Nervous Tissue 
I. THE NEURON is the structural and functional unit of the nervous system. The neuron consists of a perikaryon (cell body), dendrite, and axon, each of which contains certain components. The axon arises from an extension of the perikaryon called the axon hillock. The part of the axon between the axon hillock and the start of the myelin sheath is called the initial segment.
II. NEUROGLIAL CELLS are the supporting cells of the nervous system.
A. Central nervous system (CNS)
1. Oligodendrocytes originate in the neuroectoderm. They produce myelin.
2. Astrocytes originate in the neuroectoderm.
a. Characteristics
(1) Astroglial foot processes project to capillaries.
(2) Astrocytes contain glial fibrillary acidic protein (GFAP).
(3) Astrocytes are connected by gap junctions.
b. Functions
(1) Astrocytes form an external and internal glial-limiting membrane.
(2) They metabolize neurotransmitters.
(3) They clear the CNS interstitial fluid of K', glutamate, and a-aminobutyric acid.
(4) React to brain injury by undergoing hypertrophy and hyperplasia.
(5) They may form glial scars after injury.
3. Microglia originate in the mesoderm. They arise from monocytes, and are phago, cytic.
4. Ependymal cells originate in the neuroectoderm. They line the ventricles of the brain.
5. Tanycytes are a modified form of ependymal cell. They transport cerebrospinat fluid (CSF) to neurons in the hypothalamus.
6. Choroid epithelial cells line the ventricular surface of the choroid plexus and produce CSF.
B. Peripheral nervous system (PNS). Schwann cells originate from neural crest cells and produce myelin. They invest myelinated and unmyelinated axons of the PNS.
Table. Ultrastructural Components of Neuron
Cell Part Ultrastructural Components
Perikaryon Nucleus with prominent nucleolus, rER (Nissl substance), Golgi complex, some sER,
mitochondria, lysosomes, microfilaments (actin), neurofilaments (intermediate),
microtubules, and inclusion bodies
Dendrite Similar to the perikaryon Axon Some sER, mitochondria, neurofilaments
(intermediate), microtubules, and neurose cretory vesicles No rER (Nissl substance),
no Golgi complex, no lysosomes
rER = rough endoplasmic reticulum; sER = smooth endoplasmic reticulum.
III. BLOOD-BRAIN BARRIER
A. Represents an anatomic and physiologic separation of the blood from the CNS intersti~ tial fluid.
B. Consists of tight junctions between nonfenestrated endothelial cells with few pinocytotic vesicles.
C. Does not exist in some areas of the CNS:
1. Median eminence
2. Neurohypophysis
3. Lamina terminalis (supraoptic crest of the hypothalamus)
4. Pineal gland
5. Area postrema
6. Choroid plexus
IV. BLOOD-CSF BARRIER consists of tight junctions between choroid epithelial cells of the choroid plexus.
V. NERVE DEGENERATION AND REGENERATION
A. Peripheral nervous system
1. Degeneration. Anterograde (wallerian) degeneration of the axon and myelin sheath occurs distal to the site of injury. Macrophages infiltrate to remove cellular debris. Chromatolysis (loss of rER), movement of the nucleus to the periphery, and an increase in volume of the perikaryon occurs.
2. Regeneration. Schwarm cells proliferate and form a cord that is penetrated by the growing axon. The axon grows at a rate of 3 mm/day until it reaches its effector site (e.g., skeletal muscle). If the axon does not penetrate the cord of Schwann cells, the axon will not reach its effector site.
B. Central nervous system. Regeneration does not occur in the CNS.
V1. NODES OF RANVIER are gaps in the myelin sheath formed by spaces between successive oligodendrocytes (in CNS) or Schwann cells (in PNS) along the length of the axon. Nodes of Ranvier contain Na+ ion channels, and are sites where action potentials are generated by membrane depolarizations.
