Nerve Tissue PDF

NERVE TISSUE Lec. Gözde Öğütçü Near East University Department of Histology and Embryology [email protected]  Nerve Tissue Controls and integrates the functional activities of the or...

NERVE TISSUE Lec. Gözde Öğütçü Near East University Department of Histology and Embryology [email protected]  Nerve Tissue Controls and integrates the functional activities of the organs and organ systems Three basic functions : 1.sensing changes with sensory receptors 2.interpreting and remembering those changes 3.reacting to those changes with effectors ANATOMICAL ORGANIZATION Central Nervous System (CNS) Brain (cerebrum) Cerebellum Spinal cord (Medulla Spinalis) Peripheral nervous system (PSS) Ganglions Autonomic (sympathetic and parasympathetic) ganglia Dorsal root (spinal or sensory) ganglia Nerves Cranial nerves (those emerge from brain) Spinal nerves (those emerge from spinal cord) Nerve endings  NERVOUS SYSTEM Formed by a network of many billion nerve cells(neurons) and many more supporting cells called glial cells. Nerve tissue is distributed throughout the body as an integrated communications network. Cells in both central and peripheral nerve tissue are of two kinds: ✓Neurons ✓Glial cells Features of Nerve Tissue ▪ Nervous tissue contains nerve cells (neurons), neuroglia cells and very few connective tissues. ▪ Nerve cells are separated from connective tissue by a wall called blood brain barrier. ▪ Nerve cells do not exhibit mitosis(non-divided). Neuroglia cells exhibit mitotic activity.  ORIGIN OF NERVE TISSUE CELLS CNS neurons and central glia, except microglial cells, are derived from neuroectodermal cells of the neural tube. microglia cells are derived from mesodermal macrophage precursors, specifically from Granulocyte/monocyte progenitor (GMP) cells in bone marrow. PNS ganglion cells and peripheral glia are derived from the neural crest. NEURON Cell body (Perikaryon, soma): Contains nucleus & cytoplasm Projections a. Dendrites Multiple, receives stimuli b. Axon Single, conducts impulses away Synaptic boutons are small swellings that are found at the terminal ends of axons.  CELL BODY(SOMA) The cell body (perykarion) is the dilated region of the neuron that contains a large, euchromatic nucleus with a prominent nucleolus and surrounding perinuclear cytoplasm. The perinuclear cytoplasm contains abundant rough-surfaced endoplasmic reticulum and free ribosomes. On light microscopy, the rough endoplasmic reticulum with rosettes of free ribosomes appears as small bodies, called Nissl bodies  Nissl bodies, ribosomes are rare in dendrites It is not found in the axon The area where the axon exits the trunk is called the axon hillock (the area where the action potential begins).  DENDRITES Dendrites, which receive information from other neurons at specialized areas of contact called synapses. The dendrites are projections that branch many times, forming small, tree-shaped structures protruding from the cell body that provide locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a polarity—meaning that information flows in this one direction.  AXON The axon arises from the cell body as a single thin process, much longer than the dendrites. Its thickness is directly related to conduction velocity, which increases with axonal diameter. The portion of the axon between the cell body and the beginning of the myelin sheath is the initial segment. The axonal cytoplasm is called axoplasm.  Neurons are divided into 3 according to the number of extensions radiating from their bodies: 1-Multipolar neurons: 1 axon 2 or more dendrites 2-Bipolar neuron: 1 axon, 1 dendrite: located in taste, smell, hearing and vision organs 3-Unipolar (pseudounipolar neuron): 1 axon divided into 2: found in sensory neurons  Synapses Synapses are specialized junctions between neurons that facilitate the transmission of impulses from one (presynaptic) neuron to another (postsynaptic) neuron. Synapses between neurons may be classified morphologically as: axodendritic, occurring between axons and dendrites; axosomatic, occurring between axons and the cell body; or axoaxonic, occurring between axons and axons  Synapses are classified as chemical or electrical. Chemical synapses: Conduction of impulses is achieved by the release of chemical substances (neurotransmitters) from the presynaptic neuron. Electrical synapses: Common in invertebrates, these synapses contain gap junctions that permit movement of ions between cells and consequently permit the direct spread of electrical current from one cell to another  Electrical synapses Transition occurs through Gap Junctions.  GLIAL CELLS Support neuronal survival and activities and are 10 times more abundant than neurons in the mammalian brain. There are six major kinds of glial cells, four in the CNS and two in the PNS.  Functions of neuroglial cells Provides support and protection to neurons Myelination Repairing neuron damage Regulating the CSF (cerebrospinal fluid) content of the CNS To ensure metabolic transmission and exchange between vessels and neurons  GLIAL CELLS In the CNS In the PNS ✓Schwann cells ✓ Astrocytes ✓Satellite cells ✓ Oligodendrocyte ✓ Microglia ✓ Ependymal cells  SCHWANN CELLS Envelop nerve fibers in the peripheral nervous system. Schwann cell produces a myelin sheath. assist in the regeneration of damaged fibers Allows for faster action potential propagation along an axon in the PNS  SATELLITE CELLS – Cuboidal or squamous glial cells of ganglia arranged around the cell bodies of neurons – Function; metabolic and mechanical support to ganglia cells.  THE PERIPHERAL NERVOUS SYSTEM Main components of the peripheral nervous system (PNS) are the nerves, ganglia and nerve endings. Peripheral nerves are bundles of nerve fibers (axons) individually surrounded by Schwann cells and connective tissue.  Nerve Fibers A nerve fiber is the basic structural and functional unit of peripheral nerves As axons course through body tissues, they are associated with Schwann cells. The axon with its associated Schwann cells forms a nerve fiber.  MYELINATED FIBERS The myelin sheath is a spiral layer of insulation around a nerve fiber. Myelin is a large lipoprotein complex, its composition is like that of plasma membranes in general. 20% protein and 80% lipid (phospholipids, glycolipids, and cholesterol)  MYELINATION IN PNS Myelinated axons: Schwann cells wrapping repeatedly around an axon to form the multilayered myelin sheath. Unmyelinated axons: Several unmyelinated axons wrapping by one cytoplasm of schwann cell. Although one Schwann cell can myelinate only one axon, several unmyelinated axons can be enveloped by a one Schwann cell. Myelination of PNS axons Schwann cell engulfs one portion along the length of a large diameter axon. Schwann cell membrane fuses around the axon and one thin extension of schwann cell elongates greatly and wraps itself repeatedly around the axon to form multiple compacted layers (myelin sheat). Myelin layers are rich in lipid and provide insulation and facilitate formation of action potential along the axolemma. One Schwann cell is responsible for the formation of one internodal length of myelin.  Myelin sheath formation a) The axon initially lies in a groove on the surface of the Schwann cell. b) The axon is surrounded by a Schwann cell. Note the two domains of the Schwann cell,the adaxonal plasma-membrane domain and abaxonal plasma-membrane domain. The mesaxon plasma membrane links these domains. The mesaxon membrane initiates myelination by surrounding the embedded axon. c) A sheetlike extension of the mesaxon membrane then wraps around the axon, forming multiple membrane layers. d) During the wrapping process, the cytoplasm is extruded from between the two apposing plasma membranes of the Schwann cell, which then become compacted to form myelin. The outer mesaxon represents invaginated plasma membrane extending from the abaxonal surface of the Schwann cell to the myelin. The inner mesaxon extends from the adaxonal surface of the Schwann cell (the part facing the axon) to the myelin  Schmidt-Lanterman (Myelin cleft) The axon enveloped by the myelin sheat,which, in addition to membrane, contains some Schwann cell cytoplasm in spaces called Schmidt-Lanterman(myelin cleft) between the major dense lines of membranes. This cytoplasm moves along the myelin sheat, opening temporary spaces(clefts) that allow renewal of some membrane components as needed for maintanance of the sheat.  Nodes of Ranvier Between adjacent Schwann cells on an axon the myelin sheat shows small nodes of Ranvier(nodal gaps) specialized regions in the axonal membrane that are not insulated by myelin. Act as a partial barrier to the movement of materials in and out of the periaxonal space between the axolemma and the Schwann sheat.  Myelination saltatory conduction Internodal segment http://www.youtube.com/watch?v=DJe3_3XsBOg Unmyelinated axons Small diameter axons are engulfed by one Schwann cell. The axons are seperated and each becomes enclosed within its own fold of Schwann cell surface. NO myelin is formed by wrapping Small diameter axons utilize action potentials whose formation and maintanence do not depend on the insulation provided by the myelin sheat required by large diameter axons.  Unmyelinated Nerve In unmyelinated nerves, axons are surrounded by Schwann cell cytoplasm Astrocyte: relationship with blood vessel and axon-myelin Protoplasmic astrocytes are more prevalent in the outermost covering of brain called gray matter. These astrocytes have numerous, short, branching cytoplasmic processes. Fibrous astrocytes are more common in the inner core of the brain called white matter. These astrocytes have fewer processes, and they are relatively straight  Blood brain barrier Allows tighter control the passage of substances moving from blood into the CNS tissue. Main structural component of the BBB is capillary endothelium and surrounded by the basement membrane. Limiting layer of perivascular astrocytic feet that envelops the basement membrane of capillaries in most CNS regions contributes to the BBB  NERVE ORGANIZATION Axons and Schwann cells are enclosed within layers of connective tissue – Endoneurium: Each nerve axon, surrounded by the endoneurium. This is a thin, protective layer of connective tissue. – Perineurium: Each nerve fascicle containing more axons, is enclosed by the perineurium, a connective tissue having a lamellar arrangement in seven or eight concentric layers. Regulate diffusion into the fascicle make up blood-nerve barrier – Epineurium: The epineurium is the outermost layer of dense connective tissue enclosing the (peripheral) nerve.  ENDONEURIUM Thin loose connective tissue rich in reticular fibers Mast cells, macrophages and a few fibroblasts are present Surrounding individual fibers. In contact with basal lamina of Schwann cell PERINEURIUM Dense irregular connective tissue Surrounds a group of fibers(fascicles) Specialized to contribute to blood- nerve barrier Two or more cell layers thick EPINEURIUM Dense irregular connective tissue Surrounds the entire nerve from outside  PERIPHERAL NERVE Establish communication between centers in the CNS and the sense organs and effectors (muscles, glands…) They generally contain both afferent(sensory) and efferent(motor) fibers.  Nerve Fibers Afferent fibers carry information from internal body regions and the environment to the CNS. Efferent fibers carry impulses from the CNS to effector organs  GANGLIONS Ovoid structures Cluster of cell bodies of neurons located outside the CNS Surrounded by a connective tissue capsule Serve as relay stations to transmit nerve impulses, at least one nerve enters and another exits from each ganglion. Direction of the nerve impulse determines whether the ganglion will be a sensory or an autonomic ganglion.  GANGLIONS 1. Sensory ganglia: i. Dorsal root ganglia (Spinal Ganglia) :Those emerge from spinal cord ii. Ganglia associated with cranial nerves (V, VII, VIII, IX, X) :Those emerge from the brain 2. Autonomic ganglia: i. Sympathetic chain ganglia ii. Parasympathetic ganglia  SENSORY GANGLIA Receive afferent impulses that go to the Central Nervous System. Neurons of these ganglia are pseudounipolar and relay information from the ganglion’s nerve endings to the gray matter of the spinal cord. Satellite cells: glial cells of ganglia arranged around the cell bodies of neurons Little connective tissue containing blood vessels  AUTONOMIC GANGLIA Found within autonomic nerves. The ganglia contain postganglionic visceral efferent neurons that receive synaptic input from preganglionic visceral efferent neurons. Autonomic ganglion contains multipolar neuron cell bodies with eccentric nuclei (white arrows); axons (green arrows) arise from each cell body. Satellite glial cells (red arrows) are sparse and form poorly-defined capsules around individual cell bodies Comparison of the Histology of the Ganglion Satellite cells are small, cuboidal cells Supports neurons, provides electrical insulation Provides metabolic change It is a Schwann cell analog and does not synthesize myelin.  Degeneration Degeneration of an axon distal to a site of injury is called anterograde (Wallerian) degeneration, The axon is damaged, is axonal swelling followed by its disintegration. This leads to breakdown of the axonal cytoskeleton. The cell body of the injured nerve swells, and its nucleus moves peripherally. Initially, Nissl bodies disappear from the center of the neuron and move to the periphery of the neuron in a process called chromatolysis.  In the PNS, even before the arrival of phagocytotic cells at the site of nerve injury, Schwann cells initiate removal of myelin debris. Resident macrophages become activated after nerve injury. They migrate to the site of nerve injury, proliferate, and then phagocytize myelin debris. The efficient clearance of myelin debris in the PNS is attributed to the massive recruitment of monocyte-derived macrophages that migrate from blood vessels and infiltrate the vicinity of the nerve injury When an axon is injured, the blood–nerve barrier is disrupted along the entire length of the injured axon, which allows for the influx of these cells into the site of injury. Response of a nerve fiber to injury.  A key difference in the CNS response to axonal injury relates to the fact that the blood–brain barrier is disrupted only at the site of injury and not along entire length of the injured axon. This limits infiltration of monocyte-derived macrophages to the CNS and dramatically slows the process of myelin removal, which can take months or even years. Although the number of microglial cells increases at sites of CNS injury, these reactive microglia cells do not possess the full phagocytotic capabilities of migrating macrophages. The inefficient clearance of myelin debris is a major factor in the failure of nerve regeneration in the CNS. Another factor that affects nerve regeneration is the formation of a glial (astrocyte-derived) scar that fills the empty space left by degenerated axons.  Schematic diagram of response to neuronal injury within peripheral and central nervous systems limited disruption of the Divisions and dedifferentiation of blood–brain barrier Schwann cells and disruption of restricts infiltration of the blood–nerve barrier along monocyte-derived entire length of the injured axon. macrophages and This allows massive infiltration of dramatically slows the monocyte-derived macrophages, process of myelin removal. which are responsible for the In addition, appoptosis process of myelin removal. Rapid of oligodendrocytes, an clearance of myelin debris allows inefficient phagocytic for activity of microglia, and axon regeneration and the formation of an subsequential restoration of astrocyte-derived scar lead blood–nerve barrier. to failure in nerve regeneration in the CNS.