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NROC69H3 (7)

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University of Toronto Scarborough
Rutsuko Ito

Lecture 1 Notes Organizational principles of the mammalian CNS Dealing with complexity 11 12 Human brain: Contains approximately 10 - 10 neurons. Each communicates with 1000-10000 others through synapses How to study the human brain: 1) defining the levels of organization. 2) identifying mechanisms and structures within these levels so that generalizations can be made Systems level of brain organization Serial organization: Information that flows sequentially through different brain structures. For example: Sensory receptors  Thalamus  Primary sensory cortex  Polymodal association cortex  Prefrontal cortex  Premotor cortex  Motor cortex  Spinal motor neurons Parallel organization : information that is broken down / segregated into 'channels' and transmits information in parallel with other distinct aspects of information. Hierarchical organization and independence Hierarchy: Cortical regions assert a top down control over 'subcortical' structures. Lower centers of the brain is still capable of functioning independently Striatalnigralstriatal dopamine pathway: Projections from the shell of the striatum (a major component of the basal ganglia) innervate areas of the ventral tegmental area  which in turn projects 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. In this way, ventral striatal regions influence more dorsal striatal regions via spiraling SNS projections. Topographic organization Topography: Sensory receptors located close together project to neurons in the thalamus and cortex that are physically close together. This is reflected in the sensory and motor homunculi reflected in the sensory and motor cortices, as well as the retinotopic and tonotopic maps of their respective cortices. Neuron - basic structural and functional unit of the CNS Information flow: Generation of action potentials across chains of connected neurons Dendrites: Region where one neuron receives connections from other neurons. Dendrites receive synapses of many types and from many different presynaptic neurons. Cell body / Soma: Contains the nucleus and the other organelles necessary for cellular function Axon: a key component of nerve cells over which information is transmitted from one part of the neuron (usually the cell body) to the terminal regions of the neuron (terminal boutons) . Axons can be rather long, extending up to a meter in sensorimotor nerve cells Terminal bouton / Synapse : The terminal region of the axon where a neuron forms a connection with another and conveys information through the process of synaptic transmission Synaptic transmission: The process through which information is propagated via a presynaptic neuron, the synapse, and a postsynaptic neuron. Types of neurons Projection neurons: Have long axons and project to other areas of the brain (or body). Projection neurons are usually excitatory. Interneurons: Have shorter axons (or no axons) and remain within a specific brain region. Interneurons are usually inhibitory Electrical activity in neurons The membrane potential Electrical potential (Voltage): Generated by a separation of ionic charges across an impermeable phospholipid cell membrane due to a charge gradient across the plasma membrane. The passage of ions across the membrane is permitted only by virtue of ion channels (passive and active) Passive ion channels: Allows for ions to move across the membrane passively (without the expense of energy) due to concentration / electrical gradient Active ion channels: Ions are actively pumped in and out against the concentration / electrical gradient via the expense of ATP (energy) Fick's law of diffusion: 1) Ions will flow down a concentration gradient (high to low) until a point of equilibrium is reached. 2) As ions move down a concentration gradient, the movement of charged ions creates a charge gradient such that the ions will then begin to move in the opposite direction. Equilibrium potential: The point at which electrical and chemical forces are equal and opposite. Also known as the reversal potential for an ion. At this point, there will be no net flow of ions Nernst equation: By knowing the 1) temperature, 2) concentrations of the ion inside and outside, 3) valence of the ion, we can work out the equilibrium of each ion (given the gas constant = 8.315 / faraday's constant = 96.485).  E ion= RT/zF x ln([ion]o/[ion]i Application of Nernst equation: A miniscule change in the outside concentration of K+ causes a huge change in the V :mIncreasing the [K+] outsidey 1 mM further depolarizes the E frKm -103 mV to -88.6 mV. Consequently, increasing the [K+] inside 1 mM yields a non-negligible change in V m GHK equation: Deviations in the Nernst equation attributed to the fact that the cell membrane is permeable to more than one type of ion prompted the need for a more accurate equation derivation. The GHK equation is the weighted average of Nernst potentials for multiple ions, using relative permeability as a weighing factor.  V = RTmF x ln (p [K] +Kp [oa] +Na [Clo / pClK]+ip [Ka]+ip [Na] i Cl o) Application of GHK equation: GHK underlines the importance of relative permeability of the cellular membrane to different ion species in determining the membrane potential. Thus, if the cellular membrane is equally permeable to K+ and Na+ ions, then the V would mosm likely be somewhere between the equilibrium potentials for K+ and Na+ Resting membrane potential Resting membrane potential: At rest, the cellular membrane is dominated by its permeability to K+. However, the membrane potential is NOT equal to the equilibrium potential of K+. This is because the membrane is also permeable to Na+ and Cl- (as calculated by the GHK equation). The resting potential is also maintained by the activity of Na+/K+ ATPase pumps which actively pump out 3 Na+ out and 2 K+ in for each ATP molecule. K+/Na+ levels: K+ levels are kept high internally and low externally. [ACTIVE] / Na+ levels are kept high externally and low internally [ACTIVE]. There also appears to be a constant background 'leak' of K+ ions through K+ leak channels that pass larger outward than inward currents under physiological conditions [PASSIVE] Action potential Action potential: In neurons, electrochemical and physical stimuli can evoke large transient changes in the membrane potential called action potentials or nerve impulses. These impulses act as signals of information in the CNS. The impulses are caused by the movement of ions (mainly Na+/K+) across the membrane to redistribute charge. It is important to note that an action potential generation is primarily driven by electrical changes rather than concentration changes. Peak of action potential: Characterized by Na+ ion permeability (i.e. conductance, g increaseNa. Na+ ions flow INTO the neuron, bringing the membrane potential (V ) closer mo the equilibrium potential of Na+ ions. Due to the influx of Na+ ions, the inside of the cell becomes more positive than the outside (depolarization) Falling phase of action potential: The membrane becomes permeable to K+ ions and K+ ions flow OUT of the neuron, bringing about repolarization (i.e., restoring the negative charge on the inside). This phase is also known as the ABSOLUTE refractory period. Absolute refractory period: When another action potential cannot be generated in the same neuron, however strong the stimulus. APs cannot travel past one another, since they cannot pass through the other's absolute refractory period. Hyperpolarization / Undershoot: Repolarization typically undershoots the resting potential to around - 90 mV, causing the cell to become hyperpolarized as increase in K+ permeability is relatively long lasting and equilibrium potential of K+ is usually lower than the resting membrane potential. This is also known as the RELATIVE refractory period. Relative refractory period: Only a strong, above-threshold stimulus will trigger an action potential. Ion channel activity during an action potential Analysis of ion channel activity: The shape and time course of an AP is determined by the activation and inactivation properties of voltage gated Na+/K+ channels. Both channels open in response to depolarization but have different activation properties. Opening of voltage gated K+ channels are generally slower (characteristic of the falling phase) Importance of Na+ current in AP Na+ channel activation: [STUDY] Reduction in extracellular Na+ concentration reduces the amplitude of the AP. Support to the notion that at the peak of action potential, the membrane becomes highly permeable to sodium. In contrast, Na+ channel inactivation is the basis of the refractory period. Importance of K+ current in AP K+ channel activation: The diversity of K+ channels give rise to diversity in the types of K currents observed in the brain. They are responsible not only for the repolarization of the action potential, bu
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