The nerve roots for the L3 and L4 nerves pass through and leave the spinal column under the third and fourth lumbar vertebrae, respectively. The spinal cord does not run through these vertebrae, but the pair of L3 nerve roots exits between the L3 and L4 vertebrae, and the L4 nerve roots exit between the L4 and L5 vertebrae, the American Association of Neurological Surgeons reports.
Spinal disks are present between vertebrae and absorb shock. An injury or degenerative changes due to aging can cause a disk to herniate, meaning that the gel-like material in the center of the disk bulges out into the spinal canal. This bulging area can irritate the spinal nerve roots, reports the Mayfield Clinic.
Since the lumbar spine is flexible and bears much of the weight of the torso, it is prone to disk herniation. While injury to the L3-L4 section is less common than injuries to the L4-L5 or L5-S1 sections lower in the spine, about 5 percent of disk herniations do occur in this section, explains Spine-health.
If the disk herniation occurs between the L3 and L4 vertebrae, it can compress the L3 nerve roots and cause pain, weakness and numbness in the front inner thigh and down into the shin, says American Family Physician. It can also cause a diminished reflex in the knee.