Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C71
On the origin of sense of force and weight
R. Fitzpatrick1, B. L. Luu1, J. D. Cole3, B. L. Day2
1. Neuroscience Research Australia, Sydney, New South Wales, Australia. 2. Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, London, United Kingdom. 3. Centre of Postgraduate Medical Research and Education, University of Bournemouth and Poole Hospital, Southampton, United Kingdom.
As a muscle is weakened by fatigue or partial paralysis, the increase in the motor command needed to lift a weight is thought to explain the increasing subjective heaviness of the lifted object (1). In these studies we show that peripheral signals normally underlie this sense of exerted force. With different fatiguing contractions we halved the force output of the thumb flexor muscles, which were then used to lift an object. For normal subjects, this resulted in objects feeling the same or lighter, consistent with the expected effects of the conditioning contractions on the sensitivity of peripheral receptors and contrary to the common view that perceived heaviness would increase. In contrast, for two deafferented subjects the perceived heaviness of the lifted object approximately doubled, in keeping with the theory of a central-signal associated with the motor command. In another experiment we completely paralysed the forearm muscles with curare and then allowed them to recover to half of their force output. This resulted in objects feeling lighter when lifted by the semi-paralysed thumb, even though the motor command to the motoneurones must have been greater. This is readily explained by reduced peripheral reafference associated with the lift, caused by the prolonged paralysis of muscle spindle intrafusal fibres. We conclude that peripheral signals, including a major contribution from muscle spindles, give rise to the sense of exerted force. In concept, however, reafference from peripheral receptors may also be considered a centrally generated signal. These results therefore challenge the distinction between central- and peripheral-based perception, and the concept that muscle spindles contribute only to perception of limb position and movement.
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