Nerve fibres. Classification. Structure of myelinated and non-myelinated nerve fibres. Nerve endings. Concept on synapse, its structure

Processes of nerve cells covered by layers are called nerve fibers.

According to structure of layers there are:

1.myelinated (a)

2.non-myelinated (б).

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).

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.

Нервная ткань входит в состав нервной системы, функционирующей по рефлекторному принципу, основным компонентом которого яв-ся рефлекторная дуга. Рефлекторная дуга представляет собой цепь нейронов, связанных друг с другом синапсами и обеспечивающих проведение нервного импульса от рецептора чувствительного нейрона до эфферентного окончания на органе.

Самая простая рефлекторная дуга состоит из двух нейронов – чувствительного и двигательного. Во многом между чувствительными и двигательными нейронами включены вставочные нейроны(ассоциативные).