White matter tracts
Well-defined columns of myelinated axons (white matter) surrounding columns of neural cell bodies (grey matter) may be divided functionally into fascicles or tracts. These tracts are made up of ascending and descending nerve projections that transmit information to and from the cerebral cortex. Many of these pathways have a somatotopic arrangement. Damage to these tracts may have devastating consequences, but also provide insight into the function and organization of these complex tracts.
By the fourth week of gestation, the neural plate, derived from the ectoderm of the embryonic disk, folds to form the neural tube. The cells at the edges of the neural plate will form the neural crest, which will proliferate into the dorsal root ganglion and spinal ganglia of the peripheral nervous system. Cells of the neural crest also give rise to Schwann cells, glial cells, and pia mater, among many other cell types. Primitive neuroepithelial cells line the interior of the neural tube and rapidly proliferate into neuroblasts and glioblasts that migrate to the intermediate zone (mantle zone) and form the grey matter of the spinal cord. Unipolar and bipolar neuroblasts extend to the marginal zone, where they become myelinated and form the white matter of the spinal cord.
The spinal cord is a dense bundle of neurons extending the length of the spinal canal from the medulla oblongata to conus medullaris, around the first or second lumbar vertebrae. Fascicles of neurons, the cauda equina, continue inferiorly to reach their corresponding nerve roots. The spinal cord is enclosed by the dura, arachnoid mater, and pia mater, separated by the subarachnoid and subdural spaces. Pia and glial fibers form the filum terminale which originates at the base of the conus medullaris and tethers the cord to the dura at the caudal end of the vertebral canal. Cervical and lumbar enlargements of the cord accommodate the augmented neural mass essential for innervation of the upper and lower extremities.
A central canal, contiguous with the fourth ventricle, is filled with cerebrospinal fluid. The spinal cord is divided into two symmetrical halves by the anterior median fissure and a shallow posterior median sulcus. Anterior and posterior spinal nerve rootlets exit the spinal cord at the anterolateral sulcus and posterolateral sulcus, respectively. The anterior white commissure at the floor of the anterior median fissure, anterior to the gray commissure, accommodates crossing neurons to the contralateral side of the body.
The white matter of the spinal cord is so named for the population of myelinated nerve fibers. White matter is arranged, anatomically, into columns (funiculi), and further subdivided by function into tracts. The anterior white column lies between the anterior median fissure and anterior horns of gray matter (also demarcated by the anterolateral sulcus).
The lateral white column lies between the horns of gray matter (also demarcated by the anterolateral sulcus and posterolateral sulcus). The posterior white column, lying between the postero-median sulcus and the posterolateral sulcus, may be further subdivided into the fasciculus gracilis (medial) and the fasciculus cuneatus (lateral).
White Matter Tracts
White matter columns may be functionally divided into white matter tracts. Each tract has a specific function and a somatotopic arrangement. Some tracts decussate (cross-over) across the anterior white commissure, others in the midbrain or not at all. White matter tracts may transmit ascending or descending impulses. The white matter tracts also contain glial cells (astrocytes and oligodendrocytes) positioned along fascicles of myelinated and unmyelinated axons. Nomenclature of the tract generally indicates the origin and termination of the tract (i.e. spinothalamic tract originates in the spinal cord and ascends to the thalamus).
Five types of neurons populate the white matter:
- long ascending fibers of spinal neurons (spinothalamic tract),
- long descending neurons of supraspinal neurons (corticospinal tract),
- afferent fibers from pseudounipolar neurons of the dorsal root ganglions (posterior column),
- fibers from motor neurons of the ventral grey columns exiting via ventral nerve roots,
- spinal interneurons (propriospinal neurons).
The posterior column (dorsal funiculus), contains the ascending projections of pseudounipolar neurons of the posterior root spinal ganglia. The tracts transmit:
- proprioception (joint position)
- fine touch
- two-point discrimination information to the medulla oblongata
These pathways cross in the medulla, therefore the tracts transmit ipsilateral sensory information. The columns are bordered by the posterior grey column and postero-median septum. In the cervical region, the column is divided into the fasciculus cuneatus and fasciculus gracilis, transmitting information from the upper and lower body, respectively.
Posterior column nerves terminate in the respectively-named nuclei of the lower medulla oblongata: the nucleus cuneatus and nucleus gracilis. The second-order neurons then cross the midline via the internal arcuate tract in the medulla oblongata and terminate in the posterior thalamus. Third-order neurons then project to the somatosensory cortex. As in most tracts, neurons have a somatotopic arrangement with the lower body in the medial aspect of the column and upper body at the lateral aspect of the column.
The spinothalamic tract is responsible for conveying:
- crude touch
- pressure information
Sensory neurons are received via the dorsal roots, and ascend a short distance in the posterolateral tract (or Lissauer’s tract) to synapse in the posterior grey column. Spinothalamic neurons then decussate via the white commissure, and thus contain contralateral sensory information within the spinal tract.
The spinothalamic tract may be further subdivided into the anterior spinothalamic tract, conveying light touch information, and the lateral spinothalamic tract, conveying pain and temperature information. Fibers also show somatotopic arrangement, with the lower body positioned laterally and upper body medially. Spinoreticular neurons are found intermingled with spinothalamic tract and carry pain information to the reticular system of the brainstem.
The dorsal spinocerebellar tract runs in the posterolateral column, adjacent to the lateral corticospinal tract and dorsolateral column. It conveys:
- touch information to the cerebellum
Sensory afferents from muscle, skin, and Golgi tendon organs, pass through the dorsal root to synapse in the dorsal nucleus (Clark’s column). The dorsal spinocerebellar tract contains the second-order neurons that ascend from the ipsilateral dorsal nucleus.
Whereas, neurons in the ventral spinocerebellar tract, concerned with coordination of movement and posture, primarily decussate through the white commissure. In the cervical regions, the dorsal nucleus is replaced with the accessory cuneate nucleus, and the ascending second-order neurons form the cuneocerebellar tract, lying adjacent to the dorsal spinocerebellar tract.
Originating in the deeper layers of grey matter, axons cross the white commissure and ascend superficially in the anterior column to the olivary nuclei. These neurons carry proprioceptive and pain signals, and are primarily associated with balance regulation.
The corticospinal tract (pyramidal tract) originates in the motor cortex, passes through the internal capsule, and forms a discrete bundle, the pyramid, within the medulla oblongata. Each pyramid contains nearly a million fibers. Three-quarters of the nerves are myelinated where myelination is not complete until age 2. At the medullary pyramids, most of the fibers decussate (the pyramidal decussation) and continue as the lateral corticospinal tract.
The remaining, uncrossed fibers, continue to descend as the anterior corticospinal tract, and later decussate through the anterior white commissure. Corticospinal neurons synapse with somatic spinal nerves in the anterior grey column, projecting through ventral nerve roots to muscle groups. The lateral corticospinal tract occupies a large, somatotopically-organized area between the posterior grey column and the spinothalamic tract. The anterior corticospinal tract occupies a smaller area in the anterior column, lining the anteromedian fissure.
The vestibulospinal system consists of two distinct tracts that facilitate quick reflexive movements in response to sudden changes in body position (i.e. falling). Fibers from the medial vestibular nucleus descend in the medial vestibulospinal tract in the anterior column, both crossed and uncrossed, and synapse with lower motor neurons in the cervical cord. These fibers exert an inhibitory influence on muscles in the neck and upper back.
Beginning at the lateral vestibular nucleus, on the floor of the fourth ventricle, the lateral vestibulospinal tracts descend in the ipsilateral anterior column. These neurons have a stimulatory effect on extensor muscles of the back and limbs, while exerting an inhibitory influence on the corresponding flexor muscles.
The rubrospinal tract originates from the red nucleus in the midbrain, crosses at the ventral tegmental decussation and descends in the lateral column, partially intermingled with the corticospinal tract. The rubrospinal tract influences motor movement by stimulating distal flexor muscles and inhibiting extensor muscles, with some fibers directly innervating motor neurons. This tract is underdeveloped in humans compared to animals, in favor of a substantially developed corticospinal system.
Reticulospinal system originates from the reticular formation (tegmental field) of the medulla oblongata and the pons. The medial reticulospinal tract is a heterogenous collection of both crossed and uncrossed neurons that descend in the anterior and anterolateral columns and influence posture and movement in response to external stimuli.
The lateral reticulospinal tract is associated with both pain perception and motor function. The tectospinal tract, arising from the superior colliculus,and the interstitial spinal tract, arising from the interstitial nucleus of Cajal from the vestibulospinal tract, are also involved with control of movement.
The hypothalamospinal tract, arising from the paraventricular nucleus of the hypothalamus, extends to parasympathetic and sympathetic preganglionic cells in the posterior grey column. These pathways are variably influenced by oxytocin, vasopressin, and dopamine.
Catecholaminergic tracts responsive to epinephrine, norepinephrine, and dopamine arise from the locus coeruleus and medulla oblongata, and innervate presynaptic ganglia in the posterior and intermediate grey columns.
Raphespinal tracts arise from the raphe nuclei; they are influenced by serotonin, and involved in nociception and movement.
Propriospinal neurons comprise a significant amount of the mass in the white and grey matter tracts of the spinal cord. These interneuron tracts are ascending and descending, crossed and uncrossed, fibers that originate and terminate in spinal grey matter. They play a variety of essential roles including somatic reflexes, autonomic, and vasomotor functions.
Syringomyelia is a rare fluid-filled cyst, typically a dilation of the central canal, and most commonly occurs in the setting of Chiari malformation (a congenital malformation of the cerebellum). Small central lesions in the spinal cord may affect decussating fibers of the spinothalamic tract at the white commissure, while sparing other descending and ascending tracts.
This results in a loss of pain and temperature sensation in the affected dermatome, with sparing of vibration, pressure, and position sensation. Larger lesions may also cause segmental motor neuron weakness, and loss of vibratory and position sense below the lesion.
Rare in the antibiotic era, tabes dorsalis is an isolated dorsal column lesion associated with tertiary syphilis. Onset can be more than twenty years from the primary infection. Sensory ataxia, the loss of proprioception, and vibratory sensation, and severe lancinating pain results from involvement of the dorsal columns and dorsal roots. Sensory ataxia, due to loss of proprioceptive input, is demonstrated by Romberg’s sign (inability to maintain posture with feet together and eyes closed).
Complete hemisection of the spinal cord, classically by penetrating trauma or tumor, results in ipsilateral loss of motor function (disruption of the corticospinal tract), vibration and proprioception (disruption of the posterior column) below the level of the lesion, and contralateral loss of pain and temperature (disruption of the spinothalamic tract) below the lesion. This pattern occurs because the spinothalamic tract, corticospinal tract, and the posterior column neurons decussate, but do so at different levels.
The second-order neurons of the posterior column decussate in the medulla (internal arcuate fibers), as do the first-order neurons of the corticospinal tract; therefore, hemisection of the cord will cause ipsilateral loss of function at that level.
The second-order neurons of the spinothalamic tract travel one or two levels cephalad and then cross at the white commissure. Disruption of the corticospinal tract may present as spastic paralysis with the Babinski sign (paradoxical dorsiflection of the foot upon stroking the sole). Coexisting injury to the sympathetic chain in a cervical injury results in Horner Syndrome (miosis, ptosis, and anhidrosis affecting the ipsilateral face).
Posterolateral Sclerosis (B12 deficiency)
Severe vitamin B12 deficiency may result in degeneration of the posterior and lateral white matter tracts. Symptoms include weakness or spastic paralysis, hyperactive reflexes, and a Babinski sign, due to involvement of the corticospinal tract. Degeneration of the posterior column results in impaired proprioception, two-point discrimination, and vibratory sense.
Treatment is vitamin B12 administration, but function may be slow to recover, if at all. Severe B12 deficiency is rare in healthy individuals with a normal diet. In addition to neurologic symptoms, severe B12 deficiency is usually accompanied with folate deficiency and anemia. Milder forms of B12 deficiency are more common, caused by:
- reduced intake (i.e. vegan diet)
- impaired absorption (i.e. weight loss surgery, Crohn’s disease)
- autoimmune diseases (i.e. pernicious anemia)
- certain medications (i.e. metformin)