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Lecture

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Department
Neuroscience
Course
NROC69H3
Professor
Michael Inzlicht
Semester
Winter

Description
LECTURE 1  The human brain has about 100 millions of neurons and each of those neurons connect with 1000-10,000 other neurons through synapses  Therefore, the brain contains about 100 quadrillion synapses and they can produce incalculable number of neuronal ensembles or circuits  By looking at the monkey‟s visual cortex connections, we can conclude that the brain is highly organized  All neurons are interconnected with each other Levels of brain organization: 1. Behavioral system 2. Interregional system 3. Regional circuits 4. Neurons 5. Synapse 6. Molecule/ion channels 7. Genes  Every neuronal circuit is unique and they can be classified into certain types  These circuits can be found in different brain regions and across different species System level of brain organization  Serial organization is when information flows sequentially through different brain structures in series Example: action potentials encoding from visual field to retina, to thalamus, to primary visual cortex to visual association cortex  Parallel organization is when information is segregated into “channels” that transmit neuronal information in parallel Example: parallel becomes important in vision  it is when brain divides what it sees in four components: color, motion, shape, and depth. These are individually analyzed and then compared to stored memories, which helps the brain identify what you are viewing. Brain will then combine all of these information into the field of view that you see and comprehend  There‟s also parallel cortico-striatal-thalamic loops, which subserves motor, spatial, visual and affective information processing  Hierarchical organization and independence is generally thought as the cortical regions are assert a top-down control over “subcortical” structures However, it doesn‟t mean that the lower centers of the brain can‟t function independently  many behaviors that are habitual or automated do not require the engagement of higher center of the brain For example: “walking” has minimal input from higher centers and walking is mediated by spinal reflexes and its automated, it is needed since it will free the brain to work for other activities like **chewing gum while walking. ** Hierarchical can be seen in different brain regions but also in different neural circuitries such as stritalnigralstriatal dopamine pathways as well as forming closed striatonigralstriatal loops, projections from the shell of striatum ( major input of basal ganglia) innervate areas of the ventral tegmental area that is turn project to the striatal region adjacent to the shell, the core region. The core in turn projects to areas of the substantia nigra which then sends a dopamine projection to the more dorsal parts of the striatum. This way the ventral striatal regions influence more dorsal striatal regions via spiraling SNS projections. Refer to the diagram  Topographic organization, which simply means that the sensory receptors located close together as in the touch receptors in adjacent areas of skin projects to neurons in the thalamus and cortex that are also physically close together Example: Homonculus 1. The somatosensory cortex has a topographical representation of the whole body, forming a somatotopic map of the body surface 2. Motor neuron controlling closely spaced muscles are located together in adjacent areas of the motor cortex 3. The auditory cortex is organized as a tonotopic map where the neurons sensitive to sequential frequencies of sounds are arranged in order Circuit organization One neuron excited and activates the next neuron Inhibitory neuron in the middle, inhibits the next neuron attached, instead of exciting it Convergence: many neurons come to one neuron or Divergence: one neuron projects to many neurons the lateral inhibitory neurons inhibits the lateral neurons Excited neuron, excites the next neuron, the excites a inhibitory neuron that inhibits the first excitory neuron from getting excited * common Excitory neuron excites the next neuron and the post neuron then excites the pre neuron Local circuit organization in hippocampus Basic function of a neuron  Dendrites are the region where one neuron receives connections from other neurons  The cell body or the soma contains the nucleus and other organelles necessary for cellular function  The axon, is where the information is transmitted from one part of the neuron to the terminal region of the neuron; can be up to a meter long  The terminal bouton or synapse is the terminal region of the axon and its where one neuron forms a connection with another and conveys information through the process of synaptic transmission Types of Neuron  Neurons can come in many different sizes and shapes and can be grouped into a few broad categories  Basic division : projection neurons and interneurons  Projection neurons have long axons and projects to other areas of the brain or anywhere in the body ** usually excitory  Interneurons have shorter axons and remain within a specific region of the brain ** usually inhibitory Electrical activity in Neuron  Every neuron have a separation of electrical charges across its phospholipid membrane  the extracellular fluid has excess cation and the intracellular fluid has extra anion and they are both separated by the plasma membrane  An unequal distribution of the charges across the membrane results in a membrane potential (voltage)  The passage of ions across the membrane is permitted by virtue of ion channels, some of which are passive (allowing ions to passively move across a gradient ** no energy is used, concentration gradient allow the movement of ions) and some of which are active (ions are pumped in/out against a gradient ** energy needed since ions move low conc. to high conc. against gradient)  According to Fick‟s law of diffusion  ions will flow down a concentration gradient high to low until a point of equilibrium is reached  Through diffusion K+ move outside of the membrane, and as a result causing the inside of the membrane a negative charge  as the inside become more negative, the negative electrical charge will pull the k+ inside the cell again  eventually the counter balance equals the same force of diffusion pulling the K+ out  equilibrium potential despite the concentration gradient  The charges at which the electrical and chemical forces are equal and opposite is called the equilibrium potential and there will be no net flow of ions.  K+  E is -103 mv  Nernst equation :  If [k] out is raised then the Vm will have a large change however, if the [k] in is increased then there‟s very small change in the Vm  Any change in the Vm will drive K+ ions to move across the membrane at a rate proportional to the difference between the membrane potential and the equilibrium potential  larger movement will cause larger difference btw equilibrium potential and membrane Potential  Squid Vm  measure and predicted vm difference since the cell membrane is permeable to other ions as well but mostly Na+  GHK equation weighted average of Nernst potentials for multiple ions using relative permeability as weighing factor - P represents the relative permeability of the membrane  GHK underlies the relative permeability of the cellular membrane to different ions species to determine the membrane potential therefore, if the cellular membrane is equally permeable to K+ and Na+ ions, then the Vm would be probably between the equilibrium potentials for K+ and Na+ Ohm‟s Law Resting membrane potential  Point at which there is no net current flow across the membrane  At resting membrane potential  cellular membrane dominated to K+ permeability  The resting potential is also maintained through the activity of Na/K+  It is done through ATPase pumps which actively pump out 3 molecules of Na+ and 2 K+ molecules for each ATP  K+ levels are kept high internally and low externally  There‟s a constant background „leak of K+ ions through K+ leak channels‟ that pass larger outward than inward currents under physiological condition  Cortical pyramidal cells = -75 mv; thalamic relay neurons = -65 mv to -55 mv ; retinal photoreceptor cells = -40 mv Action potential  In neurons  electrical, chemical and physical stimuli can evoke large transient changes in the membrane potential called action potential or nerve impulses  These impulses are caused by movement of ions across the membrane to redistribute charge  At the peak of actions potentials Na+ becomes permeable  The fundamental task of a nerve cell is to receive, conduct and transmit signals  Neurons propagate signals in the form of action potential  The actions potentials are transported through the axons  The membranes in the axons are Na + voltage gated channels  When an action potential passes by, sodium channels open and influx of Na+ ions occur, the inside of cell becomes more positive than the outside depolarization  Na+ ions rush into the axon, and further depolarizing its membrane  The Na+ channels then close into a inactivated state but also refractory to reopening  the falling phase of ions flow out of the neuron, bringing the repolarization, restoring the negative charge on the inside ; also known as absolute refractory period  no other action potentials c
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