Studies on limb muscles have demonstrated that fatigue initiates an inhibitory reflex originating from the muscle which reduces the range of motoneuron firing rates that can be elicited by voluntary effort. Less direct evidence indicates this reflex operates also in fatigue of respiratory muscles. Reduced firing rates optimize force generation by minimizing fatigue onset at peripheral sites; thus fatigue is generally less evident during voluntary compared with stimulated contractions. For limb muscles this reflex prevents motoneuron firing rates exceeding those required for maximum motor unit tetanic activation, relation to muscle contractile slowing, without necessarily limiting force production. In contrast during diaphragmatic fatigue, twitch occlusion and EMG studies indicate that reduced motor drive is a major factor in the decline in voluntary force generating capacity. While the existence of a strong inhibitory reflex has been well demonstrated, it remains unclear whether this only involves spinal neural processes or requires higher brainstem involvement; or whether changes in intrinsic cortical and/or spinal neural excitability (central fatigue) also influence motoneuron firing rates. Changes in excitability of brainstem motor pathways will be assessed by cortical stimulation and by recording the incidence of F-waves elicited by stimulation of motor nerves. Mechanisms involved in fatigue-induced reflex inhibition will be examined. Motoneuron firing rates did not change when muscle speed was reduced by length changes or cooling. Thus metabolic causes are implied. In fatigue from non-ischemic low intensity intermittent contractions, no appreciable metabolic changes accompanied fatigue. Reflex inhibition will be tested under these conditions to compare with previous measurements made only during sustained MVCs in which large metabolic changes are seen. Similar tests will be performed during fatiguing contractions induced after either small and large diameter fibers have been blocked. The central distribution of inhibitory input within the spinal cord will be examined by recording maximum motoneuron firing rates from one muscle before and after an adjacent muscle has been fatigued. In each study the responses during ischemic sustained MVCs will be compared with those seen during intermittent low intensity exercise in which force is reduced by similar amounts, but more slowly under aerobic conditions.

Agency
National Institute of Health (NIH)
Institute
National Heart, Lung, and Blood Institute (NHLBI)
Type
Research Project (R01)
Project #
5R01HL030026-10
Application #
3341045
Study Section
Respiratory and Applied Physiology Study Section (RAP)
Project Start
1982-09-30
Project End
1995-06-30
Budget Start
1992-07-01
Budget End
1993-06-30
Support Year
10
Fiscal Year
1992
Total Cost
Indirect Cost
Name
John B. Pierce Laboratory, Inc.
Department
Type
DUNS #
City
New Haven
State
CT
Country
United States
Zip Code
06519
Thomas, C K; Johansson, R S; Bigland-Ritchie, B (2006) EMG changes in human thenar motor units with force potentiation and fatigue. J Neurophysiol 95:1518-26
Breugelmans, J G; Bazzy, A R (1997) Developmental differences in endplate response to P-type calcium channel blockade in the rat diaphragm. Brain Res Dev Brain Res 101:277-81
Bigland-Ritchie, B; Rice, C L; Garland, S J et al. (1995) Task-dependent factors in fatigue of human voluntary contractions. Adv Exp Med Biol 384:361-80
McKenzie, D K; Bigland-Ritchie, B; Gorman, R B et al. (1992) Central and peripheral fatigue of human diaphragm and limb muscles assessed by twitch interpolation. J Physiol 454:643-56
Thomas, C K; Bigland-Richie, B; Johansson, R S (1991) Force-frequency relationships of human thenar motor units. J Neurophysiol 65:1509-16
Thomas, C K; Johansson, R S; Bigland-Ritchie, B (1991) Attempts to physiologically classify human thenar motor units. J Neurophysiol 65:1501-8
Thomas, C K; Bigland-Ritchie, B; Westling, G et al. (1990) A comparison of human thenar motor-unit properties studied by intraneural motor-axon stimulation and spike-triggered averaging. J Neurophysiol 64:1347-51
Westling, G; Johansson, R S; Thomas, C K et al. (1990) Measurement of contractile and electrical properties of single human thenar motor units in response to intraneural motor-axon stimulation. J Neurophysiol 64:1331-8
Thomas, C K; Johansson, R S; Westling, G et al. (1990) Twitch properties of human thenar motor units measured in response to intraneural motor-axon stimulation. J Neurophysiol 64:1339-46
Woods, J J; Furbush, F; Bigland-Ritchie, B (1987) Evidence for a fatigue-induced reflex inhibition of motoneuron firing rates. J Neurophysiol 58:125-37

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