Which spinal nerves innervate the quadratus lumborum?

The quadratus lumborum is primarily innervated by the twelfth thoracic and the upper three to four lumbar spinal nerves. This muscle, located in the lower back, plays a key role in stabilizing the pelvis and lumbar spine, as well as assisting in lateral flexion of the trunk and extension of the lumbar spine.

The twelfth thoracic spinal nerve (T12) is crucial because it provides the initial innervation to the muscle, while the upper lumbar spinal nerves (L1, L2, and possibly L3 and L4) further contribute to its motor function. This anatomical connection is significant for movements involving the lower back and contributes to its overall function in posture and movement.

The other choices reference nerve roots that do not directly innervate the quadratus lumborum. For example, the lower thoracic and sacral spinal nerves do not adequately provide the specific innervation required for this muscle. Additionally, the first and second cervical spinal nerves and mid-cervical spinal nerves are far removed from the lumbar region, making them irrelevant for innervating the quadratus lumborum. Thus, the connection of this muscle to the specified spinal nerves is essential for its proper function in the lumbar region.