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PSYC62H3 (143)


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Suzanne Erb

PSYC62: Drugs and the Brain Lecture 3: Neuronal transmission and conduction (Chapter 4 and 5) Overview  Neuronal Conduction Transmission → involved in communication of the CNS  Neuroactive Ligands → major families that occur naturally in the CNS Cells Types in the CNS  Two basic types of cells that comprise the CNS → DIFF looks and functions: (1) Neurons and (2) Glial Cells Neurons  Primarily messengers - main feature to communicate with other cells is due to the axon (the conveyer)  Represent the families of cells that receive info and transmit info  Main feature: the axon represents main conducting of neuron  Billions of neuron that comprise the CNS, it difficult to estimate how many there are Glial Cell  Many more glia cells part for the CNS than neurons (about 10 to 15 more times than the amount of neurons)  Glia form the membrane that surround the BBB -  Functions: metabolic support protect blood barrier, myelin sheath on axons  Involved in scavenging debris and clean up after cell death  They are a major part of the composition ofthe myelin - they guide migration of neurons  Help vesicle support in the cleft  They provide nutrition role  In the past it was thought that they played a pure supportive role but that is no longer the case  play imp role in genesis of many disease states : depression, chronic pain paralysis, Alzheimer's Estimating neuron numbers in the CNS  There are a lot of neurons in the CNS (about 85 billion)  Density varies from region to region  Cerebral cortex comprised to 12-15 billion neuron  The cerebellum comprises of 70 billion neuron  Densely packed in the cerebellum of neurons  Somewhere 80-90 billion neurons that comprise the CNS The Main Features of Nerve Cells  Presynaptic neuron - the neuron that is conveying information to other neurons (the messenger) o Main transmitters of info o The cell that receives the information is then the postsynaptic neuron  Soma (cell body) - main metabolic centre - dendrites and the axon come out of the soma o Gives rise to dendrites and axons  Dendrites - serve as the main receiving apparatus - they contain the binding sites o Main apparatus of input o Info conveyed across synapse activate  Axon - main conducting unit and can be very long (1 meter) o Capable of conveying messages over long distances o Axon give rise to a special region called the axon hillock - the convergence of signal on the axon hillock will be determine whether it will fire a action potential o longer process o Axon cilic: info conveyed here are converged o Action potential: electrical surrounded by myelin sheath  Surrounding the axons are myeline sheath - serves as a insulation and allows the propogation of the transmission to be much quicker  Little gaps in myeline called nodes of ranvier - the action potential jumps across these nodes  The degeneration of the myelin could cause many degenerative diseases such as MS o Speeds up rate of propogation of action potential o Granvier are gaps between the myelin sheath o Axons terminates into terminal branches  At the end of each the termianl endings - the chemicals are stored and released to convey messages to the postsynpatic neuron - neurotransmitters in the terminal endings get's released  Synaptic cleft is the space that sepeartes the terminal edning of the presynpatic neuron and the dendrites of and the postsynpatic neuron receives info Two major processes involved in communication between neurons  Conduction - how action potentials are fired and the principles of action potential generation and propagation (fired) o Action potential is all or none - it's like a gun - when the minimal threshold is met, the action potential is fired at the same rate each time o Once those chemicals bind, they induce changes that are electrical - but the initial is chemical - graded (temporarily changes in potential but those changes can vary considerably and can be inhibitory or excitatory - these processes are intermittently related  Transmission - how cells and neurons talk to each other o Primarily chemical, refer to process whereby chemical released in postsynaptic o Once chemical bind on postsynaptic, cause momentary changed, causes electrical component, communication is chemical o Graded: chemicals act on postsynaptic cause variable changes → variable changes converge at action pillar CONDUCTION TRANSMISSION • Changes within neurons to allow information • Changes within one neuron caused by the transmission within neurons. release of chemicals from adjacent neurons. • Electrical • Chemical • “All-or-none” • Graded Neuronal conduction  Conduction boils down to the relative concentration of 4 ions inside versus outside the cell and shifts  These ions need to be carried through specialized channels (potassium, A (protein ions), Sodium, and Chlorine  When the cell is at rest, more sodium outside, high chloride outside - that relative distribution at rest gives in to polarity - and the inside is slightly more negative inside relative to the outside at rest (by-70mV) - could be because of the large positive sodium outside  Why doesn't the ions diffuse at rest? there are three mechanisms that stop that from happening  Relative distribution of four island on either side of cell membrane  Relative conc. shift when an action potential occurs  Resting potential arise from negatively large potassium Neuronal Conduction: Forces maintaining Resting Potential 1. Selective permeability: the ability of some molecules to pass much more freely through the membrane than others. o Other molecules such as Sodium does not have access to the sodium channels at rest and therefore sodium is Prevented from entering the cell - potassium and chloride can move diffusively o Cell at rest, potassium (small ions), chloride (high concentration outside cell), permit the ion to pass slowly, and relatively slowly o Because one moving in and one moving out of the cell 2. Sodium-potassium pump: transports three sodium ions out of the cell while simultaneously drawing two potassium ions into the cell, to result in a net movement of positive ions out of the cell. o Overall a net output of one positive charge and the selective permeability stop the sodium from coming back into the cell - so this pump and the selective permeability work together o Specialized mechanism in membrane wall that permits 3 sodium ions out and 2 potassium ions inside the cell simultaneously (See image below) 3. Electrical and concentration gradients for potassium: the differences in positive and negative charges and potassium concentrations, respectively, across the membrane o There is a electrical and concentration gradient for potassium at reset - the electrical gradient is more powerful and therefore the concentration of potassium is greater inside the cell than outside the cell o At rest, two gradients are at work on potassium (electric: into the cell, concentration: potassium is high concentration inside than outside and flow outside) o Relatively more potassium inside than leaving outside the cell  If the concentration gradient won out and more potassium cells move out , the then -70 will be much more negative Neuronal conduction Neuronal conduction (image below)  At rest, there are going to be minor fluctuations and move around the -70mV depending on signals reaching the cell  When it's reached at the -60mV, the action potential will fire and every time will have the same magnitude  The first event is the opening of sodium channels so sodium flows into the cell and that causes a dramatic increase in the membrane potential in the positive direction  B - the sodium channels close, the potassium channels open and potassium flow out of the cell and the speed at which it flows out of the cell is so great that we get a reversal in membrane potential and we overshoot and becomes hyperpolarized (much lower than the resting potential)  Two phases whether another action potential can be generated - absolute refractory period (it doesn't matter how much excitatory signals is getting, it will not fire another action potential) - the other phase will allow another action potential but only when there is enough excitatory signals Neuronal conduction: Propagation of an action potential  The signal propagates in a serial fashion from one end to another end and each region is each its own action potential basically Adjacent sections become depolarized  Signal travels along axon and arrive at particular area, sodium rush into cell, and potassium rush out of the cell Two major processes involved in communication between neurons CONDUCTION TRANSMISSION • Changes within neurons to allow information • Changes within one neuron caused by the release of transmission within neurons. chemicals from adjacent neurons. • Electrical • Chemical • “All-or-none” • Graded The main features of nerve cells  There are five steps of neurotransmission: 1. Synthesis 2. Storage 3. Release 4. Postsynaptic reception 5. Elimination Neurotransmission  Neurotransmission involves graded postsynaptic potentials. o Ligand cause momentary changes in the membrane and changes on particular binding  Postsynaptic potentials (produced by ligands) can be excitatory (EPSP) or inhibitory (IPSP) o EPSP - depolorization at the site of action o IPSP - hyperpolarization at the site of action  A neuron’s excitability is determined by the sum of its EPSP’s and IPSP’s at any given time.  EPSP’s and IPSP’s are summated through two process
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