3.2.Topic content.

Structural and functional unit of nervous system and nervous tissue is defined as neuron or nerve cell.

Neuronal organization main principles (by R.Kahal):

1. Neuron with its processes represents one morphological whole.

2. Neurons are similar genetically, they are origined from one neuroblasts.

3. Neuron is one functional integrity.

4. Excitement alongside neuron is distributed in 1 direction – from dendrites to axon.

5. Neuron represents trophyc integrity. Axons death is observed at neurons bodies removal. Axon cutting also leads to neurons death.

6. Neuron participates in pathological reactions as one a whole. Destructive changings engulf a whole neuron.

Neuronal theory.

Mentioned principles are in the basement of CNS structure neuronal theory. According to this theory, CNS structure and function are defined by multiplied interconnected neurons with discrete features.

Also CNS represents continuous structure like syncitium (it represents more in CNS of Non-Vetebrata (Hydras and other Coelenterata) and Amphibias.

Heterogenous neurons in their interconnections allow human CNS participate in millions various reactions. From hundreds to thousands different synapses are located on separate neurons bodies. Separate synapses function with different mediators and are connected to specific postsynaptic chemical reactions (see below).

Excitement process in neurons can be explained by electric and chemical theory.

Electrical theory.

Morphological-functional neuronal variery lies on its base. There is so-called “light axonic colliculus” or “light spot” at the place of axons divergence. This neuron locus is charged positively comparatively to neurons membranes other locusi (they have negative charge).

At excitement of synapses located on neurons dendrite and body potentials difference is increased between “axons colliculus” and rest neuron part. It gives life to the axon spike activity at definite level reaching. The more is gradient of potentials difference between neuron body and “axon colliculus”, the more frequent is initial axonic impulsation.

Chemical theory

This theory connects neurons excitement processes with specificity of mediators, oligopeptides and ther biologically-active substances released from synapses. You will study below about excitable (noradrenaline, acetylcholine, glutamate) and inhibitory (gamma-aminooleic acid, glycine) mediators in synapses.

Also neurons excitement process depends on the specificity of postsynaptic chemical reactions which are distributed to the neurons nuclei genetic apparatus.

NEURONS CLASSIFICATION

1. Depending on number of poles:

a) unipolar neurons – are present only in embryonic stage in human beings, from the single pole, both the processes - axon and dendrite arise;

b) pseudounipolar – they have one axon and one dendrite but both come from one neuron body pole; example – spinal ganglii;

c) bipolar neurons - have two poles, one for axon and another for dendrite; example – retina, cochlea spiral ganglium;

d) multipolar neurons - have many poles, one of which gives rise to the axon and all the other poles give rise to dendrites; biggest neurons number belong to this group.

2. Depending on function:

1. motor or efferent neurons - carry the motor impulses from central nervous system to the peripheral effector organs like muscles, glands, blood vessels, etc.:

• neurons of autonomic nervous system;

• alha-motoneurons – innervate extrafusal muscular fibers (out of muscular spindles);

• gamma-motoneurons – innervate intrafusal muscular fibers (inside muscular spindles);

2. sensory or afferent neurons - neurons carry the sensory impulses from periphery to the central nervous system;

3. associative (99,8% of all) – they bind different-typed neurons.

P.S. Muscular spindles – are muscles receptors percepting muscle length and its contraction speed.

3. Depending on length of axon:

a) Golgi Type I Neurons - have long axons, the cell body of these neurons is in central nervous system and their axons reach remote (far) peripheral organs;

b) Golgi Type II Neurons - have short axons, are present in cerebral cortex and spinal cord, cerebellar Purkin’e cells belong to them.

4. Depending on mediator:

a) adrenergic – contains noradrenalin;

b) cholinergic – acetylcholine;

c) dopaminergic – neuromediator dopamine;

d) peptidergic – contain one of neuropeptides:

• substance P;

• neuropeptide Y;

• calcitonin-gene-related peptide and so on.

NEURONS STRUCTURE

1. Nerve cell body

2. Dendrite

3. Axon

4. Myelin sheath or membrane

5. Neurilemma

Dendrites are conductive in nature and transmit impulses towards the nerve cell body. Dendrites are shorter processes terminating mostly near nerve cell body.

Axon is longer process of nerve cell. Axon may extend for a long distance away from nerve cell body. Length of the longest axon is about one meter.

NEUROGLIA

It is located between neurons. These cells, gliocytes, are known as supporting cells. The name origin: Greek – “neuron” – “nerve” and “glia” – “glue”.

Functions:

1. supporting;

2. trophic;

3. demarcating;

4. barrier;

5. secretory;

6. protective.