Special Histology and Embryology: Practical Course - V. K. Naphanyuk 2001

Nervous System
Autonomic Nervous System
Spinal Cord

The Spinal Cord (medulla spinalis) is composed of Gray matter (substantia grisea) and White matter (substantia alba).

In cross-section, the gray matter resembles the letter H or a butterfly. The protrusions of the gray matter are referred to as horns. These include the anterior or ventral horns (cornu ventrale), posterior or dorsal horns (cornu dorsale), and lateral horns (cornu laterale).

The gray matter of the spinal cord consists of clustered multipolar Neurons, neuroglial Cells, unmyelinated fibers, and fine myelinated fibers.

The multipolar Nerve Cells (neurocytes) of the gray matter are classified as ROOT, fascicular, or internal (intercalary) cells. The axons of root cells extend beyond the spinal cord as part of the anterior roots. The axons of fascicular cells form white matter tracts that connect individual nuclei or segments of the spinal cord with one another and with corresponding nuclei in the Brain.

The processes of intercalary cells terminate in synapses within the gray matter of the spinal cord. Clusters of neurocytes that share a common Morphology and function are called nuclei.

The anterior horns contain medial and lateral groups of motor nuclei. These nuclei contain fascicular cells whose axons form the anterior roots of the spinal cord.

The posterior horns contain the following nuclei:

1. Within the horn itself lies the proper Nucleus of the posterior horn. The axons of these cells cross to the opposite side via the gray commissure and enter the anterolateral tract of the white matter. Deeply located fibers of this tract reach the Diencephalon (a part of the Brainstem), terminating in the thalamus, while a superficially located group of fibers forms the anterior spinocerebellar tract, which terminates in the Cerebellum.

2. In the medial part of the Base of the posterior horn is the dorsal nucleus (Clark's Column). The axons of its cells form the posterior spinocerebellar tract, which is part of the lateral funiculus on the same side and terminates in the cerebellum.

3. In the region of the apex of the posterior horn, one distinguishes the spongy layer (stratum spongiosum), which contains numerous small nerve cells, and the gelatinous substance (substantia gelatinosa), which is rich in glial elements and sparse in nerve cells. The nerve cells (neurocytes) of the posterior horn nuclei are multipolar and small in size.

In the intermediate zone, there is the medial intermediate nucleus; the axons of its nerve cells enter the white matter and join the ipsilateral anterior spinocerebellar tract. In the lateral intermediate, or sympathetic, nucleus, the axons of the neurocytes leave the spinal cord as part of the anterior roots, after which they branch off to form the so-called white communicating Branches of the Sympathetic trunk.

In addition to these nuclei, the gray matter contains numerous diffusely scattered fascicular neurocytes whose axons do not extend beyond the spinal cord. They are localized within the WHITE MATTER OF the spinal cord and around the gray matter, forming tracts of white matter that complete local reflex arcs. The white matter is formed by nerve fibers, predominantly myelinated ones. These fibers are the axons of neurocytes from both the spinal cord and other divisions of The Nervous system. Each funiculus (anterior, lateral, and posterior) contains several bundles of nerve fibers. As a rule, each bundle has a single functional purpose. Short conduction pathways—representing the intrinsic apparatus of the spinal cord—and long conduction pathways, which provide communication between the brain and the spinal cord, are distinguished. In turn, long pathways are subdivided into ascending and descending. Ascending pathways conduct impulses toward the brain, whereas descending pathways conduct impulses away from the brain.

The white matter surrounds the gray matter and is divided by its horns into three pairs of funiculi, or columns: anterior, lateral, and posterior. On the anterior surface, the anterior median fissure divides the white matter, while on the posterior surface, the posterior median septum divides it into right and left halves. The two halves are connected via the white and gray commissures.

As already noted, the intrinsic white matter tracts are located immediately around the gray matter. These are short conduction pathways present in all funiculi—posterior, lateral, and anterior. The long pathways lie external to them.

Ascending long pathways. The posterior funiculi contain: medially, the fasciculus gracilis (Goll's tract); laterally, the fasciculus cuneatus (Burdach's tract). These tracts are formed by the axons of pseudounipolar neurocytes of the spinal ganglia. These axons enter the spinal cord as part of the posterior roots.

The lateral funiculi contain ascending pathways: the ventral spinocerebellar tract and the spinothalamic tract. These pathways are formed by the axons of cells located in the gray matter of the spinal cord.

The descending long pathways are represented by two systems: pyramidal and extrapyramidal. The pyramidal pathways are formed by the axons of neurocytes in the Cerebral Cortex. Within the spinal cord, the pyramidal pathways comprise the anterior funiculus (anterior corticospinal tract) and a portion of the lateral funiculus (lateral corticospinal tract). The latter is located between the posterior spinocerebellar tract and the lateral intrinsic tract of the spinal cord. The extrapyramidal pathways are formed by the axons of neurocytes in the motor nuclei of the brainstem: the red nucleus, vestibular nuclei, and reticular formation nuclei. In the spinal cord, the extrapyramidal pathways are located in the lateral funiculi, ventral to the lateral pyramidal tract. All pyramidal and extrapyramidal pathways terminate on the neurocytes of the anterior horn nuclei of the spinal cord. The axons of the anterior horn neurocytes exit the spinal cord as part of the anterior roots.

Microscopically, the nerve fibers of adjacent pathways are nearly indistinguishable from one another; therefore, when studying and viewing the slide, it is recommended to use a diagram simultaneously.

Slides for Study

Slide 1. Cross-section of the spinal cord (Fig. 1).

First, examine the slide with the naked eye and sketch its outlines.

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Fig. 1. Cross-section of the spinal cord. Silver impregnation. x 30:

1 — posterior median septum; 2 — anterior median fissure; 3 — anterior root; 4 — anterior gray commissure; 5 — posterior gray commissure; 6 — spongy layer; 7 — gelatinous substance; 8 — posterior horn; 9 — reticular formation; 10 — lateral horn; 11 — anterior horn; 12 — proper nucleus of the posterior horn; 13 — dorsal nucleus; 14 — nuclei of the intermediate zone; 15 — lateral nucleus; 16 — nuclei of the anterior horn; 17 — Meninges

Low magnification. The spinal cord consists of two symmetrical halves separated anteriorly by a deep ventral median fissure and posteriorly by a connective-tissue dorsal median septum. At the periphery of the organ, locate the light-colored white matter, and internally, the darker gray matter.

The grey matter on a transverse cross-section of the spinal cord resembles a butterfly. Narrower dorsal horns and broader ventral horns can be distinguished. Between them lies the intermediate zone of grey matter, with its lateral extension known as the lateral horn. The ventral horn houses the largest neurons of the spinal cord, which form motor nuclei divided into lateral and medial groups. In the intermediate zone, one can distinguish the medial intermediate nucleus and the lateral intermediate nucleus (sympathetic in origin), located in the lateral horns. At the base of the dorsal horn lies the medial thoracic nucleus, and dorso-lateral to it is the proper nucleus of the dorsal horn. The right and left halves of the grey matter are connected by the grey commissure, which is traversed by the central canal of the spinal cord. In the white matter, the ventral, lateral, and dorsal funiculi can be identified.

High magnification. Under high magnification, the myelinated fibers of the white matter appear as follows: the axon resembles a dark dot, while the myelin sheath appears as a light circle, which is a result of myelin dissolution during tissue Processing prior to embedding.

Label the following on the diagram: 1) posterior median septum; 2) anterior median fissure; 3) anterior root; 4) anterior grey commissure; 5) posterior grey commissure; 6) spongiosa layer; 7) substantia gelatinosa; 8) posterior horn; 9) reticular formation; 10) lateral horn; 11) anterior horn; 12) proper nucleus of the posterior horn; 13) dorsal nucleus; 14) nuclei of the intermediate zone; 15) lateral nucleus; 16) nuclei of the anterior horn; 17) meninges.

Slide 2. Spinal ganglion (Fig. 2).

Low magnification. Locate the spinal ganglion, which is situated along the course of the posterior root. Large nerve cells with pale nuclei—pseudounipolar neurocytes of afferent origin—are located at the periphery of the ganglion. Their axons form the posterior (sensory) root of the spinal cord. The anterior root is formed by the axons of motor nerve Cells of the spinal cord and, at some distance from the spinal ganglion, it merges with the dendrites of the pseudounipolar neurocytes to form a mixed (sensory and motor) spinal nerve.

High magnification. Locate the capsule of small ganglionic gliocytes (mantle cells) surrounding the neurocytes, featuring nuclei that are rounder and denser than those of the neurocytes. Thin Connective Tissue layers surrounding the neurocytes are identified by dense nuclei with compact Chromatin. Along the course of the myelinated fibers of the ganglion and roots, elongated neurolemmocyte nuclei are observed, which are larger and paler than connective tissue nuclei. Externally, the ganglion and roots are surrounded by a connective tissue capsule.

Label the following on the diagram: 1) capsule of the spinal ganglion; 2) pseudounipolar neurocytes; 3) satellite cells (mantle cells); 4) nerve fibers; 5) connective tissue layers.

Fig. 2. Spinal ganglion. Hematoxylin and eosin staining. x 400:

1 — capsule of the spinal ganglion; 2 — pseudounipolar neurocyte; 3 — satellite cells (mantle cells); 4 — nerve fibers; 5 — connective tissue layers

Control Questions

1. Sources of development of spinal ganglia and ganglia of the Autonomic nervous system.

2. Types of reflex arcs. Structure of their components.

3. Morphofunctional Features of neurons and gliocytes in spinal ganglia.

4. Where do the axons of pseudounipolar neurons of spinal ganglia lead, and what do they form?

5. The pathway of the dendrites of pseudounipolar neurons of spinal ganglia, and what structure do they form?

6. Structure of nerve trunks.

7. Morphological and histochemical differences between adrenergic and cholinergic structures of the autonomic nervous system.

8. Development of the spinal cord.

9. STRUCTURE OF THE spinal cord.

10. Nuclei of the grey matter of the spinal cord, their topography, and function.

11. Structure of the white matter of the spinal cord.

12. Where do the axons of neurocytes of the motor nuclei in the ventral horns of the spinal cord lead, and what structures do they form?

13. Ascending and descending pathways.

Electron micrograph 1. Nerve and glial cells of an autonomic ganglion. х 8000:

1 — nucleus of a multipolar nerve Cell; 2 — nuclei of satellite glial cells; 3 — cell membrane of the nerve cell; 4 — Cytoplasm of satellite glial cells; 5 — Mitochondria; 6 — lipid inclusions; 7 — nerve fibers (adapted from De Robertis)

Case Study Problems

1. Two micrographs show an intramural and an extra-organ nerve ganglion containing multipolar nerve cells. What is the Functional Significance of these ganglia? What Types of Nerve cells are distinguished within them According to the functional Classification?

2. Microscopic examination of the posterior root of the spinal cord reveals Cytology/practical/65.html">Myelinated nerve fibers. Where do they originate? The processes of which cells form the axis cylinders within these fibers?

3. Poliomyelitis involves damage to the spinal cord and impaired skeletal Muscle Function. The destruction of which neurons can account for this phenomenon? Which component of the reflex arc is thus disrupted?

4. In an experimental animal, the ventral roots of the spinal cord are damaged. Which Functions are impaired?

5. In an experimental animal, the pseudounipolar neurons of the spinal ganglia are destroyed. Which component of the reflex arc is blocked?

6. Microscopic examination of the spinal cord reveals degeneration (damage) of the nerve fibers in the posterior funiculi of the white matter. Damage to which nerve cells could cause this? Which processes of these nerve cells form the axis cylinders of the nerve fibers in the posterior funiculi?

Sample Examination Questions

1. Spinal cord. Development, structure, and function. Intrinsic apparatus of the spinal cord.

2. Nuclei of the gray matter of the spinal cord. Morphofunctional characteristics; METABOLISM/2.html">THE CONCEPT OF a spinal cord segment.

3. Spinal ganglia. Development, structure, function, and connections.

4. Autonomic nervous system. Autonomic ganglia: localization, structure, and function.