General characteristics and topography
At the microscopic level, white matter appears as a homogeneous pattern consisting of cross-sections of myelinated fibers and their axons. However, the functional anatomy of this zone is extremely heterogeneous: nerve fibers are grouped into specific bundles and tracts, each occupying its own strictly defined territory.
Anatomically, the white matter is divided by the horns of the gray matter and glial septa into three paired funiculi (funiculi):
- Posterior funiculi: separated by the posterior median septum. Ascending sensory pathways predominate within them.
- Lateral funiculi: contain mixed tracts (ascending and descending pathways are represented equally).
- Anterior funiculi: separated by the deep anterior median fissure. Descending motor pathways primarily run here.
Proper fasciculi (zone around the gray matter)
The region of white matter immediately adjacent to the gray matter forms the system of proper fasciculi (fasciculi proprii). These include axons of diffuse bundle cells as well as processes of cells from the interstitial nucleus of Cajal.
The main task of these fibers is to connect adjacent (higher and lower) segments of the spinal cord. The processes of these pathways end in synapses on the motor neurons of the anterior horns. This organization provides oligosegmental reflexes, allowing the spinal cord to coordinate local reactions without the direct participation of the brain.
Pathways of the posterior and lateral funiculi
In the posterior funiculi (funiculus posterior), practically only ascending fibers run — processes of sensory neurons from the spinal ganglia. They ascend without decussation and without relaying in the spinal cord, reaching the medulla oblongata (nucleus gracilis and nucleus cuneatus).
- Fasciculus gracilis (Goll's fasciculus) — occupies a medial position.
- Fasciculus cuneatus (Burdach's fasciculus) — is located laterally.
They enter the system of spinocortical pathways and are responsible for conscious proprioceptive (muscle-joint) and tactile sensitivity.
The lateral funiculi (funiculus lateralis) have a more complex structure. In their lateral part lie ascending tracts formed by axons of association neurons from the posterior horns and the intermediate zone:
- Spinocerebellar pathways (posterior tract of Flechsig and anterior tract of Gowers) — convey information for unconscious movement coordination.
- Lateral spinothalamic tract — runs to the thalamus, providing a conscious reaction to pain and temperature stimuli.
- Spinotectal pathway — heads toward the midbrain.
In the medial part of the lateral funiculi run descending tracts whose axons descend from the brain to the motor neurons of the anterior horns. These include: the lateral corticospinal tract (from the cortex for conscious movements), the rubrospinal tract of Monakow (from the red nucleus), the olivospinal tract of Bechterew–Helweg (from the olivary nucleus), and the thalamospinal tract.
Conducting pathways of the anterior funiculi
In the anterior funiculi, there is only one major ascending pathway — the anterior spinothalamic tract. It heads toward the thalamus and participates in the formation of reflex arcs for conscious reactions to tactile stimuli (touch, sensation), unlike the lateral tract, which conducts pain and temperature.
The rest of the space in the anterior funiculi is occupied by descending motor tracts ending on the motor neurons of the anterior horns. 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 decussate in the pyramids of the medulla oblongata.
- From the midbrain: tectospinal tract (from subcortical visual and auditory centers). It provides 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 maintaining muscle tone).
Briefly: answers to topical questions
What is the anatomical and functional difference between the fasciculus gracilis and the fasciculus cuneatus?
The anatomical difference between the fasciculi lies in their topography within the posterior funiculus and the region of the body from which they conduct impulses.
| Characteristic | Fasciculus gracilis (fasciculus gracilis) | Fasciculus cuneatus (fasciculus cuneatus) |
|---|---|---|
| Topography in the posterior funiculus | Located medially | Located laterally |
| Origin of impulses | From the lower limbs and trunk below the 4th thoracic segment | From the upper limbs and trunk above the 4th thoracic segment |
Both fasciculi run in the posterior funiculi and conduct impulses from muscles and joints (proprioception, muscle-joint sensitivity) and from tactile receptors.
How does the decussation of the anterior corticospinal tract differ from that of the lateral corticospinal tract?
The main difference lies in the level at which the nerve fibers cross to the opposite side.
| Characteristic | Lateral corticospinal tract | Anterior corticospinal tract |
|---|---|---|
| Level of decussation | In the medulla oblongata (pyramidal decussation) | Segmentally at the level of the spinal cord segment |
| Localization in the spinal cord | Lateral funiculi of the spinal cord | Anterior funiculi of the spinal cord |
Thus, the lateral tract is formed by crossed fibers in the brainstem, whereas the anterior tract descends as a direct (uncrossed) pathway and decussates directly within the white commissure of the spinal cord.
From which nuclei does the vestibulospinal tract originate, and what is its function?
The vestibulospinal pathways originate from the vestibular nuclei and are represented by two tracts.
- Lateral vestibulospinal tract — originates from the lateral vestibular nucleus (Deiters' nucleus). Function: facilitation of extensor reflexes and maintenance of the muscle tone required for balance.
- Medial vestibulospinal tract — originates from the medial vestibular nucleus. Function: alteration of neck muscle tone in response to changes in head position; presumably participates in regulating balance reflexes through balancing arm movements.
To memorize the topography of the posterior funiculus tracts: "Goll runs medially, Burdach walks laterally." The fasciculus gracilis lies medially, and the fasciculus cuneatus lies laterally.
Class and exam questions
What is the functional difference between the lateral and anterior spinothalamic tracts?
The lateral pathway is responsible for transmitting pain and temperature sensitivity. The anterior pathway transmits tactile sensitivity impulses (touch and sensation).
Where do the neurons of the fasciculus gracilis and fasciculus cuneatus synapse?
These pathways ascend within the posterior funiculi without relaying and without decussating in the spinal cord. They terminate exclusively in the corresponding nuclei of the medulla oblongata.
What function does the tectospinal tract perform?
It transmits impulses from subcortical visual and auditory centers in the midbrain to the motor neurons of the spinal cord, providing protective startle reflexes to sudden visual or auditory stimuli.
What else to study on this topic
- Cytoarchitectonics of the interstitial nucleus of Cajal
- Mechanism of startle reflex formation
- Differences between Flechsig's and Gowers' spinocerebellar tracts
- Spinocortical pathways: information integration
- Influence of the reticulospinal formation on motoneuron tone
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