Article

Sympathetic outflow enhances the stretch reflex response in the relaxed soleus muscle in humans.

National Institute of Occupational Health, Dept. of Physiology, Lersø Parkallé 105, DK-2100 Copenhagen, Denmark.
Journal of Applied Physiology (impact factor: 3.75). 04/2005; 98(4):1366-70. DOI:10.1152/japplphysiol.00955.2004
Source: PubMed

ABSTRACT Animal experiments suggest that an increase in sympathetic outflow can depress muscle spindle sensitivity and thus modulate the stretch reflex response. The results are, however, controversial, and human studies have failed to demonstrate a direct influence of the sympathetic nervous system on the sensitivity of muscle spindles. We studied the effect of increased sympathetic outflow on the short-latency stretch reflex in the soleus muscle evoked by tapping the Achilles tendon. Nine subjects performed three maneuvers causing a sustained activation of sympathetic outflow to the leg: 3 min of static handgrip exercise at 30% of maximal voluntary contraction, followed by 3 min of posthandgrip ischemia, and finally during a 3-min mental arithmetic task. Electromyography was measured from the soleus muscle with bipolar surface electrodes during the Achilles tendon tapping, and beat-to-beat changes in heart rate and mean arterial blood pressure were monitored continuously. Mean arterial pressure was significantly elevated during all three maneuvers, whereas heart rate was significantly elevated during static handgrip exercise and mental arithmetic but not during posthandgrip ischemia. The peak-to-peak amplitude of the short-latency stretch reflex was significantly increased during mental arithmetic (P < 0.05), static handgrip exercise (P < 0.001), and posthandgrip ischemia (P < 0.005). When expressed in percent change from rest, the mean peak-to-peak amplitude increased by 111 (SD 100)% during mental arithmetic, by 160 (SD 103)% during static handgrip exercise, and by 90 (SD 67)% during posthandgrip ischemia. The study clearly indicates a facilitation of the short-latency stretch reflex during increased sympathetic outflow. We note that the enhanced stretch reflex responses observed in relaxed muscles in the absence of skeletomotor activity support the idea that the sympathetic nervous system can exert a direct influence on the human muscle spindles.

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Keywords

3-min mental arithmetic task
 
Achilles tendon
 
arterial blood pressure
 
beat-to-beat changes
 
bipolar surface electrodes
 
enhanced stretch reflex responses
 
human muscle spindles
 
human studies
 
Mean arterial pressure
 
mean peak-to-peak amplitude
 
muscle spindle sensitivity
 
peak-to-peak amplitude
 
percent change
 
short-latency stretch reflex
 
soleus muscle
 
soleus muscle evoked
 
stretch reflex response
 
sympathetic nervous system
 
sympathetic outflow
 
three maneuvers
 

Nis Hjortskov