White matter of the spinal cord

Substantia alba medullae spinalis

Also known as: spinal white matter, funiculi, dorsal columns, lateral funiculus, anterior funiculus, spinal tracts

The peripheral mantle of myelinated axons surrounding the spinal grey matter. It is organised into posterior, lateral and anterior funiculi, which contain the ascending and descending tracts.

Overview

The white matter on each side is divided by the dorsal and ventral root entry zones into three funiculi.

• The posterior funiculus, between the posterior median sulcus and the dorsal horn, contains the dorsal columns: the medial fasciculus gracilis (lower body, below T6) and the lateral fasciculus cuneatus (upper body, T6 and above).
• The lateral funiculus carries the lateral corticospinal and rubrospinal tracts, the lateral spinothalamic tract, the dorsal and ventral spinocerebellar tracts and the lateral reticulospinal tract.
• The anterior funiculus carries the anterior corticospinal, vestibulospinal, tectospinal and medial reticulospinal tracts, the anterior spinothalamic tract and the medial longitudinal fasciculus.

The anterior white commissure, in front of the grey commissure, is where spinothalamic fibres decussate.

Tracts are somatotopically layered, which explains the clinical patterns of compression. In the dorsal columns, sacral fibres lie medially and cervical fibres laterally. In the lateral corticospinal and spinothalamic tracts the order is reversed: sacral fibres lie laterally (superficially) and cervical fibres medially, next to the grey matter. An expanding intramedullary lesion therefore affects cervical fibres first and may leave sacral sparing, whereas extramedullary compression affects sacral fibres first.

The amount of white matter increases from caudal to rostral, because ascending tracts accumulate fibres and descending tracts have not yet terminated. Cervical cross-sections therefore have the most white matter. Within a tract, fibres with the same destination run together, and propriospinal fibres (the fasciculus proprius) form a thin layer around the grey matter linking segments.

Location

Surrounds the grey matter throughout the cord; thickest in the cervical region.

Parts & subdivisions

Posterior funiculus (dorsal columns)

Fasciculus gracilis (medial; T7 and below, lower limb) and fasciculus cuneatus (lateral; T6 and above, upper limb). They carry fine touch, vibration and conscious proprioception and remain ipsilateral throughout the cord.

Lateral funiculus

Contains the lateral corticospinal tract (crossed, voluntary motor), rubrospinal tract, lateral spinothalamic tract (pain and temperature), dorsal and ventral spinocerebellar tracts (superficial) and lateral reticulospinal tract.

Anterior funiculus

Contains the anterior corticospinal tract (uncrossed), anterior spinothalamic tract (crude touch), lateral and medial vestibulospinal tracts, tectospinal tract, medial reticulospinal tract and medial longitudinal fasciculus.

Anterior white commissure

The decussation of the second-order spinothalamic axons and of some anterior corticospinal fibres. It is selectively damaged by syringomyelia and central cord lesions.

Dorsolateral fasciculus (Lissauer tract)

A thin bundle at the dorsal horn tip in which incoming pain and temperature fibres ascend or descend 1–2 segments before synapsing.

Anatomy

Arterial supply

Peripheral (circumferential) branches of the anterior and posterior spinal arteries via the pial arterial plexus (vasocorona). The posterior spinal arteries supply the dorsal columns.

Histology

Myelinated axons with oligodendrocyte myelin, fibrous astrocytes and interfascicular oligodendrocytes, with radial pial septa carrying vessels.

Clinical significance

Subacute combined degeneration (vitamin B12 deficiency)

Symmetrical demyelination of the dorsal columns (loss of vibration and proprioception, positive Romberg, sensory ataxia) and lateral corticospinal tracts (spastic weakness, extensor plantars), often with peripheral neuropathy (absent ankle jerks). Seen in pernicious anaemia, metformin use and nitrous oxide abuse. Treat with B12.

Tabes dorsalis

Tertiary syphilis damages the dorsal roots and dorsal columns, causing sensory ataxia (stamping gait, positive Romberg), lightning pains, areflexia, Charcot joints and Argyll Robertson pupils.

Multiple sclerosis and transverse myelitis

Demyelinating plaques in the cord white matter cause partial, often asymmetric sensory levels, spastic paraparesis, bladder urgency and Lhermitte sign (an electric shock down the spine on neck flexion, from dorsal column lesions).