A flexible spine for use in one or more surgical instruments including a catheter and/or sheath of a robotic instrument system. The spine includes an elongate body that defines a central lumen and that is a unitary structure having a plurality of discrete sections, each of which has a distinguishing structural attribute that differentiates it from the other sections. Such distinguishing structural attributes may include, without limitation, materials, material attributes, shapes, sizes and/or attributes related to apertures in a wall of the elongate body, such as a number, shape, size, spacing and degree of overlap of such apertures. The arrangement of discrete, structurally different sections results in varying flexibility of the elongate spine and of corresponding sections of a surgical instrument incorporating the spine.