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)

Nissl bodies

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 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

pseudounipolar neurones

multipolar 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 S 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, Ca 2+ 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 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 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 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 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 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

31 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

32. 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

33 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