Processes of nerve cells covered by layers are called nerve fibers.
According to structure of layers there are:
1.myelinated (a)
2.non-myelinated ().
Processus of nerve cell in nerve fibre is called
axial cylinder (1).
Non-myelinated nerve fibers
They are present in structure of vegetative nervous system.
Non-myelinated nerve fibre consists of a few axial cylinders and layers formed by neurolemmocytes.
With the help of electron microscope in non-myelinated fiber mesaxon is seen.
Mesaxon is dublicator of plasmalemma of neurolemmocyte, on which axial cylinder is suspended.
Myelinated nerve fibres are present both in CNS and PNS.
They are thicker, than non-myelinated.
Myelinated fiber consists of one axial cylinder (1), myelin sheath and neurilemma.
Myelin sheath contains great number of lipids.
In it periodically narrow light lines incisures (4) of myelin are present.
Through definite intervals (1-2milimeters) parcels of fiber without myelin sheath are seen.
These are nodes of Ranvier (4).
Parcel of fiber between two nodes is called internode segment.
Neurilemma or Schwann sheath is external peripheral layer of myelinated fibre.
It contains cytoplasm and nuclei of neurilemmocytes.
Nerve endings
Nerve fibres end by terminal apparatus - nerve endings.
There are 3 groups of nerve endings:
1.terminal apparatus forming interneuron synapses and realizing connection of neurons with each other;
2.effector endings (effectors) passing impulse onto tissues of working organ;
3.receptor (sensitive).
Effector nerve endings are of 2 types:
1.motor these are terminal apparatus of axons of motor cells of somatic or vegetative nervous system. With their help nerve impulse is passed onto tissues of working organs (nerve-muscular endings in transversal-stripped muscles, motor nerve endings in smooth muscles);
2.secretory they represent terminal thickenings of terminals or thickenings along nerve fiber containing synaptic vesicles (cholinergic).
Receptors
Receptor nerve endings are receptors spread in whole organism.
They percept different irritations from external environment and from internal organs.
There are 2 types of receptors:
1.exteroreceptors external these are acoustic, optic, olfactory, taste and tactful;
2.interreceptors internal these are visceroreceptors (they signalize about condition of internal organs) and vestibular proprioreceptors (receptors of supporting-motor apparatus).
Synapse
Synapse is specialized contact between neurons, where passing of impulse from one cell to another occurs.
According to localization synapses are:
1.axo-dendritic;
2.axo-somatic;
3.axo-axonic.
Interneuronic synapses
Polarization of passing of nerve impulse along chain of neurons is determined by their specialized contacts synapses.
Depending on method of passing of impulse synapses can be chemical and electrical.
Chemical synapse
Chemical synapses pass impulses onto other cell with the help of special biologically active substances neuromediators placed in synaptic vesicles (acetylcholin, noradrenalin, dophamin, serotonin, glicin, histamine).
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Electrical synapse
Electrical synapses in nervous system of mammals are present rarely.
Their structure corresponds to fissure-shaped contact.
In synapses with chemical passing are:
1) presynaptic part;
2) synaptic fissure;
3) postsynaptic part.
Presynaptic part
Presynaptic part is formed by parcel of plasmalemma of the first neuron.
It contains accumulations of mitochondria and synaptic vesicles.
Synaptic vesicles are filled by neuromediator excreted into synaptic fissure and taking part in passing of excitation onto postsynaptic part.
Synaptic fissure
Synaptic fissure is located between pre- and postsynaptic parts of synapse.
Its width is 20-30 nanometers.
Postsynaptic part
Postsynaptic part is parcel of plasmalemma of the second neuron which percepts neuromediators.
It contains receptor zones for perception of corresponding mediator.
42. Morphological substratum of reflex activity of nervous system (concept on simple and compound reflex arcs). Role of synapses in polarization of reflex arc. Neuron theory.
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