Structural and functional organization of the nervous tissue

Neurons.

There are more than 10 billion neurons in the Nervous System. Each neuron has a cell body, consisting of a nucleus and surrounding cytoplasm, which is called perikaryon and usually some cell processes - dendrites and axon. The size, shape, number and mode of branching of the cell processes may be different.

The nucleus is large, pale, spherical, or slightly ovoid, and usually centrally placed within the perikaryon. The cytoplasm of the nerve cell is crowded with filamentous, organelles. The combination of euchromatic nucleus, large nucleolus, prominent Golgi apparatus, Nissl substance, mitochondria is indicative of the high level of anabolic activity for the maintaining these large, nondividing cell.

That is all organelles are present and there are specific organelles are as follows: neurofibrils and chromophilic substance.

Neurofibrils. When the nerve cell is impregnated with silver, the neurofibrils appear as slender strands (threads) coursing through the cytoplasm of the perikaryon from one dendrite into another or into the axon. In electron micrographs can be visible that they are formed by aggregations of slender neurofilaments, about 100 Å in diameter. In dendrites and axon these filaments usually lie parallel to the long axis of the process. The neurofibrils constitute the support and drain system of neurons and their processes.

Chromophilic substance (Nissl body ). It appears as deeply basophilic masses (clumps) in the perikaryon. In EM Nissl bodiis are seen to consist of massed cisternae of rough ER. Ribosomes are attached to the outer surface of the membranes, it is the basophilic region. Notes – Nissl bodies are abundant throughout the cytoplasm, including the dendrites, but is usually absent in the axon, beginning from axon hillock. The form, size and distribution of the Nissl bodies are very considerably in different types of neurons. The lot of Nissl bodies shows high standard of the synthetic processes, providing the support of mass of perikaryon, cell processes and their functions. Under the different physiological conditions, such as rest and fatigue, or in certain pathological states, Nissl bodies change their appearance. In hard pathological states may be chromatolysis – the Nissl bodies disappear.

Mitochondria are everywhere, intermingling with Nissl bodies and neurofibrils. Their number varies from cell to cell and in different parts of the some cell, they are especially numerous in axon endings.

Centrosome is seldom observed in light microscopic preparations. Since neurons do not proliferate, the role of this organelle in the adult nerve cell is unknown.

Processes of neurons. The cytoplasmic processes of the nerve cells are their most remarkable features. In almost all neurons there are two kinds – the dendrites and the axon.

Dendrites always receive the information (irritation) from the inner our outer environment or from another cell and conduct it to cell body. They may have nearest or remote arborizations, usually contain Nissl bodies, neurofibrils and mitochondria.

Whereas there are usually several dendrites, there is only one axon to each neuron. The axon carries the response of the neuron in the form of a propagated action potential to other cells or to the functional organs. This cell process often arises from a small conical elevation on the perikaryon devoids of Nissl bodies, called the axon hillock.

The dendrites usually have many branches, while the axon may be branched or no branched. All processes complete of the nerve endings.

So, in the cytoplasm of both processes there are all elements of cytoplasm, only Nissl bodies are absent in axon.

The neurofibrils in both processes provide the way for substances – from endings to perikaryon – the retrograde stream (backward transport) in dendrites, and from perikaryon to endings – the axonal stream. Stream of substance transport has speed from 2-4 mm/day to 400 mm/day.

The cytolemma of the processes is the material basis for the conducting of the nerve impulses. Note, impulse travels along the dendrite – to the cell body, along the axon – from the cell.

There are several classifications of the nerve cells:

The nerve cells are classified according to the number of processes they bear (morphological classification). These cells are as follows. 1. Unipolar neuron contains only one process – axon. True unipolar cells are found only in early embryonic stages. 2. Bipolar neuron has two processes – one is axon and one is dendrite. Typical bipolar neuron is located in the retina, nasal epithelium. In addition in craniospinal ganglia neurons have a single process, which arises and divides like the letter “T”. One branch is directed to the periphery as dendrite and another traveling to the CNS – as axon. This single process usually runs a considerable distance before bifurcating, sometimes enveloping the cell body. These neurons are thus called pseudounipolar (3). 4. Multipolar neurons bear several dendrites and one axon. They form the most numerous type in the NS.

Fig.3. Classifications of nerve cells

Another classification may be called functional. On the basis of functional and anatomic relations neurons fall into three groups. 1. Sensory (first order or afferent) neurons, which are so situated and constructed as to percept irritation arising from within or outside the organism and to send impulses to the next (association) neurons. The dendrites of these cells always finish with the receptors.

2. Association (second order, intercalated) neurons, which are links between sensory neurons and neurons of the third group. Their processes participate in the synapses formation. Usually there is a lot of intercalated neurons in the chains of neurons in the nervous system. More than 99.9% of the CNS cells belong to intercalated neurons.

3. Motor (third order or efferent) neurons, which convey (transmit) impulses to muscles or glands, stimulating them to action. The axons of the motor neurons are always finished with the motor nerve endings.

The nerve cell processes form nerve fibers, from which the nerves are constituted, terminated with the nerve endings.

But first we must consider the neuroglia – the complex of the additional cells of the nervous tissue, which participate in the formation of the nerve fibers and nerve endings.