General Characteristics and Topography
At the microscopic level, white matter appears uniform, consisting of cross-sections of myelinated fibers and their axis cylinders. However, the functional anatomy of this region is highly heterogeneous: nerve fibers are grouped into specific bundles and tracts, each occupying a strictly defined area.
Anatomically, the white matter is divided by the gray matter horns and glial septa into three pairs of funiculi (funiculi):
- Posterior funiculi: Separated by the posterior median septum. They consist predominantly of ascending sensory pathways.
- Lateral funiculi: Contain mixed tracts (ascending and descending pathways are represented equally).
- Anterior funiculi: Separated by the deep anterior median fissure. They primarily carry descending motor pathways.
Fasciculi Proprii (Zone Surrounding the Gray Matter)
The region of white matter immediately adjacent to the gray matter forms the system of proper bundles (fasciculi proprii). These contain axons of diffuse funicular neurons as well as processes of cells from the interstitial nucleus of Cajal.
- The primary function of these fibers is to connect adjacent (superior and inferior) segments of the spinal cord. The processes of these pathways terminate in synapses on anterior horn motor neurons. This organization provides oligosegmental reflexes, enabling the spinal cord to coordinate local responses without direct input from the brain.
Pathways of the Posterior and Lateral Funiculi
The posterior funiculi (funiculus posterior) contain almost exclusively ascending fibers—the central processes of sensory neurons from the dorsal root ganglia. They ascend without crossing and without synapsing in the spinal cord, reaching the medulla oblongata (the nucleus gracilis and nucleus cuneatus).
- Fasciculus gracilis: Occupies a medial position.
- Fasciculus cuneatus: Occupies a lateral position.
They form part of the dorsal column-medial lemniscal system and mediate conscious proprioception (muscle-joint sense) and tactile sensation.
The lateral funiculi (funiculus lateralis) are more complex. Their lateral portion contains ascending tracts formed by axons of association neurons from the posterior horns and intermediate zone:
- Spinocerebellar tracts (posterior spinocerebellar tract of Flechsig and anterior spinocerebellar tract of Gowers) transmit information for unconscious movement coordination.
- Lateral spinothalamic tract travels to the thalamus, mediating conscious responses to pain and temperature.
- Spinotectal tract projects to the midbrain.
The medial portion of the lateral funiculi contains descending tracts whose axons descend from the brain to motor neurons in the anterior horns. These include the lateral corticospinal tract (from the cortex for voluntary movement), the rubrospinal tract of Monakow (from the red nucleus), the olivospinal tract of Bechterew-Helweg (from the inferior olivary nucleus), and the thalamospinal tract.
Pathways of the Anterior Funiculi
The anterior funiculi contain only one major ascending pathway—the anterior spinothalamic tract. It projects to the thalamus and participates in reflex arcs for conscious tactile sensation (touch), unlike the lateral tract, which carries pain and temperature.
The remaining space in the anterior funiculi is occupied by descending motor tracts that terminate on anterior horn motor neurons. They are classified by their origin:
- From the cerebral cortex: Anterior corticospinal tract. Unlike the lateral corticospinal tract, this pathway is direct and does not cross over in the medullary pyramids.
- From the midbrain: Tectospinal tract (from subcortical visual and auditory centers). It mediates rapid startle reflexes in response to unexpected visual and auditory stimuli.
- From the medulla oblongata: Vestibulospinal tract (from vestibular nuclei for balance regulation) and reticulospinal tract (from the reticular formation for muscle tone maintenance).