Histology of Nervous Tissue

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Histology Of nervous tissue By Madiha Dandachy

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Histology of nervous tissue:

Histology of nervous tissue Done by: Madiha dandachi Amwaj osta 1

Nervous tissue:

Nervous tissue consists of : - nerve cells : Neurons - associated with supporting cells : Neuroglia. 1- Neurons : transmit electrical impulses from one site in the body to another, and receive and process information 2- Neuroglia : are non-conducting cells that are in intimate physical contact with neurons . . 2

neurons:

neurons - Basic unit of nervous system - like muscle cells , possess electrical excitability , the ability to respond to a stimulus and convert it into an action potential. - Provide most of the unique functions of the nervous system such as sensing , thinking , remembering , controlling muscle activity and regulating glandular secretions . - It Consists of: - a cell body. - a single axon. - several dendrites 3

Neuron:

Neuron 4

Cell body:

Cell body also called Perikaryon or soma, plural somata . It is basically a cell nucleus surrounded by cytoplasm. Nuclei of nerve cells are: large, round and euchromatic with a single prominent nucleolus. Cytoplasm of nerve cell bodies is abundantly supplied with: - masses of rough endoplasmic reticulum - numerous Golgi bodies - lots of smooth endoplasmic reticulum - many mitochondria -extensive cytoskeletal elements : Nissel bodies ( clusters of RED containing : ARN ) and various filaments: neurofibrils 5

Cell body :

Cell body nerve fiber is a general term for any neural process ( extension ) that emerges from the cell body of a neuron and it consists of an axon and myelin sheath (if present) in the nervous system There are nerve fibers in CNS and PNS It may be myelinated and/or unmyelinated (in CNS ) It is made up of many nerve cell fibers bound together b y connective tissue , It has 3 parts : - Epineurium - Perineurium - E ndoneurium 6

NERVE CELL FIBER:

NERVE CELL FIBER - Epineurium : A sheath of dense connective tissue surrounds the nerve. Blood vessels of various sizes can be seen in the epineurium. - Perineurium : A sheath penetrates the nerve to form the perineurium which surrounds bundles of nerve fibers. - E ndoneurium : which consists of a thin layer of loose connective tissue, surrounds the individual nerve fibers. 7

Axon:

Axon specialized for conducting signals from one nerve cell to another or to muscle fiber or gland cell. Long , thin , cylindrical projection that often join the cell body at a cone-shaped elevation called: The axon Hillack. due to lack of RER, protein synthesis doesn’t occur in axon. Axons with myelin are called myelinated axons. Myelin is formed by support cells (Schwann Cells in the peripheral nerve system, oligodendroglia in the CNS) wrapping around the axons. cytoplasm is called Axoplasm surrounded by plasma membrane called Axolemma . Collaterals are branches along the axon. Axon terminals are the many fine ending. 8

Dendrites:

D endrites receiving or input portions of the neuron. Short , tapering and hightly branched. their cytoplasm contains nissel bodies(clusters of RER), mitochondria and other organelles. 9

Classification of neurons :

Classification of neurons Neurons display great diversity in size and shape . 1- Structural classification : according to the number of process extending from the cell body. A- Mutipolar neurons : - have several dendrites and one axon. - Neurons of brain and spinal cord. 10

Classification of neurons:

Classification of neurons B- Bipolar neurons : - have one dendrite and one axon. - found in the retina of the eyes , in the inner ear and in the offactory area of the brain 11

Classification of neurons:

Classification of neurons c- Unipolar neurons : - have dendrites and one axon - these neurons are more appropriately called: pseudounipolar neurons because they begin in the embryo as bipolar neurons. - Functions : sensory receptors that detect a sensory stimulus such pain , touch and pressure 12

Classification of neurons:

Classification of neurons 2- Functional classification : according to the direction in which the nerve impulse ( AP) is conveyed with respect to the CNS. a- Sensory or afferent neurons : Either contain sensory receptors at their distal ends or are located just after receptos that are separate cells . Once an approprimate activates a sensory receptor , the sensory neuron forms a AP in its axon and the AP is conveyed into the CNS through cranial or spinal nerves. Most sensory neurons are unipolar in structure 13

Sensory neurons:

Sensory neurons 14

Classification of neurons:

Classification of neurons b- Motor or efferent neurons : Convey AP away from the CNS to effectors ( muscles and glands) in the PNS through cranial or spinal nerves . Most motor neurons are mutipolar in structure. 15

Classification of neurons:

Classification of neurons c- Interneurons or association neurons : are located within the CNS between sensory and motor neurons. integrate ( process ) incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. Most interneurons are mutlipolar in structure. 16

Classification of neurons:

Classification of neurons d- Purkinje cells : These cells are some of the largest neurons in the human brain with an intricately elaborate dendritic arbor, characterized by a large number of dendritic spines aligned like dominos stacked one in front of the other. 17

Classification of neurons:

Classification of neurons e- Pyramidal cells: One of the main structural features of the pyramidal neuron is the triangular shaped soma or cell body, after which the neuron is named. Other key structural features of the pyramidal cell are a single axon , a large apical dendrite, multiple basal dendrites, and the presence of dendritic spines 18

Classification of neurons:

Classification of neurons 3-Neurotransmitter production classification : Neurons differ in the type of neurotransmitter they manufacture. Some examples are : Cholinergic neurons—acetylcholine. ABAergic neurons—gamma amino butyric acid Glutamatergic neurons—glutamate Dopaminergic neurons—dopamine Serotonergic neurons—serotonin 19

Supporting cells:

Supporting cells 2- Supporting cells : Neuroglia or " Glia " -Do not generate or propagate AP. -Forms the half of the CNS. - Acts as glue that hold nervous system together. - Smaller than neurons and more numerous. -Can divide and multiply in mature CNS. 20

Supporting cells:

Supporting cells The four main functions of glial cells are : to surround neurons and hold them in place to supply nutrients and oxygen to neurons to insulate one neuron from another to destroy and remove the carcasses of dead neurons. 21

Central nervous system:

Central nervous system • There are Four types of neuroglial cells : a) Oligodendrocytes : responsible for forming and maintaining the myelin sheath often found in rows between axons The myelin sheath around axons is formed by concentric layers of oligodendrocytes plasma membrane One oligodendrocyte may myelinate one axon or several. 22

Oligodendrocyte:

Oligodendrocyte 23

Central nervous system:

Central nervous system b) Astrocytes : the largest of the neuroglia. - have microfilaments, which allow it to wrap around blood capillaries creating BBB(blood barrier brain) which restricts the movement of substances between blood and interstitial fluid of CNS provides physical and metabolic support for nerve cells.-Help maintain the apropiate chemical environment for the generation of nerves impulse . 24

Astrocytes:

Astrocytes 2 Types : Protoplasmic A : short branching processes and are found in Gray Matter. Fibrous Astrocytes : have many long unbranched processes and are located in white Matter. 25

Central nervous system:

Central nervous system c) Microglia : the smallest of the glial cells originate in bone marrow and enter the CNS from the blood which are the phagocytes of the CNS 26

Central nervous system :

Central nervous system d) Ependymal cells : cuboidal to columnar cells arranged in single layer that possess microvilli and cilia they line the ventricles of the brain and central canal of the spinal cord. 27

Peripheral nervous system:

Peripheral nervous system There are two types of neurological cells : Schwann cells: are responsible for the myelination of axons wraps itself, jelly roll-fashion, in a spiral around a short segment of an axon An axon's myelin sheath is segmented because it is formed by numerous Schwann cells arrayed in sequence along the axon. similar to oligodendrocytes but schwann cells myelinate single axon while oligodendrocytes myelinate several axons. 28

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Peripheral nervous system:

Peripheral nervous system • Satellite cells : -flat cells that surround the cell bodies of neurons of PNS ganglia -regulate the exchange of materials between neuron cell bodies and interstitial fluid - In paravertebral and peripheral ganglia, nerve cell processes must penetrate between satellite cells to establish a synapse. 30

myelination:

myelination The myelin sheath is formed by multilayered lipid and protein that covers axons. The sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction. The amount of myelin increases from birth to maturity . 31

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Two types of neuroglia produce Myelin : 1- Shwann cells : begin to form myelin sheaths around axons during fetal development . the outer nucleated cytoplasmic layer of the Schwann cell which encloses the myelin sheath is the neurolemma . It's found only around axons in the PNS . Neurolemma help in regeneration of axon when it's injured by forming a regeneration tube that guides and stimlulates regrowth of the axon. 32

Schwann cells:

Schwann cells 33

myelination:

myelination 2- Oligodendrocyes : myelinates parts of several axons A neurolemma is not present because the oligodendrocyte cell body and nucleus do not develop the axon. 34

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The Central Nervous System 35

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The central nervous system ( CNS ) is the part of the nervous system that integrates the information that it receives and coordinates the activity of all parts of the body. It contains the majority of the nervous system and consists of the brain and the spinal cord . Some classifications also include the retina and the cranial nerves in the CNS . 36

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The CNS contains two areas : Areas containing  nerve cell bodies, their myelinated and unmyelinated processes and supporting (glial) cells are called grey matter . It appears grayish rather than white because the Nissl bodies impart a gray color and there is little or no myelin in these areas. Areas containing predominantly myelinated axons but also some unmyelinated axons and glial cells are referred to as white matter . The whitish color of myelin gives white matter its name. - Blood vessels are present in both white and gray matter. 37

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In the brain, the grey matter is exterior to the white matter, the reverse is the case in the spinal cord . 38

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The Peripheral N ervous System 39

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The peripheral nervous system ( PNS ) consists of the cranial and spinal nerves and ganglia outside of the brain and spinal cord . The main function of the PNS is to connect the central nervous system (CNS) to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull. 40

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The peripheral nervous system may be subdivided into : The sensory Division : ( afferent): contains Somatic and visceral sensory nerve fibers and conducts impulses from receptors to the CNS. The Motor Division : ( efferent) : contains motor nerves fibers and conducts impulses from the CNS to effectors ( muscles and glands ). 41

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The Motor Division subdivided also into to : The Somatic nervous system : SNS : contains somatic motor nerves .Its actions are largely voluntary and conducts impulses from the CNS to skeletal muscles only. The Autonomic nervous system : ANS : contains visceral motor nerves .Its actions are largely involuntary and conducts impulses from the CNS to cardiac muscles , smooth muscles and glandular epithelium. 42

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It also differs from the sensory-somatic system by using two groups of motor neurons to stimulate the effectors instead of one. The first is the preganglionic neurons that arise in the CNS and run to a ganglion in the body. Here they synapse with postganglionic neurons which run to the effector organ (cardiac muscle, smooth muscle, or a gland). 45

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The Autonomic nervous system further subdivided into : Sympathetic Division : mobilizes body systems during activity and the stimulation of the sympathetic branch of the ANS prepares the body for emergencies: for " fight or flight " (and, perhaps, enhances the memory of the event that triggered the response). Parasympathetic Division : conserves energy and returns the body functions to normal after they have been altered by sympathetic stimulation. In times of danger, the sympathetic system prepares the body for violent activity. The parasympathetic system reverses these changes when the danger is over. 46

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The Synapses 48

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Ultastructural characteristics of synapses : The synapse is a specialized junctional complex by which axons and dendrites emerging from different neurons intercommunicate. Synapses are essential to neuronal function: neurons are cells that are specialized to pass signals to individual target cells, and synapses are the means by which they do so. At a synapse, the plasma membrane of the signal-passing neuron (the presynaptic neuron) comes into close apposition with the membrane of the target ( postsynaptic ) cell. Both the presynaptic and postsynaptic sites contain extensive arrays of molecular machinery that link the two membranes together and carry out the signaling process. In many synapses, the presynaptic part is located on an axon , but some presynaptic sites are located on a dendrite or soma . 49

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Synapses are : axodendritic : from axon to dendrite. axosomatic : from axon to cell body. axoaxonic : from axon to axon. 50

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The two types of synapses : Electrical and Chemical differ both structurally and functionally : 1-Chemical synapses : the presynaptic neuron releases a chemical called a neurotransmitter that binds to receptors located in the postsynaptic cell, usually embedded in the plasma membrane. The neurotransmitter may initiate an electrical response or a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron.The connection made at a chemical synapse contains a small synaptic cleft between the connect neurons. The AP may not be conveyed across the cleft as there is no membrane in the cleft. 51

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Steps of synapse transmission : 1- AP on the pre-synaptic neuron. 2- Opening of voltage-gated calcium channels 3- increase in [ca2+] 4- migration and fusion of vesicles containing the neurotransmitter 5- neurotransmitter release 6- diffusion of the neurotransmitter in the synaptic cleft 7- binding of neurotransmitter to receptors on the post-synaptic cell 8- change in the permability ( membrane potential ) of the post-synaptic cell. 54

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2-Electrical synapses : AP conduct directly between adjacent cells through structures called Gap junctions. Each gap junction contains a hundred or so tubular connexons. As ions flow from one cell to the next one through connexons the AP spreads from cell to cell. We find electrical synapses in the CNS. 55

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ES have two main advantages : 1- Faster communication : because AP conduct directly though gap junctions ES are Faster then CS. The AP passes directly from presynaptic cell to postsynaptic cell. 2-Synchronization : ES can synchroniza the activity of a group of neurons . a large number of neurons can produce AP in unision if they are connected by gap junctions. 57

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Neuronal Signaling 58

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T he purpose of the nervous system is to transfer information from the PNS to the CNS, process the information in the CNS, and send back information to the PNS. This transfer of information from the external environment, through neurons, and back again to the external environment is known as neuronal signaling . 59

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A summary of neuronal signaling: Before a neuron receives a signal, it is in a resting state . Neurons receive signals in two forms: Chemical changes. This is done via neurotransmitters or chemical elements in the environment. Physical changes. Examples include touch receptors in the skin or photoreceptors in the retina. These signals cause ionic fluctuations in the neuron’s plasma membrane which creates an electrical current flow in the neuron . This current flow travels down the axon (perhaps long-distance through action potential , explained below) . When the current reaches the terminal boutons , neurotransmitters are released to other neurons or the environment. 60

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The difference between the inside and outside of a cell is due to the effect of the Sodium-Potassium pump.The Voltage is the measurement used to quantify the difference in charge. The difference in voltage of the cell membrane is called : Membrane potential difference or membrane potential. 61

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Neuron’s Resting State In its resting state, an electrical gradient is maintained across the neuron’s membrane, thereby creating a resting membrane potential . 62

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When neuron is not sending a signal. The inside of the neuron is negative relative to the outside. The concentrations of the different ions attempt to balance out on both sides of the membrane, but fail because the cell membrane allows only some ions to pass through channels (ion channels). At rest, potassium ions (K + ) can cross through the membrane easily but chloride ions ( Cl - )and sodium ions (Na + ) have a more difficult time crossing. There is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential . 63

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The resting membrane potential of a neuron is about 70 mV (mV=Mille volt) - this means that the inside of the neuron is 70 mV less than the outside . At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron. 64

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Electrical signals in neuron Ion channels: -Ions move across the plasma membrane down their Electrochemical Gradient. -They open and close due to gates. -Within an excitable cell's outer cell membrane which opens and closes. 65

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- Four types: a) Leakage channels: -Randomly alternate between open and close (more permeable to K than Na). b) Voltage gated channels: -Opens in response to a change in Membrane Potential. -participates in the generation and conduction of AP. c) Ligand -gated channels or chemical gated : -Opens and closes in response to a specific chemical stimulus (hormones, neurotransmitters). d) Mechanically gated channels : -Opens or closes in response to mechanical stimulation in the form of vibration (sound waves, pressure, tissue stretching). 66

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The gated-channels of interest here are: Voltage gated-channels. Chemically gated-channels: only open when the appropriate chemical is present. 67

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Action potential Occurs when a neuron sends information down an axon, away from the cell body. For any given neuron, the size of the action potential is always the same. There are no big or small action potentials in one nerve cell - all action potentials are the same size. 68

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Steps: A stimulus first causes sodium channels to open. There are many more sodium ions on the outside. The inside of the neuron is negative relative to the outside. Sodium ions rush into the neuron. Sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. The action potential goes back toward -70 mV (a repolarization ). The action potential actually goes past -70 mV (a hyperpolarization ) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to -70. 69

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Phases of Action Potential: Depolarizing phase : during which the negative membrane potential becomes less age negative 0 mV. Achieved by allowing Na + in. Repolarizing phase : during which the membrane potential is restored to normal resting state -70 mV. Hyperpolarizaton : the membrane became even more negative (more polarizaed ).Achieved by allowing Cl - in. 71

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The Propagation of an AP along an axon : -Action Potential are in the same sizes. -The larger the grated potential that led to the AP , the more AP are fired. And the information is coded by the Frequency of action potentials. 73

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We are Thankful For Your Attentions 74