Effect of spinal cord injury and its lesion level on stretch reflex modulation by cold stimulation in humans
Introduction
There have been several studies on the neurophysiological effects of cooling of a lower limb in able-bodied humans (Knutsson and Mattsson, 1969, Lee and Warren, 1974) and humans with spinal cord transection (Lightfoot et al., 1975). Lightfoot et al. (1975) examined the effect of cold stimulation on the site at which stretch reflex (SR) was measured (soleus muscle) in humans with complete spinal cord transection between cervical (C) 5 and thoracic (T) 10 and demonstrated that SR amplitude was reduced. They attributed the reduction in SR amplitude to decreased firing of the muscle spindle due to the direct cooling effect. Similar findings were obtained in studies on SR modulation of quadriceps and triceps surae muscles in able-bodied humans (Knutsson and Mattsson, 1969, Lee and Warren, 1974), but Knutsson and Mattsson (1969) found that there was a transient increase in SR amplitude during the first 2–4 min of a 20-min cold application and then a gradual decline in several subjects. The mechanism(s) underlying the modulation of SR amplitude by cold stimulation in able-bodied humans and humans with spinal cord injury (SCI) has not been fully elucidated.
Cold stimulation of the contralateral leg enables elimination of the direct cooling effect on the muscle at which SR is measured. If SR amplitude is modulated, this modulation may reflect the effect of afferent inputs induced by cold stimulation on the stretch reflex circuit. There is evidence for SR modulation by this kind of mechanism during cold stimulation in able-bodied humans, and the effect has been found to be facilitatory (Kamibayashi et al., 2009). We hypothesized that SR amplitude is modulated by the effect of afferent inputs induced by cold stimulation of the contralateral leg on stretch reflex circuits even in humans with SCI but that the SR response is different among able-bodied humans and humans with SCI of various lesion levels because the extent of central nervous systems participating in SR modulation varies with and without SCI and depending on the lesion level.
The purpose of the present study was to determine how short-latency SR amplitude in the soleus muscle is modulated by cold stimulation applied on the thigh of the contralateral leg in able-bodied individuals and in humans with SCI of various lesion levels (C6 to T12). In the present study, we chose spinal cord-injured humans with complete paralysis of the legs (ASIA grade: A) as subjects in order to minimize effects of afferent inputs from the legs to the supraspinal center and supraspinal descending commands on spinal circuitry. During an experimental test, arterial blood pressure and heart rate (HR) were simultaneously measured, because circulatory responses have been shown to relate with SR response in able-bodied humans (Hjortskov et al., 2005, Kamibayashi et al., 2009). In the SCI population, circulatory responses to stimuli imposed on the paralyzed area may be different in individuals with SCI at or above T6 and in individuals with SCI at or below T10 (Ogata et al., 2009). Therefore, we paid special attention to the difference in SR responses in these two groups.
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Participants
Twelve subjects with clinically complete SCI (ASIA grade: A) participated in the experiment along with 9 able-bodied subjects. Table 1 summarizes their physical characteristics. All subjects with SCI were at least 8 months post-injury and none were taking medications likely to affect the results of our study such as antispasticity medication and cardiovascular medication. When individuals with SCI were divided into a group with injury level at or above T6 (n = 8) and a group with injury level at
Electromyographic responses
The average latency of all reflex electromyographic responses across subjects was 44 ± 6 ms (33–59 ms), indicating that the responses were short-latency spinally mediated reflexes.
SR amplitude at CS1 was lower than the baseline in 8 of the 12 subjects with SCI (45–99% of baseline), and 6 of these 8 subjects had lesion levels at or above T6. SR amplitude at CS2 was higher than the baseline in all of the subjects with lesion level at or below T10 (115–140% of baseline), whereas SR amplitude was lower
Discussion
We determined how soleus SR amplitude is modulated by cold stimulation applied to the skin of the contralateral leg in able-bodied individuals and individuals with SCI of various lesion levels. The main findings of the present study were as follows: SR amplitude was increased by cold stimulation in the able-bodied group, SR amplitude tended to increase in the group with SCI at or below T10, SR amplitude was decreased by cold stimulation in the group with SCI at or above T6, and the decrease in
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgement
This study was supported by a grant from the Japan Society for the Promotion of Science for Young Scientists.
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