Serotonin affects movement gain control in the spinal cord.

A fundamental challenge for the nervous system is to encode signals spanning many orders of magnitude with neurons of limited bandwidth. To meet this challenge, perceptual systems use gain control. However, whether the motor system uses an analogous mechanism is essentially unknown. Neuromodulators, such as serotonin, are prime candidates for gain control signals during force production. Serotonergic neurons project diffusely to motor pools, and, therefore, force production by one muscle should change the gain of others. Here we present behavioral and pharmaceutical evidence that serotonin modulates the input-output gain of motoneurons in humans. By selectively changing the efficacy of serotonin with drugs, we systematically modulated the amplitude of spinal reflexes. More importantly, force production in different limbs interacts systematically, as predicted by a spinal gain control mechanism. Psychophysics and pharmacology suggest that the motor system adopts gain control mechanisms, and serotonin is a primary driver for their implementation in force production.

Effect of antispastic drugs on motor reflexes and voluntary muscle contraction in incomplete spinal cord injury.

OBJECTIVE: To investigate the effects of antispastic drugs baclofen and tizanidine on reflexes and volitional tasks. DESIGN: Double-blind, placebo-controlled, crossover, before-after trial, pilot study. SETTING: Research laboratory in a rehabilitation hospital. PARTICIPANTS: Men with chronic (>6mo) motor incomplete spinal cord injury (N=10) were recruited for the study. INTERVENTIONS: Tizanidine, baclofen, and placebo were tested in this study. Agents were tested in separate experimental sessions separated by >1 week. MAIN OUTCOME MEASURES: Reflex and strength were measured before and after the administration of a single dose of each intervention agent. Electromyographic and joint torque data were collected during assessments of plantar flexor stretch reflexes, maximum contraction during motor-assisted isokinetic movements, and maximum isometric knee extension and flexion. RESULTS: Reduced stretch reflex activity was observed after the administration of either tizanidine or baclofen. We observed that tizanidine had a stronger inhibitory effect on knee extensors and plantar flexors whereas baclofen had a stronger inhibitory effect on the knee flexors. The effects of these drugs on strength during isometric and isokinetic tasks varied across participants, without a consistent reduction in torque output despite decreased electromyographic activity. CONCLUSIONS: These results suggest that antispastic drugs are effective in reducing stretch reflexes without substantially reducing volitional torque. Differential effects of tizanidine and baclofen on reflexes of flexors and extensors warrant further investigation into patient-specific management of antispastic drugs.

Feasibility for developing cardiovascular exercise recommendations for persons with motor-complete paraplegia based on manual wheelchair propulsion; A protocol and preliminary data.

BACKGROUND: The Center for Disease Control, American Heart Association, and American College of Sports Medicine recommendations for duration and intensity of exercise are based on the amount of energy expenditure required to maintain cardiovascular health in able body individuals; 1000 Kilocalories (Kcals) per week of energy expenditure has been demonstrated to achieve this effect. Manual wheelchair propulsion (MWP) represents a practical and accessible form of exercise for individuals with paraplegia. OBJECTIVE: To describe a method to determine the duration of MWP required to expend 1000 Kcals, when performed by individuals with paraplegia due to motor-complete spinal cord injury (SCI). STUDY DESIGN: Cross-sectional study. SETTING: Rehabilitation Research Laboratory. PARTICIPANTS: Sixteen adults with motor complete T3-T12 paraplegia (body mass index < 35, duration of paraplegia > 3 months). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Indirect calorimetry during MWP was measured in order to calculate caloric expenditure per minute. These data were used to calculate the number of minutes of MWP required to expend 1000 Kcal in one week. RESULTS: During MWP, participants expended 3.3 ± 1.0 Kcal/minute. Based on this figure, 1000 Kcal of energy expenditure in one week would require 303 minutes of MWP per week, or 43.3 minutes per day, 7 days per week. CONCLUSIONS: Our data suggest that it is feasible to create a practical and accessible exercise recommendation based on manual wheelchair propulsion for individuals with paraplegia due to motor-complete SCI. Larger studies are needed in order to develop accurate exercise recommendations for persons with SCI.

Energy Cost of Lower Body Dressing, Pop-Over Transfers, and Manual Wheelchair Propulsion in People with Paraplegia Due to Motor-Complete Spinal Cord Injury.

BACKGROUND: Energy required for able-bodied individuals to perform common activities is well documented, whereas energy associated with daily activities among people with spinal cord injury (SCI) is less understood. OBJECTIVE: To determine energy expended during several basic physical tasks specific to individuals with paraplegia due to motor-complete SCI. METHODS: Sixteen adults with motor-complete SCI below T2 level and duration of paraplegia greater than 3 months were included. Oxygen consumption (VO2), caloric expenditure, and heart rate were measured at rest and while participants performed lower body dressing (LBD), pop-over transfers (POTs), and manual wheelchair propulsion (MWP) at a self-selected pace. These data were used to calculate energy expenditure in standard metabolic equivalents (METs), as defined by 1 MET = 3.5 mL O2/kg/min, and in SCI METs using the conversion 1 SCI MET = 2.7 mL O2/kg/min. RESULTS: VO2 at rest was 3.0 ± 0.9 mL O2/kg/min, which equated to 0.9 ± 0.3 standard METs and 1.1 ± 0.4 SCI METs in energy expenditure. LBD required 3.2 ± 0.7 METs and 4.1 ± 0.9 SCI METs; POTs required 3.4 ± 1.0 METs and 4.5 ± 1.3 SCI METs; and MWP required 2.4 ± 0.6 METs and 3.1 ± 0.7 SCI METs. CONCLUSIONS: Resting VO2 for adults with motor-complete paraplegia is 3.0 mL O2/kg/min, which is lower than standard resting VO2 in able-bodied individuals. Progressively more energy is required to perform MWP, LBD, and POTs, respectively. Use of the standard METs formula may underestimate the level of intensity an individual with SCI uses to perform physical activities.