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Lecture 1.docx

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Michael Inzlicht

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Lecture 1: Organizational Principles of the Mammalian CNS o The human brain is highly complex and contains approximately 10^11 to 10^12 neurons, and each communicates with 1000 – 10,000 others through synapses  Thus, the brain contains roughly 10^15-10^16 synapses that produce an incapable number of neuronal ensembles or circuits  The brain is highly organized and there is a map of interconnected areas in the visual cortex of a monkey alone o The study of complex systems requires two steps: 1. Defining levels of organization 2. Identifying recurrent themes, motifs, structures, and mechanisms within these levels so that generalizations can be made o In order to understand the brain, you must consider all the levels of brain organization.  Next step is to find repeating patterns or mechanisms within each level from which to draw generalizations  These circuit types are found in many different brain regions and across different species – thus we can study the brains of mice, rats, worms, frogs, etc. a. Behavioral systems b. Interregional circuits c. Local (regional) circuits d. Neurons e. Dendritic trees f. Synaptic microcircuits g. Synapses h. Molecules and ions i. Genes o There are two system levels of brain organization: serial and parallel organization 1. Serial organization – information flows sequentially through different brain structures in series.  Example: trains of action potentials encoding the visual field travel from the retina, to the thalamus, to the primary visual cortex, to visual association cortex and so on 2. Parallel organization – information segregated into “channels” that transmit information in parallel. These channels convey distinct aspects of the information.  Parallel cortico-striatal-thalamic loops, which sub serve motor, spatial, visual and affective information processing, also exist o Another principle of brain organization is that of hierarchy.  Generally cortical regions are thought to assert a top-down control over “subcortical” structures such as the basal ganglia and thalamus when situation calls  Lower centers of the brain can function independently as well, many behaviors that are automated and habitual do not require the engagement of higher center of the brain  Example: walking mediated by spinal reflexes with minimal input from higher centers – frees the brain for other activities  Hierarchy can be seen between different neural circuitries as well such as the strialnigralstriatal dopamine pathways  Forms closed striatonigralstriatal loops, prjections from the shell of the striatum (major component of basal ganglia) innervate areas of the ventral tegmental area that in turn project to the striatal region adjacent to the shell, the core region.  Core in turn projects areas of the substantia nigra which then sends a dopamine projection to the more dorsal parts of the striatum  Thus ventral striatal regions influence more dorsal striatal regions via spiraling SNS projections o The brain is also organized in a topographical nature of the projection patterns between brain regions  Means that sensory receptors located close together project neurons in the thalamus and cortex that are also physically close together a. Somatotopic map – somatosensory cortex has a topographical representation of the whole body b. Motor – motor neurons controlling closely spaced muscles are located together in adjacent areas of the motor cortex c. Retinotopic map – retina projects to this map of the visual field of both the thalamus and the visual cortex d. Tonotopic map – auditory cortex is organized in this map, where neurons sensitive to sequential frequencies of sound are arranged  Homonculus – topographical representation of the body in the motor cortex or somatosensory cortex o Neuron is the basic structural and functional unit of the CNS – contain basic features: 1. Dendrites – the region where one neurons receives connections from other neurons 2. The cell body (soma) – contain the nucleus and the other organelles necessary for cellular function 3. Axon – key component of nerve cells over which information is transmitted from one part of the neuron (ex. cell body) to the terminal regions of the neuron  Axons can be rather long extending up to a meter or so in some human sensory and motor nerve cells 4. Terminal bouton (synapse) – terminal region of the axon and it is here where one neuron forms a connection with another and conveys information through the process of synaptic transmission o There are several different types of neurons within the brain  Neurons come in a myriad of sizes and shapes and can be grouped into a few broad categories: projection neurons and interneurons  Projection neurons – have long axons and project to other areas of the brain (or anywhere in the body)  Usually excitatory  Interneurons – have shorter axons (or no axons) and remain within a specific brain region  Usually inhibitory (but there are exceptions)  Neurons may also be grouped by morphological similarities o Important to understanding neural organization is the concept of information flow which is done through the electrical activity in neurons  All cells have an electrical potential (voltage) across the plasma membrane due to a charge gradient, generated by a separation of ionic charges across an impermeable phospholipid cell membrane  Passage of ion across membrane is conduced through ion channels, some of which are passive and some of which are active (ions are pumped in/out against a gradient)  If the cell membrane was only permeable to one type of ion (assume all passive) then two forces would determine the movement of those ions across the cell membrane: electrical and chemical a. Fick’s law of diffusion – ions will flow down a concentration gradient (high to low) until a point of equilibrium b. Ions move down a concentration gradient – movement of charged ions creates a charge gradient such that the ions will then begin to move in opposite direction  Point at which electrical and chemical forces are equal and opposite is called the equilibrium potential (reversal potential) – for that ion, and at this point, there will be no net flow of ions  For potassium ions, the equilibrium potential is -103mV c. Additional forces that determine passive distribution of ions across the membrane – include: ambient temperature of the solution in which the cell is bathed, and the energy associated with separating given quantity of charge (Faraday’s constant)  Taking all these factors to consideration, Nerst created the “Nerst equation” in 1888 to describe the membrane potential (Vm) at which the net flow of a particular ion is zero (Eion)  IF you know the temperature, concentrations of the ion inside and outside, and valence of the ion, we can work out the E for each ion.  What will happen if we raise the concentration of the potassium ions outside by 1mM?  A small change in the outside concentration of potassium causes a huge change in Vm!  What will happen if we raise the concentration of the potassium ions inside by 1mM?  A small change in the inside concentration of potassium does not cause such a huge change in Vm.  Note that any change in Vm driven by concentration changes will in turn drive ions to move the membrane at a rate proportional to the difference between the membrane potential and the equilibrium potential (electrical gradient)  Experimental evidence for these concepts came in the 1950s when scientists isolated a giant axon of a squid to investigate the ionic basis of the membrane and action potential  Rolling out the content of the axoplasm – established the concentrations of various ion species in the squid axon to be as follows  Hodgin and Keynes (1954) systematically changed the concentrations of potassium ions in the bathing and measured the resultant changes in Vm of the giant axon  They predicted that Vm changes in accordance with the Nerst equation, but what they saw in reality was a deviation from the predictions of Nernst equation  Deviation can be attributed to the fact that the cell membrane is not just permeable to potassium ions, but also to other ions – sodium and chloride o In order to reflect the fact that the weighted mixture of all the ionic currents flowing across the membrane determines the membrane potential at any one time, the Goldman Hodgkin-Katz equation was formulated.  The GHK is essentially the weighted average of Nernst potentials for multiple ions, using relative permeability as weighing factor  Underlies importance of relative permeability of the cellular membrane to different ion species in determining membrane potential o When a neuron is at rest – cellular membrane dominated by its permeability to potassium ions.  The membrane is also permeable to sodium and chloride ions, and the resting membrane potential is pulled towards ENa and ECl.  Resting potential is also maintained by the activity of sodium-potassium ATPase pumps which actively pump out 3 molecules of sodium and 2 potassium molecules in for each ATP molecule, and potassium levels are kept high internally, and low externally.  There also appears to be a constant background leak of potassium ions through potassium leak channels that pass larger outward than inward currents under physiological conditions  Values of resting membrane potentials differ between neuronal types i. Cortical pyramidal cells = -75mV ii. Thalamic relay neurons = -65mV to -55mV iii. Retinal photoreceptor cells = -40mV o In most cells, the resting membrane potential is not changed or is changed only slightly by physiological stimuli. In neurons, however, chemical, and physical stimuli evoke large transient changes in the membrane potential called action potentials or nerve impulses  These changes in membrane potential act as signals of information in the CNS which are caused by movement of ions across the membrane to redistribute charge  At the peak of the action potential, the membrane becomes permeable to sodium ions and the sodium ions flow IN to the neuron, bringing the membrane potential (Vm) closer to the equilibrium potential of sodium ions  The influx of sodium ions, the inside of the cell becomes more positive th
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