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Nervous tissue Department of Histology and Embryology Medical college in Three Gorges University Content Neuron Synapses Neuroglia Nerve fiber Nerve ending Outline More pay attention to the microstructure of neuron and its classification Master the microstructure of the synapses Know the micrograph and function of glia Familiar the microstructure of nerve fiber Nervous tissue is composed of nerve cells and neuroglia. The specialized cells that constitute the functional units of the nervous system are called neurons. The function of neuron:generate nerve impulses in response to stimuli and transmit them along cellular processes. Neuroglia or glia: neurons are supported by a special kind of connective tissue within the brain and spinal cord, that is called neuroglia,it also located in the PNS. Function: support, protect, connect Nervous system Central nervous system Peripheral nervous system Brain and spinal cord The nerves and their associated ganglia. Neuron Microstructure LM: EM: Function Classification 1.Microstructure of neuron Dendrites Cell body Axon Cell membrane Cell body Cytoplasm Neurons Nucleus Processes or neurites Dendrites Axon Cell body: Perikaryon The cell body,soma, is the part of neuron that contains nucleus and surrounding cytoplasm, also called perikaryon. It is the trophic center of the neuron. The protein and enzymes synthesis in this area. Where is cell body Cell body: Perikaryon Position: only in grey matter in CNS which also contains dendrites and axons starting from or ending on the cell bodies,ganglia in PNS Shape:They can be pyramidal, spherical, ovoid or pear-shaped. Size: Measuring 5-150 um in diameter. Microscopic examination Cell body: Perikaryon (1)Cell membrane: the structure is as the same as the normal cell. It functions in getting the stimuli and integration and conducting the nerve impulse. (2) The nucleus is large and pale with H-E stain,prominent nucleoli are very clear. nucleolus nucleus Cell body: Perikaryon (3)Cytoplasm: the cytoplasm has some distinctive characteristics not seen in other cells. The cytoplasm is basophilic and full of neurofibrils. Neurofibril Nissl body H-E stains Silver nitrate Cell body: Perikaryon Nissl bodies: The cytoplasm shows the presence of a granular material that stains intensely with basic dyes; this material is the Nissl substance (also called Nissl bodies or granules). Nissil body 细颗粒样的尼氏体 axon hilllock Neurofibrils Cell body: Perikaryon Neurofibrils are thin black fibers observed in LM with silver nitrate slides, which is composed of microtubule and filaments in EM. EM synapse RER ribosome pigment mitochondria microtube EM: rough surfaced endoplasmic reticulum. The presence of abundant granular endoplasmic reticulum is an indication of the high level of protein synthesis in neurons. Mitochondria, SER,lysosomes,Golgi complexes,ribosome etc. The proteins are needed for maintenance and repair, and for production of neurotransmitters and enzymes. Neurites or processes The processes arising from the cell body of a neuron are called neurites.Most neurons give off a number of short branching processes called dendrites and one longer process called an axon. Dendrites Cell body collaterals Axon Nissl body Neurofibril Dendrites Axon H-E stains Silver nitrate 不同形态 Dendrites The dendrites are characterized by the fact that they terminate near the cell body. They are irregular in thickness, and Nissl granules extend into them. They bear numerous small spines which are of variable shape. Axon Every neuron has only one long thin process or axon which arises from a special region or axon hillock, which is devoid of Nissl bodies. It carries the impulse received by the neuron to distant region. axon hilllock An axon may have not much branches than that of dendrites. If branches, that arise near the cell body and lie at right angles to the axon are called collaterals. At its termination the axon breaks up into a number of fine branches called telodendria which may end in small swellings (terminal boutons. The axon is identified according to the axon hillock with LM.The part of the axon just beyond the axon hillock is called the initial segment. Neurites or processes Dendrites Axons many one short long irregular in thickness Nissl granules spines impulse towards the soma uniform in diameter No Nissl substance axon hillock away from the cell body Identification of dendrites from axon 1) morphology 2) Axon hillock in LM 3) Dendrites have microtubule associated protein (MAP-2 ) in immunocytochemically, not present in axons. distinguished from axons 2. Classification of neuron Dendrites Cell body Axon • 1)According to the number of the processes unipolar,or pseudounipolar neuron bipolar neuron multipolar neuron Unipolar neuron multipolar neuron bipolar neuron Unipolar neuron Spinal ganglia Unipolar neuron Satellite cells Bipolar neuron Retina of eye axon hilllock multipolar neuron According to the size of cell body and the length of axon: According to Cajal (1889): Golgi type I neurons: long axon Golgi type II neurons: short axon •According to their function: Sense ( afferent )neurons: Interneurons Motor( efferent ) neurons • The classification of neurons: • According to the number of process The shape of the cell body is dependent on the number of processes arising from it. The most common type of neuron gives off several processes from the cell body is, therefore, multipolar. Some neurons have only one axon and one dendrite and are bipolar. Another type of neuron has a single process (which is highly convoluted). After a very short course this process divides into two. One of the divisions represents the axon; the other is functionally a dendrite, but its structure is indistinguishable from that of an axon. This neuron is described as unipolar, but from a functional point of view it is to be regarded as bipolar. (To avoid confusion on this account this kind of neuron has been referred to, in the past, as a pseudounipolar neuron. Depending on the shapes of their cell bodies some neurons are referred to as stellate (star shaped) or pyramidal. • According to the neurotransmitter they release • Cholinergic neurons: acetylcholine • Aminergic neurons: adrenaline, non • Peptidergic neurons: neuropeptids The Synapse 突触: Concept: Synapses are highly specialized intercellular junctions which link the neurons of each nervous pathway. Similar intercellular junctions link neurons and their effector cells such as muscle fibers;where neurons synapse with skeletal muscle they are referred to as neuromuscular junction or motor end plate. Cell body Processes synapse • Classification of synapses: According the constitution: axodendritic synapse axosomatic synapse axoaxonal synapse dendro-axonic dendro-dendritic somato-somatic synapse somato-dendritic synapse Structure of synapse: (1) The axon may terminate in a single bulblike end called a presynapse bouton (or synaptic bag). There are several microstructure in the synapse bag: synapse vesicle,microtubules Mitochondria and lysosomes Dense presynaptic membrane which is broken up into several bits (or dense projection )in EM. (2) synapse cleft: Delicate fibres or granular material may be seen within the cleft. On either side of the cleft there is a region of dense cytoplasm. (3)postsynaptic cleft: On the postsynaptic side the dense Cytoplasm is continuous and is associated with a meshwork of filaments called the synaptic web. The thickened areas of membrane on the presynaptic and postsynaptic sides constitute the active zone of a synapse. Neurotransmission takes place through this region. The postsynaptic process may also show membranous structures of various shapes, microtubules, filaments and endoplasmic reticulum. synaptic cleft postsynaptic elements presynaptic elements postsynaptic membrane presynpatic membrane visicle mitochondria Synaptic cleft presynaptic elemen Neuroglia • Neuroglia: Within the central nerve system: Oligodendrocytes astrocytes microglia ependymal cells • Oligodendrocytes,small cells that are active in the formation and maintenance of myelin in the CNS • Astrocytes, cells that provide physical and metabolic support for the neurons of the CNS, Astrocytes are of two types, protoplasmic and fibrous • Microglia, inconspicuous cells with small,dark, elongated nuclei that possess phagocytotic properties • Ependymal cells, column-shaped cells that line the ventricles of the brain and the central canal of the spinal cord • Peripheral neuroglia: Schwann cells are active in the formation and maintenance of myelin in the PNS. Satellite cells:The neuronal cell bodies of ganglia are surrounded by a layer of small cuboidal cells called satellite cells. (spinal and autonomic ganglia) • Myelinated and non-myelinated nerve fibers In the peripheral nervous system, all axon are enveloped by highly specialized cells called Schwann cells which provide both structure and metabolic support. In general, small diameter axon (e.g. those of the autonomic nervous system and small pain fibers) are simply enveloped by the cytoplasm of Schwann cells. • These nerve fibers are said to be non- myelinated. Large diameter fibers are wrapped by a variable number of concentric layers of the Schwann cell plasma membrane forming a myelin sheath; such nerve fibers are said to be myelinated. • Within the central nerve system, myelination is similar to that tin the peripheral nervous system except that the myelin sheaths are formed by cells called oligodendrocytes. • Nerve fibers: These are process of neurons and are collected to form nerve trunk. In longitudinal section: The nerve fiber is seen to consist of the central axis cylinder or axon arising from the neuron. It is surrounding by layer myelin which consist of phospholipids which acts as an insulator. • It is interrupted at places. At these points, the points are called nodes of Ranvier. Outside the myelin sheath is a thin cell of Schwann, which are neurilemma.The cells of neurilemma are also known as cells of Schwann, which are neuroectodermal in origin. These cells are responsible for laying down the myelin sheath of the peripheral nerves. Myelin sheath Axon internode Nodes of Ranvier 施-万切迹 • Transverse section • epineurium: nerve truck • perineurium: Each of fascicles endoneurium:each nerve fibers. Peripheral nerve ending • Exteroceptive receptors: Free Nerve Endings Lamellated Corpuscles (of Pacini) Tactile Corpuscles (of Meissner) Proprioceptive receptors: Muscle Spindles When the terminals of sensory nerves do not show any particular specialization of structure they are called free nerve endings. Such endings are widely distributed in the body: connective tissue,epithelial lining of the skin, cornea, alimentary canal, and respiratory system. Lamellated Corpuscles (of Pacini) : Pacinian corpuscles are circular or oval structures. These are much larger than tactile corpuscles. They may be up to 2 mm in length, and up to 0.5 mm across. They are found in the subcutaneous tissue of the palm and sole, in the digits, and in various other situations. Tactile Corpuscles (of Meissner) : These are small oval or cylindrical structures seen in relation to dermal papillae in the hand and foot, and in some other situations. These corpuscles are believed to be responsible for touch. Muscle Spindles These are spindle-shaped sensory end organs located within striated muscle .The spindle is bounded by a fusiform connective tissue covering (forming an external capsule) within which there are a few muscle fibres of a special kind. These are called intrafusal fibers. • Intrafusal fibers contain several nuclei that are located near the middle of the fiber. nuclear bag fiber nuclear chain fibers • Motor end plate In most neuromuscular junctions the nerve terminal comes in contact with a specialized area near the middle of the muscle fiber. This area is roughly oval or circular, and is referred to as the sole plate. The sole plate plus the axon terminal constitute the motor end plate. Neurons vary considerably in the size and shape of their cell bodies (somata) and in the length and manner of branching of their processes. The cell body varies in diameter from about 5 um, in the smallest neurons, to as much as 120 um in the largest ones.