Histology of the Nervous System PDF

Histology The Nervous System Chapter 9 Dr. Sundus Shalabi, MD, PhD Faculty of Medicine Arab American University-Palestine [email protected] Outline Introduction The nervous system is the most complex system in the body. Formed by a network of bi...

Histology The Nervous System Chapter 9 Dr. Sundus Shalabi, MD, PhD Faculty of Medicine Arab American University-Palestine [email protected] Outline Introduction The nervous system is the most complex system in the body. Formed by a network of billions of nerve cells (neurons), all assisted by many more supporting cells called glial cells (Gr. glia, glue). 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. Structural organization of the nervous system: Or Anatomical Organization Functional organization of the nervous system  Cells in the nervous system Two kinds of Cells in both CNS and PNS: 1. Neurons, which typically have numerous long processes. 2. Glial cells (Gr. glia, glue), which have short processes. Functions: Support and protect neurons Participate in many neural activities Neural nutrition Defense of cells in the CNS.  Introduction The usual response of neurons to environmental changes (stimuli) usually by changing the ionic gradient inside and outside the cell. Cells which have this characteristic are called excitable or irritable. Neurons usually reacts to a stimulus by reversal of the ionic gradient (membrane depolarization) which spreads and propagated. This propagation called action potential, the depolarization wave, or the nerve impulse. The nerve impulse, is capable of traveling long distances. The nervous system continuously stabilizes the intrinsic conditions of the body by these impulses.  Development of nerve tissue Origin : from the outermost of embryonic layers called ectoderm. Nerve tissue development usually begins at the 3rd week of pregnancy. Under the influence of The notochord the overlying layer of ectodermal cells thickens as a bending neural plate, with a medial neural groove and lateral neural folds. The sides of this plate fold upward and grow toward each other medially then within a few days fuse to form the neural tube (give rise to the CNS). As the folds fuse and the neural tube separates from ectoderm (that will form epidermis), a large population of developmentally important cells, the neural crest (give rise to the PNS), separates from the neuroepithelium and becomes mesenchymal. Neural crest cells migrate extensively and differentiate as all the cells of the PNS, as well as a number of other non-neuronal cell types. General Embryology - Detailed Animation On Neurulation (youtube.com)  Neurons Considered the functional unit of the nervous system both central and peripheral. Most neurons have three main parts : 1.The cell body (also called the perikaryon or soma): Contains the nucleus and most of the cell’s organelles Function: serves as the synthetic or trophic center for the entire neuron. 2.The dendrites The numerous elongated processes extending from the perikaryon Function: receives stimuli from other neurons at unique sites called synapses. 3.The axon (Gr. axon, axis) A single long process ending at synapses Function: generate and conduct nerve impulses to other cells (eg, nerve, muscle, glands). A plasma membrane of a neuron is called neurolemma. Neuron structure  Neurons classification Classified according to the number of processes extending from the cell body : 1. Multipolar neurons one axon and two or more dendrites, are the most common. Found in the CNS and the efferent division of the PNS. 2. Bipolar neurons one dendrite and one axon. (eg: the sensory neurons of the retina, the olfactory epithelium, and the inner ear). 3. Unipolar or pseudounipolar neurons 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. Found primarily in the afferent division of the PNS. 4. Anaxonic neurons, with many dendrites but no true axon, do not produce action potentials, but regulate electrical changes of adjacent CNS neurons. Neuron classification  Neurons classification Classified functionally into: 1. Sensory neurons are afferent receiving stimuli from receptors throughout the body. 2. Motor neurons are efferent, sending impulses to effector organs such as muscle fibers and glands which is divided into 2 types: somatic neurons: under voluntary control and typically innervate skeletal muscle. autonomic neurons :control involuntary activities of glands ,cardiac muscle &most of smooth muscles. Interneurons: establish relationships among other neurons, forming complex functional networks or circuits in the CNS. They are either multipolar or anaxonic. comprise 99% of all neurons in adults Neurons classification/functional Neurons classification Because the fine processes emerging from cell bodies are rare to be seen in sections of nervous tissue, it is difficult to classify neurons structurally by microscopic inspection. In the CNS: neuronal cell bodies (perikarya) occurs in the gray matter while axons concentrated at the white matter. In the PNS neuronal cell bodies (perikarya) are found in ganglia and in some sensory regions and axons are bundled in nerves.  1. Cell Body (Perikaryon or Soma) Contains the nucleus and surrounding cytoplasm with organelles, exclusive of the cell processes. Function: acts as a trophic center, producing most cytoplasm for the processes. Nucleus: unusually large, euchromatic nucleus with a prominent nucleolus, indicating intense synthetic activity. 1. Cell Body (Perikaryon or Soma) Cytoplasm: Contains numerous free polyribosomes and highly developed RER, indicating active production of both cytoskeletal proteins and proteins for transport and secretion. Regions with concentrated RER and other polysomes are basophilic and are distinguished as chromatophilic substance (or Nissl substance, Nissl bodies)→abundant in large nerve cells such as motor neurons. Golgi apparatus only found in the cell body , while mitochondria can be found through out the cell abundant mainly at the axons. 1. Cell Body (Perikaryon or Soma) Microtubules, actin filaments and intermediate filaments are abundant Intermediate filaments are called neurofilaments in this cell type (neurons). Some nerve cell bodies also contain inclusions of pigmented material, such as lipofuscin, consisting of residual bodies left from lysosomal digestion.  2. Dendrites Dendrites (Gr. dendron, tree) are typically short, small processes emerging and branching off the soma. Each neuron has numerous dendrites that increase the receptive area of the cell. Function : signal reception and processing sites on neurons. In the CNS, most synapses occur on dendritic spines. Dendritic spines: Dynamic membrane protrusions along the small dendritic branches, visualized with silver staining. Act as initial processing site for synaptic signals. Very important in the constant changes of the neural plasticity that occurs during embryonic brain development and underlies adaptation, learning, and memory postnatally. 2. Dendrites  3. Axons Most neurons have only one axon, typically longer than its dendrites. Axonal processes vary in length and diameter according to the type of neuron. The plasma membrane of the axon is often called the axolemma and its contents are known as axoplasm. originate from a pyramid-shaped region of the perikaryon called the axon hillock. the axolemma has concentrated ion channels that generate the action potential. the 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.  3.Axons Axons generally branch less profusely than dendrites, but do undergo terminal arborization. Each small axonal 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. Branch of an axon is called axon collateral. Axoplasm contains mitochondria, microtubules, neurofilaments, and transport vesicles, but very few polyribosomes or cisternae of RER, features that emphasize the dependence of axoplasm on the perikaryon. 3.Axons  3.Axons There is a bidirectional transport of large and small molecules along the axon: Anterograde transport: Movement of organelles from cell body along axonal microtubules via kinesin from the perikaryon to the synaptic terminals. Retrograde transport : in the opposite direction along microtubules via dynein carries certain other macromolecules, such as material taken up by endocytosis (including viruses and toxins), from the periphery to the cell body. Move fairly rapidly, at rates of 50-400 mm/day. 3.Axons  These 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. Synaptic communication Function: Converts an electrical signal (nerve impulse) from the presynaptic cell into a chemical signal that affects the postsynaptic cell. They usually act by releasing neurotransmitters that bind specific receptor proteins to either open or close ion channels or initiate a 2nd messenger cascades.  Synaptic components 1. The presynaptic axon terminal (terminal bouton) contains mitochondria and numerous synaptic vesicles from which neurotransmitter is released by exocytosis. 2. The postsynaptic cell membrane contains receptors for the neurotransmitter, and ion channels or other mechanisms to initiate a new impulse. 3. The synaptic cleft A 20- to 30-nm-wide intercellular space that separates these presynaptic and postsynaptic membranes.  Synaptic components Morphological types of synapses Morphological types of synapses  Types of neurotransmitters 1. Acetylcholine found at neuromuscular junctions and some synapses of the CNS. 2. Certain amino acids (often modified), such as glutamate and γ- aminobutyrate (GABA). 3. Monoamines, such as serotonin (5-hydroxytryptamine or 5-HT) and catecholamines, such as dopamine, all of which are synthesized from amino acids. 4. Small polypeptides, such as endorphins and substance P Glial cells and neural activity  Support neuronal survival and activities. 10 times more abundant than neurons in the mammalian Glial cells brain. and Origin: progenitor cells of the embryonic neural plate. neuronal Surrounds neuronal cell body axons and dendrites. activity The processes of both the glial cells and neurons forms a fibrous network surrounding the cells of the CNS resembling collagen fibers called neuropil. Glial cells and neuronal activity  Oligodendrocytes The predominant cells of the CNS white matter(because of the lipid in the myelin sheaths). Extend many processes, each of which becomes sheetlike and wraps repeatedly around a portion of a nearby CNS axon. During wrapping cytoplasm leaves the cells forming compacted layers of cell membrane collectively termed myelin. An axon’s full length is covered by the action of many oligodendrocytes. The resulting myelin sheath. Myelin sheath electrically insulates the axon and facilitates rapid transmission of nerve impulses. Sheath and processes usually cannot be stained so cannot be seen by routine light microscope.  Astrocytes Origin:progenitor cells in the embryonic neural tube The most numerous glial cells of the brain. Astrocytes (Gr. astro-, star + kytos) have a large number of long radiating, branching processes. Proximal regions of the processes are reinforced with bundles of intermediate filaments made of glial fibrillary acid protein (GFAP). Distally the processes lack GFAP. Processes forming a network of delicate terminals contacting synapses and other structures. Terminal processes of a single astrocyte typically occupy a large volume and associate with over a million synaptic sites  Astrocytes types 1. Fibrous astrocytes, with long delicate processes, are abundant in white matter 2. Protoplasmic astrocytes: those with many shorter processes and predominate in the gray matter. The highly variable and dynamic processes mediate most of these cells’ many functions. Astrocytes communicate directly with one another via gap junctions, forming a very large cellular network for the coordinated regulation of their various activities in different brain regions.  Astrocytes functions 1. Extend processes that associate with/or cover synapses affecting the formation, function, and plasticity of these structures. 2. Regulate the extracellular ionic concentrations around neurons, with particular importance in buffering extracellular K+ levels. 3. Guide and physically support movements and locations of differentiating neurons during CNS development. 4. Extend fibrous processes with expanded perivascular feet that cover capillary endothelial cells and modulate blood flow and help move nutrients, wastes, and other metabolites between neurons and capillaries. 5. Form a barrier layer of expanded protoplasmic processes, called the glial limiting membrane, which lines the meninges at the external CNS surface. 6. Filling tissue defects after CNS injury by proliferation to form an astrocytic scar. Astrocytes Neurons, neuropil, and the common glial cells of the CNS.  Ependymal cells Columnar or cuboidal cells that line the fluid-filled ventricles of the brain and the central canal of the spinal cord. In some CNS locations, the apical ends of ependymal cells have cilia, which facilitate the movement of cerebrospinal fluid (CSF), and long microvilli, which are likely involved in absorption. Joined apically by apical junctional complexes like those of epithelial cells, but there is no basal lamina. The basal ends of ependymal cells are elongated and extend branching processes into the adjacent neuropil. Ependymal cells  Microglia Origin: From circulating blood monocytes, belonging to the same family as macrophages and other antigen- presenting cells. Small cell bodies from which radiate many long, branched processes. They are less numerous than oligodendrocytes or astrocytes but nearly as common as neurons in some CNS regions. Evenly distributed throughout grey and white matter. Relatively static while their processes continuously probe and interact with neuropil, synapses, and other cells in an area up to 10-fold that of the cell body. Usually not visualized with H&E but can be seen by Immunohistochemistry using antibodies against cell surface antigens of immune cells. Function : 1. Remove apoptotic bodies and debris from damaged or remodeled synapses by phagocytosis 2. Constitute the major mechanism of immune defense in the CNS Glial cells of PNS  Schwan cells (neurolemmocytes ) Origin: from precursors in the neural crest. Found only in the PNS and considered as the counterpart to oligodendrocytes of the CNS, having trophic interactions with axons and most importantly forming their myelin sheathes. Unlike an oligodendrocyte, a Schwann cell forms myelin around a portion of only one axon.  Satellite Cells of Ganglia Origin: from the embryonic neural crest. Small satellite cells form a thin, intimate glial layer around each large neuronal cell body in the ganglia of the PNS. Satellite cells exert a trophic or supportive effect on these neurons, insulating, nourishing, and regulating their microenvironments. Satellite Cells of Ganglia  Central nervous system (CNS) Three major structure are comprising the CNS: 1. The cerebrum. 2. The cerebellum. 3. The spinal cord. CNS tissue is completely covered with a connective tissue layer called meninges. It contains very small amount of collagen making it relatively soft and easily damaged by injuries.  Central nervous system (CNS) Arranged into two areas (based on lipid-rich myelin): White matter: ▪ Main components are myelinated axons often grouped together as tracts, and the myelin-producing oligodendrocytes. ▪ Also contains Astrocytes and microglia with very few neuronal cell bodies. ▪ Most white matter is found in deeper regions called cerebral nuclei. ▪ Peripheral in the spinal cord. Gray matter: ▪ Contains abundant neuronal cell bodies, dendrites, astrocytes, and microglial cells, and is where most synapses occur. ▪ Makes up the thick cortex or surface layer of both the cerebrum and the cerebellum. ▪ Deeper than the white matter, H-shaped mass at the spinal cord. Cerebral cortex 6 layers Pyramidal cells  Cerebellar cortex Organized within three layers: Molecular layer: A thick outer layer has much neuropil and scattered neuronal cell bodies. Purkinje cells layer: A thin middle layer consists only of very large neurons. These are visible even in H&E-stained sections, and their dendrites extend throughout the molecular layer as a branching basket of nerve fibers. Granular layer: A thick inner layers contains various very small, densely packed neurons and little neuropil. Cerebellum  Spinal cord In cross sections, the white matter is peripheral, and the gray matter forms a deeper, H-shaped mass. The two anterior projections of this gray matter called the anterior horns, contain cell bodies of very large motor neurons whose axons make up the ventral roots of spinal nerves. The two posterior horns contain interneurons, which receive sensory fibers from neurons in the spinal (dorsal root) ganglia. Near the middle of the cord, the gray matter surrounds a small central canal, which develops from the lumen of the neural tube, is continuous with the ventricles of the brain, is lined by ependymal cells, and contains CSF.  Spinal Cord CSF-filled central canal (C). In the spinal cord, the gray matter is internal, forming a roughly H-shaped structure that consists of two posterior (P) horns (sensory) and two anterior (A) (motor) horns, all joined by the gray commissure around the central canal. (a) The gray matter contains abundant astrocytes and large neuronal cell bodies, especially those of motor neurons in the ventral horns. (b) The white matter surrounds the gray matter and contains primarily oligodendrocytes and tracts of myelinated axons running along the length of the cord. (Center X5, a, b X100; All silver-stained) (c) With H&E staining, the large motor neurons (N) of the ventral horns show large nuclei, prominent nucleoli, and cytoplasm rich in Nissl substance, all of which indicate extensive protein synthesis to maintain the axons of these cells that extend great distances. (d) In the white commissure ventral to the central canal, tracts (T) run lengthwise along the cord, seen here in cross section with empty myelin sheaths surrounding axons, as well as small tracts running from one side of the cord to the other. (Both X200; H&E  Difference between white and gray matter A cross section of H&E-stained spinal cord shows the transition between white matter (left region) and gray matter (right). The gray matter has many glial cells (G), neuronal cell bodies (N), and neuropil; white matter also contains glia (G) but consists mainly of axons (A) whose myelin sheaths were lost during preparation, leaving the round empty spaces shown. Each such space surrounds a dark-stained spot that is a small section of the axon. (X400)  The meninges Three meningeal layers of connective tissue laying between the bone and nervous tissue called: the dura, arachnoid, and pia maters. Dura mater: Also called tough mother, it is the most external thick layer consist of dense irregular connective organized as an outer periosteal layer continuous with the periosteum of the skull and an inner meningeal layer. These two layers are usually fused, but along the superior sagittal surface and other specific areas around the brain, they separate to form the blood-filled dural venous sinuses. Around the spinal cord, the dura mater is separated from the periosteum of the vertebrae by the epidural space. Meninges arrangement around the brain  Arachnoid The arachnoid (Gr. arachnoeides, spider web-like). Has two components: 1. A sheet of connective tissue in contact with the dura mater. 2. A system of loosely arranged trabeculae composed of collagen and fibroblasts, continuous with the underlying pia mater layer. Surrounding these trabeculae is a large, sponge-like cavity, the subarachnoid space, filled with CSF (provide protection and cushion like effect).  Arachnoid The connective tissue of the arachnoid is avascular because it lacks nutritive capillaries The arachnoid and the pia are intimately associated and often considered as a single membrane called pia-arachnoid. In some areas, the arachnoid penetrates the dura mater and protrudes into blood-filled dural venous sinuses located there these CSF filled protrusions called arachnoid villi and function as sites for absorption of CSF into the blood of the venous sinuses  Pia mater It is the tender mother, the inner most layer of meninges. Consists of flattened, mesenchymally derived cells closely applied to the entire surface of the CNS tissue. Does not contact the neurons cells directly usually separated by the glial limiting membrane, or glia limitans (a layer of astrocytic processes). Both (astrocytic layer and the pia) form a physical barrier separating CNS tissue from CSF in the subarachnoid space. Blood vessels penetrate CNS tissue through long perivascular spaces covered by pia mater, although the pia disappears when the blood vessels branch to form the small capillaries, these capillaries remain completely covered by the perivascular layer of astrocytic processes.  The blood brain barrier Functional barrier that controls tightly the passage of substances from the blood into CSF and vice versa. Structural components: 1. The capillary endothelium, in which the cells are tightly sealed together with well-developed occluding junctions, with little or no transcytosis activity, and surrounded by the basement membrane. 2. The limiting layer of perivascular astrocytic feet, which closely envelops the basement membrane of the continuous capillaries in most CNS regions.  The blood brain barrier(BBB) Function : 1. Regulates passage of molecules and ions from blood to brain. 2. Protects neurons and glia from bacterial toxins, infectious agents, and other exogenous substances. 3. Helps maintain the stable composition and constant balance of ions in the interstitial fluid required for normal neuronal function. Sites not covered with BBB: 1. Hypothalamus where plasma components are monitored. 2. Posterior pituitary that releases hormones. 3. Choroid plexus where CSF is produced.  Choroid plexus It’s a folded highly vascular tissue projecting(villi) into the large ventricles of the brain. Found in the roofs of the 4th and 3rd and some parts of the two lateral ventricles. Each villus contains a thin layer of well-vascularized pia mater covered by cuboidal ependymal cells. Function: to remove water from blood and release it as the CSF.  Choroid plexus Characteristics of CSF: Clear fluid produced continuously. Completely fills the ventricles, the central canal, the subarachnoid and perivascular spaces. Contains ions (Na,K,Cl),low protein content. Very sparse cells only lymphocytes. Arachnoid villi provide the main pathway for absorption of CSF back into the venous circulation. There are very few lymphatic vessels in CNS tissue. Choroid plexus  Peripheral nervous system (PNS) Main components of the peripheral nervous system (PNS) : the nerves, ganglia, and nerve endings. Nerve fibers: They are similar to tracts in the CNS, containing axons enclosed within sheaths of glial cells specialized to facilitate axonal function. axons usually are sheathed by Schwann cells or neurolemmocytes. The sheath may or may not form myelin around the axons, depending on their diameter. Classified into : Myelinated nerve fibers. Unmyelinated nerve fibers.  Myelinated nerve fibers As axons of large diameter grow in the PNS, they are engulfed along their length by a series of differentiating schwann cells and become myelinated nerve fibers. Myelin sheath Composed mainly of lipid bilayers and membrane proteins. myelin is a large lipoprotein complex that, like cell membranes, is partly removed by standard histologic procedures. serves to insulate axons and maintain a constant ionic microenvironment most suitable for action potentials.  Myelinated nerve fibers By TEM, the myelin sheath appears as a thick electron-dense axonal covering in which the concentric membrane layers may be visible. The prominent electron-dense in the sheath, the major dense lines, represent the fused, protein-rich cytoplasmic surfaces of the Schwann cell membrane. Along the myelin sheath, these surfaces periodically separate slightly to allow transient movement of cytoplasm for membrane maintenance; at these myelin clefts (or Schmidt-Lanterman clefts), the major dense lines temporarily disappears.  Myelinated nerve fibers Between adjacent Schwann cells on an axon, the myelin sheath shows small nodes of Ranvier (or nodal gaps),where the axon is only partially covered by interdigitating Schwann cell processes. At these nodes, the axolemma is exposed to ions in the interstitial fluid and has a much higher concentration of voltage-gated Na+ channels, which renew the action potential and produce salutatory(L. saltare, to jump) of nerve impulses, their rapid movement from node to node. The length of axon ensheathed by one Schwann cell, the internodal segment, varies directly with axonal diameter and ranges from 300 to 1500 μm.  Myelinated nerve fibers Schmidt-Lanterman clefts: contain Schwann cell cytoplasm that was not displaced to the cell body during myelin formation. This cytoplasm moves slowly along the myelin sheath, opening temporary spaces (the clefts) that allow renewal of some membrane components as needed for maintenance of the sheath. Nodes of Ranvier or nodal gap. Interdigitating processes extending from the outer layers of the Schwann cells (SC) partly cover and contact the axolemma at the nodal gap. This contact acts as a partial barrier to the movement of materials in and out of the periaxonal space between the axolemma and the Schwann sheath. The basal or external lamina around Schwann cells is continuous over the nodal gap. The axolemma at nodal gaps has abundant voltage- gated Na+ channels important for impulse conductance in these axons. Myelination of nerve fibers  Unmyelinated nerve fibers The smallest diameter axons of peripheral nerves are still enveloped within simple folds of Schwann cells. These very small unmyelinated fibers do not however undergo multiple wrapping to form a myelin sheath. In unmyelinated nerves, each Schwann cell can enclose portions of many axons with small diameters Due to the absence of myelin sheath nodes of Ranvier are not seen along unmyelinated nerve fibers. have evenly distributed voltage-gated ion channels; their impulse conduction is not saltatory and is much slower than that of myelinated axons. Unmyelinated nerve fibers  Nerve organization In the PNS, nerve fibers are grouped into bundles to form nerves. Nerves have a whitish, glistening appearance because of their myelin and collagen content. In large nerves: Axons and Schwann cells are enclosed within layers of connective tissue. Immediately around the external lamina of the Schwann cells is a thin layer called the endoneurium, consisting of reticular fibers, scattered fibroblasts, and capillaries. All bundled together as fascicles by a sleeve of perineurium, containing flat fibrocytes with their edges sealed together by tight junctions.  Nerve organization From two to six layers of these layers regulate diffusion into the fascicle and make up the blood-nerve barrier that helps maintain the fibers’ microenvironment. Externally, peripheral nerves have a dense, irregular fibrous coat called the epineurium, which extends deeply to fill the space between fascicles. Small nerves: Consist of one fascicle & found in many organs within their connective tissue. Nerves can be afferent/sensory or efferent/motor. Nerves also can be motor,sensory or mixed (most common). Peripheral nerve connective tissue: Epi-, peri-, and endoneurium.  Ganglia Ovoid structures containing neuronal cell bodies and their surrounding glial satellite cells supported by delicate connective tissue and surrounded by a denser capsule. Serve as relay stations to transmit nerve impulses, at least one nerve enters, and another exits from each ganglion. According to the direction of the nerve impulse ganglia divided into sensory or autonomic ganglion.  Receive afferent impulses that go to the CNS. Associated with both cranial nerves (cranial ganglia) and the dorsal roots of the spinal nerves (spinal ganglia). Supported by a distinct connective tissue capsule and an Sensory internal framework continuous with the connective tissue Ganglia layers of the nerves. These neurons are pseudounipolar and relay information from the ganglion’s nerve endings to the gray matter of the spinal cord via synapses with local neurons.  Autonomic ganglia Autonomic nerves control the following: 1. The activity of smooth muscle. 2. The secretion of some glands. 3. Heart rate. 4. Involuntary activities by which the body maintains a constant internal environment (homeostasis).  Autonomic ganglia The type of neurons usually is multipolar neurons. Some are located within certain organs, especially in the walls of the digestive tract, where they constitute the intramural ganglia. The capsules of these ganglia may be poorly defined among the local connective tissue. Has two neuron circuit: 1. Preganglionic fiber, is located in the CNS usually its axon forms a synapse with post ganglionic fibers, the neurotransmitter of theses fibers is acetylcholine. 2. Postganglionic fibers multipolar neuron located in a peripheral ganglion system.  Autonomic nervous system has two parts Sympathetic nervous system. Parasympathetic nervous system. In the medulla and midbrain and Neuronal cell bodies of in the sacral portion of the spinal preganglionic fibers are located cord. in the thoracic and lumbar segments of the spinal cord Post ganglionic neurons located in very small ganglia always Post ganglionic neurons located located near or within the in the small ganglia along the effector organs (ex:walls of vertebral column intestine). Ganglia  Neural plasticity and regeneration Despite its general stability, the nervous system exhibits neuronal differentiation and formation of ❖ The potential of new synapses even in adults. neural stem cells to allow tissue regeneration and Controlled by several growth factors produced by functional recovery both neurons and glial cells in a family of proteins within the CNS called neurotrophins. components is a subject of intense investigation. Neuronal stem cells are present in the adult CNS, located in part among the cells of the ependymal.  Neural plasticity and regeneration Astrocytes do proliferate at injured sites. These growing astrocytes can interfere with successful axonal regeneration in structures such as spinal cord tracts. In much simpler peripheral nerves, injured axons have a much greater potential for regeneration and return of function. The onset of regeneration is signaled by changes in the perikaryon: this process called chromatolysis as in follows: 1. the cell body swells slightly. 2. Nissl substance is initially diminished. 3. the nucleus migrates to a peripheral position within the perikaryon.  The proximal segment of the axon close to the wound degenerates for a short distance, but begins to grow again distally as new Nissl substance appears and debris is removed. Neural The new Schwann cells align to serve as guides for plasticity and the regrowing axons and produce polypeptide factors that promote axonal outgrowth. regeneration Motor axons reestablish synaptic connections with muscles and function is restored.  Neural plasticity and regeneration Regeneration in peripheral nerves. Thank you!