Tail muscles become slow but fatigable in chronic sacral spinal rats with spasticity

Abstract

Paralyzed skeletal muscle sometimes becomes faster and more fatigable after spinal cord injury (SCI) because of reduced activity. However, in some cases, pronounced muscle activity in the form of spasticity (hyperreflexia and hypertonus) occurs after long-term SCI. We hypothesized that this spastic activity may be associated with a reversal back to a slower, less fatigable muscle. In adult rats, a sacral (S2) spinal cord transection was performed, affecting only tail musculature and resulting in chronic tail spasticity beginning 2 wk later and lasting indefinitely. At 8 mo after injury, we examined the contractile properties of the segmental tail muscle in anesthetized spastic rats and in age-matched normal rats. The segmental tail muscle has only a few motor units (<12), which were easily detected with graded nerve stimulation, revealing two clear motor unit twitch durations. The dominant faster unit twitches peaked at 15 ms and ended within 50 ms, whereas the slower unit twitches only peaked at 30-50 ms. With chronic injury, this slow twitch component increased, resulting in a large overall increase (>150%) in the fraction of the peak muscle twitch force remaining at 50 ms. With injury, the peak muscle twitch (evoked with supramaximal stimulation) also increased in its time to peak (+48.9%) and half-rise time (+150.0%), and decreased in its maximum rise (-35.0%) and decay rates (-40.1%). Likewise, after a tetanic stimulation, the tetanus half-fall time increased by 53.8%. Therefore the slow portion of the muscle was enhanced in spastic muscles. Consistent with slowing, posttetanic potentiation was 9.2% lower and the stimulation frequency required to produce half-maximal tetanus decreased 39.0% in chronic spinals. Interestingly, in spastic muscles compared with normal, whole muscle twitch force was 81.1% higher, whereas tetanic force production was 38.1% lower. Hence the twitch-to-tetanus ratio increased 104.0%. Inconsistent with overall slowing, whole spastic muscles were 61.5% more fatigable than normal muscles. Thus contrary to the classical slow-to-fast conversion that is seen after SCI without spasticity, SCI with spasticity is associated with a mixed effect, including a preservation/enhancement of slow properties, but a loss of fatigue resistance.

FIG. 1

Twitch properties of the 14th ventrolateral segmental tail muscle of old normal rats and of rats with spasticity after chronic sacral spinal cord injury in response to supramaximal stimulation pulses. Representative twitches from normal (–––) and chronic spinal (—) animals show that peak twitch force is much greater in chronic spinal animals (A). When these twitches are normalized to peak twitch force, it is clear that contraction and relaxation are both prolonged in spastic tail muscles from chronic spinal animals (B), and not just larger. Time to peak twitch (C) is the time required to achieve peak twitch force beginning at the onset of force production. Twitch half-rise time (D) is the time required to achieve one-half of peak force (see 0.5 on vertical scale of B) beginning at onset of force production. Twitch force (E) is taken as the absolute maximal twitch force, and force:mass ratio (F) is the ratio of twitch force to muscle mass. In this and subsequent figures, white bars are normal animals and black bars are chronic spinals. *Significant difference from old normal rats and †significant difference from young normal rats in this and subsequent figures (P< 0.05). Values shown are means ± SD.

FIG. 2

Representative traces from both old normal (A) and chronic spinal (B) rats show that twitch contractions of segmental tail muscles can be separated into twitches of their constituent motor units. With nerve stimulation of slowly increasing intensity, discrete increases in twitch force were seen (a–g in A and a–h in B), representing successive recruitment of individual motor units. Means of successive motor unit twitches that make up the muscle twitch were subtracted as indicated to reveal constituent motor units. For example, c − b = 1 in A and d − c = 1 in B. Motor units are arranged from fastest (1) to slowest (6 or 7) twitch contraction time. Note that slowest units in the chronic spinals reached their peak force ~50 ms after force onset, whereas faster units completed their twitch by this time. Thus twitch force at 50 ms (TT₅₀), normalized to peak twitch force, shows the overall “slowness” of a tail muscle (C). Bar graph format as in Fig. 1. Significance accepted at P< 0.05.

FIG. 3

Maximum rates of twitch rise and twitch decay determined from the derivative of twitch. Absolute max rise rate (A) is maximum rate of rise of twitch force development in the twitch, and absolute max decay rate (B) is maximum rate of muscle relaxation after peak twitch force. Normalized rise (C) and decay (D) rates are rates of rise and decay of twitch normalized to peak force. Bar graph format as in Fig. 1. Significance accepted at P < 0.05.

FIG. 4

Representative tetani and their relationships to twitch contractions are shown for old normal rats (A) and chronic spinal rats (B). Tetani were elicited with supramaximal stimulation at 200 Hz for 500 ms. Twitches were produced by single, supramaximal stimulation pulses delivered 1,200 ms before and 1,200 ms after the onset of tetanus. In A and B, twitch before and twitch after are shown on an enlarged scale to the right of the full-scale traces. Tetanic force (C) is the peak force achieved during a single tetanus, and normalized tetanus (D) is the ratio of tetanic force to muscle mass. Note that tetanic force production is significantly smaller in tail muscles from chronic spinal rats (C). However, when the ratio of tetanic force to muscle mass is calculated, decrease in tetanic force with spinal cord injury seems to be accounted for by muscle mass atrophy (D). Tetanus half-rise time (E) is time required to reach one-half of peak tetanic force. Tetanus half-fall time (F) is time required after the end of tetanic stimulation for muscle force to be reduced by half. Posttetanic potentiation (G) is the ratio of peak twitch force after a tetanus to peak twitch force before a tetanus. Twitch:tetanus ratio (H) is the ratio of peak force of the nonpotentiated twitch (twitch before) to peak force of the tetanus. Bar graph format as in Fig. 1. Significance accepted at P < 0.05.

FIG. 5

Fusion properties of segmental tail muscles. A: forces developed during supramaximal tetanic stimuli (300 ms) at frequencies from 10 to 200 Hz are shown for chronic spinal rats (●, error bars upward), old normal rats (○, error bars downward), and young normal rats (□, error bars upward). Note that symbols for old normal rats largely overlay those for young normal rats because values for these groups are nearly identical. Values are normalized to maximum tetanic force at 200 Hz. B: F₅₀ is the frequency required to reach half-maximal tetanic force (0.50 on vertical scale in A). Force frequency curve for each animal was fit to a sigmoidal function (r²≥ 0.9 for all sigmoidal fits), and F₅₀ was calculated for each animal using its sigmoidal function. In chronic spinal rats relative to normal rats, F₅₀ is achieved at a significantly lower stimulation frequency. Bar graph format as in Fig. 1. Significance accepted at P < 0.05.

FIG. 6

Fatigue with repeated contractions. Muscles were fatigued by producing 300-ms tetanic contractions (each with supramaximal stimulation at 40 Hz), repeated once per second for 100 s. Representative traces show the 1st and 100th tetani of the test for segmental tail muscles in old normal rats (A) and chronic spinal rats (B). Insets: ongoing change in peak tetanic force over 100 sweeps. Note that peak force in repeated 40-Hz trains potentiates slightly before declining, as shown in insets of A and B, and declines more in chronic spinal rats. Note also that force scales in A and B differ. To quantify fatigability, fatigue index (C) was calculated as the ratio of tetanic force produced in the 100th contraction to tetanic force produced in the 1st contraction. Both forces were measured relative to baseline force before the 1st contraction. Bar graph format as in Fig. 1. Significance accepted at P < 0.05.