Nervous Tissue Lecture (PDF)

Nervous Tissue Lecture Ass.Prof.Dr. Noha Ramadan Elswaidy  -Anatomically, the nervous system is divided into: Central nervous system (CNS):Brain &Spinal cord. Peripheral nervous system (PNS): Peripheral nerves & Ganglia. -Structurally, Nervous tissue consists of two types...

Nervous Tissue Lecture Ass.Prof.Dr. Noha Ramadan Elswaidy  -Anatomically, the nervous system is divided into: Central nervous system (CNS):Brain &Spinal cord. Peripheral nervous system (PNS): Peripheral nerves & Ganglia. -Structurally, Nervous tissue consists of two types of cells: 1-Nerve cells (Neurons): which transmit nerve impulses and control the function of different organs. 2-Supporting cells :(Glial or Neuroglia) These cells are involved in nutrition, support, insulation, protection of neurons. in C N S: Neuroglial cells in PNS:- Schwann cells around axon. -Satellite cells around cell bodies in ganglia. Neurons The neuron is the structural and functional unit of the nervous system  Classification of neurons A. According to the number of the processes that arise from the nerve cell body: 1- Pseudo-unipolar nerve cell: e.g. spinal ganglion. 2- Bipolar nerve cell. e.g. olfactory epithelium. 3- Multipolar nerve cell. Multipolar nerve cells, are sub-classified according to the shape of the cell body into: Polygonal nerve cells: e.g. sympathetic ganglia Pyriform nerve cell. e.g. purkinji cells in cerebellum Pyramidal nerve cell. e.g. cerebral cortex Polygonal Pyriform Pyramidal B. Neurons are classified according to their function: 1- Motor neurons. carry motor impulses from CNS or from ganglia to effector organs (e.g. muscles & glands). 2- Sensory neurons. carry sensory stimuli from peripheral sensory cells and organs to CNS. 3- Inter-neurons. short neurons that connect sensory and motor neurons. Structure of the Neurons Cell body (perikaryon). Dendrites. Axon 1. Cell body (perikaryon) ❖Nucleus: large, central and pale stained. It has extended chromatin and prominent nucleolus. organelles ❖Cytoplasm: contains cytoskeleton inclusions  Organelles ❖ Nissl’s bodies or granules. By L/M: large basophilic granules scattered in perikaryon and dendrites but not in axon and axon hillock. They are stained with toluidine blue. By E/M: RER, free ribosomes and polyribosome. ❖ Golgi complex : around the nucleus. ❖ Mitochondria. ❖ Lysosomes. No Centrosomes i.e. Mature nerve cell does not divide.  Cytoskeleton Neurofilaments (intermediate filaments): stained by silver Microtubules. Their function is supportive (maintain shape of neuron) and share in transport of impulses Inclusions  lipofuscin pigment, melanin granules, glycogen and lipid droplets. Cell body (perikaryon)  2. Dendrites: - They are usually multiple and short. - Their cytoplasm is similar to that of perikaryon, but they are devoid of Golgi complex. - They contain mitochondria, RER(Nissl’s granules), neurofilaments and microtubules. - They carry the impulse toward cell body. 3. Axon: carries the nerve impulses away from the cell body. Axon arise from the perikaryon by a short pyramidal shaped region called “hillock”, which does not contain Nissl’s granules. It is always single, long and cylindrical. The plasma membrane called “axolemma”, and its cytoplasm called “axoplasm”; which contains mitochondria, microtubules and neurofilaments.  Nissl’s granules are present in ……….. And …………… Nissl’s granules are not present in…………. Nucleus and Golgi apparatus are present in………………… and not present in……………….  Neuroglial cells Neuroglia in C.N.S. is similar to connective tissue in other organs. They are protective and supportive in function. They are: 1.Astrocytes : They are star shape branched cells with numerous processes end with feet like expansions on blood vessels. They support neurons and blood vessels.  Two types are present: A- Fibrous astrocytes in white matter. Their processes are slender, longer and fewer. B-Protoplasmic astrocytes in gray matter. Their cytoplasm is granular and their processes are shorter, thicker with many branches. 2.Oligodendrocytes: They are smaller than astrocytes with few short processes with darkly stained nuclei. They form myelin sheath in C.N.S 3.Microglia: Small in size with elongated dark nuclei, few in number, Their processes are short with small thorny branches. They are present in both gray and white matter. They are phagocytic cells. 4.Ependymal cells: They line the cavities (ventricles) of brain and central canal of spinal cord. They are cuboidal in shape. Their luminal surface has microvilli and cilia 2 1 4 3  Nerve Fibers Consist of axons enveloped by special sheaths derived from ectoderm. The nerve fiber may have one or two sheaths: A. Cellular sheath (Neurolemmal sheath), - In (PNS) this sheath formed by Schwann’ cells. - It is a cellular tube surrounds the axon outer to the myelin sheath. - In (CNS), the cellular sheath is formed by ‘’Oligodendrocytes.’’ B. - Myelin sheath (Non cellular sheath), - It is an interrupted fatty tube that directly surrounds axons. -Consists of cholesterol ,phospholipids and fatty acids. - In (PNS) it is formed by Schwann’ cells, while in (CNS) it is formed by the processes of oligodendrocytes. -Functions: conduction and insulation of impulses. So the nerve fibers in P.N.S may be -Myelinated -Non myelinated  Myelinated fibers in PNS Plasmalemma of Schwann cells winds and form concentric layers around axon. Layers of membranes unite forming myelin sheath.  Myelin sheath shows gaps: nodes of Ranvier. Distance between 2 nodes: internode segment (one Schwann cell). Myelination of nerve fibers in C.N.S by oligodendrocytes The nerve trunk:(peripheral nerve): The longitudinally arranged parallel myelinated nerve fibers are grouped in bundles to form nerve trunk. It has an external sheath of dense C.T. called epineurium. Each bundle is surrounded by dense C.T. coat called perineurium. Each nerve fiber with its sheathes of myelin and Schwann’ cell is enveloped by a layer of loose C.T. called endoneurium. L/M: T.S. nerve trunk In H&E stained section: Each nerve fiber shows acidophilic axon in the center surrounded by thin acidophilic neurolemmal sheath because myelin sheath(fat) is dissolved during staining. In osmic acid stained section: only myelin sheath is stained and appears as black circle. LM nerve trunk H&E LM nerve trunk osmic acid  Ovoid. Nerve Ganglia surrounded by C.T. capsule. collection of nerve cell bodies and glial cells (satellite cells). outside CNS. Types of ganglia : Cranio-spinal (sensory): Cranial ganglia. Spinal ganglia. Autonomic ganglia(motor): Both sympathetic and parasympathetic have the same basic structure. Thin or nonmylinated Spinal ganglion Sympathetic ganglion Differences between Cranio-spinal and Autonomic ganglia Autonomic ganglion (Sympathetic) Cranio-spinal ganglion  Structure Cranio-spinal Autonomic e.g. Sympathetic ganglion capsule Thick and poorly vascular Thin and vascular Nerve cell pseudounipolar Multipolar & star shaped Size of cell Variable in size Small and equal in size Cell bodies Contain central rounded Contain pale eccentric nucleus with prominent nucleus with prominent nucleolus nucleolus Arrangement Arranged in groups scattered in the ganglia Number Few in number numerous Satellite cell Surrounding the cells Surrounding the cells forming forming complete circle incomplete circle The neuroglial cell of ganglia Nerve fiber Thickly myelinated present Thin or non-myelinated between the cell group Thank You