NERVOUS TISSUE A tissue whose functional cells have higher degree of irritability and conductivity.

Introduction: The nervous system is formed of the following organs: Brain Spinal cord Nerves Ganglia (sing. Ganglion) Tissue and Organs of the nervous system are basically formed of the neurons. Neurons are structural and functional units of the nervous system which have higher degree of irritability and conductivity. Other associated or supporting cells of the nervous system are known as neuroglia or glial cells. These include microglia, schwann cells, oligodendrocytes and ependymal cells. Neurons and glial cells have their intercellular substance known as neuropile more pronounced in the brain and spinal cord.

Neurons: This is a nerve cell (basic unit of NS) and all of its processes. Typical neuron has two parts; cell body (Perikaryon or soma) and processes. Cell body contains a nucleus and all necessary organelles for metabolic activity. Processes are for impulse conduction; an axon (long and only one per cell) conducts the impulses away from the cell body while the dendrites (short and one or more per cell) conduct towards the cll body.

Neuron cont……… Neurons are known to have one axon and one or more dendrites. Sometimes a rare adult type neuron (unipolar – single process neuron) is found. This contains several branches for reception and others are axonal hence the name Pseudounipolar Bipolar neurons are found in the retina, cochleal and vestibular ganglia, and olfactory epithelium. Most of the neurons are multipolar with numerous dendrites and an axon.

Neuron cont……… Functionally, neurons are motor controlling effector organs such as muscles and glands, or sensory which receive sensory stimuli both exteroreceptive and interoreceptive, and a third group is internuncial (interneurons or intermmediate) which connect other neurones to establish complex functional circuits or chain of neurons (see diagram below). Nerve cell bodies in the CNS are found in gray matter (peripherally in the brain and centrally in the s/cord). The white matter contains only neuron processes and the supporting cells (glial cells or neuroglia).

Neuron cont……. The diagram showing the lowest functional level by a simple reflex arc.

PERIKARYON (SOMA or CELL BODY) This is formed by the nucleus and cytoplasm and is capable of reception of stimuli generated by other nerve cells, also is a nourishing centre of the cell supplying the oganelles and the processes. Orientations of the processes determine the size and shape of the soma in respect to the region it is found. It is globular in pseudounipolar, elongated and spindle-shaped in bipolar and varying from stellate to pyramidal to globular in multipolar cells. Nuclei are mostly centrally situated in the soma with one or more nucleoli indicating active synthesis in the cell. Organelles are all seen except the poorly developed centrioles, as the neurons are incapable undergoing cell divisions

Soma cont…….. Characteristically, perikaryon presents basophilic components called nissl bodies in the cytoplasm which are associated with the granular ER. Nissl granules are absent in the axon and axon hillock. Other prominent features are neurofibrils which extend from the cytoplasm to the processes. Inclusions are fat droplets, glycogens in embryonic nerve cells not in adults, pigment granules, lipofuscin granules (increase with age) and melanin brownish-black incertain regions of the CNS.

Nerve cell processes These are cytoplasmic extensions of the nerve cell body whose function is to receive sensory impulses – Dendrites, and conducting impulses away from the soma – Axon or axis cylinder. Dendrites are mostly covered by numerous, small, spine-like processes called dendritic spines. These are for synaptic contacts. Axons or axis cylinders usually occur singly per neural cell, arising from a soma at a region called axon hillock. An extension from the hillock get narrowing before reaching an area where myelination usual begins. Myelination of axon are perfected by the oligodendrocyte cells in the CNS or the Schwann cells in the PNS. This is a procee of synthesizing and wrapping of myelin sheath around the axis cylinder.

A cell may be myelinated or non myelinated. Processes cont…… A cell may be myelinated or non myelinated. Axon usually terminate in twig-like branching called telodendria which their small swellings –terminal boutons serve for synaptic contacts. SYNAPSES are junctions between one nerve cell and another. Different parts of a nerve cell can receive stimululi and/or send impulses accordingly. (Ref; soma, axon and dendrite). Synapses are formed of the components from synaptic knobes. From the presynaptic knobe , are presynaptic membrane, synaptic vesicles with neurotransmitters and numerous presynaptic mitochondria. From the postsynaptic knobe are post synaptic membranes, post synaptic receptors and post synaptic mitochondria.

Synapse cont……

SUPORTING CELLS: NEUROGLIA or GLIAL CELLS Neuron = nerve Glia = glue These cells are non-neuronal and function to (bind together) facilitate the proper functioning of the nerve cells in several ways like support and stability, nourishment, protection and maintanance. Neuroglia are relatively smaller than neurons but in themselves there are large sized cell – Macroglia and small sized – Microglia. Macroglia include Astrocytes, Oligodendrocytes Schwann cells and Ependymal cells. Microglia is a Reticuloendotherial types of cell in the NS.

Glial cells cont……… Astrocytes (or astroglia) Are star-shaped cells. Their processes are often in contact with a blood vessel (perivascular foot processes). Astrocytes provide mechanical and metabolic support to the neurones of the CNS. They participate in the maintenance of the composition of the extracellular fluid. they may be involved in the removal of transmitters from synapses and the metabolism of transmitters. Astrocytes are also the scar-forming cells of the CNS.

Oligodendrocytes (or oligoglia) Glial cells cont……… Oligodendrocytes (or oligoglia) Have fewer and shorter processes. Oligodendrocytes form myelin sheath (see below) around axons in the CNS and are the functional homologue of peripheral Schwann cells. Oligodendrocytes has many active processes and may, in contrast to Schwann cells in the periphery, form parts of the myelin sheath around several axons. That mean one Oligodendrocyte is capable of myeliniting many axons at a time.

Glial cells cont…… Ependymal cells. The entire NS develops as a hollow cylinder – neural tube. Its cavity remains in adult as the ventricles of the brain and the central canal of the spinal cord. These cavities are lined by the Ependymal cells (specialized epithelial cells of the NS) which retain the epithelial characteristic present early in the embryo. Cells are ciliated and in adult they appear to be of cuboidal epithelia type.

Glial cell cont……. Ependymal cells seen lining the nervous cavity.

Are small cells with complex shapes. Glial cells cont…… Microglia: Are small cells with complex shapes. Microglia are, in contrast to neurones and the other types of glial cells, of mesodermal origin. They are derived from the cell line which also gives rise to monocytes, i.e. macrophage precursors which circulate in the blood stream. In the case of tissue damage, microglia can proliferate and differentiate into phagocytotic cells.

Connective tissue involvement in the nerve tissue There are organization of connective tissue around the nerves cells. Nerve cell bodies and fibres are wrapped by a delicate connective tissue sheath called endoneurium, nerve cells form bundles or fasciculi which are wrapped by perineurium and the bundles or fasciculi are forming a nerve which is in turn wrapped by a tough connective tissue sheath - epineurium. This continues with the deep fascia.