In a medical instrument for insertion into an examination subject, as well as a medical examination device or treatment device employing such an instrument, the instrument has an elongated instrument body formed by a number of successively arranged rigid sections, with respective, successive sections being connected to one another via articulated joints which can be angled relative to one another. At least one optical fiber supplied with light is conducted along the instrument body, and at least one fiber Bragg grating is fashioned in the optical fiber in a region adjacent to a joint which is deformed to a degree corresponding to angling of the adjacent sections to the joint, thereby modulating the light in the optical fiber.