Histological Constituents of the Nervous System

Abstract

The histological constituents of the nervous system include neurons which can be classified based (a) on the number of cell processes as unipolar, bipolar, pseudounipolar neuron, and multipolar, (b) types of neuronal connections as projection neurons or interneurons. The synapse, composed of the presynaptic membrane, synaptic cleft, and postsynaptic membrane, can be excitatory or inhibitory. The classification of synapses (asymmetric, symmetric, axo-dendritic, axo-somatic, axo-axonal) was widened by considering the tripartite synapse and the tetrapartite synapse or “synaptic quadriga.” Many genes are involved in the regulation of synaptic proteins dealing with presynaptic neurotransmitter release, postsynaptic proteins, and trans-synaptic communication. A description of nerve fibers and peripheral nerve is provided.

Glial cells include astrocytes, oligodendrocytes, microglia, ependymal cells, tanycytes, and choroid epithelium. Astrocytes (protoplasmic and fibrous/fibrillary) fulfill trophic and reparatory functions and play important roles in brain plasticity, development, structural, vascular and metabolic processes, maintenance of the extracellular milieu and control of CNS environment, signaling, and response to injury. The astrocytopathology is manifold. Oligodendrocytes form the myelin sheath of axons in the central nervous system. Oligodendrocytes which are found closely apposed to neuronal perikarya in the gray matter are known as the perineuronal satellite cells, while the interfascicular oligodendrocytes are those seen in the white matter. Microglia are highly specialized tissue macrophages of the brain which function as defense cells of the nervous system secluded behind the blood–brain barrier. They are present as ramified microglia, activated microglia, and phagocytic microglia. Ependymal cells line the ventricular system. Tanycytes are a special form of ependymal cells found in the third ventricle and at the floor of the fourth ventricle. Myelin is mainly made up of lipids (70% of its dry weight) and of proteins (30% of its dry weight). Of clinical importance are various myelin proteins, i.e., myelin basic protein (MBP), proteolipid protein (PLP), myelin-associated glycoprotein (MAG), myelin oligodendrocyte glycoprotein (MOG), 2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNP), and myelin oligodendrocyte basic protein (MOBP).

The histological makeup of the meninges (dura mater, arachnoidea, pia mater, dural sinuses), choroid plexus, vessels (artery, arteriole, capillary, venule and vein, and endothelium) is described. The concepts of a glymphatic system as well as of the neurovascular unit composed of neurons, glia, endothelial cells, vascular smooth muscle, and immune cells are provided.