Nervous tissue Nervous tissue -Neurons -Glial cells CNS: oligodendrocytes, astrocytes, ependymal cells, microglia PNS: satellite cells, Schwann´s cells Synapse Myelin sheat Nervous tissue Výsledek obrázku pro nervous tissue cells Pic14.jpg (102351 bytes) Neuron •Nerve cells are very variable in appearance, shape and size •- cell body - called soma or perikarion •- processes extending from the nerve cell to communicate with other cells • there are two types of processes: • dendrites (receive impulses ) • axons (neurits) (transmit impulses ) •All nerve cells have one axon, which is usually the longest process that extends from the cell and one or more (hundreds) dendrites(shorter and thicker than the axon) •Synapse - the junction where a nerve cell communicates with another nerve cell or an effector cell (eg. muscle fibre), can be chemical and electric •Neurotransmitter – substance at the terminal part of the axon with chemical synapses releases substances which acts on the membrane of the other cell • • f14-3a_structures_in_a__c illu_neuron Main parts of neuron ·Dendrites ·Cell body ·Axon (neurit) ·Axon terminal Výsledek obrázku pro neurons Perikaryon •Cell body – PERIKARION: contains nucleus and most cytoplasm with organelles: •nucleus – round or oval, very light, with prominent nucleolus •rough ER (called Nissl´ substance) – involved in synthesis of proteins (neurotransmitters) •other usual organelles (mitochondria, Golgi apparatus, lysosomes) •neurofibrils – neurofilaments and neurotubules •pigment lipofuscin • NERV03[1] cnemnns nucleus nucleolus Nissl substance dendrite perikaryon (pyramidová buňka z cortex cerebri) Perikaryon Cells of Cajal Cortex of brain Purkynje cells cerrebellum Spinal ganglia neuron Pyramidal cells Cortex of brain Motoric neurons Spinal cord Dendrites •DENDRITES – input structure – receive signals; number of dendrites: one – several hundreds •short, branched processes with structure similar to perikarion (cytoplasm + organelles + neurofibrils) •incoming signals summate to initiate action potential highly branched tree structure • Výsledek obrázku pro neurons dendrites Classification of neurons according to number of processes (dendrites): 1.Multipolar neuron – several dendrites extend from body found in brain & spinal cord 2.Bipolar neuron – one dendrite and one axon (in retina of eye) 3.Unipolar neuron – one process only, link to axon (sensory neurons) 4.Pseudounipolar neuron – one short process divides later into dendrite and axon (spinal ganglia) fig16-4 typyneuronu image008 3_09 ORG11 1 2 3 1, 2 – multipolar. neurons: Nissl bodies 3 – pseudounipolar neurons:lipofuscin AXON – only one ·no protein synthesis here ·Trigger zone - where nerve impulses arise ·Axon hillock – the cone-shaped base of the axon, its cytoplasm is free of rER (Nissl substance) ·Axons terminal - end with fine branching with „terminal boutons“ – mitochondria and synaptic vesicles containing neurotransmitters ·Axon hillock and terminal are not covered with oligodendrocytes (in CNS) or Schwann cells (in PNS) ·Serves for impulses transmission and for axonal transport of neurotransmitters and nutrients Classification of neurons according to length of axon: 1.Golgi type I – long axon (up to 1 m) – somatic motor neurons 2.Golgi type II - short axon (in μm) Classification of neurons according to function: 1.sensitive neurons – (afferent) conduct informations from receptors to CNS 2.motor neurons – (efferent) conduct infirmations from CNS to effector cells: somatomotor to skeletal muscle and visceromotor to smooth muscle cells, cardiomyocytes or glandular cells 3.interneurons (97 %) synapse asym -Axodendritic (1) -Axosomatic (2) -Axoaxonal (3) Dendrodendritic synapses image179 neuromuscular_junction Chemical Synapses ·Presynaptic cell releases neurotransmitters from synaptic vesicles ·Act on the postsynaptic cell (help initiate AP) ·Neurotransmitters can excite or inhibit ·Neurotransmitters (acetylcholine, serotonin, norepinepherine and epinephrin, dopamine, GABA, …) ·Neurotransmiter must be removed to prevent continual firing of neurons ·Enzymatically - acetylcholineresterase ·Many pharmaceuticals and drugs modulate this effect ·Cocaine block removal of dopamine Electrical Synapses ·Without synaptic vesicles; synaptic cleft – only 2 nm thick ·Depolarizating wave continues from presynaptic to postsynaptic membrane ·Morphologically (in electron microscope) it looks like communicatin intercellular connection: gap junction (nexus) SUPPORT CELLS -essential to the function and survival of nerve cells - -CNS and PNS each have their own specific types of support cells Support cells in the CNS: -general term for support cells in the CNS is glia or neuroglia -here are four types of neuroglial cells. Oligodendrocytes - myelin-secreting cells of the CNS Astrocytes - provide physical and metabolic support for nerve cells Microglia (microglial cells) - phagocytes of CNS Ependyma (ependymal cells) - lining brain cavities and central canal in spinal cord Výsledek obrázku pro glial cells f14-6_cellular_organiza_c CENTRAL GLIA Astrocytes the largest of the neuroglial cells with processes that extend between neurons and blood vessels -ends of the processes expand to form end feet, which cover large areas of the outer surface of the blood vessel or axolemma -play a role in the movement of metabolites and wastes to and from neurons -may be involved in regulating the tight junctions in the capillaries that form the blood-brain barrier -cover the bare areas of neurons, at nodes of Ranvier and synapses -two kinds of astrocytes are identified protoplasmic fibrous astrocytes, contain prominent bundles of intermediate filaments, but the filaments are more numerous in fibrous astrocytes. (fibrous astrocytes are more prevalent in white matter, protoplasmic ones in grey matter) Výsledek obrázku pro astrocytes NERV05 Microglia - the smallest of the glial cells, with short twisted processes -are the phagocytes of the CNS, considered part of the mononuclear phagocytic system -are believed to originate in bone marrow and enter the CNS from the blood -In the adult CNS only in small numbers, but proliferate and become actively phagocytic in disease and injury. Ependymal cells -cuboidal to columnar cells in one layer lining the fluid-filled -brain ventricles and central canal (canalis centralis) in spinal cord -is involved in cerebrospinal fluid production in som regions (choroid plexus) Výsledek obrázku pro microglia Oligodendrocytes -have few processes, found in rows between axons - -the myelin sheath around axons is formed by concentric layers of oligodendrocytes plasma membrane -each oligodendrocyte gives off several tongue-like processes that contacts axon, where each process wraps itself around a portion of the axon, forming an internodal segment of myelin !!! -one oligodendrocyte may myelinate one axon or several, in the CNS, nodes of Ranvier (between myelinated regions) are larger than those of the PNS,and the larger amount of exposed axolemma makes saltatory conduction more efficient Unmyelinated axons in the CNS are truly bare, that is they are not embedded in any glial cell process. (In contrast to the situation in the PNS, described below.) cnsglial Výsledek obrázku pro nervous tissue cells Support cells in the PNS: -support cells of the PNS : satellite cells and Schwann cells Satellite cells -surround the cell bodies of the neurons in ganglia - small cuboidal cells form a complete layer around the nerve cell body, but only their nuclei are visible in routine preparations -controll microenvironment around the nerve cell body, providing electrical insulation and a pathway for metabolic exchange - nutrition and isolation of neurons in ganglia Schwann cells - responsible for the myelination of axons in the PNS ( Schwann cell wraps itself, jelly roll-fashion, in a spiral around a short segment of an axon. During the wrapping, cytoplasm is squeezed out of the Schwann cell and the leaflets of plasma membrance of the concentric layers of the Schwann cell fuse, forming the layers of the myelin sheath. An axon's myelin sheath is segmented because it is formed by numerous Schwann cells arrayed in sequence along the axon. The junction where two Schwann cells meet has no myelin and is called the node of Ranvier (the areas covered by Schwann cells being the internodal regions). F8-3B Sheaths of axons: fig16-3 Many axons are wrapped in a lipid-rich covering called myelin. This myelin sheath insulates the axon from the surrounding extracellular component and increases the rate of electrical conduction. The myelin sheath is discontinuous at intervals called the nodes of Ranvier. The area covered with myelin is called internodal area (internodium). In myelinated axons, the voltage reversal (that is, the impulse propagation) can occur only at the nodes, and the impulse "jumps" from node to node. This is called saltatory conduction. In unmyelinated axons, the impulse is conducted more slowly, moving as a wave of voltage reversal along the axon. The lack of Schwann cell cytoplasm in the concentric rings of the myelin sheath is what makes it lipid-rich. Schwann cell cytoplasm is however found in several locations. There is an inner collar of Schwann cell cytoplasm between the axon and the myelin, and an outer collar around the myelin. The outer collar is also called the sheath of Schwann or neurilemma, and contains the nucleus and most of the organelles of the Schwann cell. The node of Ranvier is also covered with Schwann cell cytoplasm, and this is the area where the plasma membranes of adjacent Schwann cells meet. (These adjacent plasma membranes are not tightly apposed at the node, so that extracellular fluid has free acess to the neuronal plasma membrane.) Finally, small islands of Schwann cell cytoplasm persist within successive layers of the myelin sheath, these islands are called Schmidt-Lanterman clefts. Myelination (development of myelin sheath): myelination Not all nerve fibres is the PNS are covered with myelin, some axons are unmyelinated. In contrast to the situation in the CNS, unmyelinated fibres in the PNS are not completely bare, but are enveloped in Schwann cell cytoplasm. The Schwann cells are elongated in parallel to the long axis of the axons, which fit into grooves on the surface of the Schwann cell. One axon or a group of axons may be enclosed in a single groove. Schwann cells may have only one or up to twenty grooves. Single grooves are more common in the autonomic nervous system. NERV06 Myelinani PERIFERNÍ GLIE: SCHWANNOVY BUŇKY wobbly1 wobbly2 getimage F8-3B condmyel epen100he EllenG1 introd6 f2_06 07b80e00 Cortex cerebri – Pyramidal cells – multipolar neurons Cortex cerebri – Pyramidal cells – multipolar neurons Cerebellum Cerebellum – Purkynje cell Cerebellum - Nissl bodies Nisslova substance – Purkyňovy buňky mozečku Axon hillock Nissl bodies Kořenové buňky předního míšního rohu psa Medulla spinalis Somatomotoric multipolar neurons Somatomotoric multipolar neurons Myelinated axons Medulla spinalis – ependymal cells Ganglion spinale Pseudounipolar neurons Satellite cells Vegetative ganglion – ganglion cells (multipolar neurons), satellite cells Ganglion cells with lipofuscin Peripheral nerve – longitudinal section Node of Ranvier Myelin sheaths with nodes of Ranvier – peripheral nerve (OsO4) [USEMAP] Axon terminals Motor end-plate Muscle fibers Motor end-plates in motor unit Motor end-plates (localization of acetylcholinesterase) stavba_svalu_small Axons with Schwann sheath Nucleus of Schwann cell Myelin sheath wobbly2 Presynaptic ending synapse NERVE TISSUE Slides: Pyramidal cell (75, 76. Cortex cerebri) Purkinje cell (77. Cerebellum) Nissl substance (78. Cerebellum) Somatomotoric multipolar neuron (79. Medulla spinalis) Pseudounipolar neuron (81. Ganglion spinale) Axon with myelin and Schwann sheath (84, 86. Peripheral nerve) Myelin sheath (87. Peripheral nerve) Motor end plate (acetylcholinesterase detection) Atlas EM: Nucleus (3) and cytoplasm of neuron (55) Axons with sheaths (56, 58) Presynaptic endings (57)