05 Nerve Tissue AND Nervous System notes lecture PDF

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Human Histology | FEU Manila MT 202405. NERVOUS TISSUE AND THE NERVOUS SYSTEMInstructor/Prepared by: Dr. Marisa RubioHuman Nervous System The most complex system in the body. Formed by a network of many billion nerve cells, neurons. o All assisted by many more supporting cells called glial cells. Th...

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Human Histology | FEU Manila MT 2024

05. NERVOUS TISSUE AND THE NERVOUS SYSTEM Instructor/Prepared by: Dr. Marisa Rubio

Human Nervous System • • •

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The most complex system in the body. Formed by a network of many billion nerve cells, neurons. o All assisted by many more supporting cells called glial cells. The two major divisions: o Central nervous system (CNS) ▪ brain and spinal cord. ▪ overall “command center”, processing and integrating information. o Peripheral nervous system (PNS) ▪ Nerves (cranial, spinal, and peripheral nerves) and ganglia ▪ Receives and projects information to and from CNS; mediates some reflexes. Cells in both central and peripheral nerve tissue are of two kinds: neurons. o They have numerous long processes and various glial cells (Gr. glia, glue), which have short processes, support and protect neurons, and participate in many neural activities, neural nutrition, and defense of cells in the CNS. Neurons respond to environmental changes (stimuli) o Excitable or irritable o Membrane depolarization – neurons react promptly to stimuli with a reversal of the ionic gradient. Action potential – this reversal of the ionic gradient spreads from the place that received the stimulus and is propagated across the neuron’s entire plasma membrane. o Nerve impulse – capable of traveling long distances along neuronal processes, transmitting such signals to other neurons, muscles, and glands. All cells maintain a gradient called electrical potential,

Functions: o Stabilizes the intrinsic conditions of the body o Maintains behavioral patterns The General Organization of the Nervous System • The nervous system (CNS and PNS) consists of the following: 1. Sensory division (afferent) A. Somatic – sensory input perceived consciously (eyes, ears, skin, fascia, joints, and musculoskeletal structures) B. Visceral – sensory input not perceived consciously (internal organs and cardiovascular structures. 2. Motor division (efferent) A. Somatic – motor output controlled consciously or voluntarily (skeletal muscle effectors) B. Autonomic – motor output not controlled consciously (heart or gland effectors) • Autonomic motor nerves, comprising the autonomic nervous system (ANS), all have pathways involving two neurons: 1. Preganglionic neurons – with the cell body in the CNS 2. Postganglionic neurons – with the cell body in a ganglion. • The ANS has two divisions: 1. Parasympathetic division – its ganglia within or near the effector organs, maintains normal body hemeostasis 2. Sympathetic division – its ganglia close to the CNS and controls the body’s responses during emergencies and excitement. • Enteric nervous system – located in the wall of the digestive tract.

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Neurons • • •

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The functional unit in both CNS and PNS Neurolemma – special name of neuron for the cell membrane The three main parts of neurons: o Cell body (perikaryon or soma) – 150 um in diameter; contains the nucleus and most of the cell’s organelles and serves as the synthetic or trophic center for the entire neuron. o Dendrites – are numerous elongated processes extending from the perikaryon and specialized to receive stimuli from other neurons at unique sites called synapses. o Axon – single long process ending at synapses specialized to generate and conduct nerve impulses to other cells; may also receive information from other neurons, information that mainly modifies the transmission of action potentials to those neurons. Classification according to the number of processes (nerve fibers) extending from the cell body: o Multipolar neurons – each with one axion and two or more dendrites, are the most common. o Bipolar neurons – one dendrite, and one axon, comprise the sensory neurons of retina, the olfactory epithelium, and the inner ear. o Unipolar or pseudounipolar neurons – include all other sensory neurons, each have a single process that bifurcates close to the perikaryon, with the longer branch extending to a peripheral ending and other toward the CNS. o Anaxonic neurons – with many dendrites but no true axon, do not produce action potentials, but regulate electrical changes of adjacent CNS neurons

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Nervous components can also be subdivided functionally: o Sensory neurons are afferent – receiving stimuli from receptors throughout the body. o Motor neurons are afferent – sending impulses to effector organs (muscle fibers and glands). o Somatic motor nerves – are under voluntary control and typically innervate skeletal muscle o Autonomic motor nerves – control the involuntary or unconscious activities of glands, cardiac muscle, and most smooth muscle. Interneurons – establish relationships among other neurons; forms circuits in the CNS. Gray matter – where most neural perikarya occur White matter – where axons of gray matter are concentrated 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.

Cell Body (Perikaryon or Soma) • Contains the nucleus and surrounding cytoplasm, exclusive of the cell processes. • Primarily a trophic center • Are in contact with a great number of nerve endings conveying excitatory or inhibitory stimuli generated in other neurons. • A typical neuron has a unusually large, euchromatic nucleus with a prominent nucleolus, indicating intense synthetic activity. • Binuclear nerve cells are sometimes seen in sympathetic and sensory ganglia. • Chromatophilic substance (Nissl bodies) – clumps of basophilic materials in the cytoplasm (RER and free ribosomes). o Amount varies according to the type and functional state of the neuron and is particularly abundant in large nerve cells such as motor neurons.

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Golgi apparatus is located ONLY in the cell body. Mitochondria can be found throughout the cell and are usually abundant in the axon terminals. Neurofilaments – unique protein subunits that form intermediate filaments. o They are also referred to as neurofibrils by light microscopists. Lipofuscin – inclusions of pigmented material which consists of residual bodies left from lysosomal digestion. Neurons also contain microtubules.

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Dendrites • Typically short, small processes emerging and branching off the soma. • Covered with many synapses o Making dendrites the principal signal reception and processing sites on neurons. • Most nerve cells have numerous dendrites, which considerably increase the receptive area of the cell. • The large number and extensive arborization of dendrites allow a single neuron to receive and integrate signals from many other nerve cells. • Dendrites become much thinner as they branch, with cytoskeletal elements predominating in these distal regions. • Dendritic spines – where most synapses on dendrites occur o These are dynamic membrane protrusions along the small dendritic branches, visualized with silver staining and studied by confocal or electron microscopy. o Serve as the initial processing sites for synaptic signals and occur in vast numbers. o Its morphology depends on actin filaments and changes continuously as synaptic connections on neurons are modified. ▪ These changes are key importance in the constant changes of the neural plasticity that occurs during embryonic development and underlies adaptation, learning, and memory postnatally.

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Cell bodies are large with long axons to maintain them since these axons contain most of the neuron’s cytoplasm. Axolemma – the plasma membrane of the axon Axoplasm – the contents of axolemma. o Contains mitochondria, microtubules, neurofilaments, and transport vesicles o Very few polyribosomes or cisternae of RER Axon hillock – a pyramid-shaped region of the perikaryon where axons originate. Initial segment – site of the axon where 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. o Just beyond the axon hillock has several types of ion channels that are important in generating the action potential. Axons branch less profusely but undergo terminal arborization Collaterals – major branches of axons of interneurons and some motor neurons that end at smaller branches with synapses influencing the activity of many other neurons. Each small axonal branch ends with dilation called a terminal bouton o This contacts another neuron or non-nerve cell at a synapse to initiate an impulse in that cell. If an axon is severed from its cell body, its distal part quickly degenerates and undergoes phagocytosis. Bidirectional transport of small and large molecules occur within axons. 1. Anterograde transport – organelles and macromolecules synthesized in the cell body move along the axonal microtubules (kinesin) from the perikaryon to the synaptic terminals. 2. 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 body cell.

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.

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Nerve Impulses (Action Potential) • A nerve impulse or action potential, travels along an axon like a spark moves along an explosive’s fuse. • Electrochemical processes initiated at the axon hillock when other impulses received at the cell body or dendrites meet a certain threshold. • This is propagated along the axon as a wave of membrane depolarization produced by voltage-gated Na+ and K+ channels in the axolemma that allow diffusion of these ions into and out of the cytoplasm. • In unstimulated neurons, ATP-dependent Na-K pumps and other membrane proteins maintain an axoplasmic Na+ concentration only onetenth of that outside of the cell and a K+ level many times greater than the extracellular concentration. o This produces a potential electrical difference across the axolemma of about –65 mV, with the inside negative to the outside. This difference is the axon’s resting potential. • Stimulated neuron -> ion channels open -> sudden influx of extracellular Na -> resting potential from -65 mV to +30mV making the cell interior positive compared to the extracellular environment provided by the insulating glial cells; this shift is the beginning of the action potential or nerve impulse. • The 30mV potential rapidly closes the Na channels and opens the K channels. • Action potential propagates along the axonal membrane, producing the nerve impulse. • When an action potential arrives at the nerve ending, it promotes discharge of stored neurotransmitter that stimulates or inhibits another neuron or a non-neural cell, such as a muscle or gland.

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Components of synapse o Presynaptic axon terminal (terminal bouton) – contains mitochondria and numerous synaptic vesicles from which neurotransmitter is released by exocytosis. o Postsynaptic cell membrane – contains receptors for the neurotransmitter, and ion channels or other mechanisms to initiate a new impulse. o Synaptic cleft – a 20 to 30 mm wide intercellular space that separates these presynaptic and postsynaptic membranes.

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o Synaptic Communication • Synapses – are sites where nerve impulses are transmitted from one neuron to another, or from neurons and other effector cells. • Ensures that the transmission is unidirectional. • Synapses convert an electrical signal (nerve impulse) from the presynaptic cell into a chemical signal that affects the postsynaptic cell. • They act by releasing neurotransmitters, which are small molecules that bind specific receptor proteins to either open or close ion channels or initiate second messenger cascades.

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Neurotransmitters from excitatory synapses cause postsynaptic Na+ channels to open, and the resulting Na+ influx initiates a depolarization wave in the postsynaptic neuron or effector cell. At inhibitory synapses neurotransmitters open Cl- or other anion channels, causing influx of anions and hyperpolarization of the postsynaptic cell, making its membrane potential more negative and more resistant to depolarization. Once used, neurotransmitters are removed quickly by enzymatic breakdown, endocytosis mediated by specific receptors on the presynaptic membrane. ▪ Prevents an undesirable sustained stimulation of the postsynaptic neuron.

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Major components of a synapse

Glial Cells and Other Neuronal Activity •

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Types of synapses

Glial cells – support neuronal survival and activities. o Most of these develop from progenitor cells of the embryonic neural plate. o In the CNS, glial cells surround both the neuronal bodies, which are often larger than it, and the processes of axons and dendrites occupying the spaces between neurons. o They substitute for cells of connective tissue to support neurons and create immediately around those cells microenvironments that are optimal for neuronal activity. Neuropil – network of fine cellular processes emerging from neurons and glial cells; resembles collagen by light microscopy.

Oligodendrocytes • Oligios – small, few; dendron – tree; kytos – cell • Origin: Neural tube • Location: CNS, predominantly white matter • Function: myelin production, electric insulation for neurons in the CNS • They extend many processes, each of which becomes sheetlike and wraps repeatedly around a portion of a nearby CNS axon. o This produces myelin

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The produced myelin sheath electrically insulates the axon and facilitates rapid transmission of nerve impulses. The predominant glial cells in white matter are found only in the CNS oligodendrocytes. o white matter is white because of the lipid concentrated in the wrapped membrane sheaths. Routine light microscope staining – processes are not visible; usually appear as small cells with rounded, condensed nuclei and unstained cytoplasm.

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Astrocyte • Origin: progenitor cells in the embryonic neural tube • Location: CNS • Structural support, repair processes, blood-brain barrier, metabolic exchanges • Have a large number of long radiating, branching processes. • proximal regions of the astrocytic processes are reinforced with bundles of intermediate filaments made of glial fibrillary acid protein (GFAP)

o serves as a unique marker for this glial cell. Has a large number of radiating processes; unique to the CNS o Fibrous astrocytes – relatively few long processes and are in the white matter o Protoplasmic astrocytes – many short, branched processes, are found in the gray matter.

Functions: o Extending processes that associate with or cover synapses, affecting the formation, function, and plasticity of these structures. o Regulating the extracellular ionic concentrations around neurons, with particular importance in buffering extracellular K+ levels. o Guiding and physically supporting movements and locations of differentiating neurons during CNS development. o Extending 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.

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Forming a barrier layer of expanded protoplasmic processes, called the glial limiting membrane, which lines the meninges at the external CNS surface. o Filling tissue defects after CNS injury by proliferation to form an astrocytic scar. Perivascular feet – important for the ability of astrocytes to regulate vasodilation and transfer of O2, ions, and other substances from the blood to the neurons. Glial limiting membrane – a layer formed by other expanded processes w/c lines the pia matter. When the CNS is damaged, astrocytes proliferate to form cellular scar tissue (which often interferes with neuronal regeneration. Glial fibrillary acid protein (GFAP) – makes up the bundles of intermediate filaments that reinforces the processes of all astrocytes o Most common source of brain tumors. o

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Ependymal Cells • Low columnar or cuboidal cells that line the ventricles of the brain and central canal of the spinal cord • No basal lamina lamina; basal ends are elongated and tapered with extending processes that branch and penetrate the adjacent neuropil. • Responsible for production of this fluid in specialized ventricular tissues of the choroid plexus.

Microglia • Less numerous than astrocytes and oligodendrocytes • Evenly distributed throughout the gray and white matter. • Small cells with short irregular processes • Migrate through the neuropil, analyzing the tissue for damaged cells and invading microorganisms.

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Secrete a number of immunoregulatory cytokines and constitute the major mechanism of immune defense in CNS tissues. Originates from circulating blood monocytes; same family as macrophages and other antigen-presenting cells. Nuclei can be recognized in routine H&E preparations; dense elongated structures.

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When activated – retract their processes and assume morphologic characteristics as macrophages, become phagocytic and act as antigenpresenting cells.

Satellite Cells of Ganglia • From the embryonic neural crest • Form a covering later over the large neuronal cell bodies in PNS ganglia. • Exert a trophic or supportive role.

Schwann Cells (neurolemmocytes) • Only found in the PNS • Have trophic interactions with axons and allow for their myelination like the oligodendrocytes of the CNS. • One neurolemmocyte forms myelin around a segment of one axon while oligodendrocytes branch and sheath parts of more than one axon.

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Central Nervous System • Principal structures: o Cerebrum o Cerebellum o Spinal cord o No connective tissue; soft, gel-like organ • Meninges – connective tissue layers which cover the CNS. • Has little collagen, making it relatively soft and easily damaged by injuries affecting the protective skull or vertebral bones. White Matter • Main components o Myeline axons – the myelin-producing oligodendrocytes • No neural cell bodies, but microglia are present. Gray Matter • Contains abundant: o Neuronal cell bodies o Dendrites o Initial unmyelinated portions of axons o Astrocytes, and microglial cells • Where synapses occur o Prevalent at the surface or cortex of the cerebrum and cerebellum • Nuclei – aggregates of neural cell bodies forming islands of gray matter embedded in the white matter.

Cerebral Cortex • Six layers with most neurons arranged vertically. • Most abundant neurons o Efferent pyramidal neurons which come in many sizes. • Function: o Integration of sensory information and the initiation of voluntary motor responses. • Coordinates muscular activity throughout the body. • Three layers: o Molecular layer – outer layer o Purkinje cells – very large neurons; central layer o Granular layer – inner layer; formed by very small neurons (the smallest in the body), which are packed together densely.

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 darkstained spot that is a small section of the ...