Nervous Tissue PDF

NERVOUS TISSUE The nervous tissue is ectodermal in origin.  Anatomically: the nervous system is divided into: A- Central nervous system [C.N.S.] consisting of the: 1-Brain inside the skull. 2-The spinal cord inside the vertebral column. B- Peripheral nervous system [...

NERVOUS TISSUE The nervous tissue is ectodermal in origin.  Anatomically: the nervous system is divided into: A- Central nervous system [C.N.S.] consisting of the: 1-Brain inside the skull. 2-The spinal cord inside the vertebral column. B- Peripheral nervous system [P.N.S.] composed of: 1- Nerves. 2- Nerve ganglia. 3- Nerve endings.  Structurally: Nervous tissue consists of two types of cells: 1- Nerve cells (Neurons): The human nervous system contains more than 100 millions of nerve cells (neurons). 2- Glial cells (neuroglia): They are for support ,protection, nutrition of neurons in CNS. 1-Nerve cells (NEURONS) The neuron is the structural and functional unit of the nervous tissue. Functions of the neurons: 1- Reception and transmission of stimuli. 2- Triggering of certain cell activities. 2 Neurons consist of 2 parts: 1-Cell body (perikaryon). 2-Processes: -Dendrites. -Axon. 3   Structure of the Neuron I-Cell body (perikaryon): -It is the part of the neuron that contains the nucleus and surrounding cytoplasm. -Perikaryons are present only in gray matter of C.N.S and ganglia in P.N.S and some sensory regions (e.g olfactory mucosa). It contains: Nucleus: It is single, spherical and central with fine dispersed euchromatin and prominent nucleolus. Cytoplasm: It is mildly basophilic containing most of the cell organoids and inclusions. 1) Nissl's Bodies or Nissl's Granules: By L/M, the cytoplasm contains large basophilic granules with spotty distribution called Nissl's Granules. By E/M ,they consist of highly developed rER arranged in parallel cisternae with free ribosomes and polyribosomes in between. Nissl,s granules are scattered in the perikaryon and extend into the dendrites but not in the axon. After injury or exhaustion of the nerve cell Nissl,s Granules disappear due to their breaking down and this is called (chromatolysis). 2) The Golgi complex: consists of multiple stacks around the nucleus. 3) Mitochondria: They are in all parts of the neuron. 4) Neurofilaments (intermediate filaments): They are in all parts of the neuron and can be stained with silver. 4  5) Microtubules: They are in all parts of the neuron. Their function is supportive (maintain the shape of the neuron) and share in transport of impulses. 6) Lysosomes. 7) Cell inclusions: Stored food as lipid and glycogen granules. Pigments as lipofuscin pigments in old nerve cells and melanin pigments in nerve cells of the substantia nigra of the midbrain. II- processes : Dendrites and Axons 1-Axon: Axon arises from the perikaryon by a short pyramidal shaped region called "the axon hillock". Which does not contain Nissl,s granules. It gives branches terminally by giving terminal arborizations in PNS. The plasma membrane of the axon is called the "axolemma" and its cytoplasm is known as "axoplasm". The axoplasm contains mitochondria, microtubules, neurofilaments and axoplasmic vesicles. 2-Dendrites: Dendrites are usually multiple and short. They divide like the branches of a tree. Their cytoplasm is similar to that of perikaryon but they are devoid of Golgi complexes. they contain mitochondria, Nissl's bodies, neurofilaments and microtubules. They carry impulses to the cell body. 5   Classification of Nerve cells (Neurons) : A) According to the number of processes that arise from the nerve cell body, neurons are classified into: 1-Unipolar nerve cell: unipolar neuron only has one nerve process extending from the cell body, neurons only occur in invertebrates. 2- Pseudo-unipolar nerve cell: The cell body has one process which divides into two branches (axon and dendrites) e.g. neurons of spinal ganglion. 3- Bipolar nerve cell :which has two processes e.g. bipolar neurons of retina & olfactory mucosa. 4- Multipolar nerve cell: which have more than two cell processes, one process being the axon and others are dendrites. 6 B) According to function, neurons are classified into: 1-Motor neurons (efferent): carry motor impulses from CNS to effector organs as muscles and glands. 2-Sensory neurons (afferent): which receive sensory stimuli from the receptors and carry them towards the CNS. 3-Inter-neurons: They establish relationships among other neurons. 7   Nerve Fibers They consist of axons enveloped by special sheath of ectodermal origin. The nerve fiber may have one or two sheaths; A) Cellular sheath (neurolemmal sheath): - In PNS most axons are covered by cellular sheath called neurolemmal sheath which is formed of Schwann's cells. Functions of cellular sheath: formation of myelin sheath. It is important for regeneration of the nerve fibers after trauma. B) Myelin Sheath:  It is an interrupted tube that directly surrounds axons.  It is formed by Schwann's cells in PNS  Function of myelin sheath is to help in conduction and insulation of nerve impulses. 8 9  Types of Nerve Fibers: A) According to the length of the fiber: 1) Golgi type I: They are very long fibers. 2) Golgi type II: They are short fibers. B) According to their sheaths: 1)Myelinated. 2)Non-myelinated.  SYNAPSE Definition: -Synapse is the site of contact between two neurons or between neurons and other effector cells (e.g. muscle and glands). Types of Synapses: 1-Axodendritic (the commonest type) between axon of a neuron and dendrite of another neuron. 10 2- Axosomatic between the axon of a neuron and the cell body of another neuron. 3- Axoaxonic (The rarest type) between the axon of a neuron and the axon of another neuron.  Nerve Ganglia A nerve ganglion is a collection of nerve cell bodies outside the CNS. Ganglia are ovoid structures surrounded by C.T. capsules containing neuronal cell bodies and glial cells. 11   According to the direction of the nerve impulse, the ganglia are of two types: 1- Cranio-spinal Ganglia (Sensory): -Unipolar cell bodies are found in spinal ganglia (on dorsal roots) and in ganglia on the roots of cranial nerves. -There are no synapses within these sensory ganglia. -The impulses are transmitted from the ganglia to C.N.S. 2- Autonomic Ganglia : -Multipolar neurons are found in autonomic ganglia The impulses are transmitted from C.N.S. to the ganglia. -They constitute the ganglia of sympathetic and parasympathetic system (intramural ganglia). 12  Differences between Cranio-spinal ganglia and Sympathetic ganglia Structure Cranio-spinal ganglia Sympathetic ganglia 1-The capsule Thick Thin. 2-Nerve cells shape They are pseudounipolar rounded They are multipolar and star cells. shaped cells. 3- Nerve cells size Their cell bodies are variable in The cells are small and of size mostly large (120 µm) but equal size (20-45 µm). some are small (20 µm). 4- Nerve cells nucleus Central. Eccentric. 5- Nerve cells In groups. Scattered. arrangement 6- Nerve cells number Few. Numerous 7-Satellite cells (capsular Complete layer Incomplete layer cells ) 8- Nerve fibers Thickly myelinated. Thinly or non myelinated. 9-Synapses No synapses. Synapses are present. 13  2-NEUROGLIA Neuroglia or glial cells are the second type of cells present in C.N.S. Types of neuroglia in C.N.S. 1- Astrocytes 2- Oligodendrocytes 3- Microglia 4- Ependymal cells Astrocytes, oligodendrocytes and ependymal cells are ectodrermal while microglia are mesodermal in origin. These cells can be stained with silver or gold. 1- Astrocytes: They are ectodermal in origin. Astrocytes are star-shaped cells with multiple long processes. Some of these processes extend to form foot-like expansions which attach to the wall of blood capillaries (vascular foot). The nucleus: single, central, rounded, pale with fine chromatin. The cytoplasm is mildly basophilic and contains the usual organoids, intermediate filaments and lysosomes which are called gliosomes. 14 2-Oligodendrocytes: They are ectodermal in origin. They are present in the white and grey matter of the C.N.S. These cells have a few short processes that wrap around the axons in CNS producing a myelin sheath. The nucleus: single, central, oval and small which is moderately stained. The cytoplasm is scanty forming a thin rim around the nucleus. It contains mitochondria, ribosomes, rER and Golgi. 15 3-Microgolia: Mesodermal in origin and derived from blood monocytes. They are small elongated cells with branching processes arising from each pole of the cell. The nucleus is single central oval dense. The cytoplasm is basophilic and contains the usual cell organoids and especially lysosomes. 16