Sensation of respiratory force following low cervical spinal cord transection

The psychophysical technique of magnitude scaling was used to assess the sensation of respiratory muscle force in patients with low cervical spinal cord transection. The exponents of the power function relationship between load magnitude and sensation intensity during graded elastic and resistive ventilatory loading in the quadriplegic patients were 0.51 and 0.54, respectively. These values were significantly lower than those in normal subjects (P less than 0.05). This impairment in load sensation could not be attributed to differences in either the duration or magnitude of the forces generated by muscles of inspiration during loading, since the power function relationships between airway pressure and sensation intensity corrected for inspiratory duration were also significantly lower in the quadriplegic patients compared with the normal subjects. In contrast the perception of respiratory force during inspiratory maneuvers against a closed airway, as determined by both magnitude estimation and production tests, was the same in normal subjects and quadriplegics. These results suggest that afferent signals from rib cage receptors are important in shaping the intensity of the sensation during submaximal ventilatory loading. During maximal loading, i.e., airway occlusion, when respiratory muscle tension changes proportionally with the motor command, centrally generated efferent command signals may be employed to subserve the sensation of respiratory muscle force when inputs from rib cage muscle receptors are not available.