SYSTEMIC HISTOLOGY III OVERVIEW OF THE HISTOLOGY OF THE CENTRAL NERVOUS SYSTEM - PowerPoint Presentation

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SYSTEMIC HISTOLOGY IIIOVERVIEW OF THE HISTOLOGY OF THE CENTRAL NERVOUS SYSTEMCentral nervous system comprises the brain and the spinal cord with nervous tissue making up the system. This system is the most intricate system in human body.The nervous tissue consists of rich network of billions of nerve cells in both the CNS and PNS nervous tissue develops from ectoderm(neural tube and neural crest) They are excitable for processing information and generating various responses

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Nervous system can be divided into central and peripheral nervous system. The focus of the lecture is placed on the central nervous system.The nervous tissue entails both the neural cells or neurons and neuralgial or glial cellsNeurons are anatomical and functional units of the nervous tissue Glial cells are supporting cells of the nervous tissueBoth neurons and glial cells are located in the grey matter of the central nervous system

their processes or

fibres form the white matter of the system

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Histology of the Elements of the Nervous System This focuses on the histology of nervous tissue which comprises of the neurons and glial cellsThe neuron - Also known as nerve cell. This is the structural and functional unit of nerve tissue. Nerve cell has three components which are cell body or perikaryon, dendrite and axon. Cell body

- which is the

trophic or synthetic center for the whole nerve cell and is also receptive to stimuli.

Dendrite - which are multiple elongated processes specialized in receiving stimuli from the environment, sensory epithelial cells, or other neurons

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Axon - which is a single process specialized in generating and conducting nerve impulses to other cells (nerve, muscle, and gland cells)The neuroglia Also known as glial cellsThese are supporting cells of the nervous tissue. They are tenfold more than the nerve cells. They do not conduct nerve impulses. they are four; namely astrocytes

,

oligodendrocytes, microglia, and

ependymal

cells.

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Histological Methods for Studying Cellular Elements of Nerve Tissue H&E – this technique demonstrates the nuclei, cell bodies and their cytoplasmic constituents. Nissl method for RNA and are useful to highlight the arrangement of neurones, glial cells and blood vessels.

Myelin methods

, e.g. Weigert-Pal or

Luxol

fast blue, are routinely used in studying the arrangement of grey matter and white matter in the brain stem and spinal cord, as well as abnormalities of myelination

. With myelin methods, the white matter stains strongly, the grey matter remaining unstained.

Immunohistochemical

method

using antibodies against proteins specific to neurones, astrocytes, oligodendroglia or microglia. It is usseful in pathological diagnoses of the diseasses of nervous system.

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Heavy metal impregnation methods, applied to thick tissue sections, demonstrate overall cell morphology, especially the pattern of branching of axons and dendrites, permitting study of neuronal interconnections. These methods are rarely used todayUseful Terminology in CNSA cortex : The extensive, diffuse arrangement of neurones over the surface of the brainA nucleus : An arrangement of neuronal cells as a discrete unit A column : An arrangement of neuronal cells running along the spinal cord

Neurite : is either an axon or dendrite in undifferentiated form

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Mynelinated axons are called nerve fibres. Bundles of nerve fibres form a fascicle or tractGray matter: Rich in neuronal and glial cell bodiesWhite matter: Compact bundles (tracts, fascicles) of myelinated axons and supporting oligodendrocytes

Neuropil

: Hypocellular

, fibrous meshwork of

unmyelinated neuronal and glial

cell processes

Lemniscus

: a bundle of secondary sensory

fibres in the brainstem e.g. lateral lemnisus, etc.Ganglion : collection of cell bodies located outside the CNS e.g spinal ganglion7

Morphology of Cellular Elements of CNSThe neuron It has a cell body and two processes.The processes consist of an axon and dendrites neuron can be classified into different types based on; Size : which can range from smallest -sized neuron known as granular neuron to largest sized- neuron called pyramidal neuron Shape or morphology : pyramidal, flask, stellate, or granular shape Functional role: motor/efferent, sensory/afferent and interneuronsPresumed discoverer e.g. Purkinje cell of the cerebellum

e.g

Czech Anatomist Jan Evangelista Purkyně

in 1839

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Morphologically, neurons can be generally classified as;Unipolar or formally called pseudounipolar neurons which have a spherical cell body with single process that bifurcates . They are sensory.e.g numerous sensory ganglia of cranial and spinal nerves. Bipolar neurons : are spindle-shaped, with one process at each end of the cell

(e.g., cochlear and vestibular peripheral ganglia and olfactory and retinal receptor cells)

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Multipolar neurons : they are the most common types of neurons in human nerve tissue . They have one axon and many dendritic processes., e.g., autonomic ganglia and the enormous population of cells in the central nervous system)10

11Individual Neuronal Cells Granular cell: this neuron has very small cell bodies, it accounts for most of the neurons in human brainIt can be found in the granular layer of cerebellum, dentate gyrus of hippocampus, Superficial layer of the dorsal layer of the dorsal cochlear nucleusThe olfactory bulb the cerebral cortex Damage to these neurons can lead to abnormal functions

Big white arrows

show the Granule cells of the Cerebellum

Small black arrow reveals Purkinje cell

12Purkinje cells : from above photomicrograph, Purkinje neurons are: some of the largest neurons in human brain elaborate dendritic arbor with abundant dendritic spines found in Purkinje layer of cerebellum the major output of all motor coordination in the cerebellar

cortex

clinically, damage to Purkinje cells can lead to lesions such as

Spinocerebellar

ataxiasAutism

Intention tremor, etc

13Pyramidal neurons they are example of multiple neurons, which are the largest in size(also called Beltz cells) have conic shaped soma with a single axon and numerous dendritic arborization and spineCharacterized by excitatory projection neurons they can be found in Layer III and V of cerebral cortex for motor function prefrontal cortex for cognition

hippocampus

the

amygdala

The

corticospinal

tract

14Photomicrograph of pyramidal neurons of layer III of cerebral cortex. (A) shows numerous pyramidal cells in low magnification. (B) shows a single neuron with abundant dendritic arborizations. (C) reveals a pyramidal dendrite with abundant spines

15Neuroglia Also known as glial cells which are nonneuronal and mitotic and are far more numerous but smaller than the neurons. they occupy spaces between the neurons and their fibres except where blood vessels are dense network of fibres from both the neuron and glial cell bodies fill up the interneuronal space which is called the

neuropil

As pointed earlier, they are four

(A)

Astrocytes

originated from the neural tube as the neurons

they are the most numerous

glial cells in CNSNumerous processes radiate from the astrocyte soma which are invisible in routine light microscopy. however, with gold staining, astrocytes can be classified based on their amounts of their processes into two Fibrous astrocytes have few and straight processes (located in white matter) and protoplasmic astrocytes which have numerous and branching processes(located in grey matter)

16 All astrocytes have intermediate processes that contain intermediate filaments of glial fibrillary acidic protein (GFAP) and antibodies against this protein can be demonstrated to identify the cells. Functionally, Astrocytes provide structural support, they repair neuronal damage, they possess perivasscular feet for blood-brain barrier,

they are involved in metabolic exchanges

17 (B) OligodendrocytesAlso originated from neural tube and can be found only in the CNS they extend short and extensive branching processes that elaborate myelin sheath around several axons (up to 30 axons) they are prominent in white matter of CNS where they myelinate axonsThe sheath thus provides electrical insulation Analogous to the Schwann cells that myelinate individual axons in the nerves of the PNS. Two types; Interfascicular

oligodendrocytes (found in white matter) and satellite cells (found in gray matter)

18(C )Microglia ( or Hortega cell) this is derived from bone marrow as momocytes and migrated to CNS. They are small cells with short irregular processes they are less common than astrocytes and oligodendrocytes but evenly distributed in both white and grey matter, accounting for about 10-15% of cells in the brain they are described as resident macrophages that provide active immune defense in the CNS

Microglia

19Ependymal cellsline the central canal of the spinal cord and ventricles of the brain.• They vary from cuboidal to columnar in shape which possess cilia only in embryologic stages in man.• include choroid epithelial cells of the choroid plexus The choroid plexus cells produce cerebrospinal fluid (CSF) and are interconnected by tight junctions that constitute the blood–CSF barrierEpendymal cells seen in the third ventricle