Neuroglia

In any sections of the nervous system, prepared by ordinary histological methods, small nuclei are seen scattered among the nerve cells and their processes. They are nuclei of the neuroglial cells. It is known 2 types of neuroglia – macroglia and microglia (see fig.1). Macroglia divides into ependyma, astrocytes, oligodendrocytes. Macroglia is of ectodermal origin, while the precursor of microglia is monoblast.

Ependyma lines the central canal of the NS. These are elongated cells, that look like columnar epithelium. They contain rather long processes are directed into the brain matter and cilia, which are directed into the brain cavity. These cells provide the exchange of material between the brain and the cerebrospinal fluid and also produce the cerebrospinal fluid.

Astrocytesare of two varieties. The protoplasmic astrocyte has numerous thick processes. Many processes attach to blood vessels and to the pia mater. The fibrous astrocyte is distinguished by long, thin and branched expansions. These cells are also often attached to blood vessels by means of their processes. Protoplasmic asctrocytes are found in gray matter and fibrous astrocytes in white mater between nerve fibers. The both types of these cells provide the nutrition and support for the nerve cells and their processes in CNS.

Oligodendrocytes are smaller; their few, brief and slender processes have few branches. They always surround nerve cells and processes, in the peripheral Nervous System (ganglia, plexuses, nerves) are called Schwann cells and provide the support and form covers of processes in the CNS and PNS.

Microglia. They are small cells with few short extensions. Microglial cells are scattered everywhere throughout the brain and spinal cord and can migrate and provide phagocytosis.

So, the whole neuroglia provide the mechanical support to neurons and their processes, their nourishment and isolation of the nerve impulses. With ability to phagocytosis some cells (microglia) provide protective function. Neuroglial cells proliferate in some regions and they are actively involved in regeneration of the nerve fibers after injury.

Nerve fibers. The nerve fibers are composed of the nerve cells processes and certain sheaths. All processes are enclosed by a sheath of Schwann cells, which invest (surround) them almost from their beginning to near their peripheral terminations (nerve endings).

There are two types of nerve fibers – myelinated and unmyelinated.

The unmyelinated fiber is the cell process which is invaginated into the cytoplasm of Schwann cells. That is in time of fiber formation the Schwann cells cover or surround the process. Sometimes several processes may invaginate into the cytoplasm of one Schwann cell.

The myelinated fiber has two membranes – myelin and the Schwann sheaths. The electron microscope shows that myelin is actually part of the Schwann cell, consisting of spirally wrapped layers of its surface membrane. The nature of myelinated fiber sheaths is better understood by considering the mode of its formation.

The process lying near the Schwann cell, invaginates the cytoplasm of the Schwann cell. At that time the process comes to be suspended by a fold (duplication) of cell membrane. This fold is called mesaxon. The mesaxon becomes elongated and comes to be spirally around the process (rotates around the process), which is thus surrounded by several layers of cell membrane. These layers of the mesaxon form a rather thick sheath around the process – this is the myelin sheath and it consists of lipids and proteins (as each cell membrane). Outside the myelin sheath forms a layer of Schwann cell cytoplasm – displaced and flattened cytoplasm with nucleus and organelles, which is called Schwann sheath or neurilemma. Each process is related to a large number of Schwann cells over its length. One Schwann cell provides the myelin sheath for a short segment of the process. At the junction of two segments there is a short gap (interruption) in the myelin sheath. It is the node of Ranvier – it is the place between neighboring Schwann cells along the length of the process and it is the place where myelin sheath is interrupted.

Also at the slide we can see that the myelin of each segment is interrupted by oblique cone shaped discontinuities. They are the incisures or clefts of Schmidt-Lantermann, consisting of the Schwann cell cytoplasm and providing a path for conduction of the metabolites into the myelin sheath and axon.

So, Schwann cells are necessary for the life and function of the processes of nerve cells, running different distance from the cell body, while myelin – for the insulation and increase of speed of conducting nerve impulse.

Myelin nerve fibers belong to the somatic nerves, they are long nerves, and amount of the myelin layers depends upon the distance they run. Unmyelinated nerve fibers innervate viscera.

Nerve endings. They are synapses and peripheral nerve endings; they are sensory and motor.

Synapse. The Nerves System consists of complex chains of neurons, arranged to transmit nerve impulse from one neuron to another. The site of transmission is the synapse. The place of contact may be between axon and dendrite – a)axodendritic s., between axon and body of the another cell - b)axosomatic, and frequently may be contact between two axons – c) axoaxonic.

In the place of synapse can be visible the end of axon expands and forms rounded enlargement as bouton (bud), closes to the dendrite or cell body of the other neuron. In these endings there are many mitochondria, lysosomes, synaptic vesicles with neurotransmitters. There are pre- and postsynaptic membranes, separated by a narrow extracellular cleft, that is a synaptic cleft.

From a physiological standpoint the synapse may be excitatory or inhibitory.

During the transmission of the nerve impulse the neurotransmitter is released from synaptic vesicles into the synaptic cleft.

The neurotransmitters are as follows: adrenaline, acetylcholine, dophamine. A few synapses in the SNC are electrical synapses.

2) Sensory nerve ending (afferent, receptors). It is the ending of dendrite of sensory neuron, which receives irritation from inside or outside the body. There are 3 classifications of sensory nerve endings.

	Nerve endings
Sensory	Synapse	Motor (motor end plate)
	Sensory
Exteroceptors	Free	Mechanoreceptors
Interoceptors	Encapsulated	Thermoreceptors
Proprioceptors	Muscle spindles	Photoreceptors
		Chemoreceptors
	Synapse
Exitatory	Axodendritic	Chemical
Inhibitory	Axocomatic	Electrical
	Axoaxonic

Fig.4. Classification of nerve endings

First classification is by the location in the body. 1. Exteroceptors – are the nerve terminals, which are stimulated by the external environment.

2. Interoceptors are located in and transmit impulse from viscera.

3. Proprioceptors are found in muscles, tendons, joints, which give information about movement and position of the body.

The type of stimuli makes another classification – there are chemoreceptor, mechanoreceptor, photo-, thermo-.

Third classification is by the types of structures.

1. Free nerve ending - is the simplest type. It consists of terminal branches of the sensory neuron dendrite with slight enlargements. They are situated in stratified epithelium of cornea, tendon and are touch, pain receptors – mechanoreceptors.

2. Encapsulated nerve endings contain terminals of the sensory neuron dendrites are surrounded with glial cells and may be enclosed within a connective tissue capsule: They are: a) the tactile corpuscles of Meissner– lie under the epithelium of skin and have the capsule is made up of several layers of glial cells (they are touch receptors). b) Corpuscles of Pacini are found in the deeper dermis or around the viscera. There is myelinated nerve fiber in the middle, intermediate zone is from glial cells, and outermost layer is formed of several layers of connective tissue. This corpuscle looks like onion and is pressure receptor is the most numerous on the feet and hands.

3 Muscle spindles. They are located in striated muscle, are spindle shaped (fusiform shaped) and are the proprioceptive receptors. The terminal of the sensory neuron dendrite is surrounded by connective tissue capsule, within which there are two muscle fibers, called intrafusal, and around are located extrafusal.

Some intrafusal fibers look like bag with big amount of nuclei and called nuclear bag fibers and they are connected with the extrafusal fibers. Other intrafusal fibers have a row of nuclei and called nuclear chain fibers.

The sensory fibers are wound spirally around the nuclear region of intrafusal fibers, other are located away from the nuclear region.

Muscle spindles provide information to the brain about the extend and rate of stretching of muscle.

3. The motor endings. In the striated muscles the end of motor neuron axon forms Motor end plate. Near the striated muscle fiber the axon loses its myelin sheath and divides into the branches. The axon terminal is rich in mitochondria and vesicles with the acetylcholine, similar to synapse. The sarcolemma curves in this region and forms numerous folds. Nerve endings in smooth muscle are simpler. At the point of contact the axon contains vesicles with noradrenaline or acetylcholine.

Response of the neurons to injury. Two important responses of the neuron to injury are the progressive degeneration of the axon severed from its cell body and the morphological changes, which occur in the perikaryon of a neuron after axonal transection.

When an axon is severed, trauma occurs at both cut edges. Proximally, primary degeneration extends only a short distance. Damage is soon repaired and new axonal sprouts appear. At about two days the distal axon part begins to fragment and dissolve and the myelin sheath begins to disintegrate. The Schwann cell cytoplasm remains after axonal and myelin degeneration forms a tube filled with fluid and scattered fragments. This place called band. Soon followed the axonal regeneration in the form of fine protoplasmic sprouts. Regenerating fibers grow along the band of Schwann cells.

Functional recovery depends on reestablishment of appropriate sensory and motor connections at the periphery. It means that trauma occurs not in one axon, but in the nerve, which is usually mixed and contains a lot of sensory and motor nerve fibers. Following the regeneration of the peripheral nerve the axon, myelin and Schwann cell slowly return to their normal size and condition.

Retrograde changes in the cell body of origin of a severed axon are as follows: retrograde chromatolysis – it is disappearance of the Nissl body, dispertion of ribosomes and membranes of rER. Retrograde chromatolysis begins about one day after axonal injury and is sometimes referred to as axonal reaction, it reaches its peak in about two weeks. Similar pathologic changes in the nerve cell body may occur for other reactions.