INTERNAL STRUCTURE

 

Internally, the spinal cord is composed of a central area of gray matter surrounded by white matter. The gray matter is shaped like butterfly wings that communicate through the gray commissure. This is located dorsally and ventrally to the central canal. It is formed by unmyelinated fibers involved in crossed reflexes. On each side, the gray matter is divided into a dorsal horn (sensory) and a ventral horn (motor). At the level of the thoracic and lumbar spinal segments, there is a small lateral horn or intermediate gray substance, formed by the bodies of preganglionic neurons of the sympathetic system. At the level of the sacral segments, the sacral preganglonic parasympathetic neurons have their body also located at the intermediate gray mater. The funiculi[1] are formed by white matter made of ascending and descending axons. The two dorsal funiculi (left and right) are separated by the dorsal median septum, identifiable externally by the dorsal median sulcus. The ventral funiculi (left and right) are partially separated by the ventral median fissure but communicate by the white commissure for myelinated axons. This commissure is located dorsally to the ventral median fissure and ventrally to the gray commissure. The lateral funiculi are situated between the dorsal and ventral horns on each side. Most of the ascending tracts undergo some mixing in the spinal cord as they are not clearly defined except for the gracilis and cuneate fascicles (G. Skerrit, G. King’s applied anatomy of the central nervous system of domestic mammals. 2nd edition 2018). However, these proprioceptive fascicles are also mixed with other nociceptive tracts.

The tracts in the white matter of the spinal cord are the followings:
- Fasciculus gracilis[2]
- Fasciculus cuneatus[3]
- Fasciculus proprius[4]
- Medullary reticulospinal tract[5]
- Rubrospinal tract[6]
- Lateral pyramidal tract[7]
- Ventral pyramidal tract[8]
- Dorsal spinocerebellar tract[9]
- Ventral spinocerebellar tract[10]
- Spinothalamic tract[11]
- Spinotectal tract[12]
- Pontine reticulospinal tract[13]
- Medial vestibulospinal tract[14]
- Lateral vestibulospinal tract[15]
- Medial tectospinal tract[16]

- Lateral tectotegmentospinal tract[17]

- Rostral spinocerebellar tract[18]

- Spinovestibular tract[19]

- Spinocervicothalamic tract[20]

 

The neurons of the gray matter of the spinal cord can be ordered in nuclei or laminae. The first classification does not account for the neurons that are dispersed between the nuclei but it may be easier for a better understanding.

The spinal cord nuclei are identified as follows:

- Lateral motor nuclei[21]
- Medial motor nuclei[22]
- Motor nucleus of the accessory nerve[23] (from C1 to C7)
- Thoracic nucleus or Stilling-Clark nucleus[24] (from T1 to L3-L4)
- Lateral cervical nucleus[25] (C1 and C2)
- Marginal nucleus[26] (also called dorsomarginal nucleus)
- Substantia gelatinosa[27]
- Nucleus proprius[28]
- Intermediolateral nucleus[29]
- Intermediomedial nucleus[30]
- Sacral parasympathetic nucleus[31]

The neurons of the gray matter of the spinal cord are also be arranged in 10 laminae (Rexed’s laminae):

-

- Lamina I: Corresponds to the dorsomarginal nucleus

- Lamina II: Corresponds to Rolando's gelatinous substance

- Laminae III and IV: Correspond to the nucleus propius. Receive nociceptive, tactile and mechanical fibers

- Laminae V and VI: Receive the rubrospinal and corticospinal tracts, and visceral afferences. Plate VI is the origin of the fasciculus propius.

- Lamina VII: It is the widest. It is made up of interneurons that control motor neurons. It is the site of the thoracic nucleus and neurons of the autonomic nervous system.

- Lamina VIII: It is made up of alpha and gamma neurons whose axons exit from the ventral root to axial muscles.

- Lamina IX: It is made up of alpha and gamma neurons whose axons exit from the ventral root to axial and appendicular muscles.

- Lamina X: Consists of gray matter that surrounds the central canal. It is the central intermediate  substance and it is involved in visceral input from the ventral funiculum.

 

Along the length of the spinal cord there is a central canal filled with cerebrospinal fluid (CSF) and lined by ependymal cells. It extends rostrally (to become the fourth ventricle of the brain stem) and caudally (at the level of the conus medullaris to form a small ventricular dilatation called the terminal ventricle).

[1] From the Latin funiculus, “cord string”.

[2] The fasciculus gracilis is formed by proprioceptive fibers from the pelvic limb that end in the gracilis nucleus in the medulla oblongata.

[3] The fasciculus cuneatus is located laterally to the fasciculus gracilis. It is made of proprioceptive fibers from the thoracic limb. It ends in the medial cuneate nucleus in the medulla oblongata.

[4] The fasciculus proprius is formed by fibers located close to the gray matter of the spinal cord. Its fibers connect different spinal cord segments.

[5] The fibers of the medullary reticulospinal tract originate in the reticular formation of the medulla oblongata and descend bilaterally in the lateral funiculus of the spinal cord.

[6]  The rubrospinal tract is formed by fibers that connect the red nucleus with the motor neurons of the brain stem and the spinal cord. Its fibers cross in the ventral portion of the tegmental decussation and descend in the lateral funiculus. It reaches laminae V. VI and VII in the spinal cord. The rubrospinal tract is almost absent in humans.

[7] The lateral pyramidal (lateral corticospinal) tract is formed by fibers that descend from the motor cerebral cortex. The majority of the fibers cross the midline at the decussation of pyramids in the medulla oblongata and descend in the lateral funiculus. They reach motor neurons of the ventral horn in the spinal cord. Axons from the sensory cortex project to laminae III to VI, and axons from the motor cortex project to laminae VI to X.

[8] The ventral pyramidal (ventral corticospinal) tract is formed by a few fibers that do not decussate at the level of decussation of pyramids and descend in the ventral funiculus, crossing the midline in the white commissure of the spinal cord to reach the motor neurons of the contralateral ventral horn. In dogs, most of the axons (50%) of the lateral and ventral corticospinal tract reach cervical level, 20% the midthoracic portion of the spinal cord, and the rest (30%) the lumbosacral region. In the cat the pyramidal system reaches the entire spinal cord. In horse, goat, cow, deer and sheep the corticospinal fibers reach only the cervical segments. However, in humans, this corticospinal tract is well developed.

[9] The dorsal spinocerebellar tract is located dorsally in the lateral margin of the lateral funiculus. It is formed by axons coming from the ipsilateral thoracic nucleus (Clarke's) that reach the cerebellum through the caudal cerebellar peduncle. It transmits proprioceptive information from the pelvic limbs.

[10] The ventral spinocerebellar tract is located in the lateral margin of the lateral funiculus, ventrally to the dorsal spinocerebellar tract. It is formed by axons from neurons located in the contralateral thoracic nucleus (Clarke's) at the base of the dorsal horn, between T1 and L4, that decussate in the white commissure and ascend to enter the cerebellum through the rostral cerebellar pedunculus. The ventral spinocerebellar tract carries proprioceptive fibers from the pelvic limb.

[11] The spinothalamic tract is located in the lateral funiculus. It originates in the substantia gelatinosa and mostly of its fibers decussate in the white comissure. It transmits sensory stimuli (nociceptive, tactile and thermal) to the contralateral thalamus. Collaterals from this tract are projected to the ascending activating reticular system in the brain stem.

[12] The spinotectal tract is formed by fibers from the nucleus proprius in the dorsal horn. They cross the midline in the white commissure and ascend in the ventral funiculus to reach mesencephalic nuclei (rostral colliculus, red nucleus, reticular formations, periaqueductal gray matter) and the thalamus. It transmits stimuli from mechanoreceptors and nociceptors (pain).

[13] The pontine reticulospinal tract is formed by fibers from the pontine reticular formation that descend in the ipsilateral ventral funiculus to end in ipsi- and contralateral neurons of the ventral horn.

[14] The medial vestibulospinal tract is formed by axons from the rostral, medial and caudal vestibular nuclei that reach cervical spinal cord segments. The fibers of the medial vestibulospinal tract travel in the medial longitudinal fasciculus in the brain stem, and in the ventral funiculus in the cervical spinal cord

[15] The lateral vestibulospinal tract is formed by axons that descend from the lateral vestibular nucleus to ipsi- and contralateral motor neurons of the ventral horn of the entire spinal cord.

[16] The medial tectospinal tract is formed by fibers from the colliculi, decussate in the dorsal portion of the tegmental decussation and descend in the ventral funiculus of the cervical segments of the spinal cord. It is responsible for reflex movements of the head and neck started by visual and auditory stimuli.

[17] The lateral tectotegmentospinal tract (lateral tectospinal tract) is formed by fibers coming from the mesencephalic tectum and tegmentum that cross in the dorsal portion of the tegmental decussation and descend in the lateral funiculus to reach cervical spinal segments and the intermediate gray matter of the first four thoracic segments. It is involved in the sudden turning of the head towards an auditory or visual stimulus and activation of the sympathetic preganglionic neurons.

[18] The rostral spinocerebellar tract is formed by axons from a column of neurons located in the ventral portion of the dorsal horn of the cervical segments. It ascends ipsilaterally in the lateral funiculus, medial to the ventral spinocerebellar tract, to reach the ipsilateral cerebellum through the rostral cerebellar peduncle.

[19] The spinovestibular tract is formed by ascending proprioceptive fibers, in the ventral funiculs, to the caudal vestibular nucleus.

[20] The spinocervicothalamic tract is located in the lateral funiculus. It is involved in nociceptive and tactile pathways. It reaches the lateral cervical nucleus and, then decussates to join the medial lemniscus. It is important in carnivores. 

[21] The lateral motor nuclei are formed by neurons located laterally in the ventral horn. They innervate ventral hypaxial muscles. They are well developed in the cervical and lumbar intumescences because they form the brachial plexus and the lumbosacral plexus respectively.

[22] The medial motor nuclei are absent between L7 and S1. Their axons innervate muscles of the trunk.

[23] The motor nucleus of the accessory nerve is located in the dorsal part of the ventral horn between the cervical spinal cord segments C1 and C6/C7.

[24] The thoracic nucleus is formed by neurons located at the base of the dorsal horn.

[25] The lateral cervical nucleus is located in the first two cervical segments of the spinal cord, surrounded by white matter. It is formed by neurons from the cervicospinothalamic tract, being a relay station for nociceptive (pain) and tactile pathways.

[26] The marginal nucleus is formed by neurons located on the dorsal surface of the dorsal horn along the length of the spinal cord. It is involved in the transmission of nociceptive (pain) stimuli. It was described by the German neurologist Heinrich Lissauer (1861-1891).

[27] The substantia gelatinosa extends along the entire length of the spinal cord. It overlaps with the nucleus of the spinal tract of the trigeminal nerve in the medulla oblongata. It receives fibers that transmit nociceptive (pain), tactile and thermal stimuli.

[28] The nucleus proprius is a group of neurons located deep in the substantia gelatinosa. It extends along the entire length of the spinal cord. It receives axons from neurons of the substantia gelatinosa, from the dorsal roots and from ascending pathways of the spinal cord. It is the site of origin of ascending tracts for noxious stimuli except for the spinothalamic tract.

[29] The intermediolateral nucleus forms the lateral horn between spinal segments T1 and L3. It is formed by sympathetic preganglionic neurons of the autonomic nervous system.

[30] The intermediomedial nucleus is located in the medial part of the lateral intermediate substance. It is formed by interneurons of the autonomic nervous system.

[31] The sacral parasympathetic nucleus is located in a mid-dorsal position in the intermediate gray matter of the sacral spinal cord segments. It is formed by preganglionic parasympathetic neurons of the autonomic nervous system.

 

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23] The motor nucleus of the accessory nerve is located in the dorsal part of the ventral horn between the cervical spinal cord segments C1 and C6/C7.

[24] The thoracic nucleus is formed by neurons located at the base of the dorsal horn.

[25] The lateral cervical nucleus is located in the first two cervical segments of the spinal cord, surrounded by gray matter. It is formed by neurons from the cervicospinothalamic pathways, being a relay station for nociceptive (pain) and tactile pathways.

[26] The marginal nucleus is formed by neurons located on the dorsal surface of the dorsal horn along the length of the spinal cord. It is involved in the transmission of nociceptive (pain) stimuli. It was described by the German neurologist Heinrich Lissauer (1861-1891).

[27] The substantia gelatinosa extends along the entire length of the spinal cord. It overlaps with the nucleus of the spinal tract of the trigeminal nerve in the medulla oblongata. It receives fibers that transmit nociceptive (pain), tactile and thermal stimuli.

[28] The nucleus proprius is a group of neurons located deep in the substantia gelatinosa. It extends along the entire length of the spinal cord. It receives axons from neurons of the substantia gelatinosa, from the dorsal roots and from ascending pathways of the spinal cord. It is the site of origin of ascending tracts for noxious stimuli except for the spinothalamic tract.

[29] The intermediolateral nucleus forms the lateral horn between spinal segments T1 and L3. It is formed by sympathetic preganglionic neurons of the autonomic nervous system.

[30] The intermediomedial nucleus is located in the medial part of the lateral intermediate substance. It is formed by interneurons of the autonomic nervous system.

[31] The sacral parasympathetic nucleus is located in a mid-dorsal position in the intermediate gray matter of the sacral spinal cord segments. It is formed by preganglionic parasympathetic neurons of the autonomic nervous system.