Histology of nerve system

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Histology of nerve system

  1. 1. Nerve Tissue & Nervous System Assoc. Prof Dr. Karim Al-Jashamy IMS/MSU 2010
  2. 2. Nerve Tissue & the Nervous System:  The human nervous system, by far the most complex system in the human body, is formed by a network of more than 100 million nerve cells (neurons), assisted by many more glial cells.  Each neuron has, on average, at least 1000 interconnections with other neurons, forming a very complex system for communication.  Neurons are grouped as circuits. Like electronic circuits, neural circuits are highly specific combinations of elements that make up systems of various sizes and complexities.  Nerve tissue is distributed throughout the body as an integrated communications network.  Anatomically, the nervous system is divided into the central nervous system, consisting of the brain and the spinal cord, and the peripheral nervous system, composed of nerve fibers and small aggregates of nerve cells called nerve ganglia
  3. 3.  Structurally, nerve tissue consists of two cell types: nerve cells, or neurons,  Usually show numerous long processes, and several types of glial cells which have short processes, support and protect neurons, and participate in neural activity, neural nutrition, and the defense processes of the central nervous system.  Neurons react promptly to stimuli with a modification of electrical potential that may be restricted to the place that received the stimulus or may be spread (propagated) throughout the neuron by the plasma membrane. This propagation, called the action potential, or nerve impulse, is capable of traveling long distances; it transmits information to other neurons, muscles, and glands.
  4. 4. Central Nervous System (CNS)  The CNS consists of the brain (encephalon), which is enclosed in the skull, and the spinal cord, which is contained within the vertebral canal.  Nervous tissue of the CNS does not contain connective tissue other than that in the three meninges (dura mater, arachnoid membrane and pia mater) and in the walls of large blood vessels.  Collagenous fibers or fibrocytes/blasts are consequently not observed, which is quite unlike other tissues. Because of the absence of connective tissue, fresh CNS tissue has a very soft, somewhat jelly-like consistency.  The major classes of cells that make up the nervous tissue are nerve cells, neurones, and supporting cells, glia.
  5. 5. The Central Nervous System  The central nervous system consists of the cerebrum, cerebellum, and spinal cord. It has almost no connective tissue and is therefore a relatively soft, gel- like organ.  When sectioned, the cerebrum, cerebellum, and spinal cord show regions that are white (white matter) and that are gray (gray matter).  The differential distribution of myelin in the central nervous system is responsible for these differences: The main component of white matter is myelinated axons and the myelin-producing oligodendrocytes.  White matter does not contain neuronal cell bodies.
  6. 6. Meninges
  7. 7. Meninges  The delicate, innermost, mesh- like layer of the meninges. The pia mater closely envelops the entire surface of the brain, running down into the fissures of the cortex.  It joins with the ependyma which lines the ventricles to form choroid plexuses that produce cerebrospinal fluid. In the spinal cord, the pia mater attaches to the dura mater by the denticular ligaments through the arachnoid membrane.
  8. 8. Gray and White Matter  Microscopically, the CNS contains 2 neural elements:  Neuron cell bodies (clusters are known as nuclei)  Nerve fibers (axons) in bundles called tracts.  Viewed macroscopically, CNS tissues can be distinguished by color:  Gray matter consists of somata, dendrites, and unmyelinated axons.  White matter consists primarily of myelinated axons.
  9. 9.  Gray matter contains neuronal cell bodies, dendrites, and the initial unmyelinated portions of axons and glial cells.  Gray matter is prevalent at the surface of the cerebrum and cerebellum, forming the cerebral and cerebellar cortex  whereas white matter is present in more central regions. Aggregates of neuronal cell bodies forming islands of gray matter embedded in the white matter are called nuclei  In the cerebral cortex, the gray matter has six layers of cells with different forms and sizes. Neurons of some regions of the cerebral cortex register afferent (sensory) impulses; in other regions, efferent (motor) neurons generate motor impulses that control voluntary movements.  Cells of the cerebral cortex are related to the integration of sensory information and the initiation of voluntary motor responses
  10. 10. Cerebral Cortex  Allows for sensation, voluntary movement, self- awareness, communication, recognition, and more.  Gray matter!  40% of brain mass, but only 2-3 mm thick.  Each cerebral hemisphere is concerned with the sensory and motor functions of the opposite side (contralateral side) of the body.
  11. 11. CEREBELLAR CORTEX Impregnated with silver nitrate 1 - molecular layer 2 - ganglionic layer (Purkinje cell layer) 3 - granular cell layer 4 - Purkinje neurons (cells) 5 - fold of the cortex 6 - white matter CEREBELLAR CORTEX Stained with H&E 1 - molecular layer 2 - ganglionic layer (Purkinje cell layer) 3 - granular cell layer 4 - Purkinje neurons (cells)
  12. 12.  Silver-stained section of cerebral cortex showing many pyramid-shaped neurons with their processes and a few glial cells. The cerebellum (H&E) does not reveal the unusually large dendritic of the Purkinje cell,
  13. 13.  Section of the cerebellum with distinct Purkinje cells. One Purkinje cell shows part of its rich dendritic arborization.
  14. 14.  The gray matter of the spinal cord showing several motor neurons with their basophilic bodies (Nissl bodies). Nucleoli are seen in some nuclei. The neurons are surrounded by a mesh of neuronal and glial processes. PT stain. Medium magnification
  15. 15. Cerebellum • Lies inferior to the cerebrum and occupies the posterior cranial fossa. • 2nd largest region of the brain. • 10% of the brain by volume, but it contains 50% of its neurons • Has 2 primary functions: 1. Adjusting the postural muscles of the body • Coordinates rapid, automatic adjustments, that maintain balance and equilibrium 2. Programming and fine-tuning movements controlled at the subconscious and conscious levels • Refines learned movement patterns by regulating activity of both the pyramidal and extrapyarmidal motor pathways of the cerebral cortex • Compares motor commands with sensory info from muscles and joints and performs any adjustments to make the movement smooth
  16. 16. Cerebellum  Cerebellar cortex contains huge, highly branched Purkinje cells whose extensive dendrites can receive up to 200,000 synapses.  Internally, the white matter forms a branching array that in a sectional view resembles a tree – for this reason, it’s called the arbor vitae
  17. 17.  In cross sections of the spinal cord, white matter is peripheral and gray matter is central, assuming the shape of an H.  In the horizontal bar of this H is an opening, the central canal, which is a remnant of the lumen of the embryonic neural tube.  It is lined with ependymal cells. The gray matter of the legs of the H forms the anterior horns. These contain motor neurons whose axons make up the ventral roots of the spinal nerves.  Gray matter also forms the posterior horns (the arms of the H), which receive sensory fibers from neurons in the spinal ganglia (dorsal roots).
  18. 18. Spinal Cord  Spinal cord neurons are large and multipolar, especially in the anterior horns, where large motor neurons are found
  19. 19.  Cross section of the spinal cord in the transition between gray matter (below) and white matter (above).  The gray matter contains neuronal bodies and abundant cell processes, whereas the white matter consists mainly of nerve fibers whose myelin sheath was dissolved by the histological procedure. PT stain. Medium magnification.
  20. 20.  Section of spinal cord gray matter. The meshwork of cell neuron and glial processes appears distinctly.  The small nuclei are from glia cells. Note that these cells are more numerous than neurons. H&E stain. Medium magnification.
  21. 21.  Ganglion cells will typically be several times larger than other cells in the ganglia  The perikaryon is very large and surrounds a large and light nucleus. Only the cells immediately surrounding the ganglion cells as one flattened layer are satellite cells.  Ganglion cells are of course in contact with other parts of the nervous system and with the peripheral tissues which they innervate. Consequently, nerve fibers will be visible close to or within the ganglion.
  22. 22. Peripheral Nerve  longitudinal H&E stained sections it is possible to identify the axon running in its myelin sheath, nodes of Ranvier and Schwann cell nuclei.  Components of the connective tissue elements, which accompany the nerve, should be visible and identifiable in both longitudinal and transverse sections.  transversely cut preparations give a good picture of the axon in the middle of a ring-like structure (sometimes fussy), which represents the remains of the myelin sheath.  Due to their small size and the lack of a myelin sheath, type C fibres are very difficult to detect in either osmium or H&E stains.

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