Tracts also contain afferent and/or efferent fibres, which may be sensory, motor or mixed. Each tract follows a specific path within the brain and spinal cord and can also carry both somatic &/or autonomic information.
Nucleus – small masses of neuronal cell bodies located inside the CNS.
Ganglion – small masses of neuronal cell bodies located inside the PNS.
Neuroglia
The neuroglia comprise well over 90 % of the cells in the nervous system. Neuroglial cells provide structural support, nourish, protect & insulate axons, maintain the extracellular space and internal ionic environment and phagocytize debris. They are very active in the participation in operation of the nervous tissue. Neuroglia are smaller than neurons and do not propagate or generate action potentials. There are 6 types of neuroglia. The 1 st 4 are associated with the CNS and the last 2 are associated with the PNS. 1. Ependymal cells line the ventricles of the brain and the central canal of the spinal cord. They are columnar cells with apical cilia. Modified ependymal cells form the epithelium of the choroid plexus which secretes, monitors & circulates cerebral spinal fluid. Ependymal cells form the blood-brain barrier. 2. Astrocytes are star-shaped cells characterized by numerous delicate branching processes. They provide support & strength and monitor chemical environments. They contribute to the blood-brain barrier. They are capable of phagocytosis and form glial scars in the event of CNS injury.
3. Oligodendrocytes are found in the white matter of the CNS where they form the myelin sheath.
4. Microglia are the smallest of the glial cells and are rather rare. They migrate to sites of injury or degeneration and are capable of phagocytosis.
5. Schwann cells are found in the PNS where they form the myelin sheath and support unmyelinated axons. They are also capable of phagocytosis.
6. Satellite cells surround cell bodies in the PNS and regulate exchange of materials.
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