General morphofunctional characteristic of nerve tissue. Sources of development. Neurons. Morphological and functional classification. Structure of neuron. Neurosecretory cells

Characteristic of nerve tissue

Significance of nerve tissue in organism is determined by basic properties of nerve cells (neurons) to percept irritation, to get condition of excitation, to produce impulse and pass it.

Nerve tissue is the basis of structure of organs of nervous system providing regulation of the all tissues and organs, their integration in the organism and connection with surroundings.

Development of nerve tissue

Nerve tissue is derived from dorsal thickening of ectoderm – nerve lamina.

Neurons of spinal ganglia, neurons of vegetative ganglia and pigment cells of skin are derived from nerve comb.

Glioblasts of nerve tube are source of development of ependimocytes, astrocytes and oligodendrocytes.

Neurons of ganglia of cranial nerves (V, VII, IX, X) are developed from neural placoda and nerve comb.

From neural placoda neurolemmocytes (Schwann cells) are developed.

Structure of nerve tissue

• Nerve tissue consists of neurons and neuroglia.

• Neuroglia provides existence and functioning of neurons and realizes supporting, trophic, limiting, secretory and protective functions.

• Nerve tissue is derived from dorsal thickening of ectoderm – nerve lamina.

• Neurons or neurocytes consist of body (pericaryon) and processes – axon (3) and different number of branched dendrites (1).

• Axon is processus along which impulse passes from body of cell.

• Dendrite is branched processus passing impulse to body of neuron.

Morphological classification of neurons

According to number of processes there are neurons:

1.unipolar

2.bipolar

3.multipolar

4.pseudounipolar

Unipolar neurons have just 1 axon.
Neuroblasts have unipolar shape.

Bipolar neurons have axon and dendrite.
They are localized in sense organs.

Multipolar neurons have axon and many dendrites. Great number of neurons, neurons of CNS and of vegetative ganglia are multipolar.

Pseudounipolar neurons have 1 processus further dividing into axon and dendrite. Pseudounipolar neurons are present in spinal ganglia.

Functional classification of neurons

Depending on function there are neurons:
1.Receptor (or sensitive, sensory, afferent). They percept impulse.

2.Associative (or intercalated). They realize connection between neurons.

3.Effector (or efferent, motor). They pass impulse onto tissues of working organs, impel them to action.

Structure of neuron

• Neurons contain one nucleus located at the center.

• Shape of nucleus is round.

• Chromatin is dispersed.

• In nucleus 1 or 2 big nucleoli are located.

• Cytoplasm is rich is organelles – Golgi complex, ER, mitochondria, lysosomes, components of cytoskeleton.

Chromatophilic substance

• During staining of nerve tissue by anilin dyes in cytoplasm of neurons basophilic blocks and granules appear.

• This is chromatophilic substance.

• Also it is called tigroid substance or Nissl substance.

• Chromatophilic substance is accumulation of granular ER, free ribosomes and polysomes.

• This substance is localized in pericaryons and dendrites of neurons.

Structure of neuron

• Corresponding to high specificity of functional activity neurons have specialized plasmalemma.

• Plasmalemma of neurons is able to pass excitation.

• Golgi complex in neurons is seen as accumulation of different shape rings, twisted threads and granules.

• Cellular center is located between nucleus and dendrites.

• Mitochondria are present in body of neuron and in the all processes.

Cytoskeleton of neurons

• During impregnation of nerve tissue by silver in cytoplasm of neurons neurofibrils are revealed.

• They form compact net in pericaryon of cell and are oriented parallel in dendrites and neurites.

• Neurofilaments and neurotubules correspond to neurofibrils.

Secretory neurons

• All the neurocytes are able to synthesize and secrete biologically active substances, particularly mediators.

• But there are neurocytes specialized predominantly for realizing of this function.

• These are secretory neurocytes.

• They are large neurons with neurosecretion in cytoplasm.

• Neurosecretions realize role of neuroregulators.

• Neurons of hypothalamus possess neurosecretory function.