Lecture 9
General medicine_2nd semester
Nerve tissue
Structure, classification and function of neurons.
Synapse
Neuroglial cells - types and function
Sheathes of nerve fibres. Conduction of nerve
impulses
Outline of development of the nerve tissue
nervous tissue is the most important tissue of the body
it is widely distributed and with few minor exceptions all organs contain
nervous 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 parts
of the body so that a suitable response may occur
the 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 agents
with 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 terminal
arborizations - 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 long
the part of the axon that joins to the cell
body is cone-shaped - axon hillock
the 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 branches
terminal 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
mitochondria
the 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, in
the spiral ganglion of the cochlea, in the
vestibular ganglion)
special bipolar neurones have occur in the
middle layer of the cerebellar cortex - have
been described by PURKINJE - have flaskshaped 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
pseudounipolar neurones
multipolar neurones
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 synapse
presynaptic 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 CNS
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 migrate
and 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 is
propagated 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 two
internal 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 lines
both 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 so
that known appearance of myelin (dense lines) occurs
the 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 nucleiSchwann's cells
an internode of the myelin sheath always corresponds with one (single)
Schwann cell!!
in unmyelinated fibers is one
single Schwann cell common
for more axons
Neuroglia
= cells with supporting, metabolic, protective and phagocytic function in the nerve tissue
central glial cells: astrocytes, oligodendrocytes, microglia and ependyma
peripheral glial cells: cells of Schwann, satellite cells
Astrocytes
are the largest of the neuroglial cells
two 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 fibrous
ones in the white matter
Oligodendrocytes
Astrocytes
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
Microglia
are the smallest of glial cells
they have very small, elongated cell bodies: from each end of cell a thick process projects that
branches freely in the gray matter
the 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 central
spinal canal
cells are of cuboidal or columnar shape with
kinocilia
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
Outline of development of 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 side
by 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 brain
the narrower caudal one develops in the spinal cord
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