Tissue composition and general organization
Anatomically, the white matter (substantia alba) consists of more than just neural elements. Its composition includes:
- a system of myelinated and unmyelinated nerve fibers;
- neuroglial cells;
- blood vessels;
- a small amount of connective tissue.
Nerve fibers form compact groups known as fascicles (fasciculi). The left and right halves of the spinal cord are not isolated from each other: their white matter is connected by a transverse structure — the white commissure (commissura alba). Topographically, this commissure is located strictly anterior to the central canal of the spinal cord.
Funiculi of the spinal cord
The longitudinal sulci on the surface of the spinal cord (except for the posterior intermediate sulcus) act as natural boundaries. They divide the white matter of each half into exactly three funiculi (funiculi medullae spinalis):
- Anterior funiculus (funiculus ventralis / anterior) — located between the anterior median fissure and the anterolateral sulcus.
- Lateral funiculus (funiculus lateralis) — occupies the space between the anterolateral and posterolateral sulci.
- Posterior funiculus (funiculus dorsalis / posterior) — situated between the posterolateral and posterior median sulci.
An important anatomical feature is observed in the cervical and upper thoracic regions. Here, the posterior funiculus is additionally divided by the posterior intermediate sulcus. As a result, two bundles are formed: the medial fasciculus gracilis and the lateral fasciculus cuneatus.
Tracts and propriospinal fibers
Two main types of fibers run within the white matter:
- Projection fibers (afferent and efferent) — connect the spinal cord to the brain.
- Association fibers — connect segments of the spinal cord to one another.
Association fibers form the so-called fasciculi proprii of the spinal cord (fasciculi proprii). They are divided into anterior, lateral, and posterior, and their main feature is that they densely surround the gray matter on all sides.
Several specific structures are distinguished among the propriospinal fibers:
- Dorsolateral tract (tractus dorsolateralis) — a small bundle lying between the apex of the posterior horn (gray column) and the outer surface of the spinal cord, very close to the posterior root.
- Septomarginal fasciculus (fasciculus septomarginalis) — a thin bundle of descending fibers. It lies adjacent to the posterior median fissure and is best developed in the lower thoracic and lumbar segments.
- Interfascicular fasciculus (semilunar fasciculus) (fasciculus interfascicularis / semilunaris) — also consists of descending fibers. It is hidden within the medial part of the fasciculus cuneatus and is clearly identifiable in the cervical and upper thoracic segments.
Briefly: answers to topical questions
Which projection tracts run in the anterior funiculus of the spinal cord?
Motor pathways run in the anterior funiculus of the spinal cord. These include the tectospinal, anterior corticospinal, olivospinal, vestibulospinal, and reticulospinal tracts. These pathways predominantly contain descending fibers that transmit impulses from higher centers to the spinal cord.
Which ascending tracts run in the lateral funiculus of the spinal cord?
Ascending sensory pathways run in the lateral funiculus of the spinal cord: the anterior and lateral spinothalamic tracts, the anterior and posterior spinocerebellar tracts, as well as the spinotectal tract (tractus spinotectalis), which runs alongside the spinothalamic tract in the lateral funiculi. The posterior spinocerebellar tract ascends along the lateral funiculus, while the anterior spinocerebellar tract ascends in the ventrolateral parts of the lateral funiculi.
Which descending tracts are located in the lateral funiculus of the spinal cord?
Descending motor pathways are located in the lateral funiculus of the spinal cord. The lateral corticospinal and rubrospinal tracts (tractus rubrospinalis) run in the central part of the lateral funiculi. These tracts ensure the transmission of nerve impulses from higher centers of the central nervous system.
What is the function of the propriospinal fibers (fasciculi proprii) of the spinal cord?
The fasciculi proprii (fasciculi proprii) of the spinal cord are formed by association fibers surrounding the gray matter on all sides. They connect adjacent segments of the spinal cord with motor neurons of the anterior horns. The main function of these bundles is to mediate intersegmental reflexes, i.e., establishing communication between different segments of the spinal cord.
To remember the division of the posterior funiculus: "Gracilis is medial, cuneatus is lateral." The medial fasciculus gracilis runs closer to the center, while the lateral fasciculus cuneatus lies lateral to it.
Class and exam questions
What does the white matter consist of besides nerve fibers?
In addition to myelinated and unmyelinated fibers, the white matter contains neuroglia, blood vessels, and a small amount of connective tissue.
How do the right and left halves of the white matter connect?
They are connected by the white commissure (commissura alba), which is located anterior to the central canal of the spinal cord.
In which regions does the posterior funiculus divide into two bundles?
In the cervical region and the upper half of the thoracic region. The posterior intermediate sulcus divides it into the medial fasciculus gracilis and the lateral fasciculus cuneatus.
Where is the interfascicular (semilunar) fasciculus located?
It is formed by descending fibers and is located in the medial part of the fasciculus cuneatus. It is predominantly identified in the cervical and upper thoracic segments.
What else to study on this topic
- Topography of projection afferent fibers of the spinal cord
- Topography of projection efferent fibers of the spinal cord
- Detailed structure of the anterior and lateral fasciculi proprii
- Connection of association fibers with the functional anatomy of gray matter
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