulation of motor units from the MG muscle of a control and a spinal rat, separately for FF (Fig. 2A) and FR motor units (Fig. 2B). These examples evidently show differences in the profiles of unfused tetani between fast motor units of both studied groups of rats and a lack of ‘sag’ effect in spinal rats.


Figure 1
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  		Figure 1.  Distribution of contraction times for fast and slow motor units in control rats (A) and in spinal rats (30 days after injury; B)
Open symbols, slow motor units; filled symbols, fast motor units with sag phenomenon visible at 40 Hz stimulation in intact rats; shaded symbols, fast motor units in spinal animals (note 4 shaded symbols representing MUs with ‘sag’ phenomenon visible exclusively at 20 Hz stimulation frequency). Arrows in A and B indicate border values of contraction time used for classification of motor units into fast and slow categories.

 

Figure 2
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  		Figure 2.  Comparison of unfused tetani generated by FF (A) and FR motor units (B) in control and spinal rats
Note that a ‘sag’ phenomenon is not visible in records obtained after spinal cord transection.

 
Table 1 presents mean values (± S.D.) and ranges of the analysed indices of motor unit contractile properties in both groups of rats. The time parameters of single twitch (contraction time and half-relaxation time) were significantly longer in motor units of spinal rats (P < 0.001, Student's unpaired t test; see also Table 1). Therefore, most motor units from the MG of the spinal animals became slower contracting, but unlike FR and S motor units of normal animals, they were also more fatigable. In view of this dissociation between speed of contractions and fatigability, it is difficult to use the conventional classification of motor unit types in MG muscle of spinal rats. As shown in Fig. 2, the increase of contraction time also affected the shape of tetanic contractions, causing a higher degree of fusion even at lower rates of stimulation.


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  		Table 1.  Characteristic mean values ± S.D. and range of contractile properties of motor units in intact (control) and spinal rats 1 month after complete spinal cord transection
 
After spinal cord transection, the ability of fast motor units to potentiate the force diminished significantly. As shown in Table 1, the post-tetanic potentiation tended to be reduced, but the difference was significant only in FF motor units (P < 0.001).

The mean values of forces developed during single twitch in all types of motor units in spinal animals did not change significantly in comparison to units of the control group (P > 0.1). Moreover, ratios of tetanus forces to muscle weight were also similar for intact and spinal animals (0.088 and 0.081, respectively).

The maximal tetanic force was lower than that in controls in each type of motor unit after spinal cord transection (Table 1), so the ratio of twitch-to-tetanus forces was higher (Fig. 3). The difference between the control and spinal groups of animals was significant for all motor unit types (Table 1). A correlation was observed between CT and Tw/Tet ratio in the control group of rats (r = 0.66, P < 0.01), but 1 month after transection of the spinal cord this correlation was considerably disrupted (r = 0.31, P > 0.05).


Figure 3
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  		Figure 3.  Examples of records of single twitch and fused tetani for FF motor units of a control animal (A) and of a spinal animal (B)
The twitch-to-tetanus ratio (Tw/Tet) was 0.30 and 0.51, respectively. The maximal tetanus was recorded at 150 Hz stimulation.

 
The fatigability of all types of motor units tended to increase in rats with a transected spinal cord, as shown by the decrease of fatigue index. Nevertheless, this was statistically significant only for FR motor units (P < 0.001; Table 1). It is important to note that in intact animals the vast majority (32 out of 43) of FR motor units were fatigue resistant, with a fatigue index higher than 0.75, while in rats after spinal cord injury 21 out of 30 FR motor units were more fatigable (fatigue index within the range 0.50–0.75).

Figure 4 summarizes in three-dimensional diagrams the main changes observed in motor units of spinal rats. The relationships between the three indices of contractile properties of motor units (the contraction time, the tetanic force and the fatigue index) indicate differences in the distribution of motor unit types in both investigated populations, the control group (Fig. 4A) and the spinal animals (Fig. 4B). Two distinct groups, i.e. fast and slow motor units, could be distinguished with respect to the contraction times in control as well as in spinal rats. The diagrams also illustrate the decrease of tetanic force in all types of motor units and the decrease of the fatigue resistance, particularly in the population of FR units. The correlations between the tetanic forces and fatigue indices were found both for the control group and for spinal rats. The respective correlation coefficients amounted to –0.61 (P < 0.01) and –0.67 (P < 0.01). Finally, they demonstrate the adjustment of the contraction time border value (from 18 to 20 ms) between fast and slow units that was used to differentiate these two types of motor units in the group of rats after spinal cord transection.


Figure 4
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  		Figure 4.  Relationship between the contraction time, maximal tetanic force and fatigue index for all tested motor units in all animals from the two experimental groups: the control rats (n = 86; A) and the spinal rats (n = 78; B)
The various types of motor units are indicated as follows: shaded circles, FF (fast fatigable); filled circles, FR (fast fatigue resistant); and open circles, S (slow). The continuous bold lines show the border values of the contraction time between fast and slow motor units (18 ms in the control group and 20 ms in the group after spinal cord transection) and of the fatigue index between fatigable (FF) and resistant to fatigue units (FR and S).

 
In addition to changes in the contractile properties of Mus, we observed muscle atrophy. In control rats the mean weight of the medial gastrocnemius muscle was 817 ± 54 mg (range, 670–950 mg), whereas in spinal animals a decrease in muscle weight was observed and the mean value was 480 ± 48 mg (range, 440–550 mg). It is worth noting that the muscle weight expressed as a percentage of the animal body weight was lower in spinal rats (in the range 0.17–0.19%) than that obtained in intact animals (0.20–0.27%).


   Discussion
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 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
Our results demonstrate that 1 month after complete transection of the spinal cord the medial gastrocnemius muscle atrophied, the mean tetanic force of its motor units decreased and the mean time course of contraction changed in investigated MU groups. In all types of MUs there was also a dramatic increase of the Tw/Tet ratio. At the same time there was a reduction of mean values of the fatigue index of FR and S units resistant to fatigue (however, differences were statistically significant only for FR units).

Contraction time

Our results showing, in the MG muscle of spinal rats, prolongation of the contraction and half-relaxation times in all types of motor units are consistent with results from denervated and TTX-inactivated muscles (St-Pierre & Gardiner, 1985; Buffelli et al. 1997). They are, however, different from the results obtained in spinal cats, in which all types of motor units in the soleus muscle 4 months after spinal cord transection (Cope et al. 1986), as well as in the medial gastrocnemius 6 months after complete spinal cord transection (Mayer et al. 1984), had significantly shorter contraction times. One of the reasons for this discrepancy might be the different times between the spinal cord transection and examination of the motor units. The different fibre type composition of the muscle in rats and cats might be another explanation for the discrepancies between these two species. The feline medial gastrocnemius muscle is composed mainly of fast fatigable motor units (type IIB muscle fibres), whereas the rat medial gastrocnemius has a more balanced composition of various types of motor units, with a relatively big percentage of FR motor units (type IIA fibres; Burke et al. 1973; Kanda & Hashizume, 1989; Kanda & Hashizume, 1992; Krutki et al. 2006). The different fibre type composition of the muscles is probably related to the more active behaviour observed in rats compared to cats. The twitch prolongation described in our study is consistent with data obtained by other authors who observed prolongation of the isometric twitch contraction, i.e. longer contraction and half-relaxation times in both slow Sol and fast EDL muscles that were denervated for relatively short times (from a few days to about 4 weeks; Finol et al. 1981; Gundersen, 1985; Spector, 1985; Buffelli et al. 1997). However, with longer times of denervation (2–3 months; Hennig & Lømo, 1985; Westgaard & Lømo, 1988), while the EDL isometric twitch was still prolonged, that of Sol was shortened, thus approaching similar intermediate values of contraction times to those reported here. Talmadge et al. (2002) also observed a decrease in contraction time of the rat soleus muscle after long-term spinal cord transection. These results suggest variable responses of individual (fast or slow) muscles to the spinal cord transection.

The mechanism that may be responsible for the prolongation of the contraction time remains an open question. The time course of muscle contraction in physiological conditions is determined by several factors, including a rate of calcium release and reuptake (Pette & Staron, 1990), and the described prolongation may be associated with a prolonged Ca2+ transient, possibly caused by less efficient calcium reuptake mechanisms from the sarcoplasm.

Force properties

Our study shows no significant changes in twitch forces 1 month after spinal cord transection, but a significant decrease of maximal tetanic tensions, resulting in higher values of twitch-to-tetanus ratios. This result is partly inconsistent with data presented by Lieber et al. (1986) who indirectly demonstrated an increase in twitch forces for both fast and slow muscles (EDL and Sol) in spinal rats. These authors observed no changes in tetanic forces; however, a dramatic increase in twitch-to tetanic ratios was found. According to St-Pierre & Gardiner (1985), higher values of twitch-to-tetanus ratio are important signs of changes of contractile responses characterizing atrophied muscles. Gerrits et al. (2005) observed an increase in the twitch-to-tetanus ratio for patients with spinal cord injuries. The results obtained in studies on paralysed rat skeletal muscles (Buffelli et al. 1997), as well as experiments on motor units in spinal cats (Mayer et al. 1984; Cope et al. 1986), clearly confirm an increase of this ratio associated with a great decrease in the tetanic force. In contrast, the mechanism discussed above that is responsible for the prolongation of the twitch, i.e. prolonged Ca2+ transition, may prevent a significant decrease in the twitch force related to the muscle atrophy and is responsible for the increase of the twitch-to-tetanus ratio. In our study, the weight of the medial gastrocnemius muscle was considerably smaller, indicating its atrophy.

Our study revealed that the ability to potentiate the force, present in fast motor units in intact rats, was reduced 1 month after spinal cord transection. The post-tetanic potentiation was more reduced in the FF motor units as a result of spinal cord transection than that in the FR motor units. It has been suggested (Pette & Staron, 1990) that the post-tetanic potentiation is related to the mechanism of phosphorylation of myosin regulatory light chains, rendering actin–myosin interaction more sensitive to calcium ions released from the sarcoplasmatic reticulum. Therefore, we cannot exclude the possibility that this process might be weakened in muscle fibres of spinal rats. However, such a reduction of the post-tetanic potentiation in muscles of spinal animals has not been reported in other species. Mayer et al. (1984) reported similar values of post-tetanic potentiation in all types of motor units in gastrocnemius muscle of both intact and chronic spinal cats. It might be that these interspecies differences are related to differences in the properties of muscle fibres forming motor units of different species and the different proportion of motor unit types in gastrocnemius muscle in these animals (Stephens & Stuart, 1975; Kanda & Hashizume, 1992; Kadhiresan et al. 1996).

Fatigue properties

Our study demonstrated that fatigability of FR motor units was influenced by spinal cord transection, since they were more fatiguable in comparison to the control population. Since no significant changes were found in the fatigability of S and FF units, and the relative number of FR units was considerably diminished, it is likely that spinal cord transection influenced mainly the transformation of biochemical features of muscle fibres in FR units, shifting them to the fast fatigable type. It was reported that up to 7 months after spinal cord transection no changes of the fatigue index within individual types of motor units in cats were observed, although the mean value of the fatigue index for the whole population of motor units was significantly decreased (Munson et al. 1986). One might suggest that the observed decrease of the fatigue index of FR units only 1 month after spinal cord transection is due to their incomplete transformation. It can be expected that after a longer period this fatigue resistance would diminish to values characteristic for FF units. The probable reason of this decrease in fatigue resistance is limited motor activity of spinal rats. A similar decrease was obtained for EDL rat muscle when denervation or nerve conduction block by TTX were performed (Buffelli et al. 1997). These authors stressed the fundamental role of muscle contractile activity as a regulatory signal for muscle contractile properties, including fatigue resistance.

Profile of unfused tetanus

The ‘sag’ effect is widely described as the characteristic feature of fast motor units that is used for the classification into fast or slow units (Burke et al. 1973; Kernell et al. 1983). For the rat medial gastrocnemius, a 40 Hz stimulation frequency is used to demonstrate ‘sag’ (Grottel & Celichowski, 1990; Celichowski, 1992). However, in the present study ‘sag’ could not be used as the only criterion for classification of motor units in medial gastrocnemius muscle of spinal rats. In the vast majority of ‘fast’ motor units in rats after complete spinal cord transection, the ‘sag’ effect could not be obtained. In a few cases, ‘sag’ was observed at lower frequencies of stimulation only. In intact animals, the ‘sag’ phenomenon is related to a very effective process of summation of several contractile responses to the first few stimuli at the beginning of the tetanus (Celichowski et al. 2005). Thus, the disappearance of ‘sag’ after spinal cord transection indicates disturbances in the mechanism of summation of contractions within the unfused tetanus. This conclusion is in agreement with our results showing the increase in the twitch-to-tetanus ratio as well as a significant prolongation of contraction time after spinal cord transection. The correlation between the contraction time and ‘sag’ has been suggested previously by Carp et al. (1999).

Conclusion

In conclusion, considerable changes in the four basic contractile properties of motor units, i.e. contraction time, force, fatigability and ‘sag’, were observed in rats 1 month after spinal cord transection. The significant prolongation of single twitches affected all MUs, and in fast motor units disappearance of the ‘sag’ phenomenon could be found. Changes in the force parameters also influenced the twitch-to-tetanus ratio. However, direct relations between intracellular processes and physiological properties of muscle fibres, especially after spinal cord injury, still remain to be investigated.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Ariano MA, Armstrong RB & Edgerton VR (1973). Hindlimb muscle fiber populations of five mammals. J Histol Cytochem 21, 51–55.

Buffelli M, Pasino E & Cangiano A (1997). Paralysis of rat skeletal muscle equally affects contractile properties as does permanent denervation. J Muscle Res Cell Motil 18, 683–695.[CrossRef][Medline]

Burke RE, Levine DN, Tsairis P & Zajac FE (1973). Physiological types and histochemical profiles in motor units of the cat gastrocnemius. J Physiol 234, 723–748.[Abstract/Free Full Text]

Carp JS, Herchenroder PA, Chen XY & Wolpaw JR (1999). Sag during unfused tetanic contraction in rat triceps surae motor units. J Neurophysiol 8, 2647–2661.

Celichowski J (1992). Motor units of medial gastrocnemius in the rat during the fatigue test. I. Time course of unfused tetani. Acta Neurobiol Exp 52, 17–21.[Medline]

Celichowski J & Grottel K (1992). The dependence of the twitch course of medial gastrocnemius muscle of the rat and its motor units on stretching of the muscle. Arch Ital Biol 130, 315–325.[Medline]

Celichowski J, Pogrzebna M & Raikova RT (2005). Analysis of the unfused tetanus course in fast motor units of the rat medial gastrocnemius muscle. Arch Ital Biol 143, 51–63.[Medline]

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Cope TC, Bodine SC, Fournier M & Edgerton R (1986). Soleus motor units in chronic spinal transected cats: physiological and morphological alterations. J Neurophysiol 6, 1202–1220.

Davey DF, Dunlop C, Hoh JF & Wong SY (1981). Contractile properties and ultrastructure of extensor digitorum longus and soleus muscles in spinal cord transected rats. Aust J Exp Biol Med Sci 59, 393–404.[Medline]

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   Acknowledgements
 
The project was supported by grant no. KBN 2 P05A 092 27 from the Polish Ministry of Science and Information.





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