Neuroanatomy, Anterior White Commissure

Structure and Function

A cross-section of the spinal cord shows a butterfly-shaped area of gray matter composed of nerve cell bodies surrounded by white matter composed of myelinated nerve fibers. The anterior white commissure is a rectangularly-shaped collection of nerve fibers located directly anterior to the central canal and gray commissure, posterior to the anterior median fissure. Sensory fibers from the spinothalamic tract and motor fibers from the anterior corticospinal tract decussate in the anterior white commissure. The anterior white commissure is particularly important in the relay of contralateral sensory and motor information because of these crossing fibers.

The spinothalamic tract is a major ascending neural pathway in the sensory detection of pain, temperature, touch, and pressure. Sensory fibers synapse on cell bodies in the dorsal root ganglion. These axons of second-order neurons from the spinothalamic tract cross over at the anterior white commissure and also ascend one to three levels in the spinal cord through the Lissauer tract. After decussating, they ascend and synapse in the ventroposterolateral (VPL) nucleus of the thalamus.  Third-order neurons continue to transmit sensory information to the primary somatosensory cortex.[1]

The corticospinal tract is a major descending neural pathway for the control of voluntary movement. Most of the fibers of the corticospinal tract decussate in the medulla and form the lateral corticospinal tract. Up to 15% of fibers within the anterior corticospinal tract, however, cross at the anterior white commissure. These fibers then synapse with cell bodies of lower motor neurons in the gray matter of the contralateral ventral horn.[2] The anterior corticospinal tract communicates with lower motor neurons that innervate the axial muscles of the trunk.

Embryology

At approximately week three following fertilization, the developing embryo forms the neural groove, the shallow median indentation of the neural plate. The neural tube on the neural plate's lateral margins begins to elevate before joining to form the neural tube.  Signaling from the notochord causes the neural tube and related structures to give rise to central nervous system components. The brain arises from the cranial end of the neural tube the spinal cord from the caudal end. Neural crest cells arise from the neural ectoderm during the neurulation process, transitioning from epithelial to mesenchymal cells then migrating and giving rise to the peripheral nervous system.[3][4][5] The anterior white commissure develops as part of the central nervous system and is composed of the white matter of the spinal cord, with embryological derivation from the neuroectoderm.

Blood Supply and Lymphatics

Blood supply to the spinal cord comes from three main vessels: the anterior spinal artery and the two paired posterior spinal arteries.[6] The anterior spinal artery originates from the vertebral artery and from radicular arteries, the most important of which is known as the artery of Adamkiewicz. The anterior spinal artery provides blood supply to the anterior white commissure.

Physiologic Variants

Physiological variations of the anterior white commissure are dependent on the differences between the spinothalamic and anterior corticospinal tracts. Variations among these tracts may result in more or fewer fibers from each respective tract crossing at the anterior white commissure. The anterior corticospinal tract has significant physiological variations between individuals. In a study of 50 spinal cords, 74% had variations in the size of the right and left corticospinal tracts, with most cords having more fibers on the right.[7] Also, the anterior corticospinal tract develops later than the lateral corticospinal tract and may even be absent in certain individuals.[8] As a result, crossing fibers in the anterior white commissure may lack anterior corticospinal tract fibers between individuals.   

Surgical Considerations

Commissural myelotomy is a surgical procedure in which an incision is made in the anterior white commissure to interrupt the spinothalamic tract. This procedure aims to interrupt the pain transmission from the spinothalamic tract and leads to bilateral loss of pain. In a study of 11 patients, it was documented to significantly improve pain in 8 patients with oncogenic pain in the abdominal and pelvic regions[9]. Punctate myelotomy (limited myelotomy) is another option to interrupting the nociceptive pathway. Instead of incising the anterior white commissure, the visceral nociceptive signals are interrupted through an incision in the ventromedial dorsal columns of the spinal cord.[10]

Clinical Significance

The anterior white commissure is a clinically important region for central cord syndrome,[11] an incomplete injury that typically involves the cervical spinal cord and affects the upper more than lower extremities. Central cord syndrome can result from acute-on-chronic trauma from falls or motor vehicle collisions, especially among patients over age 50.[12] Syringomyelia, or cavitation of the cervical spinal cord originating in the central canal, begins by affecting crossing spinothalamic fibers of the anterior white commissure, causing a characteristic loss of pain and temperature sensation. Symptoms may progress to a "cape-like" distribution of numbness and weakness as surrounding structures become involved. Because it contains crossing pain and temperature fibers of the spinothalamic tract,  the anterior white commissure is also a potential therapeutic target for pain control interventions. 

Other Issues

The anterior white commissure of the spinal cord is not to be confused with another midline neural pathway with a similar-sounding name, the anterior commissure of the brain. This latter structure is a fiber bundle running posterior to the corpus callosum that is believed to be important in interhemispheric communication within the brain.[13]

Disclaimer: This scholarly activity was supported (in whole or part) by HCA and/or an HCA-affiliated entity. The views expressed in this publication represent those of the author(s) and do not necessarily represent the official views of HCA or any of its affiliated entities.