bs148h 25 October 2007Read: Text ch 48 • nervous systems• neurons• info processing - reflexes • resting & action potentials• synapses & mechanisms• transmitters• autonomic NS • CNS• depression• addiction• synaptic plasticity & learning• sex differencesNervous Systems:Nervous Systems:(from molecules to mind)Animal survival & reproduction depend on rapid & flexible responses.A diversity of nervous systems has evolved in various animal phyla. {except sponges}As in many other organ systems, animal nervous systems are remarkably similar animal nervous systems are remarkably similar at the cellular levelat the cellular level -how neurons workhow neurons work –but differ at higher levels of organization, such as the structure and function of their brains.CNSPNSPNSulnarulnarnerve nerve relays sensation from the little & ring fingers and activates many of the small muscles in the hand. If pinched against elbow - ‘crazy bone:crazy bone:’ depolarizes all axons; {later: can action potential go backwards up axon? – across synapse?}Myelinated CNS = ‘white matter’unmyelinated CNS = ‘gray matter’oligidendrocytes & glia: CNS neuroglia → myelinSchwann cells: PNS neuroglia → myelinPeripheral nervesPeripheral nerves are cable-like collections of axons, electro-chemically isolated inside myelin sheathsNeuronsNeurons& nerves& nervessynapseMultiple sclerosisMultiple sclerosis (MS) results from autoimmune destruction of myelin;impulse speed is reduced; electro-chemical ‘cross-talk’ between axons.Neuronal RoadmapNeuronal RoadmapAs the neural system develops, a distinctive network of interneuron connections is created.Neural circuit formation requires an intricate orchestration of Neural circuit formation requires an intricate orchestration of ... ... cell migration, axon guidance, cell migration, axon guidance, dendriticdendriticgrowth, growth, synaptic target selection, and synaptic target selection, and synaptogenesissynaptogenesis.Colón-Ramos et al. (p. 103) find that, in the nematode worm C. elegans, the supporting the supporting glialglialcells provide the requisite road map cells provide the requisite road map for making these connectionsfor making these connections. ... consistent with observations made in vertebrates consistent with observations made in vertebrates and highlights the importance of and highlights the importance of glialglialcells cells in specifying precise neural connectivity.in specifying precise neural connectivity.Glia Promote Local SynaptogenesisThrough UNC-6 (Netrin) Signaling in C. elegansDA Colón-Ramos et al. 2007. Science 318: 103 – 106.Glial cells are important in neural development and neural activityA cultured sensory neuron extending a growth conewith long thin filopodia, photo by Ken BalazovichMuscle length & velocity are monitored by musclemuscle--spindle stretch receptorsspindle stretch receptors. Activation of these receptors initiates the postural reflexthe postural reflex: motor neurons ofsynergists are activated (+) & those ofantagonists are inhibited (-).+-αVoluntary contractionVoluntary contractionDendrites of alpha (α) motor neuroncan have ~50k incoming synapses from brain.Gamma neuronsto spindles (not shown)inhibit the postural reflex,allowing muscle contraction.A simple functional network:the Patellar postural reflexthe Patellar postural reflex; latent chicken pox emerge from dorsal root gangliadorsal root gangliato become shinglesSo, how can you move?http://en.wikipedia.org/wiki/ReflexAdult human reflexesBaby reflexes not seen in adults All cells have a resting potentialresting potential (ionic gradients) across the plasma membrane,but neurons have voltage-sensitive permeability (voltage-gated ion channels) Membrane potentials are determined by (a) the ionic concentration differences across the membrane, and (b) the membrane's relative permeabilities to different ions. Plasma membrane Na,K-ATPase pumps maintain intracellular sodium concentration low and potassium high. In almost all resting cells, the plasma membrane is much more permeable to K+than to Na+;the membrane potential is close to the Kthe membrane potential is close to the K++equilibrium potentialequilibrium potential, that is, the inside is negative relative to the outside. {about about --70mV70mV}http://www.mhhe.com/biosci/ap/vander/student/olc/index.htmWater-soluable Ions cannot dissolve in thephospholipid plasma membrane; they must either be pumped by membrane proteins ordiffuse through ion channels, which are aqueous pores made of specific transmembraneprotein molecules. These molecular channels are selective for specific ions.The K+& Na+gradients are created by ATP-driven sodium-potassium pump and some back-flow diffusion through ion leak channelsleak channels (more by K+).Within neurons, graded potentialsgraded potentialsintegrate inputsintegrate inputs (like AM, adding analog signals)and action potentialsaction potentialstransmit decisionstransmit decisions (like FM, w/o degradation)At excitatory synapseexcitatory synapse: an excitatory postsynaptic potential (EPSPEPSP) {depolarization}{depolarization}results whenligandligand--gated sodium channels are openedgated sodium channels are opened. (ex by glutamate)At inhibitory synapseinhibitory synapse: an inhibitory postsynaptic potential (IPSPIPSP) {{hyperpolarizationhyperpolarization}}results whenligandligand--gated chloride channels are opened. gated chloride channels are opened. (ex by glycine or GABA)The postsynaptic cell's membrane potential is the result of temporal & spatial summationtemporal & spatial summation of the EPSPsEPSPs and IPSPsIPSPsat the many excitatory and inhibitory synapses on the cell.This integrates informationintegrates information; a a ‘‘decisiondecision’’at axon hillockat axon hillockcorresponds to whether depolarization >depolarization >threshold ~threshold ~--55mv55mv;if yes, triggers +FB opening of voltagetriggers +FB opening of voltage--gated Nagated Na++--KK++channelschannels. An action potential (APaction potential (AP; 2-5)) is a selfself--propagatingpropagatingunidirectional waveunidirectional waveof opening & closing of voltage-gated Na+and K+ion channels, In myelinatedmyelinatedaxonsaxons, APsmanifest saltatory {skippy} conduction;allows small diameter axons to be fast; myelin lost in multiple sclerosis (MS)Action potentials:Action potentials:+FB opening of voltage+FB opening of voltage--gated Nagated Na++& K&