Transcript Axon 轴突
Nervous tissue 神经组织 Department of Histology and Embryology Medical college in Three Gorges University Nervous tissue, composed of neurons and neuroglia, is richly supplied with blood. The specialized cells that constitute the functional units of the nervous system are called neurons（神经元）. Within the brain and spinal cord neurons are supported by a special kind of connective tissue that is called neuroglia（神经胶质细 胞）. Neurons Membrane细胞膜 neurons Cell body Cytoplasm 细胞 质 Nucleus 细胞核 Dentrites 树突 Processes or neurites Axon 轴突 • Cell body(胞体): Perikaryon The cell body,also called perikaryon, is the part of neuron that contains nucleus and surrounding cytoplasm. Microscopic examination shows that the cell bodies of neurons are located only in grey matter which also contains dendrites and axons starting from or ending on the cell bodies. They can be pyramidal, spherical, ovoid or pear-shaped. Measuring 5-150 um in diameter. (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. (3)Cytoplasm: the cytoplasm has some distinctive characteristics not seen in other cells. The cytoplasm is basophilic and full of neurofibrils. 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). When examined by EM, these bodies are seen to be composed of rough surfaced endoplasmic reticulum. The presence of abundant granular endoplasmic reticulum is an indication of the high level of protein synthesis in neurons. The proteins are needed for maintenance and repair, and for production of neurotransmitters and enzymes. Neurites: The processes arising from the cell body of a neuron are called neurites. These are of two kinds. Most neurons give off a number of short branching processes called dendrites and one longer process called an axon. Dendrites 树突 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. Dendrites can distinguished immunocytochemically from axons because of the presence in them of microtubule associated protein (MAP-2) not present in axons. • Axon 轴突: • 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. The axon may be up to one meter in length. An axon may give off a variable number of branches. Some 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 or bouton terminaux). An axon (or its branches) can terminate in two ways. 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: dendrites axons many one short long irregular in thickness uniform diameter Nissl granules No Nissl substance spines axon hillock In addition to these differences in structure, there is a fundamental functional difference between dendrites and axons. In a dendrite, the nerve impulse travels towards the cell body whereas in an axon the impulse travels away from the cell body. • 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 but this term has now been discarded). Depending on the shapes of their cell bodies some neurons are referred to as stellate (star shaped) or pyramidal. • According to the number of the processes: unipolar,or pseudounipolar neuron bipolar neuron 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 • According to the neurotransmitter they release • Cholinergic neurons: acetylcholine 胆碱能神经元：乙酰胆碱 • Aminergic neurons: adrenaline, non 胺能神经元：肾上腺素 • Peptidergic neurons: neuropeptids 肽能神经元：神经多肽 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. • Classification of synapses: According the constitution: axodendritic synapse 轴树突触 axosomatic synapse 轴体突触 axoaxonal synapse 轴轴突触 dendro-axonic 树轴突触 dendro-dendritic 树树突触 somato-somatic synapse 体体突触 somato-dendritic synapse 体树突触 Structure of synapse: (1) The axon may terminate in a single bulb-like 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. Neuroglia • Neuroglia: Within the central nerve system,supporting cells are designated neuroglia , or glial cells.the four of glial cells are: 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 神经纤维: 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神经外膜: nerve truck perineurium神经束膜: Each of fascicles endoneurium神经内膜:each nerve fibers. Peripheral nerve ending 神经末梢 • 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) ：环层小体 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) 触觉小体 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 肌梭 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 运动终板 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. Histology of cerebra, cerebellum and spinal cord Chengying Fu Nervous system Central nervous system Peripheral nervous syetem Brain and spinal cord The nerves outside the CNS and their associated ganglia. • Cerebral cortex: • The neurones of the cerebral cortex are divided into five different morphological types which are arranged in several layers. • Neurone types in the cerebral cortex: • The five types of cortical neuron: Pyramidal cells Stellate cells Cells of Martinotti Fusiform cells Horizontal cells of Cajal: Horizontal cells of Cajal Cells of Martinotti Stellate cells Pyramidal cells Fusiform cells • Layers of the neocortex: Plexiform layer Outer granular layer Pyramidal cell layer Inner granular layer Ganglionic layer Multiform cell layer Plexiform layer Outer granular layer Pyramidal cell layer Inner granular layer Ganglionic layer Multiform cell layer • 1. Plexiform layer: containing dendrites and axons of cortical neurone making synapses with one another, and small interneuron. • 2. Outer granular layer:A dense population of small pyramidal cells and stellate cells make up this thin layer which also contains various axons and dendritic connection from deeper layer. • 3. Pyramidal cell layer: • Pyramidal cell of moderate size predominate in this broad layer, the cells increasing in size deeper in the layer. • 4. Inner granular layer: This layer consists mainly of densely packed stellate cells. • 5. Ganglionic layer: large pyramidal cells and smaller number of stellate cells and cells of Martinotti make up this layer. • 6. Multiform cell layer: This is so named for the wide variety of differing morphological forms found in this layer. 1 2 3 4 5 6 Plexiform layer Outer granular layer Pyramidal cell layer Inner granular layer Ganglionic layer Multiform cell layer • Cerebellum: • The cerebellum has enormous amount of surface area due to the folds, or folia. These folds are small gyri. White matter and gray matter can be identified at low magnification. • The gray matter includes the granular cell layer, purkinje cell layer ( with Purkinje cells and cell processes), and the molecular layer. The pale region, which is most superficial, is the molecular layer. This is made up primarily of small axonal processes and a few small dark cell bodies. • These are mainly a single layer of Purknjie cell which is deep to the molecular layer, on the boundary between the molecular and granular cell layers. The Purkinje neurons are the large irregularly spaced cells without much of their broad dendritic arborization evident in this preparation. The granular cell layer is made up of many tightly packed small dense staining nerve cell bodies. • Purkinje cell is impregnated with silver in this preparation.The flask-shaped Purkinje cells are arranged in single row. Each Purkinje cell possesses several main dendrites that enter the molecular layer and direct toward surface of cerebellum. • The dendrites arborize richly in the molecular layer. The arborization is fanshaped and extends at right angles to the cerebellar folia.An axon projects out of the bottom of the Purkinje cell. • Spinal cord: • In cross section,the spinal cord exhibits a butterfly-shaped grayish tan inner substance, the gray matter surrounding the central canal and a whitish peripheral substance, the white matter. • The white matter contains only myelinated and unmyelinated axons traveling to and from other pars of the spinal cord and to and from the brain. Functionally related bundles of axons in the white matter are called tracts. The white matter contains nerve fiber that form ascending and descending tracts. • The gray matter contains neuronal cell bodies and their dendrites, along with axons and neuroglia. • The anterior and posterior prongs are referred to as ventral horns and dorsal horns respectively. They are connected by the gray commissure.