Book 4 - Self Instructional Manual for Cancer Registrars - SEER ...

SECTION MTHE NERVOUS SYSTEMThe nervous system provides the integration and control of body processes. It is customarilydivided into: 1) the central nervous system which consists of the brain and spinal cord and theircoverings which are called meninges and 2) the peripheral nervous system which consists of thenerves that connect the brain and spinal cord to the tissues and organs of the body.Nervous TissueNervous tissue is composed of three types of cells:• NeuronThe nerve cell (neuron) is the basic structural and functional unit of the nervoussystem. Each neuron consists of a cell body which contains a large nucleussurrounded by cytoplasm and processes called dendrites and axons. These processesare peculiar to neurons and are responsible for the conduction and transmission ofneural information. Dendrites carry impulses toward the cell body; a single axonconducts impulses away from the cell body. Axons (nerve fibers) in the peripheralnervous system may be covered by a myelin sheath and by neurilemma, which is amembranous sheath that covers the axon directly or covers the myelin sheath whenit is present. Not only does the myelin sheath increase the speed of nerve impulseconduction, but it insulates and maintains the axon; the neurilemma assists in theregeneration of injured axons. Nerve tissue of the brain and spinal cord has noneurilemma, and, thus, it does not regenerate. Instead, the central nervous systemcontains neuroglial cells, called oligodendrocytes, which form a myelin covering forthe brain and spinal cord.• Neuroglia (glial cells)The neuroglial cells of the nervous system provide the connective tissue which bindstogether the neurons in the central nervous system giving support and protection.They consist of astrocytes, oligodendrites, ependymal cells, and microglial cells.Most central nervous system (CNS) tumors arise in the neurogliai cells.• Schwann cellsThe Schwann cells form a sheath (neurilemma)nerve fibers.covering the axons of peripheralThe processes of the neurons (nerve fibers) may serve sensory or motor functions. Sensory nervefibers carry messages of sensation and feeling from the tissues and organs to the brain and spinal cord.Motor nerve fibers carry messages pertaining to movement from the brain and spinal cord t__qo tissuesand organs.393