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Download Structure of a Neuron Transmission of “Information” Nerve Impulse
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Recapitulation Nervous Tissue and Peripheral Nerves • Central Nervous System (CNS) – Brain and spinal cord • Peripheral Nervous System (PNS) – Peripheral nerves (cranial and spinal) – Ganglia • Cellular basis – Neurons : Generate and propagate impulses (action potentials) – Neuroglia : supporting cells ANHB 2212 – 2008 Avinash Bharadwaj Structure of a Neuron Transmission of “Information” • Electrical transmission • Body (‘soma’) – Very fast – However, not like conduction through electrical wires! – Main cytoplasmic mass with nucleus • Nucleus euchromatic, nucleolus prominent • Cytoplasm rich rER – seen as granules (Nissl granules or substance) • Chemical messengers • Branched, ‘tree-like’ processes – Dendrites – Bring information towards the soma – Across narrow gaps between cells • Single, long process – – – – Axon Takes information away from the soma Unbranched except at its end Terminal part – swellings Nerve Impulse – Fundamentals • Resting membrane potential – Interior – ve : ‘polarisation’ – Difference approx 70 mV, expressed as –70mV +++ + - - - - - -+++ - - ++ -+ • Action potential – Begins at one point, progresses like a wave – Previously depolarised regions ‘repolarise’. 70 mV More on Processes • Dendrites – Resemble the soma – Can summate small changes • Axons – Long, thin – Can only conduct action potential Exceptions exist! Processes that bring an impulse to the soma, yet structurally and functionally are like axons. 1 Communication Between Neurons Chemical Synapse • Neuron meets neuron! • ‘Synapse’ : junction • Majority of neuronal synapses use a chemical mechanism for transmission. N1 • May be between • Presynaptic membrane • Synaptic cleft • Postsynaptic membrane – Axon and a dendrite – Axon and soma – Axon and axon (Only axo-dendritic shown here) • Depolarisation, e.g. – 60 OR hyperpolarisation e.g. – 85 N2 • Vesicles • Neurotransmitter • Receptors (Not shown) • Great variety of neurotransmitters e.g. Acetylcholine, Noradrenaline, γ-amino butyric acid More on Structure • Myelin sheath of an axon • Schwann cell – – membrane rolled around, myelin deposited Speed of Conduction • • • • Non-myelinated fibres (Ø < 1.5µm) : < 2.5 m/sec Thinly myelinated (Ø < 1 – 3 µm) : 5 – 15 m/sec Medium (Ø < 5 – 15 µm) : 30 – 80 m/sec Thick (Ø < 12 – 20 µm): 70 – 120 m/sec Consider that the longest axons in the body measure about 1 metre! • Nodes of Ranvier (arrows) • Action potential ‘jumps’ from node to node (saltatory conduction) Maintenance Department! • • • • Maintenance of ‘distant’ structures Transport along axons Fast and slow Bidirectional Neurons – Shapes and Sizes – Unipolar / bipolar / multipolar – Stellate / pyramidal … and many more! – 1 to 2 µm… … 25 µm or more 2 Neuroglia Peripheral Nerves • Glia : glue • A variety of supporting cells • In CNS • Nerve fibre bundles with connective tissue • Epineurium • Perineurium • Endoneurium • Myelin sheath • “Axis cylinder” – Astrocytes – Microglia – Oligodendrocytes…. And more! • Peripheral nerves – Schwann cells – myelination • No connective tissue in CNS!! A Typical Spinal Nerve Roots Rami Dorsal Ventral (Motor) Dorsal (Sensory) With ganglion Ventral Autonomic connections … later! Last Slide 3