Skip to content
Robotic Spine Universe

Learn · Structure

The L4–L5 motion segment

Two lumbar vertebrae, the disc between them, the joints and ligaments that tie them, and the nerves that pass by. One of the two most common places for a disc to herniate, and a good place to see how bone, disc and nerve fit together.

Updated 2026-10-028 sources

Depth:

Why it matters

The lumbar vertebrae carry the greatest share of body weight, and the discs at L4–L5 and L5–S1 are the most common sites of disc herniation, which can cause sciatica [1]. Looking closely at one lumbar level shows how bone, disc, ligament and nerve share a small space.

What it is

Two vertebrae, L4 and L5, joined in front by the intervertebral disc between their bodies and behind by paired articular processes and ligaments [1].

  • The vertebral body is the front part of each vertebra and supports the body weight; lumbar vertebrae have large, thick bodies [1].
  • The disc is a fibrocartilaginous pad with a fibrous outer anulus fibrosus and a gel-like centre, the nucleus pulposus [1].
  • Behind, the paired superior articular processes of L5 join the inferior articular processes of L4, forming slightly moveable joints [1].
  • Inside the canal the posterior longitudinal ligament is attached to the back of the vertebral bodies, and the ligamentum flavum joins the laminae of the two vertebrae [1].
  • The joint between the two bodies is a cartilaginous joint: the disc unites the vertebrae strongly but still allows a limited amount of movement [2].

What it does

It bears weight and lets the lower back bend a little at each level. The nucleus pulposus has a high water content that resists compression, which makes it important for weight bearing [1].

Bending forward compresses the front of the disc and expands the back of it [1].

How it works

Nerves pass this level in two ways [3, 4]:

  1. The L4 nerve leaves the canal here, through the intervertebral foramen between L4 and L5, because each lumbar nerve emerges between the vertebra of the same name and the next one [3].
  2. The cord itself has usually ended higher up, so the canal at this level holds the cauda equina, the bundle of lower lumbar and sacral nerves on their way down to their own exits [4, 5].

The L4 and L5 nerves both join the sacral plexus, which also takes S1 to S4; the lumbar plexus takes all the lumbar nerves [3]. In the standard examination after spinal cord injury, ankle dorsiflexion tests L4 and extension of the great toe tests L5; the key sensory points are the medial malleolus for L4 and the top of the foot at the third metatarsophalangeal joint for L5 [6, 7].

Normal behaviour

With increasing age, the water content of the nucleus pulposus gradually declines [1]. The discs are thickest in the lumbar region, which carries the most body weight [1].

The bottom of the lumbar spine varies between people. In radiographs of 5941 people, 9.9 %[8] had a lumbosacral transitional vertebra: a sacralised L5 or a lumbarised S1 [8].

When it does not behave

If the back of the anulus fibrosus is weakened by injury or age, bending forward and lifting can push the nucleus pulposus out backwards through it: a herniated disc [1]. At L4–L5 or L5–S1 this can cause sciatica, pain that spreads from the lower back down the thigh into the leg [1].

Sciatica is the result of compression or irritation of the sciatic nerve or of any of the spinal nerves that form it [3]. This site describes such patterns for learning only; it cannot say what causes anyone's pain.

Common misconceptions

Misconception: A slipped disc slides out of place like a loose tile.

The disc does not move as a whole: in a herniation the gel-like nucleus pulposus protrudes through a weakened part of the anulus fibrosus [1].

Misconception: The spinal cord runs through the L4 vertebra.

In most adults the cord ends near L1, so the canal at L4 holds the cauda equina rather than the cord [5, 4].

Check yourself

Which nerve leaves the canal between L4 and L5?

The L4 nerve: lumbar nerves emerge between the vertebra with the same name and the next one [3].

What part of the disc protrudes in a herniation?

The nucleus pulposus, through a weakened posterior anulus fibrosus [1].

Which muscle function tests the L4 segment in the ISNCSCI examination?

Ankle dorsiflexion [6, 7].

Read next

References

  1. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 7.3 The Vertebral Column. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/7-3-the-vertebral-column
  2. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 9.1 Classification of Joints. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/9-1-classification-of-joints
  3. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 13.4 The Peripheral Nervous System. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/13-4-the-peripheral-nervous-system
  4. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 13.2 The Central Nervous System. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/13-2-the-central-nervous-system
  5. Nakashima H, Ito K, Katayama Y, Tsushima M, Ando K, Kobayashi K, et al.. The Level of Conus Medullaris in 629 Healthy Japanese Individuals. Journal of Clinical Medicine. 2021;10(14):3182. doi:10.3390/jcm10143182
  6. Rupp R, Biering-Sørensen F, Burns SP, Graves DE, Guest J, Jones L, et al.. International Standards for Neurological Classification of Spinal Cord Injury. Topics in Spinal Cord Injury Rehabilitation. 2021;27(2):1-22. doi:10.46292/sci2702-1
  7. Kirshblum SC, Burns SP, Biering-Sorensen F, Donovan W, Graves DE, Jha A, et al.. International standards for neurological classification of spinal cord injury (Revised 2011). The Journal of Spinal Cord Medicine. 2011;34(6):535-546. doi:10.1179/204577211x13207446293695
  8. French HD, Somasundaram AJ, Schaefer NR, Laherty RW. Lumbosacral Transitional Vertebrae and Its Prevalence in the Australian Population. Global Spine Journal. 2014;4(4):229-232. doi:10.1055/s-0034-1387808

For learning only. This page does not diagnose, predict outcomes or recommend treatment. Corrections are welcome: how to suggest one.