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V. SPINAL CORD Slides for study # 1-13 # 1, 2 & 3 Upper cervical Weil, Nissl & LFB # 4 & 5 Lower cervical Weil & Nissl # 6 & 7 Thoracic Weil & Nissl # 8 & 9 Lumbar Weil & Nissl # 10 & 11 Sacral Weil & Nissl # 11 & 13 Coccygeal Weil & Nissl General description The spinal cord is the least modified portion of the embryonic neural tube and the only part of the adult nervous system which clearly preserves a segmental character (Fig. 5.1). Up to the third month of intra-uterine life, it occupies the entire length of the vertebral canal, but in the adult it occupies only the upper two thirds of the vertebral canal. Fig. 5.1 * Why? The lower limit of the spinal cord at birth is the third lumbar vertebra. Using your slides and the drawings in this section (Figs. 5.2, 5.3, 5.4) try to familiarize yourself with the following structures: Anterior median fissure,

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posterior median sulcus, posterior intermediate septum (above T6), posterolateral sulcus. Identify sites of entry of dorsal roots and exit of ventral roots. Delineate the boundaries of the posterior, lateral and anterior funiculi. Define funiculus. How does it differ from a fasciculus. Identify the central canal lined by ependymal cells. In the adult, the central canal is often obliterated. In cross section, the cord is seen to have an H-shaped, centrally placed area of gray matter. The two halves of the gray matter are connected in the midline by a ventral and a dorsal gray commissures, below and above the central canal. Anterior to the ventral gray commissure is the anterior white commissure. You can distinguish dorsal from ventral, at any level of the spinal cord, by the relatively wide open anterior (ventral) fissure. Spinal cord levels: The spinal cord is composed of 31 segments which vary greatly in size, shape and relative amounts of gray and white matter. In all segments, a posterior and an anterior gray horns (columns) can be identified. These horns are particularly large in segments of the cord associated with the cervical and lumbar enlargements.

Why? Cervical spinal segments are the largest and contain the greatest amount of white matter?

The posterior funiculus is divided by the posterior intermediate septum. A small reticular process extends laterally from the posterior horn. Thoracic spinal segments have definite intermedio lateral cell column and a nucleus dorsalis of Clark. A reticular process may also be seen in these segments. The nucleus dorsalis is best developed at T1 through T12 levels. Lumbar segments contain a sizable amount of gray matter relatively less white matter and a nucleus dorsalis of Clark is present in the upper two segments. The well developed anterior horn has a blunt process that extends into the lateral funiculi. Sacral segments show a relatively large amount of gray matter and a small amount of white matter. Coccygeal segments are unmistakable because of their small size and small amount of white matter. Try to answer the questions in Fig. 5.2 and compare the general shape of these figures with your slides.

Fig. 5.2

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Organization of gray matter (cytoarchitecture) The gray matter of the spinal cord can be divided into a number of regions on the basis of the size, orientation, and staining characteristics of the nerve cells using a Nissl stain. This type of study is called a cytoarchitectonic analysis.

Fig. 5.3

A nucleus is a collection of cells of similar function within the central nervous system. Nucleus may form continuous longitudinal columns throughout the length of the cord, or discontinuous columns limited to certain spinal levels (Fig. 5.4). Cells in the spinal cord are located essentially in the central gray matter and best seen in Nissl and LFB preparation. Fig. 5.4. Columnar organization of the various spinal nuclear groups

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In the posterior horn identify the substantia gelatinosa. It forms a cap about the head of the posterior horn. In Nissl stained sections, the cells of the substantia gelatinosa appear as very small oval cells. Dorsolateral to the substantia gelatinosa is the nucleus posteromarginalis which can be appreciated only in Nissl preparations. Its cells are larger than those of the substantia gelatinosa but less numerous. In the central part of the head of the posterior horn are medium size cells that constitute the nucleus proprius (centrodorsalis). At the base of the posterior horn (medially) is a well defined group of large cells known as the nucleus dorsalis (of Clark). This cell group is found only between C8 and L2 and is best seen in the lower thoracic and upper lumbar segments. Fibers arising from this cell group are large and ascend to the cerebellum via the dorsal spino cerebellar tract. In the anterior horn identify the large motor neurons. These give rise to the ventral root fibers which form the spinal nerves and supply skeletal muscles. *** The ventral roots contain axons not related to motorneurons. Try to find the location of their cell bodies and their functions. Using Nissl stained sections try to compare the shape of the anterior horn at different spinal levels.

Is it related to motorneurons? Are there more than one motor neuronal group? What would be the function of each group? Fig. 5.3 may help you to find the

correct answers? In thoracic and upper lumbar segments identify the lateral horn and examine

cells of the intermediolateral cell columns. What is the functional significance of this cell column? What is the course of fibers originating in this cell group? Where do they synapse?

Sacral autonomic nuclei are scattered along the dorso-lateral surface of the anterior horn segments S-2, S-3, S-4.

Where do fibers from this cell group synapse? What division of the autonomic nervous system do they belong to?

Fig. 5.5

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Try to label the Fig. 5.5 and to make a correlation between the nuclear organization of the spinal cord (that you have seen) and the laminar organization proposed by REXED. Remember that Rexed had excellent Nissl preparations, used rather thick sections in which the grouping is more readily seen and based much of his study on transverse sections.

Fig. 5.6

Organization of the white matter The white matter of the spinal cord form three distinct columns on each side, separated from each other by the midline sulci or by the spinal roots. A tract (fasciculus) is a group of axons of similar origin, termination and function within the central nervous system. A given tract; usually named to imply its origin and termination, conveys a particular modality of sensation.

Ascending Fiber Tracts

Tracts are best studied in preparations stained with Weil or LFB.

All primary sensory neurons lie in the dorsal root ganglia. Their peripheral processes convey impulses toward the cell body and their central processes enter the spinal cord. Fibers of the dorsal root enter the posterolateral aspect of the spinal cord in small bundles. Examine the entering dorsal root fibers in Weil stained sections. Small, poorly myelinated fibers occupy the lateral part of the entry zone. (see Fig. 5.7) A group of larger, thickly myelinated fibers occupy an area medial and dorsal to the entry zone. The more medial fibers convey proprioceptive impulses. These fibers enter the spinal cord and bifurcate into long ascending and short descending branches. The long ascending branches enter the ipsilateral posterior funiculus. Identify the fasciculus gracilis and fasciculus cuneatus. At what level do you find both fasciculi?

These large ascending tracts are laminated in particular manner. What is the manner of lamination? What types of sensations are conveyed by the posterior columns? Where do these first order neurons terminate? Where do fibers in this system decussate? How does the neurologist test the posterior column system?

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Fig. 5.7

The medial division of the posterior root contributes fibers to two other ascending tracts: (1) the posterior (Dorsal) spino cerebellar tract. What prominent cell group is associated with this tract? Fibers of this tract are uncrossed and convey proprioceptive impulses (2) some fibers from the dorsal root pass to the base of the posterior horn where they synapse. Second order fibers originating from cells here cross to the opposite side of the cord and ascend as the anterior spinocerebellar tract.

Do proprioceptive impulses conveyed by the spinocerebellar tract reach conscious levels?

Identify the approximate location of the posterior and anterior spinocerebellar tracts.

Fibers in the lateral division of the spinal root are concerned with transmission

of impulses for pain, light touch, and thermal sense. These fibers enter the zone of Lissauer (Fig. 5.7) and travel up and down for variable distances before synapsing with cells of the substantia gelatinosa and nucleus centrodorsalis. Second order neurons give rise to fibers of the spinothalamic tracts which decussate in the anterior white commissure. These tracts are more difficult to identify in spinal sections because they overlap or occupy areas containing descending fibers. Fibers of the anterior and lateral spinothalamic tracts also are somatotopically arranged.

What is the arrangement? What sensory modalities are transmitted by these tracts? What is cordotomy? For what purpose might an anterolateral cordotomy be done?

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Descending Fiber Tracts

The most important descending tract of the spinal cord is the corticospinal

tract. This tract consists of all fibers which arise from the cerebral cortex, pass through the medullary pyramids and enter the spinal cord. Three divisions of this tract are recognized. (1) A crossed lateral component (2) An uncrossed anterior component and (3) an uncrossed lateral component (smallest component). In Weil stained sections of normal spinal cord it is almost impossible to accurately locate these tracts. Experimental material or sections from patients with lesions in the corticospinal tract should ideally be used. Try to identify on your slides the approximate location of this fiber tract.

The corticospinal tract originates from broad areas of the cerebral cortex, it

contains over a million fibers which range in size from 1-22. What are the neurological phenomena associated clinically with lesions of this

system? Cortical cells and their axons descending in the corticospinal tract are spoken

of in clinical terms as "upper motor neurons" in contrast to "lower motor neurons". What usefulness do these concepts serve?

The rubrospinal tract arises from the red nucleus in the midbrain. In the

spinal cord, fibers are located in the upper part of the lateral funiculus, where they partially intermingle with fibers of the corticospinal tract. Anatomical data concerning the extent of this tract in man is incomplete.

The lateral vestibulospinal tract arises exclusively from the lateral vestibular

nucleus, is uncrossed and descends in the ventral part of the lateral funiculus. Locate the approximate site of this tract. This tract is considered to exert facilitatory influences upon the reflex activity of the spinal cord and upon extensor muscle tone.

The medial longitudinal fasciculus (MLF): In the spinal cord is a composite bundle of descending fibers originating from different brainstem nuclei. Vestibular fibers in this bundle originate almost exclusively from the medial vestibular nucleus. A large number of fibers also arise from the reticular formation.

Reticulospinal tracts are organized into two principal systems: (1) the

pontine reticulospinal tract and (2) the medullary reticulospinal tract. The pontine reticulospinal tract is almost entirely uncrossed and descends mainly in the medial part of the anterior funiculus. The medullary reticulospinal tract is uncrossed (predominantly) and descends mainly in the ventral part of the lateral funiculus. Almost all of our information concerning the reticulospinal tract has been derived from experimental studies in animals. These tracts are composed of relatively fine fibers, poorly myelinated and cannot be demonstrated in Weil preparations.

Descending autonomic pathways in the spinal cord are located mainly in the

anterolateral and postero-lateral portions of the spinal cord. Information concerning these fibers, which are thought to end in relationship with autonomic cell groups of the spinal cord, is based largely upon physiological evidence.

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Important remarks:

* The student should remember that a number of other ascending and descending fiber systems have been described in the spinal cord. In this lab module, a brief description is given to tracts known to have a significant clinical relevance.

* At the end of this lab session, try to draw the appropriate tracts in Fig. 5.9, 5.10 and 5.11 and to answer the given clinical cases.

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Case Discussions

Case History # 1 An individual sustained an injury to the spinal cord which resulted in the following symptoms and signs:

On the left side On the right side 1. Loss of proprioceptive and 1. Loss of pain and thermal

vibratory sense below C-6 sense below C-7 2. Paralysis of all muscles

below C-6 3. Atrophy of the intrinsic

muscles of the hand

4. Horner's syndrome 5. Increased deep tendon reflexes in the lower extremity-Babinski sign 6. Loss of pain, thermal sense and touch on inner aspect of upper extremity, and fourth and fifth digits A. Make a simple outline drawing of the extent of the lesion at one level. B. Approximately how many segments of the spinal cord were involved by the

lesion. C. List any ascending and descending tracts which would be degenerated six

months later. On the left side On the right side Ascending Tracts Descending Tracts

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Case History # 2 A housewife sustained a severe burn on her right hand without realizing it, until the burned area was observed. No sensation of heat or pain was felt. Neurological examination a short time later revealed impaired to absent pain and thermal sense over most of the right and left upper extremities. Tactile sense was somewhat impaired in the areas of most pronounced pain and thermal loss, but position sense and vibratory sense were within normal limits. Weakness and atrophy of the intrinsic muscles of the right hand were noted. Healing of the wound was slow and the fingers remained swollen: the skin and fiber nails showed trophic changes. Although the pupil and palpebral fissure on the right side were smaller than on the left, both pupils reacted promptly to light and accomodation. In the space below make a careful outline drawing of the transverse section through the level of the lesion, drawing accurately the structures at that level and labeling them. Outline the extent of the lesion. Case # 3. Friedrichs Ataxia Objectives: Explain how our position in space and sense of place involves both the cortex and the cerebellum. What sensory pathways are involved? What do you observe when they are interrupted? A 15-year-old, single, white male student presented with a one year history of progressive unsteadiness in walking which was present during daytime as well as nighttime. He noted some loss of coordinated movements in his hands, but felt that his greatest deficit related to unsteadiness when walking. The patient noted no actual weakness, but reported some numbness in the toes. The patient lived in a small town, his mother and father were second cousins. Several aunts and uncles had "trouble with their legs" in later life, resulting in difficulty in walking. COMMENT: This patient suffers from one of the many types of hereditary (spinocerebellar) ataxia. 1. He had loss of deep tendons reflexes (DTR) in lower limbs with hypotonia. What do these findings suggest? Suggests loss of large sensory axons. 2. How does information from tendon organs and muscle spindles reach the cerebellum? 3. What pathway(s) are degenerating that could explain his symptoms?

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Spinocerebellar tracts. Friedreich's ataxia. Also involvement of the Dorsal Columns, dorsal roots, dorsal root ganglia, nucleus dorsalis, dorsal column nuclei and accessory cuneate nucleus and within 5 years the corticospinal tract. The disease is described as autosomal recessive: Mean onset is 15 yr. (range 2-50 yrs.) Trinucleotide repeats. Most affected are large primary sensory neurons of the DRG and their central and peripheral branches. In this case all the findings could also indicate just a sensory neuropathy. 4. If you did a Romberg test, what would you expect? He would fall due to decreased proprioceptive information from the degenerating dorsal columns. Students may not understand sensory ataxia, which is associated with a positive Romberg and a dorsal column lesion or a peripheral sensory neuropathy. A Romberg sign is not an isolated sign of cerebellar disease. In cerebellar disease the Romberg test does not get significantly worse when closing the eyes. Gait ataxias are almost always cerebellar either input tracts or cerebellum itself. 5. At autopsy an examination of the dorsal root ganglion showed a loss of large neurons. The largest axons innervate which receptors? Muscle spindles, Golgi tendon organs. Ia, II, Ib fibers

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Fig. 5.8. Diseases affecting the spinal cord

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Fig. 5.9 Fig. 5.10

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Fig. 5.11