White Matter


The White Matter of the spinal cord is located more superficially to the Grey Matter. It is composed of bundles, enclosed by a sheath, of  ascending and descending myelinated axons named: funiculi.

*NOTE: Grey Matter – Cell bodies; White Matter – Myelinated Axons.

The White Matter between sulcus posterolateralis and sulcus medianus posterior (fasciculus gracilis and fasciculus cuneatus) is referred to as The Posterior Funiculus.

The Anterior Funiculus is located between sulcus posterolateralis and fissura mediana anterior. At the location where ventral roots of spinal nerves exit the ventrolateral sulcus can be observed.

Sobotta: Atlas of Human Anatomy, Head, Neck, and Neuroanatomy; 2011; p. 334; Fig 12.182a

A tract represents bundles of axons that have the same origin and target. In contrast, a bundle of neurons that originate in different places and have different targets are termed fasciculi.

Three important terms:

  • “Funiculi” – bundle of axons
  • “Tracts” – bundle of axons that originate and terminate in the same region
  • “Fasciculi” – bundle of axons that originate and terminate in different regions

The image below illustrates the organisation of the first, second and third order neurons.

figure_10_06_labeled

Nerve Fasciculi.—The longitudinal fibers are grouped into more or less definite bundles or fasciculi. These are not recognizable from each other in the normal state, and their existence has been determined by the following methods: (1) A. Waller discovered that if a bundle of nerve fibers be cut, the portions of the fibers which are separated from their cells rapidly degenerate and become atrophied, while the cells and the parts of the fibers connected with them undergo little alteration. This is known as Wallerian degeneration. Similarly, if a group of nerve cells be destroyed, the fibers arising from them undergo degeneration. Thus, if the motor cells of the cerebral cortex be destroyed, or if the fibers arising from these cells be severed, a descending degeneration from the seat of injury takes place in the fibers. 

Reference:
1. Grey, Henry. 1918. Anatomy of the Human Body; IX Neurology. 3. The Spinal Cord; 29

Descending tracts in the Anterior and Lateral Funiculi:

  1. Corticospinal tract – Contains axons originating predominantly from the frontal and parietal lobes. They pass trhough the internal capsule, pedunculi cerebri of the midbrain, the pons and they form the medullary piramids (see diagram). At the border between medulla and the spinal cord the corticospinal tract undergoes an incomplete decussation to form the:
    1. Anterior Corticospinal tract
    2. Posterior Corticospinal tract
  2. Vestibulospinal tract – Axons arise from the lateral vestibular ncl. They synapse with interneurons and alpha-motoneurons, to exert a facilitatory influence on spinal mechanisms that control back and extensor muscles. They have a big influence over the cervical and lower lumbar spinal regions.
    1. Medial
    2. Lateral
  3. Reticulospinal tract – Consists of fibers from the pntine and medullary reticular formation, which mainly terminate on the internucial neurons. However some medullary axons synapse with motor neurons.
    The reticulospinal tract corresponds to regions from which inhibitory effects of the spinal motoneurons are elicited. Facilitatory effects of the spinal motoneurons are obtained from the pontine reticular formation.
  4. Tectospinal tract – The axons arise from neurons in the superior colliculus. The majority of the fibers terminate in the upper 4 cervical segments, but none end directly on alpha-motoneurons. This tract mediates reflexes of postural movements in respons to visual and auditory stimuli.
  5. Rubrospinal tract – Fibers of the rubrospinal tract originate in the magnocellular region.
  6. Interstitiospinal tract
  7. Raphespinal tract

Ascending tracts in the Anterior and Lateral Funiculi:

  1. Spinothalamic tract 
    1. Anterior – Ascending axons from ncl. proprius carry information (impulses associated with thermal and painful sensations) across the midline, through the anterior funiculus, to the thalamus.
    2. Lateral – Ascending axons from around ncl. proprius carry information from the lower limb and the upper limb (axons relatively closer to the grey matter in contrast to the axons carrying info. from the lower limb). They are responsible for the ‘light touch’ feeling.
  2. Spinoreticular tract – Bundle of axons which terminate as crossed fibers in the pontine reticular formation and predominantly as uncrossed fibers in the medullary reticular formation. They form the so called ascending reticular activating system, which plays a significant role in behaviour awareness and cortex electroactivity modulation. It is also involved in the transmission of pain and sensations originating from the internal organs.
  3. Spinotectal tract
  4. Anterior spinocerebellar tract – Makes its first appearance at the lower lumbar levels, but its neurons do not originate from such a discrete regions. At the upper pontine levels the tract enters the cerebellum by coursing along the superior cerebellar peduncle and the fibers terminate contralaterally in the anterior cerebellar vermis.
    The impusles conveyed by this tract are concerned with the co-ordinated movement and posture of the entire lower limb.
  5. Posterior spinocerebellar tract – The axons rise from ncl. thoracicus (Dorsal nucleus of Clarke. The pass through the inferior cerebellar peduncle and terminate in rostral and caudal portions of the vermis. Neurons of Clarke’s nucleus receive monosynaptic excitation from muscle spindles, Golgi tendon organs, and pressure receptors. Impulses are utilised in the fine co-ordination of posture and movement of individual limb muscles.
  6. Spino-olivary tract – Part of the spinocerebellar circuitry, conducting impulses to the cerebellum via the olive.
  7. Cuneocerebellar tract – Dorsal ncl. of Clarke is not present above C8, therefore the axons receiving impulses from the upper limb terminate on the accessory cuneate nucleus. This is said to be the homologous of ncl. thoracicus posterior. Ascending fibers give rise to the cuneocerebellar tract. They enter the cerebellum via cerebellar peduncles, and terminate in the cerebellar cortex.

Posterior Funiculus:

  1. Spinobulbar tract/fibers

 

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