ANAT2241 Histology: Basic & Systematic Nervous Tissue PDF

ANAT2241 Histology: Basic & Systematic Nervous Tissue Dr. Reza Shirazi Department of Anatomy, School of Medical Sciences Faculty of Medicine & Health, UNSW Sydney [email protected] Image by Dr John Abramyan, University of Michigan Resources Recommended reading: Textbook chapter (Wheat...

ANAT2241 Histology: Basic & Systematic Nervous Tissue Dr. Reza Shirazi Department of Anatomy, School of Medical Sciences Faculty of Medicine & Health, UNSW Sydney [email protected] Image by Dr John Abramyan, University of Michigan Resources Recommended reading: Textbook chapter (Wheather's online): Nervous System Textbook chapter (Junqueira’s Basic Histology: Text and Atlas, 15e online): Nervous Tissue Nervous System Online Support Page University of Michigan Histology and Virtual Microscopy Learning Resources Chapman Histology https://www.youtube.com/channel/UCvSvCkHjCbHn8aGd6OPno_A/featured Learning Objectives 1. To describe the parts of the neuron 2. To understand the classifications of the neurons 3. To understand the synaptic communications 4. To know the basic histological features of glial cells Nervous Tissue Nervous tissue: Distributed throughout the body as an integrated communications network Composed of: 1- Cells: I. Nerve cells/Neurons II. Supporting cells/Neuroglial cells 2- Extracellular matrix (ECM) o Very small amount Nerve cells/Neurons Respond to environmental changes (stimuli) by altering the ionic gradient that exists across their plasma membranes Neurons are excitable or irritable cells: • Cells that can rapidly change the electrical potential (ionic gradient across the cell membrane) in response to stimuli (eg, neurons, muscle cells, some gland cells) Action potential (nerve impulse): • Reversal of the ionic gradient as membrane depolarization (depolarization wave) spreading in the neuron’s plasma membrane Nerve cells/Neurons The functional unit in both the CNS and PNS Composed of 3 parts: 1- Cell body (or perikaryon or soma) 2- Neurites (nerve fibers) I. Dendrites II. Axon Cell body (Perikaryon or Soma) • Containing the nucleus and most of the cell’s organelles • Large, with a large, euchromatic nucleus and well-developed nucleolus indicating intense synthetic activity • Serves as the synthetic or trophic center for the entire neuron • Contains chromatophilic substance called Nissl substance or Nissl bodies: o Large masses of free polysomes and RER o Basophilic o Abundant in large nerve cells such as motor neurons Cell body (Perikaryon or Soma) • Golgi apparatus: located only in the cell body • Mitochondria: can be found throughout the cell and are usually abundant in the axon terminals • Microtubules, actin filaments and intermediate filaments in perikaryon and its processes (axon and dendrites): o Neurofilaments (or neurofibrils): intermediate filaments • Contain inclusions of pigmented material, such as lipofuscin Dendrites • Numerous elongated processes • Extend from the perikaryon • Specialized to receive stimuli from other neurons at unique sites called synapses • Usually covered with many synapses • Become much thinner as they branch • Contain dendritic spines in CNS: o Dynamic membrane protrusions along the small dendritic branches Axon • • Axolemma: plasma membrane of the axon Axoplasm: cytoplasm of the axon • A single long process ending at synapses • Specialized to generate and conduct nerve impulses to other cells (eg, nerve, muscle, and gland cells) Axon • Originated from axon hillock of the perikaryon • Initial segment: o Just beyond the axon hillock containing concentrated ion channels that generate the action potential • Undergo terminal arborization: o Each small axonal branch ends with a dilation called a terminal bouton Classification of the Neurons Classified in many different ways: Function: • Sensory (afferent) • Motor (efferent) • Interneuron Location: • Cortical • Spinal Neurotransmitter: • Glutamatergic • Cholinergic Morphology: • Pyramidal • Granule • Mitral OR • Multipolar • Bipolar • Unipolar or pseudounipolar • Anaxonic Morphological Classification of the Neurons 1- Multipolar neurons: • One axon and two or more dendrites • The most common including all motor neurons and most CNS interneurons Classification of the Neurons 2- Bipolar neurons: • One dendrite and one axon • Comprise the sensory neurons of the retina, the olfactory epithelium, and the inner ear Classification of the Neurons 3- Unipolar or pseudounipolar neurons: • A single process that bifurcates close to the perikaryon, with the longer branch extending to a peripheral ending and the other toward the CNS • Include all other sensory neurons Classification of the Neurons 4- Anaxonic neurons: • Many dendrites but no true axon • Found only in certain CNS interneurons • Do not produce action potentials, but regulate electrical changes of adjacent CNS neurons Synapses • Sites where nerve impulses are transmitted from one neuron to another, or from neurons and other effector cells • Transmission is unidirectional • Types of synapses: o Chemical o Electrical Chemical Synapses Components: 1- Presynaptic axon terminal (terminal bouton): • Contains mitochondria and numerous synaptic vesicles from which neurotransmitter is released by exocytosis 2- Synaptic cleft: • A 20- to 30-nm-wide intercellular space • Separates these presynaptic and postsynaptic membranes 3- Postsynaptic cell membrane: • Contains receptors for the neurotransmitter, and ion channels or other mechanisms to initiate a new impulse Morphological Types of Chemical Synapses 1- Axosomatic synapse 2- Axodendritic synapse 3- Axoaxonic synapse Glial Cells Support neuronal survival and activities 10 times more abundant than neurons in the mammalian brain Participating in the formation of neuropil of CNS: • Network of fine cellular processes emerging from neurons and glial cells • The fibrous intercellular network of CNS tissue superficially resembles collagen by light microscopy (X200; H&E staining) • N: neuronal cell body • G: glial cell • Np: neuropil (X200; Gold chloride and hematoxylin staining) Types of Glial Cells Glial cells present in CNS: 1. Astrocytes 2. Oligodendrocytes 3. Microglia 4. Ependymal cells Glial cells present in PNS: 1. Schwann cells 2. Satellite cells Oligodendrocytes • Predominant glial cells in white matter • Extend many processes, each of which becomes sheetlike and wraps repeatedly around a portion of a nearby CNS axon • Forming myelin sheath • An axon’s full length is covered by the action of many oligodendrocytes • Oligodendrocytes in routine light microscope staining: o Small cells with rounded, condensed nuclei and unstained cytoplasm Astrocytes Have a large number of long radiating, branching processes Two types: 1- Fibrous astrocytes: • With long delicate processes • Abundant in white matter 2- Protoplasmic astrocytes • With many shorter processes • Predominate in the gray matter Functions of the Astrocytes 1- Extending processes that associate with or cover synapses, affecting the formation, function, and plasticity of these structures 2- Regulating the extracellular ionic concentrations around neurons 3- 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 4- Forming a barrier layer of expanded protoplasmic processes, called the glial limiting membrane, which lines the meninges at the external CNS surface 5- Filling tissue defects after CNS injury by proliferation to form an astrocytic scar Ependymal Cells Columnar or cuboidal cells that line: 1- Fluid-filled ventricles of the brain 2- Central canal of the spinal cord Characteristics of the apical ends of the ependymal cells in some regions: 1- Cilia: facilitate the movement of cerebrospinal fluid (CSF) 2- Long microvilli: likely involved in absorption Characteristics of the basal ends of the ependymal cells in some regions: 1- No basal lamina 2- Elongated and extend branching processes into the adjacent neuropil Microglia • Consist of small cell bodies from which radiate many long, branched processes • Evenly distributed throughout specific regions of gray and white matter • Constitute the major mechanism of immune defense in the CNS, removing any microbial invaders and secreting several immunoregulatory cytokines Schwann Cells / Neurolemmocytes • Counterparts to oligodendrocytes of the CNS • Function: o Form myelin sheathes in the PNS o Unlike an oligodendrocyte, a Schwann cell forms myelin around a portion of only one axon Satellite Cells of Ganglia • Form a thin, intimate glial layer around each large neuronal cell body in the ganglia of the PNS • Function: o Trophic or supportive effect on these neurons o Insulating, nourishing, and regulating neurons microenvironments