lecture9a genmed 2nd semester - masaryk university
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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