Neuroscience 1 - Trans 4 - Neuron and Neuroglia.pdf

1A NUEROANATOMY NEURON AND NEUROGLIA DR. A. VIADO, M.D. OUTLINE...

1A NUEROANATOMY NEURON AND NEUROGLIA DR. A. VIADO, M.D. OUTLINE According to Function I. Neuron II. Structure of the Neuron  Motor - conducts impulses from the CNS to the muscles III. Nerve cell Process and glands IV. Synapse  Sensory – Receives sensory input. Conduct impulses V. Myelin Production towards the CNS. VI. Neuroglia  Interneurons – interconnectors ; Establish neuronal circuit between sensory and motor neurons. I. NEURON  Nerve cells and its processes According to size  Amitotic A. Functions  Receptive – Receive stimuli and transducer into nerve impulses for transfer to another region  Integrative – processing impulse on the higher center  Motor – Initiating motor response and transducer the impulse to an effector Dendrite – Receives information and conducting it TOWARD the cell body. Axon – Long tubular neurite that conducts impulse away from the cell body. Nerve Fibers – Combination of dendrites and axons  Golgi type I Neurons - long axons that may be 1 meter or B. Varieties of Neurons more. o Forms the long fiber tracts of the brain and spinal cord and nerve fiber of peripheral nerve.  Golgi type II Neurons – Short axon o Greatly outnumbered the type I o Star-shaped appearance o Often inhibitory in function Neuron Grouping  Cortex – Neuron form six layer on the cerebrum, three layers on the cerebellum  Unipolar Neurons – cell body has a single neurite. o The branches of this single neurite have the  Nuclei – In subcortical region (thalamus, midbrain, structural and functional characteristics of an brainstem and spinal cord) neuron form irregular cluster axon; forms central and peripheral processes. o Found in Posterior root ganglion  Ganglion – Cluster of neuron outside CNS\  Bipolar Neurons – Elongated cell body from each end of which a single neurite emerges. o Found in retinal bipolar cells and cells of the II. STRUCTURE OF THE NEURON sensory cochlear and vestibular ganglia  3 main parts: o One is a fused dendrite, the other is and axon o Cell Body  Multipolar Neurons – have a number of neuritis arising o Axon from the cell body. o Dendrites o All are dendrites except for a single axon. o Most neurons of the CNS o Fiber tracts of the brain, spinal cord and peripheral nerves. Trans 2 | Brul, Allida, Andam, Andoy 1 of 6  Neuroanatomy Neuron and Neuroglia - Belongs to cytokeratin family Microfilaments – concentrated at the periphery of the cytoplasm - Function in the formation of new cell process and retraction of old ones - Assist microtubules in axon transport Microtubules – found interspersed among the neurofilaments  Rapid transport – brought about by two motor proteins associated with ATPase site. KINESIN for antegrade (away from the cell) DYNEIN for retrograde movement.  Slow transport – bulk movement of cytoplasm. Occurs only in Antegrade direction Lysosome – Intracellular scavengers contain hydrolytic enzymes formed by budding of golgi apparatus. Centrioles – Bundle of microtubules found in immature dividing nerve cells. NERVE CELL BODY  Perikaryon / Soma – Clusters of cell bodies Lipofuscin (Pigment Material) – result of the lysosomal activity  Bounded by plasma membrane, mass of cytoplasm which - Yellow to Brown ; accumulate with age nucleus is embedded Melanin Granules – Found in the substantia nigra of midbrain. - Related to the cathecholamine synthesizing ability of the Nucleus – Stores the genes and is commonly centrally located - In mature neurons, the chromosome no longer duplicate and function only in gene expression - There is usually a single prominent nucleus - The nuclear envelope can be regarded as a special portion of the rough ER. - Transcriptionally active Cytoplasm – No centrioles, incapable of cell division - Stains basophilically due to abundance of RER and poly ribosome. (Stains: Toluidine Blue or Basic Aniline Dye)  Nissl Bodies – clumps of RER that are site of protein synthesis - Distributed throughout the cytoplasm except for the AXON HILLOCK  Microtubules and Neurofibrils – protein transport (Neurofilaments)  Chromatolysis – Disappearance of Nissl Bodies Golgi Complex – Stained by Silver Osmium method - Network of irregular wavy threads around the nucleus. - Protein storage and where CHO may be added to form glycoproteins - Active in lysosome production and in synthesis of cell membranes neurons, whose neurotransmitter is dopamine. - Important in the formation of synaptic vesicles of the axon terminals Where to find nerve cell bodies with their dendrites?  Cerebral Cortex Mitochondria – important in energy production  Inner gray of Spinal Cord (Dorsal and Ventral Horns)  Clusters: Neurofibrils – May form bundles (neurofilaments) o CNS – Basal ganglia and Thalamus - Form the main component of the cytoskeleton o PNS – Ganglia S1T2 2 of 6  Neuroanatomy Neuron and Neuroglia o Often have small protrusions, called dendritic spines PLASMA MEMBRANE that expand the dendritic surface area.  External boundary of the cell body  Serves as sites of synaptic contact with the  Site of Initiation and conduction of impulse. axon of another neuron. Excitation of the Plasma Membrane ( Nerve Impulse)  Axon – longest process of the cell body.  Stimulation of the Nerve Cells (-80mV) o Generate Nerve impulses away from the soma o Originate from:  + Na Ions diffuse in Plasma Membrane Axon hillock  distal portion  axon terminal  end bulbs  synapse with another neurons  Depolarization (+40 mV) o Devoid of nissl boddies  Varicosite – series of swelling in the terminal  Increase membrane permeability for K + ends resembling string of beads. o Larger diameter – Rapid Impulse  + K Influx o Smaller diameter – slower impulse  Resting Stage  Sarcolemma – the plasma membrane bounding the axon Refractory Period – Duration of non-excitable state  Axoplasm – cytoplasm of the axon o No Nissl granules or golgi apparatus - Influx of Cl - Causing the inhibitory stimuli producing hyperpolarization o Under normal condition, an action potential does not originate on the plasma membrane of the cell body, instead always at the initial segment. o When bundled together:  CNS : Tracts (e.g. Corpus callosum, corticospinal tract) – made up of AXONS only.  PNS: Nerves (e.g. Ulnar nerve)  Tracts – bundle of axons (nerve fiber) within the CNS (connective tissue is absent) o Found in the white matter / spinal cord  Nerves – bundle of axons that extend out from the brain as cranial nerves and from the spinal cord as spinal nerves (surrounded by connective tissue sheaths) Axons are either:  Myelinated – surrounded by insulating sheath that speeds conduction of the electrical impulses  Non-Myelinated – lacking of myelin sheath and thus conduct impulse slowly. IV. SYNAPSES  Interneuronal communication III. NERVE CELL PROCESS Types depending on site  Dendrites – Short processes of the cell body  Axodendritic (most common) o Cytoplasm resembles that of cell body.  Axosomatic o Conduct the nerve impulses towards the cell body.  Axoaxonic o Receptive input region, Electrical signals are  Dendrodrendritc conveyed as Graded Potential o Always Unmyelinated Synaptic Spine – extension of the surface of a neuron from receptive sites for synaptic contact of afferent neurons. S1T2 3 of 6  Neuroanatomy Neuron and Neuroglia Summary Dendrites ( Graded potential) Soma/body (integration) Axons (Action Potential) Axon terminal synapse Peripheral Nerve -chemical transmission -chemical transmission -Axodendritic, axosomatic -Neuromascular Junction Axoaxonic ** transmission in nerve cells occurs along the cell membrane ** within the cell : Electrical transmission ** between the cell : Synaptic transmission Ultra structure of chemical synapses  Apposed surface of the terminal axonal expansion and neurons are called : presynaptic and post synaptic separated by synaptic cleft.  The vesicles fuse with the presynaptic membrane and discharge the neurotransmitter into the synaptic cleft by a process of exocytosis. Action of neurotransmitters  Nerve impulse ++  Ca influx  Fusion of synaptic vesicles with presynaptic membrane  Ejection of neurotransmitter to the ECF in synaptic cleft  Action Neuromodulators - modulate and modify the activity of post synaptic neurons. - Control hemeostasis V. MYELIN PRODUCTION  CNS – Oligodendrocytes Electrical transmission (ionic)  PNS – Schwann Cells  One way transmission along the cell membrane.  Flow of ion along the length of a membrane CNS : Oligodendrocytes  Graded potential (Along Dendrites)  Have processes that form myelin sheath around CNS nerve  Action potential (Along Axons) fibers.  Role in the CNS  Multiple flat process that can coil around up to 60 axons  No chemical transmitter  Both myelinated and unmyelinated fibers are present in  Bidirectional CNS  Nodes of Ranvier are widely spaced Synaptic transmission  No neurilemma (Basal lamina)  Diffuse across space  Chemical transmission Myelin Sheath and Neurilemma Formation (PNS)  Cell to Cell signalling  Whittish, fatty (protein – lipid) segmented sheath around  Axon terminals (Axodendritic / Neuromascular Junction) most long axons S1T2 4 of 6 Neuroanatomy Neuron and Neuroglia  It Functions in:  End in expansions of blood vessels, and found in the outer o Protection of the axons and inner glial limiting membranes. o Electrically insulating fibers from one another  Outer glial – beneath the pia matter o Increase the speed of nerve impulse  Inner glial – beneath the ependyma lining the ventricles of transmission the brain  Formed by Schwann Cells  Functions: o Enveloped an axon with its plasma membrane o Supporting framework for the nerve cells and o Concentric layers of membrane make up the nerve fibers. myelin sheath o Scaffolding for the migration of immature o Wraps only one axon neuron  Neurilemma – remaining nucleus / cytoplasm of Schwann o Electrical insulators cells o Form barriers for the spread of neurotransmitter o Responsible for salutatory conduction substances released at synapses o Store glycogen within their cytoplasm Nodes of Ranvier o Secrete cytokines  Myelin sheath gaps between adjacent Schwann cells, occur o Structure of Blood Brain Barrier at regular intervals.  Sites where axon collaterals can emerge OLIGODENDROCYTE  Unmyelinated axons – Schwann cells enclose axons but no  found in rows along myelinated nerve fibers myelin present.  Functions: VI.NEUROGLIA o Responsible for the formation of myelin sheath o Provides the axons with insulating coat  Non excitable cells supporting the neurons o And increase the speed of nerve conduction  Smaller than neurons, comprise about half the total along the axons. volume of the brain and spinal cord  th Myelination begins at 16 week of intrauterine life  Metabolic and mechanical support  Mitotic MICROGLIA  Embryologically unrelated to the other neuroglial cells and are derived from macrophages outside cells with spine like projections  Functions: o Microglial cell in the normal brain and spinal cord appear to be inactive Resting microglial cells o Immune effector cells o Can become and antigen presenting cells o Actively phagocytic EPENDYMA  Line the cavities of the brain and the centrak canal of the spinal cord  Cuboidal or columnar in shape, contains microvilli and cilia  Cilia contributes to the flow of CSF  Divided into: o Ependymocytes – line the ventricles of the brain and central canal of the spinal cord  In contact with the CSF. Circulate and ASTROCYTES absorbs CSF rd  Small cell bodies with branching processes. o Tancytes – Line the floor of the 3 ventricle  Types: overlying the median eminence of hypothalamus o Fibrous Astrocytes – found mainly in the white  Transport chemical substances from matter CSF to the hypophysial portal system  Long, slender, smooth and NOT o Choroid epithelial cells – Covers the surface of branched containing many filaments the choroid plexus o Protoplasmic Astrocytes – Found mainly in the  The presence of tight junction prevents gray matter the leakage of CSF  Shorter, thicker and branched  Produce and secretes CSF S1T2 5 of 6 Neuroanatomy Neuron and Neuroglia  Functions: o Assist in the Circulation of CSF o Control the hormone production and secretion of CSF from the choroid plexus. EXTRACELLULAR SPACE  Gap separates the neurons and neuroglial cells  Surrounds the blood capillaries in brain and spinal cord  Pathway for exchange in ions between the blood and the neurons and glial cells. REFERENCES th 1. Snell RS,M.D. Clinical Neuroanatomy. 7 ed, updated. Philadelphia: Lipincott Williams & Wilkins, 2010. 2. Powerpoint of Dr. Viado S1T2 6 of 6