NERVOUS TISSUE | PATH OF SENSE | Medicoze

This tissues are capable of excitability and conductivity. It consists of excitable cells  and supporting cells,which in the central nervous system are the neuroglial cells and in the peripheral nervous system are the Schwann cells.The neuron is the functional unit of the nervous system and consists of a cell body and processes, usually an axon and one or more dendrites. The cell bodies are situated in

the central nervous system or in peripheral ganglia. They possess a large nucleus and well-marked cellular inclusions. The axon  begins at a small axon hillock on the cell body and carries impulses away from the cell body.This often long, slender process ends by dividing into manybranches that have small terminal knobs, boutons, related to the cell bodies or branches of other neurons. The relationship is known as a synapse and may be either facilitatory or inhibitory, depending on the neuron of origin, and possibly on the receptor area of the second neuron. A rather specialized synapse is formed when a nerve ends on a muscle fibre at a motor endplate.Axons may give off one or more short collateral branches.They are myelinated or unmyelinated. The myelin is interrupted

about every millimetre or so by a constriction called the node of Ranvier. In the peripheral nervous system each internodal segment of sheath is produced by a Schwann cell,

the nucleus of which is seen on its surface. These cells play an important role in peripheral nerve regeneration. Fibres of the peripheral nervous system are also covered by a thin

fibrous membrane, the neurilemma. In the central nervous system oligodendroglia take the place of the Schwann cells.Dendrites are usually short unmyelinated processes carrying membrane depolarizations to the cell body. The volume over which the dendrites of a single cell extend is known as the dendritic field. Afferent neurons carry information towards the central

nervous system, and efferent neurons carry instructions

away from it. Within the central nervous system afferent and efferent neurons are often connected by many intercalated neurons.

The neurons are organized to form the central and peripheral nervous systems. The former comprises the brain and spinal cord and the latter the cranial nerves, spinal nerves and autonomic nervous system .A group of neurons in the central nervous system is called a nucleus,

and outside the central nervous system (CNS) such a group is known as a ganglion. Within the CNS are neuroglial cells,variously known as astrocytes, oligodendroglia and microglia,

and these make up almost half of the brain substance.

             Astrocytes are stellate cells with large nuclei and numerous processes, which may be of the thick protoplasmic variety, as found mainly in the grey matter, or the thin fibrous variety found mainly in the white matter. Some of the processes end on blood vessels, and the astrocytes are thought to be concerned with fluid balance in the central nervous system and

with the nutrition of the neurons. Oligodendroglia are oval dark-staining cells possessing few processes. They produce

the myelin of the central nervous system. Microglia are small mobile phagocytic cells and form part of the macrophage system.

Ependymal cells are columnar in shape and line the cavities of the brain and spinal cord. In certain regions the ependyma is modified to form the choroid plexuses of the brain, which produce the cerebrospinal fluid.