PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26reductionismholismCh. 48: Nervous Systems:(from molecules to mind?)Animal survival and reproduction depends on rapid & flexible responses to changes in the environment.A diversity of nervous systems has evolved in various animal phyla. As in many other organ systems, animal nervous systems are remarkably similar at the cellular level - how neurons work - but differ at higher levels of organization, such as the structure and function of their brains.http://www-gpi.physik.uni-karlsruhe.de/pub/robert/Diplom/node5.htmlIn 1911 Ramón y Cajál came up with the idea of the neuron as the basic component of the brainthe neuron as the basic component of the brain. … a human brain contains some 100 billion ( 1011) neurons with about 1,000 to 10,000 connections each (resulting in 1014 -1015 interconnections). Although one neuron is about 106 times slower than a transistor of a computer the massively parallel processing capability of the brain gives it a much higher efficiency.… it takes about 100-200 msec for the brain to recognize a familiar face … this simple task causes great problems to the computer - if it can do it at all.{efforts at developing AI have been disappointing}Functional organization of fundamental (neural) unitsFunctional organization of fundamental (neural) unitsA Neural Network is an assembly of simple processing elements, whose functionality is loosely based on the neuron. The processing ability of the network is stored in the inter-unit connection strengths, obtained by a process of adaptation to, or learning from, a set of training patterns. {function ‘emerges’ from interactions, that evolve by selection on performance}Effector?A simple functional network:the Postural (knee-jerk) reflexthe Postural (knee-jerk) reflex; Muscle length and velocity are monitored by muscle-spindle stretch receptors. Activation of these receptors initiates the stretch reflexthe stretch reflex: motor neurons of antagonists are inhibited (-) & those of synergists are activated (+). {voluntary contractionvoluntary contraction} Tension on the stretch receptors is maintained by gamma efferent activation of the spindle muscle fibers.{alpha  efferent to power effector muscle, gamma efferent (not shown) to spindle keeps sensor stretched as effector shortens} Alpha and gamma motor neurons are often coactivated {in voluntary movement}. http://www.mhhe.com/biosci/ap/vander/student/olc/index.htm+-recall: herpes emerge fromdorsal root gangliato become shinglesAll cells have a resting potentialresting potential (ionic gradients) across plasma membrane, but neurons have voltage-sensitive permeability (voltage-gated ion channels) Membrane potentials are generated by the diffusion of ions and 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 potassium than to sodium; {see gates Fig 48.9} the membrane potential is close to the potassium equilibrium potential, that is, the inside is negative relative to the outside. {about -70mV} http://www.mhhe.com/biosci/ap/vander/student/olc/index.htmIons cannot dissolve in the phospholipid plasma membrane; they must either be pumped by membrane proteins or diffuse through ion channels, which are aqueous pores made of specific transmembrane protein molecules. These molecular channels are selective for specific ions.Within neurons, graded potentialsgraded potentials integrate inputs integrate inputs (like AM, adding analog signals) and action potentialsaction potentials transmit decisions transmit decisions (like FM, w/o degradation)At an inhibitory synapseinhibitory synapse: an inhibitory postsynaptic potential (IPSP) {hyperpolarization} results when channels to potassium are opened. At an excitatory synapseexcitatory synapse: an excitatory postsynaptic potential (EPSP){depolarization} results when channels to sodium are opened. The postsynaptic cell's membrane potential is the result of temporal and spatial summation of the EPSPs and IPSPs at the many excitatory and inhibitory synapses on the cell. {this integrates informationintegrates information; a ‘decision’ at axon hillock} http://www.mhhe.com/biosci/ap/vander/student/olc/index.htmIn myelinated nerve fibers, APs manifest saltatory {skippy} conduction. {which allows small diameter axons to be fast; invertebrates have ‘giant axons’} http://www.mhhe.com/biosci/ap/vander/student/olc/index.htmMS results from autoimmune destruction of myelinAn action potential (AP)action potential (AP) is a rapid change in the membrane potential during which the potential rapidly depolarizes and repolarizes. {the axon hillock ‘decides’ to fire off an all-or-none AP when local graded potentials integrate to above the threshold,  -50mV} APs provide long-distance transmission of information through the nervous system. {along the axon, w/o degradation}http://www.mhhe.com/biosci/ap/vander/student/olc/index.htmAPs occur in excitable membranes because these membranes contain voltage-gated sodium channelsvoltage-gated sodium channels, which open as the membrane depolarizeswhich open as the membrane depolarizes, {Fig 48.9} causing a positive feedback toward the sodium equilibrium potentialcausing a positive feedback toward the sodium equilibrium potential. {Na+ channels open & as Na+ enters, opposing electrical gradient develops}The signal from a pre- to a postsynaptic neuron is a neurotransmitter stored in synaptic vesicles in the presynaptic axon terminal. Depolarization of the axon terminal, which raises the calcium concentration within the terminal, causes the release of neurotransmitter into the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic cell; the activated receptor usually opens {chemically-gated} ion channels. http://www.mhhe.com/biosci/ap/vander/student/olc/index.htmMost graded