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Download 17-01-05 1 Golgi - stained neurons Neuronal function
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17-01-05 Golgi - stained neurons Neuronal function is intimately related to structure and structures are fantastically complex. Your brain is an organ but it is not a glorified liver. Externally recognizable parts of a typical neuron: - Soma (cell body) - Axon and axon collaterals (usually but not always) Dendrites (spiny or smooth) Presynaptic release sites (terminals, boutons and non-localized release sites) 1 17-01-05 Someneuronalmorphologiesfoundinthehumannervoussystem Cell body: also called “soma” plural “somata” main metabolic centre of neuron main (but not only) site of protein synthesis lots of mitochondria lots of endoplasmic reticulum size in vertebrates: small: 8 µm e.g. granule cells in cerebellum large: 50 µm layer V motor cortical neurons largest: 200 µm Mauthner cell in fish brainstem size in invertebrates small: 8 µm procerebral neuron in slug procerebral lobe GIANT: >600 µm R2 neuron in Aplysia (sea slug) abdominal ganglion 2 17-01-05 Dendrites: - arise from cell body - primary sites of input from other neurons: # of inputs to cell determined by size and complexity of the dendritic “arbor” - can release transmitter in some neurons - contain ribosomes which means they can potentially make proteins locally! - vary in diameter… generally thick proximal, thinner distally -contain lots of actin Dendrites: - arise from cell body - primary sites of input from other neurons: # of inputs to cell determined by size and complexity of the dendritic “arbor” - can release transmitter in some neurons - contain ribosomes which means they can potentially make proteins locally! - vary in diameter… generally thick proximal, thinner distally -contain lots of actin Axons: - arise from cell body (vertebrate neurons) - specialized for relaying electrical signals - vary in length … a few 10’s of microns, to many cm. - contain microtubules and microtubule binding proteins - relatively constant diameter in any neuron - always have specialized areas that release neurotransmitter -- terminal or en passant 3 17-01-05 axon in red, dendrites in blue An example of a local interneuron… axon branches only near the soma These structural features: axons and dendrites serve the primary functions of neurons to generate, send, receive, and integrate (process) electro-chemical signals that encode information. The physiolgical processes required for neural communication will be the focus between now and the midterm exam Neurons communicate with each other via synapses Where are they? What are they? How do they work? 4 17-01-05 It’s time to start looking at neuronal structure at a higher level of resolution – submicron scale (electron microscopy); micron scale (confocal, multiphoton, widefield microscopy 5 17-01-05 axon dendrites axon Presynaptic transmitter release site 6 17-01-05 Transmitter released onto receptors 15nm away across a synaptic cleft “Directed” synapses axon Axo-dendritic Axo-somatic Axo-axonic dendro-dendritic dendrite “Non-directed” hormonal/neuromodulatory axon Transmitter released onto receptors 1µm to mm’s away axonic (e.g. serotonergic brainstem neuron endings in cortex) dendritic (e.g. dopaminergic ventral tegmentum neurons) Dendrites of vertebrate central nervous system neurons have dendrites vertebrate brain are “spiny” smallMany branches, oftenin with small “spiny” protrusions Nimchinsky, Sabatini, Svoboda, 2002 Ann Rev Physiol 64:313-353 7 17-01-05 An example of a “structure-function” relationship: Dendritic spines and compartmentalization of calcium influx. Excess pumped out Only a little Ca escapes the spine to the dendrite Most is bound to Ca buffers, e.g. parvalbumin Ca2+ Enters through ligand-gated or voltage-gated channels Spine can compartmentalize biochemical processes Spine head can also produce amplification of local voltage changes if voltage dependent channels are in spine head. Axo-dendritic synapses Enpassantsynapse 1 µm Fiala and Harris,1999 In: Dendrites, Stuart, Spruston and Hauser eds. 8 17-01-05 Circuits are made from connected neurons Convergence…..defines by the number of inputs to a single neuron. e.g. purkinje cell up to 100,000 Divergence …. number of targets innervated by one neuron Neurons connect via synapses to create circuits: Monosynaptic, disynaptic, polysynaptic, feedforward and feedback circuit elements (draw on board….) 9