Histology of Nervous System PDF

JABIR IBN HAYYAN MEDICAL Lecture By : Alaa AL Husainy UNIVERSITY COLLEGE OF MEDICINE Lecture:1 DEPARTMENT OF HUMAN ANATOMY...

JABIR IBN HAYYAN MEDICAL Lecture By : Alaa AL Husainy UNIVERSITY COLLEGE OF MEDICINE Lecture:1 DEPARTMENT OF HUMAN ANATOMY -j45-98a is S - endocrine 18 Histology of Nervous System ↳ in all the body The nervous system is the most complex system in the body, is formed by a network of many billion nerve cells (neurons), all assisted by many more supporting cells called (glial cells). Each neuron has hundreds of interconnections with other neurons, forming a very complex system for processing information and generating responses. Nerve tissue is distributed throughout the body as an integrated communications network. Anatomically, the general organization of the nervous system has two major divisions: Central nervous system (CNS), consisting of the brain and spinal cord. Peripheral nervous system (PNS), composed of the cranial, spinal, and peripheral nerves conducting impulses to and from the CNS (sensory and motor nerves, respectively) and ganglia that are small groups of nerve cells outside the CNS. NEURONS The functional unit in both the CNS and PNS is the neuron or nerve cell. Some neuronal components have special names,such as “neurolemma” for the cell membrane. Most neurons 3 &dis - consist of three main parts: The cell body, or perikaryon, which contains the nucleus and most of the cell’s organelles and serves as the synthetic or trophic center for the entire neuron.982 869 · , · · 2 The dendrites, which are the numerous elongated processes extending from the perikaryon and specialized to receive stimuli from other neurons at unique sites called synapses. - single The axon(Gr. axon, axis), which is a single long process ending at synapses specialized to generate and conduct nerve impulses to other cells (nerve, muscle, and gland cells). Axons may also receive information from other neurons, information that mainly modifies the transmission of action potentials to those neurons. Neurons and their processes are extremely variable in size and shape. Cell bodies can be very large, measuring up to 150 μm in diameter. Other neurons, such as the cerebellar granule cells, are among the body’s smallest cells. Neurons can be classified according to the number of processes extending from the cell body: Multipolar neurons, which have one axon and two or more dendrites, Most neurons are multipolar Bipolar neurons, with one dendrite and one axon, Bipolar neurons are found in the retina, olfactory mucosa, and the (inner ear) cochlear and vestibular ganglia, where they serve the senses of sight, smell, and balance, respectively. Unipolaror pseudounipolar neurons, which have a single process that bifurcates close to the perikaryon, with the longer branch extending to a peripheral ending and the other toward the CNS, Pseudounipolar neurons are found in the spinal ganglia (the sensory ganglia found with the spinal nerves) and in most cranial ganglia. Anaxonic neurons, with many dendrites but no true axon, do not produce action potentials, but regulate electrical changes of adjacent neurons. Functionally neurons subdivided into: 1)Sensory neurons are afferent and receive stimuli from the receptors throughout the body. 2)Motor neurons are efferent, sending impulses to effector organs such as muscle or gland. 3)Interneurons establish relationships among other neurons, forming complex functional networks or circuits(as in the CNS and retina). Interneurons are generally multipolar or anaxonic and are estimated to include 99% of the neurons in the human CNS. In the CNS most neuronal perikarya occur in the gray matter, with axons concentrated in the white matter. These terms refer to the general appearance of unstained CNS tissue caused in part by the different densities of nerve cell bodies. In the PNS cell bodies are found in ganglia and in some sensory regions, such as the olfactory mucosa, and axons are bundled in nerves.  ‫ﻟﯿﺶ ‪ cell body‬ھﻮ اﻟﻠﻲ ﯾﻜﻮن ‪ more active‬او اﻟﻤﻨﻄﻘﺔ اﻟﺮﺋﯿﺴﯿﺔ ﻟﻤﻌﺎﻟﺠﺔ‬ ‫اﻻﯾﻌﺎزات ؟‬ ‫ﻻﻧﮭﺎ ﺗﺤﺘﻮي ﻋﻠﻰ ‪nucleus + colgi apparatus‬‬ ‫* ﺑﺎﻟﻨﺴﺒﮫ ‪ axon‬ﻓﺎﻟﺠﺰء اﻟﻘﺮﯾﺐ ﻣﻦ ‪ cell body‬اﺳﻤﯿﮫ ‪ axon hillock‬ھﻮ اﻟﻠﻲ‬ ‫ﯾﻮﻟﺪ ﻧﺒﻀﮫ او اﻛﺸﻦ ﺑﻮﺗﯿﻨﺸﯿﻞ ﻻن ﯾﺤﺘﻮي ﻋﻠﻰ اﻟﻘﻨﻮات اﻻﯾﻮﻧﯿﮫ ﻗﻨﻮات ﺑﻮﺗﺎﺳﯿﻮم‬ ‫وﺻﻮدﯾﻮم وﻛﺎﻟﺴﯿﯿﻮم‬ ‫وﯾﻌﺘﺒﺮ ﻣﻨﻄﻘﮫ ﻧﺸﻮء اﻻﻛﺴﻮن‬ ‫‪supporting cell of nervous system ( glial cells or neuroglia ) is the‬‬ ‫‪difference between cns & pns‬‬ ‫‪nissl substance is polysome + rough endoplasmic reticular‬‬ ‫ﺗﺤﻮي ﻋﻠﻰ اﻟﺒﺮوﺗﯿﻦ وﺗﻮﺟﺪ ﻓﻲ ‪cell body‬‬ ‫ﺗﺼﺒﻎ ﺑﺎﻟﺼﺒﻐﮫ ‪ basophilic‬ﺗﻤﯿﻞ ﻟﻠﻮن اﻻزرق‬ ‫ﻛﻤﯿﺘﮭﺎ ﺗﺨﺘﻠﻒ ﻣﻦ ﺧﻠﯿﺔ ﻻﺧﺮى‬ ‫‪ axon‬اذا ﻣﺘﻔﺮع ﺑﺸﻜﻞ اﻛﺒﺮ راح ﯾﺤﻘﻖ اﻻﯾﻌﺎز ﺑﺸﻜﻞ اﻛﺘﻒ اﻛﺜﺮ واﺳﺮع‬ ‫اﻛﻮ ﻋﻠﻰ ‪ dendratis‬ﻓﺪ ﻧﻘﺎط اﺳﻤﮭﻦ ‪ dendratis spint‬واﻟﻠﻲ ھﻨﮫ ﯾﻤﺜﻠﻦ‬ ‫ﻣﻨﺎطﻖ ارﺗﺒﺎط ال ‪ dendratis‬ﺑﺎﻛﺴﻮن اﻟﺨﻠﯿﺔ اﻟﺜﺎﻧﯿﺔ‬ ‫*‪axon without myelin sheath is abnormal‬‬ ‫ﺑﺎﻟﺴﻼﯾﺪ اﻟﻮردي‬ ‫‪ cell body + nucleus‬اﻋﺮﻓﮭﺎ ﻣﻦ ‪grey matter‬‬ ‫‪ lipid in myelin sheath in axon‬اﻋﺮﻓﮭﺎ ﻣﻦ وﺟﻮد اﻟﺪھﻮن ‪white matter‬‬  506105 Parkinson disease is a slowly progressing disorder affecting muscular activity. It is caused by - - gradual loss by apoptosis of dopamine-producing neurons whose cell bodies lie within the & 57su , u 2).15 /1999-2 nuclei of the CNS substantia nigra. Parkinson disease is treated with l-dopa. - apoptosis CVS ( % Cell Body (Perikaryon) The cell body is the neuronal region that contains the nucleus and surrounding cytoplasm, & 15 % SS exclusive of the cell processes. It acts as a trophic center, producing cytoplasm for movement into the processes, although most cell bodies also receive a great number of nerve endings conveying excitatory or inhibitory stimuli generated in other nerve cells. Most nerve cells have a generally spherical, unusually large, euchromatic (pale-staining) nucleus with a prominent nucleolus. The chromatin is finely dispersed, reflecting the intense synthetic activity of these Fin cells. Cytoplasm of perikarya often contains a highly developed RER with many parallel cisternae and neighboring regions with numerous polyribosomes, indicating active production of both cytoskeletal proteins and proteins for transport and secretion. Histologically these regions with concentrated RER and other polysomes appear as clumps of basophilic material called 53 53. chromatophilic substance (or Nissl substance, Nissl bodies). The amount of this basophilic material varies with the type and functional state of the neuron and is particularly abundant in large nerve cells such as motor neurons. The Golgi apparatus is located only in the cell body, but mitochondria can be found throughout the cell and are usually abundant in the axon terminals. Intermediate filaments are abundant both in perikarya and processes and in this cell are often called neurofilaments. Dendrites Dendrites (Gr. dendron, tree) are usually short and divided like tree branches. They are usually covered with many synapses and are the principal signal reception and processing sites on neurons. Most nerve cells have many dendrites, which increase the receptive area of the cell considerably. The arborization of dendrites makes it possible for one neuron to receive and integrate a great number of axon terminals from other nerve cells. Unlike axons, which maintain a nearly constant diameter, dendrites become much thinner as they subdivide. The cytoplasm of the dendrite base is similar to that of the perikaryon, with cytoskeletal elements predominating in the branched regions. Most synapses impinging on neurons occur on dendritic spines, which are short blunt structures projecting at points along dendrites, visible with silver staining methods. Dendritic spines occur in vast numbers and serve as the initial processing sites for synaptic signals. Axons Most neurons have only one axon, a fine cylindrical process that varies in length and diameter according to the type of neuron. Axons are usually very long processes. For example, axons of the motor neurons of the spinal cord that innervate the foot muscles may have a length of nearly 100 cm and require large cell bodies for their maintenance. Axons originate from a pyramid- shaped region of the perikaryon called the axon hillock. The plasma membrane of the axon is often called the axolemma and its contents are known as axoplasm. Just beyond the axon hillock, at an area called the initial segment, is the site where various excitatory and inhibitory stimuli impinging on the neuron are algebraically summed, resulting in the decision to propagate—or not to propagate—a nerve impulse. The axolemma of the initial segment contains various ion channels important in generating the action potential. In contrast to dendrites, the typical axon is much longer,has a constant diameter, and branches less profusely. However, the distal end of an axon forms a terminal arborization, and axons of interneurons and some motor neurons have branches called collaterals that end at synapses influencing the activity of many other neurons. Each branch ends with a dilation called a terminal bouton(Fr. bouton, button) that contacts another neuron or non-nerve cell at a synapse to initiate an impulse in that cell. Axoplasm contains mitochondria, microtubules, neurofilaments, and some cisternae of smooth ER, but essentially no polyribosomes or RER, emphasizing its dependence on the perikaryon for maintenance. If an axon is severed, its peripheral part quickly degenerates. There is a lively bidirectional transport of small and large molecules along the axon. Organelles and macromolecules synthesized in the cell body move by anterograde transportalong the axon from the perikaryon to the synaptic terminals. Retrograde transport in the opposite direction carries certain other macromolecules, such as material taken up by endocytosis (including viruses and toxins), from the periphery to the cell body. Axonal transport in both directions uses motor proteins on microtubules. Kinesin, a microtubule- activated ATPase, mediates anterograde vesicular transport, and the similar ATPase called cytoplasmic dynein provides retrograde transport. Synaptic Communication Synapses (Gr. synapsis, union) are sites where nerve impulses are transmitted from one neuron to another or from neurons and other effector cells. The structure of a synapse ensures that transmission is unidirectional. Synapses convert an electrical signal (nerve impulse) from the presynaptic cell into a chemical signal that affects the postsynaptic cell. Most synapses act by releasing neurotransmitters, which are usually small molecules that bind specific receptor proteins to either open or close ion channels or initiate second-messenger cascades. A synapse has the following components: Presynaptic axon terminal (terminal bouton) from which neurotransmitter is released by exocytosis from synaptic vesicles. Postsynaptic cell membrane with receptors for the transmitter and ion channels or other mechanisms to initiate a new impulse. Synaptic cleft intercellular space separating the presynaptic and postsynaptic membranes. Morphologically, various types of synapses are seen between neurons: 1) Axosomatic synapse; axon forms a synapse with a cell body. 2) Axodendritic; axon with a dendrite. 3) Axoaxonic; Axon with another axon, Axoaxonic synapses modulate activity of the other two types. Synaptic structure cannot be resolved by light microscopy, although components such as dendritic spines may be shown by methods such as silver precipitation.