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88. Nerve tissue enables the body to respond to changes in its external and internal stimuli = > Nervous system regulates the function of internal organs (+ endocrine and immune system ) Regulatory systems Nervous system Endocrine system Immune system Neurotransmitters Hormones Cytokines Synaptic signalisation Nerve tissue - overview • neurons are excitable cells that use the combination of electrical (membrane depolarisation) and chemical (synapse) signalling • integrated communicating network composed from the cells with processes • anatomically: central and peripheral nervous system (CNS and PNS) • embryonic origin: neuroectoderm (microglia – mesenchyme!) • regeneration: neurons in vivo in general do not divide in general x glial scar • in vitro / in experiments can neurons differentiate into neurons Neurons (nerve cells) a neuroglia (supporting cell population) Beta III tubulin and glial fibrilar acidic protein central and peripheral NS grey and white matter (2 types of nerve tissue) grey matter– perikarya, dendrites, axons, neuroglia white matter– myelinated axons + neuroglia Tissue microscopic structure: cells with processes – integrated communicating network neuroglia gap-junction neurons synapse neuron theory – contact junctions among neurons N3 - Mozeček HE • NEURON is structural and functional unit of nerve tissue Neurons and neuroglia are cells with processes 2 multipolar neurons connected synaptically biomedicalengineering.yolasite.com Nerve tissue - morphology cells : 1.neurons and 2.glial cells • extracellular material is extremely reduced replaced by glial branched process • neurons receive stimuli and conduct nerve impulse via their processes • action potential transmission to the next cell through synapses (= intercellular contacts) • extensive vasculature with variable amount of connective tissue both in CNS and PNS • blood-brain barrier in CNS Perikaryon (soma, cell body) receptor region, integration, trophic centre • nucleus • large, lightly basophilic, with dispersed chromatin, prominent nucleolus • RER • abundant → substance of Nissl - basophilic • Golgi apparatus • only in perikaryon! • cytoskeleton (neurofilaments) • mitochondria • inclusions • lipofuscin, melanin Cytoskeleton • microtubules – axonal transport (anterograde, retrograde, microtubuleassociated motor proteins used ATP – dynein, kinesin) – shape regulation • neurofilaments (intermediate filaments in neurons) – resistance against deformations – regulatory process • microfilaments (aktin) Dendrites receptor region principal recieving site • arborisation (branching) increases receptive area of the cell (100 000 contacts and more) • dendritic spines (site of synapse - postsynaptic membrane, actin microfilaments) • neurofilaments (NF-L, NF-M, NF-H), other cytoskeleton units, proteosynthetic apparatus except GA • always non- myelinated Axon conductive region • transmit stimuli in the form of action potential to other neuron or effector cell • 1 neuron has 1 axon • metabolically dependent on perikaryon • Golgi type I neurons – motor neurons of CNS with long axon (up to 1 meter) terminate on skeletal muscle • Golgi type II neurons – short axons • axon hillock, initial segment – site of action potential generation • axonal transport transport vesicles, mitochondria , proteins…by anterograde x growth factor, viruses..by retrograde Synapse effector region, unidirectional transmission of nerve impulse presynaptic (axonal) terminal postsynaptic membrane (dendritic, somatic, axonal) synaptic cleft 20 nm presynaptic cytoplasm : synaptic vesicles with neurotransmitters, microtubules+kinesin, mitochondria postsynaptic membrane of effector cell: 1. nerve cell 2. muscle cell 3. secretory cell Synaptic transmission • action potential opens calcium channels in presynaptic membrane → Ca2 influx triggers exocytosis of synaptic vesicles into synaptic cleft → reaction with receptors of postsynaptic membrane promotes depolarisation (excitatory s., neurotransmitters: acetylcholine, glutamine,serotonine) or hyperpolarisation (inhibitory s., neurotransmitters: GABA-g-aminobutyric acid, glycine) Synapses thus can excite or inhibit impulse transmission. Neuroglia “connective tissue“ of CNS • provide neurons with structural support and maintain local condition for neuronal function • forming 50% volume of CNS • 10 x more abundant than neurons • histological staining • impregnation by Ag, Au salts, histochemical technics Glia - types – in CNS 1. macroglia – astrocytes – oligodendrocytes – ependymal cells 2. Microglia - in PNS - Schwann cells - Satelite cells Astrocytes (protoplasmic, fibrous) • • the largest of neuroglia astrocytic end feet- connection to vessels and neurons providing movement of wastes and metabolites to and from neuron • regulate ionic concentration in intercellular space • contribute to blood-brain barrier (together with tight junctions of endothelal cells) mechanical support of neurons proliferation - glial scar glial fibrillary acidic protein Oligodendrocytes • smaller, ↓intermediate fibres, darker nucleus • formation of myelin sheath • myelinisation of several axons Microglia • the smallest, dark elongated nucleus • they migrate to the sites of dead cells, proliferate, phagocytes (dead cells, cell debris…) • mesoderm -derived Ependym • epiteloid arrengement – remnant of neuroepithelium of neural tube * secretory elements (cerebrospinal fluid) component of choroid plexus of brain ventricles and central canal of spinal cord Nerve fibres • axons provided by special connective tissue capsules of ectodermal origin • axons: – unmyelinated – myelinated Peripheral nerve Unmyelinated nerve fibres Myelinated nerve fibres • in PNS: axons are invaginated into cytoplasm of enveloping Schwann cell (sheath is formed by layers of modified cell membranes) x in CNS: cell process of oligondrocytes form sheath • nodes of Ranvier and internode, 1-2 mm • Schmidt-Lanterman´s incisures Myelinated nerve fibres Eg 13 - Myelinizovaný axon N8 – Ganglion HE Satelite cells Neuron Arrangement of nerve tissue in the body, CNS and PNS • CNS • clusters of neurons – cell body: layers, columns (grey matter) • bundles of axons: tracts, fasciculi, lemnisci (white matter) • (grey and white matter differentiation: according to the content of myelin- fatty) • PNS • clusters of neurons: ganglia • bundles of axons: cranial spinal peripheral nerves Neuron, neuroglia, blood vessel Nerve tissue microstructure (example 2 – brain ) Glial c. Neuron