LectureLecture 99 GeneralGeneral medicinemedicine_2nd _2nd semestersemester

Nerve tissueNerve tissue

SStructuretructure, classification and function of neurons. , classification and function of neurons. Synapse Synapse

NeuroglialNeuroglial cells cells -- types and functiontypes and function

Sheathes of nerve fibres. Conduction of nerve Sheathes of nerve fibres. Conduction of nerve impulsesimpulses

Outline of Outline of developmentdevelopment of the nerve tissueof the nerve tissue

nervous tissue is the most important tissue of the bodyit is widely distributed and with few minor exceptions all organs containnervous elements

the primary function of nervous tissue is to receive stimuli from the outside,to transform them into nervous impulses, and to convey these to other partsof the body so that a suitable response may occurthe tissue derives from the ectoderm

the nervous tissue consists of two principal types of cells: the nerve cells or neurones, and special supporting cells called neuroglia

in neurones two properties of protoplasm are developed to a great degree:irritability - the capacity for response to physical and chemical agentswith the initiation of an impulse,conductivity - the ability to transmit such an impulse from one locality to another

morphologically neurones differ from other cells of the body above all by a great diversity of shape and size of cell bodies and lengths of their processes

Structure of the neuron

the neuron consists of:

the cell body, or perikaryon (contains the nucleus and the main concentration of organelles

the dendrites (their number varies in a great range, theoretically from one to several hundreds; they are usually short and conduct impulses to the perikaryon

the axon (neurite) - it is mostly very long and always single, it conducts the impulses away from respective cell

(in the periphery the axons /in some neurone also dendrites/

run gathered together in groups termed as nerves)

twig-like branchings or terminalarborizations - the telodendria, which touch the perikarya, dendrites or axons of one or more neurons in sites called synapses

the cell body or perikaryon - pale-staining round nucleus with a prominent

nucleolus in the centre; the cytoplasm is slightly basophilic: numerous mitochondria, large

Golgi apparatus, lysosomes, microtubules, neurofilaments and inclusions are detectable in it by electron microscopy; free ribosomes and RER are often clustered and form areas known as Nissl bodies in the light microscopy - they stain with the basophilic and metachromatic dyes, e.g. with toluidine blue and thionin (red-violet)

lipofuscin - tear and wear pigment

dendrites are short and end near the cell body; they tend to branch and send off

short spinous processesthat touch the axonal endings of other neurones

dendrites contain the same organelles as the cell body proper, except the Golgi network

axon is only one for each neurone and may be often extremely longthe part of the axon that joins to the cell body is cone-shaped - axon hillockthe initial segment is a portion of the axon between the axon hillock and the point at which myelination begins - is the site of generation of nerve impulses

axon may give rise near the cell body collateral branchesterminal part of the axon is richly branched branches are thin and of knoblike shape -terminal arborizations (presynaptic knobs or terminal boutons)the axoplasm similar to axon hillock is free RER, but contains numerous neurofilaments, microtubules, synaptic vesicles, and mitochondriathe axolemma covers the surface of each axon

Classification of neurones

a) number of processes

unipolar neurones - they have only an axon

they are found in the developing nervous system,

in the adult human - rod and cone

cells, olfactory cells

bipolar neurones - spindle-shaped,

having the axon at one pole and a dendrite at the other (in the retina, inthe spiral ganglion of the cochlea, in thevestibular ganglion)

special bipolar neurones have occur in the middle layer of the cerebellar cortex - havebeen described by PURKINJE - have flask-shaped cell body, tree-like dendrite and an axon

pseudounipolar neurones - axon and dendrite come together (fuse) and leave the cell body as a single process

the process then divides in the form T,

one branch corresponds with the dendrite (from the periphery) while the other is the axon (extending centrally)

cells occurr in cranial and spinal ganglia

multipolar neurones - are the

commonest type; in general, the shape

depends chiefly on the number and

position of the dendrites - star shape

bipolar neurones – Purkinje cells

pseudounipolarpseudounipolar neuronesneurones

multipolarmultipolar neuronesneurones

Classification of neurones - continue

b) length of the axon

Golgi type I - these neurones have very long axons (from several mm to 50 cm) and are of pyramidal of stellate shape,

Golgi type II - their axons are short and end in the vicinity of the cell body, which varies greatly in size and shape (spherical, oval, pyriform, fusiform, polyhedral)

c) relation to the synapsepresynaptic and postsynaptic neurones

b) function and location

sensory neurones - convey impulses from receptor to the CNS

motor neurones - convey impulses from the CNS to the effector cells

interneurones - intercalated or central neurones - are interposed between

the sensory and the motor neurones

they form cca 97 % of the all neurones in CNSS

Synapses and transmitters; classification of synapses

synapse

is defined as the site of junction of neurones or

site of junction between the neurone and the

effector cell

serves to one-directed transmission of signals

synapses:

chemical

electrical

chemical synapse:

- a presynaptic knob or axonal

ending of one neurone

it contains besides mitochondria and

neurofilaments a great number of

synaptic vesicles, in which transmitters

are stored

- a postsynaptic membrane - there

is a membrane of the next neurone

and/or effector cell

- a synaptic cleft - there is narrow space, about 20 nm, separating above mentioned parts of each synapse

Types of transmitters:

- acetylcholine

- noradrenaline (norepinephrine, NE)- dopamine (DA)- serotonine (5-hydroxytryptamine)

- gamma amino butyric acid (GABA)- glutamic acid and glycine,

- some of peptides

- NO

classification of synapses

central (interneuronal) ones: axodendritic axosomatic axosomatodendritic

axoaxonic, dendrodendritic - are rare

peripheral ones - neurone and effector cell

on smooth muscle cells, cardiomyocytes and glandular cells –synapses small and have shape of boutons

on rhabodomyocytes = motor end plates (40–60 µm)

besides chemical synapses, in which a chemical substance mediates the transmission of the nerve impulse, there are

the electrical synapses

nerve cells are linked through a gap junction

electrical synapses are not numerous as chemical synapses

principle of transmission on synapse

when an impulse reaches axonal ending, Ca2+ ions enter the presynaptic knobs

the action of calcium causes the synaptic vesicles to migrateand to fuse with the presynaptic membrane and then discharge the transmitter into the synaptic cleft by exocytosis

the transmitter diffuses across the synaptic cleft and binds to receptors in postsynaptic membrane (membrane of dendrite or perikaryon)this process results in depolarization of the postsynaptic membrane that ispropagated to the initial segment - the site where nerve impulses are generated

Functional organization of the neuron

functionally, three parts on each neurone

are distinguished:

reception part – membranes of the all dendrites and cell body of the neurone

synaptic potential

transmission part – the initial segment and axon of the neurone

generation and propagation of nerve impulses

secretion part - all axonal endings (presynaptic knobs) of respective neurone

release od neurotransmitters

Sheathes of nerve processes

are twointernal myelin sheath and external sheath of Schwann (the

neurilemma)

processes enclosed by sheathes are usually

called nerve fibers

The myelin sheath:

- has noncellular character and is composed of

lipoprotein material (thin lamellae)

- is discontinuous, interrupted for at intervals about 0.1 to 0.6 mm by so called nodes of Ranvier

- a segment of myelin sheath between adjacent nodes of Ranvier is an internode

- thickness is 1 to 20 µm, under fresh conditions appears to be homogeneous and very refractive

by electron microscopy:the myelin sheath is composed of concentric arranged thin lamellae - major dense lines and intraperiod linesboth originated by fusion of plasma membrane of Schwann cells

or oligodendrocytes

Conduction of nerve impulses

function of the myelin sheath:the parts of axon covered by myelin appears to be insulated so that the wave of voltage reversal jumps from one node of Ranvier to the next

/in unmyelinated axons the wave of voltage reversal (impulse) is conducted continual on the axolemma/

myelinated fibers conduct nerve impulses 100- 150 x faster than umyelinated ones

is possibly bi-directionally

Development of myelin sheath

the myelin sheath develops by spiral rotation of mesaxon of Schwann cell

mesaxon is a site on Schwann cell where the plasma membrane forms parallel and pair structure connecting invaginated axon with the surface

after rotation of mesaxon the cytoplasm between the membranes is extruded sothat known appearance of myelin (dense lines) occursthe whole thickness of myelin depends on how many wrappings are made

The sheath of Schwann (neurilemma)

is of the cellular character, being composed of elongated cells with flattened nuclei-Schwann's cells

an internode of the myelin sheath always corresponds with one (single) Schwann cell!!

in unmyelinated fibers is one single Schwann cell commonfor more axons

Neuroglia

= cells with supporting, metabolic, protective and phagocytic function in the nerve tissue

central glial cells: astrocytes, oligodendrocytes, microglia and ependymaperipheral glial cells: cells of Schwann, satellite cells

Astrocytes

are the largest of the neuroglial cellstwo kinds astrocytes are distinguished: protoplasmic and fibrous

both have numerous processes that extend to blood vessels and to neurones where they expand as end feet

cells isolate neurones from the blood capillaries

the protoplasmic astrocytes are more prevalent in the gray matter while the fibrousones in the white matter

Oligodendrocytes

in white matter where are arranged in rows between the myelinated fibres, they have smaller cell body from which not numerous, thin and hardly branched processes project

oligodendrocytes produce the myelin of myelinated axons of white matter

Astrocytes

Microglia

are the smallest of glial cellsthey have very small, elongated cell bodies: from each end of cell a thick process projects that branches freely in the gray matterthe microglia cells are phagocytic and play the part of histiocytes for the central nervous system: they represent therefore essentially a defense mechanism

microglia cells differ from the others that they are of mesenchymal origin

Ependyma

epedymal cells form a lining of ventricles and centralspinal canal

cells are of cuboidal or columnar shape withkinocilia

in several locations, the ependyma is closely associated with pia mater that is extremely vascular and forms choroid plexus, it produces cerebrospinal fluid (the medial walls of lateral ventricles, the roof of the 3rd and 4th ventricles)

Schwann ' s cells

were described take part in myelin development in the peripheral nerve fibres

Satellite cells

occur in the spinal or vegetative ganglia where cell bodies of neurones surround in

single layer and separate them from the connective tissue

cells are flat and

contain deeply

stained nuclei

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Outline of Outline of developmentdevelopment of the nerve tissueof the nerve tissue

it develops from a thickened area of the embryonic ectoderm - neural plate

it occurs very early on the dorsal aspect of the embryonic disc cranially to the primitive knob reaching to the oropharyngeal membrane over the notochord

on about day 18, the neural plate begins to invaginate along the cranio-caudal axis and forms neural groove limited with neural folds on each sideby the end of the third week, the neural folds become to move together and fuse into a neural tube

the neural tube separates from the ectoderm and is then located between it and notochord

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at the time when the neural folds fuse, some neuroectodermal cells separate from them and form along the dorsal aspect of the tube single cord - called the neural crest; it soon divides in the left and right parts that migrate to the dorsolateral aspect of the neural tube

neural crest cells give rise to cells of the spinal ganglia and cells of the autonomic ganglia

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from the beginning, the proximal segment of the neural tube is broadened and corresponds to future brainthe narrower caudal one develops in the spinal cord