The Spinal Cord II

Spinal cord I | Main Anatomy Index | Blood supply of the spinal cord

• The posterior horn consists mainly of interneurons.
• The processes of these remain within the spinal cord and of tract cells whose axons collect into long ascending sensory pathways.

• This area of grey matter contains 2 prominent parts:

1. The substantia gelatinosa;
2. And the body of the posterior horn.

• This is a distinctive region that caps the posterior horn at all spinal levels.
• In myelin-stained preparations this region looks pale compared with the rest of the grey matter.

• It deals mostly with finely myelinated and unmyelinated sensory fibres that carry pain and temperature information.

• This is relatively pale staining area between the substantia gelatinosa and the surface of the cord.

• This consists mainly of interneurons and tract cells.
• These transmit many types of somatic and visceral sensory information.
• In this respect it functionally overlaps parts of the intermediate grey matter.

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• This contains the cell bodies of the large motor neurons that supply skeletal muscle.

• These alpha motor neurons (lower motor neurons) are the only means by which the nervous system can exercise control over body movements, whether voluntary or involuntary.

• Destruction of these neurons supplying a muscle, or interruption of their axons causes complete paralysis of that muscle.
• Lower motor neuron lesions cause paralysis of a type called flaccid paralysis.
• Reflex contractions can no longer be elicited, and the muscle slowly atrophies.

• Alpha motor neurons occur in groups.
• They are separated from one another by areas of interneurons.

• The groups that innervate axial muscles are medial to those that innervate limb muscles.
• In the cervical and lumbar enlargements, which contain the limb motor neurons, the anterior horns are enlarged laterally.

• Smaller gamma motor neurons are interspersed with alpha motor neurons.
• They innervate the intrafusal muscle fibres of muscle spindles.

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• This is intermediate to the anterior and posterior horns.
• It has some characteristics of both and also contains the spinal preganglionic autonomic neurons.
• In addition, at some levels it includes a distinctive region called the dorsal nucleus (of Clarke).

• From T1 through L2 or L3, the preganglionic sympathetic neurons lie in a column of cells, the intermediolateral cell column.
• This forms the pointy lateral horn of the spinal grey matter.
• Their axons leave through the ventral roots.

• In segments S2 to S4, corresponding cells form the sacral parasympathetic nucleus but do not form a distinct lateral horn.
• Their axons leave through the ventral roots and synapse on the postganglionic parasympathetic neurons for the pelvic viscera.

• This is a rounded collection of large cells located on the medial surface of the base of the posterior horn from about T1 to L2 or L3.
• It is particularly prominent at lower thoracic levels.

• This is an important relay nucleus for the transmission of information to the cerebellum.
• It also plays a role in forwarding proprioceptive information from the leg to the thalamus.
• It is considered by many to be part of the posterior horn, due to its role in sensory processing.

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• In 1952 Rexed devised a system for subdividing the grey matter of the cat's spinal cord.

The same system has since been applied to the cords of other mammals, including humans.

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• The nerve fibres in the white matter of the spinal cord are of 3 general types:

1. Long ascending fibres projecting to the thalamus, the cerebellum, or various brainstem nuclei;
2. Long descending fibres projecting from the cerebral cortex or from various brainstem nuclei to the spinal grey matter;
3. Shorter propriospinal fibres interconnecting various spinal cord levels, such as the fibres responsible for the co-ordination of flexor reflexes.

• Fibres having similar connections tend to travel together forming the various tracts of the spinal cord.

• Propriospinal fibres mostly remain in a thin shell surrounding the grey matter called the propriospinal tract or fasciculus proprius (L. fasciculus, little bundle).
• Descending tracts are found primarily in the lateral and anterior funiculi.
• Ascending tracts are found in all funiculi.

• Information that reaches the thalamus is relayed to the cerebral cortex and perceived consciously.
• Information that reaches the cerebellum is used in the regulation of motor patterns; we are not consciously aware of cerebellar activity.

• There are 2 traditionally important routes for somatic sensory information to reach the thalamus:

1. The posterior column-medial lemniscus pathway;
2. And the spinothalamic tract.

• Two generalisations may be made about ascending somatosensory pathways of the spinal cord:

1. Most somatosensory information travels in more than one pathway;
2. Tracts in the posterior half of the cord ascend uncrossed whereas those in the anterior half of the cord cross the midline as they form.

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• This refers to the entire contents of a posterior funiculus, exclusive of its share of the propriospinal tract.
• The posterior columns consist mainly of ascending collaterals of large myelinated primary afferents.
• These carry impulses from various kinds of mechanoreceptors.
• This has traditionally been considered the major pathway by which information from low-threshold cutaneous, joint, and muscle receptors reaches the cerebral cortex.

• Many of these fibres have their cell bodies in the ipsilateral dorsal root ganglia.
• Where the dorsal root enters the spinal cord, it segregates into medial and lateral divisions.

• The medial division contains large myelinated afferents.
• The lateral division contains small, finely myelinated or unmyelinated afferents.

• Fibres of the medial division enter the posterior column.
• Most of them give off numbers of collaterals to the spinal grey matter and finally terminate at some spinal level.
• Some, however, reach the caudal medulla and synapse there.

• Caudal to T6, each posterior column is an undivided bundle called the fasciculus gracilis (L. gracilis, slender).
• Rostral to T6, fibres may leave the fasciculus gracilis, but few if any are added.
• Afferents entering rostral to T6 accumulate in a 2nd bundle lateral to the fasciculus gracilis, called the fasciculus cuneatus (L. cuneus, wedge).

• A glial partition, the posterior intermediate septum, extends inward to partially separate the two.

• Those posterior column fibres that reach the brainstem synapse in the nucleus gracilis or the nucleus cuneatus (the posterior column nuclei) in the caudal medulla.

• Second order fibres arising in these nuclei cross the midline and form the medial lemniscus (L. lemniscus, ribbon).
• This is a flattened bundle of fibres that proceeds rostrally through the brainstem and terminates in the thalamus.

• These fibres arise in the thalamus (specifically in the ventral posterolateral nucleus of the thalamus, or VPL) and ascend through the internal capsule.
• They synapse mainly in the cortex of the postcentral gyrus.
• This is the primary somatosensory cortex.

• Fibres entering the posterior columns add on laterally to those already present.
• A lamination results, with layers of fibres from sacral levels most medial and layers from cervical levels most lateral.
• This arrangement is called somatotopic organisation and is characteristic of most sensory and motor pathways.

• When the primary afferents of the posterior columns terminate in the posterior column nuclei, they maintain this organisation.
• Fibres from sacral levels terminate in the most medial portions of the nucleus gracilis.
• Fibres from cervical levels terminate in the most lateral portions of the nucleus cuneatus.

• A somatotopic arrangement is found throughout the rest of this pathway.
• In this way, information from sacral segments travels through a particular part of the medial lemniscus, projects to a particular portion of the VPL, and then proceeds to a particular region of the postcentral gyrus.

• This does not mean that the sacral-to-cervical sequence remains along a medial-to-lateral line, but rather that sacral information remains segregated from cervical information at all points.

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• Collaterals of some touch and pressure-sensitive fibres in the posterior columns, as well as those of mechanoreceptive, thermoreceptive, and nociceptive fibres of the lateral division of the dorsal root, enter the posterior horn.
• They synapse in or near the substantia gelatinosa.

• Although these afferents end in the substantia gelatinosa, the latter cells do not give rise to the spinothalamic tract.
• Rather, the same afferents make additional synapses on dendrites of neurons whose cell bodies are located deeper in the posterior horn or on the surface of the substantia gelatinosa.

• These second order cells then send their axons across the midline with a slight rostral inclination to form the spinothalamic tract.

• This is one alternate pathway by which mechanoreceptive input reaches the thalamus and cerebral cortex.
• It is the primary pathway for pain and temperature information.

• The tract occupies most of the anterior half of the lateral funiculus.

• New fibres join the spinothalamic tract at its ventromedial edge, so that this tract, like the posterior columns, is somatotopically organised.
• Fibres from the most caudal segments occupy its most dorsolateral portion.
• Those from more rostral segments occupy more ventromedial portions.

• Although the spinothalamic tract carries some tactile and pressure information, a great deal also travels in the posterior column system.
• Destruction of the spinothalamic tract causes no significant tactile deficit.

• There are, however, several types of sensation (in addition to pain and temperature) subserved more or less predominantly by the spinothalamic tract.
• These are:

1. Itch (and probably tickle) sensations;
2. Pressure sensations from bladder and bowel;
3. And sexual sensations.

• However, with the exception of itch, this information is carried bilaterally.

• However, as the spinothalamic tract is the principal pathway of somatic pain sensations its destruction produces contralateral analgesia.

• An operation to destroy the tract (called a cordotomy or chordotomy) is sometimes performed on patients suffering from intractable pain.

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• Collaterals of posterior column fibres conveying tactile, pressure, and proprioceptive information (mainly the latter, from muscle spindles and Golgi tendon organs) synapse on neurons of nucleus dorsalis.

• These then send their axons into the lateral funiculus of the same side.
• This forms the posterior spinocerebellar tract.

• This tract is a curved band of fibres extending from the dorsal root entry zone to the dentate ligament.
• It lies at the surface of the spinal cord.
• Fibres in the tract project ipsilaterally to the vermis of the cerebellum through the inferior cerebellar peduncle.

• Collaterals of some of these fibres end in the nucleus gracilis.
• This provides an important route by which nonprimary afferents transmit proprioceptive information from the leg to the posterior column-medial lemniscus system.

• Since the nucleus dorsalis does not exist caudal to L2 or L3, neither does the posterior spinocerebellar tract.
• However, afferents from segments caudal to L2 or L3 ascend to that level in the fasciculus gracilis to synapse in nucleus dorsalis.
• This probab1y explains why nucleus dorsalis is large at upper lumber and lower thoracic levels

• The posterior spinocerebellar tract is principally concerned with the ipsilateral leg.

• Most spinocerebellar-type afferents that enter in cervical and upper thoracic segments do not project to nucleus dorsalis.
• They travel in the fasciculus cuneatus to a nucleus in the medulla analogous to nucleus dorsalis called the lateral (or external) cuneate nucleus.
• It is located just lateral to the nucleus cuneatus.

• Axons of these cells form the cuneocerebellar tract.
• They also project ipsilaterally to the vermis of the cerebellum through the inferior cerebellar peduncle.

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• Cells in the body of the lumbosacral posterior horn, together with cells on the lateral surface of the anterior horn (spinal border cells) give rise to the anterior spinocerebellar tract.

• The inputs to these tract cells are more complex than those of the posterior spinocerebellar tract.
• They come not only from group I muscle afferent (mainly Golgi tendon organs), but also from a wide variety of cutaneous receptors including spinal interneurons and from fibres of descending tracts.

• The tract is crossed at the level of the spinal cord, in contrast to the posterior spinocerebellar tract.

• Finally, the anterior spinocerebellar tract takes a roundabout route to the cerebellum.
• It ascends as far as the rostral pons and then turns caudally and enters the cerebellum via the superior cerebellar peduncle.
• Here, most of its fibres recross the midline before ending in the vermis of the anterior lobe.

• Thus, the fibres of the anterior spinocerebellar tract ultimately end in the cerebellum on the ipsilateral side to their origin.

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• The lateral corticospinal tract is a large, crossed, descending tract that contains the 85% of fibres from the contralateral pyramid that cross in the pyramidal decussation.
• It is also known as the pyramidal tract.
• It occupies the posterior portion of the lateral funiculus medial to the posterior spinocerebellar tract.

• Its fibres originate in the cerebral cortex (in the precentral gyrus and nearby areas).
• They descend through the cerebral peduncle, basal pons, and medullary pyramid.
• They then decussate and end in the anterior horn or intermediate grey matter.

• They terminate on the motor neurons of the anterior horn or, more often, on smaller interneurons.
• These in turn synapse on motor neurons.

• Lateral corticospinal fibres are arranged somatotopically.
• Those destined for more caudal cord levels are located more laterally.
• The fibres of the lateral corticospinal tracts usually synapse on motor neurons or interneurons that ultimately go to the distal muscles.

• The 15% of the fibres in each pyramid that do not cross in the pyramidal decussation continue into the anterior funiculus.
• This is located adjacent to the anterior median fissure as the anterior corticospinal tract.

• These fibres also terminate on motor neurons or interneurons of the anterior horn or intermediate grey matter, mainly in cervical and thoracic segments.
• Many of them cross in the anterior white commissure before synapsing.

• The term "pyramidal tract" refers to the combination of lateral and anterior corticospinal tracts.

• These fibres ultimately tend to go to the axial muscles.

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• This is an alternative route for the mediation of voluntary movement.
• It originates in the red nucleus --> crosses to the other side of the midbrain --> descends in the lateral part of the brainstem tegmentum --> travels through the lateral funiculus of the spinal cord in the company with the lateral corticospinal tract.
• It is small and rudimentary in humans.

• It arises in the lateral vestibular nucleus and projects to all levels of the ipsilateral spinal cord.
• It is located in the ventral part of the lateral funiculus.

• It is the principle route by which the vestibular system brings about postural changes to compensate for tilts and movements of the body.

• This arises mainly in the medial vestibulospinal nucleus and projects bilaterally to the cervical spinal cord.
• It is responsible for stabilising the head position as we walk around.
• This tract only goes down to the midthoracic level.

• Many secondary vestibular fibres project directly through the medial longitudinal fasciculus (MLF) to the motor neurons of the oculomotor, trochlear and abducens nuclei.
• This forms much of the basis of the vestibuloocular reflex.

Reticulospinal Tracts

Click here to go to the entry under the Reticular Formation.

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