Disclosed is an internal cable system that communicates signals in an electronic device. The errors can be corrected by having the excitation pulse cover a smaller or a larger volume than the refocusing pulses. Furthermore, the intervertebral disc may be used in the cervical, the thoracic, or the lumbar regions of the spine. The torsion spring applies a torque onto each of the cams to reduce an angular distance between the cams when a cable exhibits insufficient tension.