Muscle Specific Adaptations of Short Latency Stretch Reflexes under Different Gravity Conditions
- Paper number
IAC-07-A1.2.04
- Author
Mr. Albert Gollhofer, University of Freiburg, Germany
- Coauthor
Mr. Mark Melnyk, University of Freiburg, Germany
- Coauthor
Mr. Wolfgang Taube, University of Freiburg, Germany
- Coauthor
Mr. Maike Hodapp, Germany
- Coauthor
Mr. Michael Faist, Germany
- Coauthor
Dr. Markus Gruber, University of Freiburg, Germany
- Year
2007
- Abstract
Changes in gravity conditions result in various adaptations of the whole body. Especially under microgravity the nervous system is affected to a great extent. Until now, only the adaptation of M. soleus (SOL) H-reflex was measured under microgravity during parabolic flight experiments (1, 2). For that reason there are two notable restrictions, considering the functional interpretation of the existing results. First, the H-reflex is evoked by electrical stimulation of Ia afferents providing an estimate of alpha motoneuron excitability in the target motoneuron (MN) pool, bypassing the muscle spindle. Therefore, only the stretch reflex reflects changes in fusimotor activation, muscle or tendon properties, which are of functional importance for movement and neuromuscular control (3, 4). Second, the SOL is a postural muscle, acting to a much larger extent against gravitational forces than its antagonist the M. tibialis anterior (TA). Eliciting the TA H-reflex is much more difficult than eliciting the SOL H-reflex. Therefore, only few studies investigated short latency reflexes in TA and SOL to describe adaptations for both an antigravity muscle and its antagonist (5). Therefore, the aim of the present study was to determine the gravity dependent modulation of mechanically (stretch reflex) induced stimuli of a lower leg antigravity muscle (SOL) and its antagonist (TA). Eight healthy subjects (6 men and 2 women) participated in a parabolic flight procedure. Several gravity levels were imposed perpendicular to the floor of the aircraft. The subjects were asked to maintain an erect posture. They were fixed to an ergometer, which induced fast dorsi- and plantarflexion of the ankle joint. Thus, stretch reflexes of either SOL or TA were elicited mechanically. Electromyographic activity (EMG) of SOL and TA was recorded in both legs using a pair of surface electrodes. Position signal of ergometer pedals and EMG traces were summed up for each subject in three gravity conditions (normal gravity NG: 0,95-1,05g; hyper gravity HG: >1,7g and micro-gravity MG: -0,05-0,05g Stretch reflexes were larger during MG compared to NG in SOL muscle indicating the excitability of the SOL motoneuron pool is dependent on some gravity-related factors. This was suggested by previous studies that demonstrated reduced H-reflexes during sitting and lying compared to standing. Interestingly, this was not the case for TA stretch reflexes. In TA short latency reflexes were considerably decreased during MG. The results indicate differences in the gravity dependent modulation of short latency reflexes between an antigravity muscle and its antagonist. Acknowledgement The experiments were performed during the 41th ESA parabolic flight campaign. References 1) Sato, T., Miyoshi, T., Nakazawa, K., Yano, H., Takeoka, H. (2001) J Gravit. Physiol 8, 97-99. 2) Miyoshi, T., Nozaki, D., Sekiguchi, H., Kimura, T., Sato, T., Komeda, T., Nakazawa, K., Yano, H. (2003) Exp. Brain Res. 150, 109-113. 3) Ferris, D. P., Aagaard, P., Simonsen, E. B., Farley, C. T., Dyhre-Poulsen, P. (2001) J. Physiol 530, 167-180. 4) Gollhofer, A., Schopp, A., Rapp, W., Stroinik, V. (1998) Eur. J. Appl. Physiol Occup. Physiol 77, 89-97. 5) Hoffman, M., Palmieri, R. M., Ingersoll, C. D. (2003) Int. J Neurosci. 113, 39-46.
- Abstract document