What is the motion unit?

What is the motion unit?

• A. The entire spine
• B. A disc
• C. Two adjacent articulating surfaces and all the tissues between them
• D. A single vertebra

Answer: (C)

A motion unit is the two neighboring vertebrae with the intervertebral disc between them, along with all the tissues between them (facet joints, ligaments, joint capsules, and the attachments of surrounding muscles). This segment is where most spinal motion occurs, governed by the disc and facet joints and constrained by the ligaments within that pair of vertebrae. The entire spine is made up of many such motion units in sequence, not just one, and a single vertebra by itself or a disc alone does not constitute the unit of motion.

A motion unit is the two neighboring vertebrae with the intervertebral disc between them, along with all the tissues between them (facet joints, ligaments, joint capsules, and the attachments of surrounding muscles). This segment is where most spinal motion occurs, governed by the disc and facet joints and constrained by the ligaments within that pair of vertebrae. The entire spine is made up of many such motion units in sequence, not just one, and a single vertebra by itself or a disc alone does not constitute the unit of motion.