1. School of Physiology, Faculty of Health Sciences, University of the Witwaterstrand, 7 York Street, Parktown, Johannesburg 2193, South Africa
2. Department of Physiology, School of Medicine, Faculty of Health Sciences, University of Pretoria, Private Bag X323, Gezina, Pretoria 0031, South Africa
3. Department of Human Anatomy and Physiology, Faculty of Health Sciences, University of Johannesburg, Corner Beit and Siemert Street, Doornfontein, Johannesburg 2094, South Africa