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AHD Oct/22/08 Amal Al-Hashmi R4 Quiz • List the different parts of a neuron? • What are the functions of a neuron? • List the different types of glia cells and name one function for each? Out line 1. Structure of neurons 2. Classifications of neurons 3. Axonal transport and its clinical applications 4. Electrical properties of neurons 5. Glial cell functions and clinical application 6. Conclusion/quiz The structure of neurons • Dendrites • Cell body • Axons and axons terminals Dendrites • *Usually brch extensively in the vicinity of the cell body • *receive signals either from other neurons or from the environment via specialized receptors • *Info travels from distal to proximal along dendrites to converge at the cell body • *Small bud like extension (dendrite spines) of variety of shapes is frequently seen on the more distal brch of dendrite contact Cell body • Cell body of neuron is also called the soma or the perikaryon • Varies widely in shape depending on the neuron type • Active protein synthesis is indicated by the large size of the nucleus and its content of diffuse chromatin and at least one prominent nucleolus (site of ribosomal RNA synthesis), Cell body The cell body of multipolar neurons vary widely in shape so their profiles in tissue sections may appear fusiform , flask shaped , triangular , polygon or stellate Variations of stellate polygon are most common Bipolar have a round or oval perikaryon, with a single large process emanating from each end of the cell body The pseudo unipolar has a spherical cell body with centrally placed nucleus Axons and Axons Terminals • The axon arises from the cell body at a small elevation called axon hillock. • The proximal part of the axon, adjacent to the axon hillock, is the initial segment • The cytoplasm of the axon (axoplasm) contains dense bundles of microtubules and neurofilaments • Axons are typically devoid of ribosomes • Axon in CNS often end with fine terminals called terminal arobes which are capped with small terminal boutons • The surface area of axon can be several 1000 times the surface area of the parent cell body Classification of neurons • Multipolar 99% • Bipolar • Pseudounipolar/unipolar Type of neuron Unipolar bipolar Location of cell bodies Post root or CN ganglion Retina, olfactory epithelium,vestibular ganglion , auditory ganglion Multipolar Stellate Fusiform Pyriform Pyramidal Purkinje Mitral Chandelier Granule amacrine Many areas of CNS Hippocampal Cerebellar cortex Olfactory bulb Visual area of cerebral cortex Cerebral and cerebellar cortex retina Axonal transport Anterograde Retrograde Allows neurons to respond to molecules Slow 1mm/day Not clearly understood Continual recycling of components of axon terminal. driven by the protien dynein Fast 50-250mm/day Fast @ speed 400mm/day, driven by the protien Kinesin Implication in of axonal transport in pathogenesis of some human neurological diseases e.g. Rabies virus Clostridium tetani ( Tetanus) Axonal transport as a research tool Injecting a fluorescent substance into region containing axons terminals ( retrograde) Injecting radioactively labeled a. a into cell body ( anterograde) Electrical properties of neuron Neurons carry a negative electrical charge relative to the extra cellular fluid bathing them The plasma membrane is a semi permeable because certain ions can cross at certain times but there is not a free exchange The opening and closing of specific ion channels can be controlled by chemical signals including neurotransmitter Neurons as info receivers • Primary sensory neurons photoreceptors ( cones. rods) chemoreceptors (taste/smell, PH receptor mechanoreceptors ( vestibular, auditory, somatosensory system) thermoreceptors ( pain/temp) Neurons as info transmitters • Synapses • Location at which a process of one neuron communicates with a second neuron or a effector cell • Chemical synapse consists of presynaptic elements , postsynaptic elements and synaptic cleft • Unidirectional • The strength of effect on postsynaptic membrane is variable Glial cells • Major types in CNS are: astrocytes oligodendrocytes microglia • Major types in PNS: satellite schwann cells MQ Astrocytes • Occur through out CNS • Highly branched cells with processes that contact most of of the surface of neuronal dendrites and cell bodies as well as axonal surface • Role in structural support and response to injury • Secrete growth factors, Cholesterol and lipoprotein , • Contribute to the regulation of inflammatory processes ( cytokines and IL-1, IL6) Astrocytes • Environmental modulation • Metabolism • Regional heterogeneity • Role at the BBB Oligodendrocytes Arise during development from oligodendrocte precursor cells Occurs in both grey and white matter Main function is myelination One oligodendrocyte can myelinates 30 to 50 – nearby axons Oligodendrogial cell body located at some – distance form myelin sheath, linked by thin cytoplasmic bridge oligodendrocytes Microglia Immune effector cells of CNS Predominant cells involved in CNS inflammation Makes up to 1% of CNS cell population They have phagocytic role They secrete IL-IB, TNF, PG microglia Clinical implications Excessive secretion of Cytokines IL-IB &TNF induces endothelial cells to open the BBB allowing leukocytes infiltration into the brain parenchyma ( bacterial meningitis) some molecules secreted by activated microglial cells are neurotoxin( including glutamate, oxygen radicals and TNF ) and neurons may be directly damaged by inflammatory rxn in the CNS thus the microglial rxn maybe a 2nd cause of neuronal damage following stroke or tumor Clinical implications • release of O2 radicals by microglia and astrocytes can lead to oxidative damage • in MS , demylination results from inflammatory signaling that allows T lymphocytes to extravasate and enter CNS tissue • HIV virus leads to neuronal damage through targeting microglia cells. ?!dementia Glia-derived tumors Astrocytomas the most frequently encountered glial cell tumor Grade I astrocytoma are slowly growing, arising from white matter Grade II Slow, but more infiltrative Glia-derived tumors Grade III Rapid growing with mitotic figures grade IV Also called glioblastoma multiform Highly malignant , rapidly growing invasive tumor • Oligodendrogliomas slow growing tumors Summery • Neurons are excitable cells that are specialized for the perception of stimuli and conduction of nerve impulses • They vary in shape and size • There are two ways of axonal transportation • Chemical synapse consists of presynaptic elements , postsynaptic elements and synaptic cleft Summery Glial cells provide structural support for the nerve cells and control the environment within the CNS quiz • Define the term synapse? • List the different types of glia cells and name one function for each. • What is the role of microglial cells in CNS truma or inflammation? Thank you questions