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Lecture 5

PSYCH 1XX3 Lecture 5: Neuroscience I

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McMaster University
Joe Kim

Neuroscience I UNIT 1: INTRODUCTION TO NEUROSCIENCE  Descartes’ dualistic perspective: o Mind was seen as a separated entity existing outside of our biology, yet in control of our actions and thoughts Neuroscience studies how the biological nervous system produces mental processes.  Challenge – to understand how the biological brain produces the mental processes of the mind UNIT 2: THE NEURON Neurons are communication cells; the receptive zone receives neural signals and the transmission zone passes them on to other cells.  100 billion neurons in brain are organized into signaling pathways to communicate via synaptic transmission  Receptive zone and dendrites: receive signals from other neurons  Transmission zone and the axon: pass signals to other cells  Receptive zone starts with cell body  Cell body - # of projections called dendrites, which look like long, stretching branches of a tree  Dendrites reach out to other neurons and receive signals to be relayed through dendrites branch to cell body The neural signal is transmitted from the axon to its terminal boutons, and further to the receptive zone (dendrites) of nearby neurons  Once neuron receives signal, passed down along fiber called axon  Terminal boutons reach out and make connections with receptive zone of neurons  Glial cells: important supporting cast which are found throughout nervous system, provides structural support, nourishment and insulation  Work with neurons, resting in a bath of ions, chemicals and blood vasculature make up entirety of brain Neural communication requires that certain ions move across the cell membrane depending on the charge of the cell  Selective permeability: preferentially allowing different ions to pass through with various levels of ease  Protein channels: act as passage ways for ions to pass through (important: potassium and sodium channels)  Resting potential (-70mv): add up all charges, the starting baseline for the differing concentrations of ions produces an electrical imbalance between the outside and the inside of the neuron  Electrostatic force (chloride): the repulsion between ions of the same charge. One of forces controlling resting potential of neuron  Diffusion (sodium): tendency for molecules to distribute themselves evenly through a medium. One of forces controlling resting potential of neuron Movement of potassium ions (2) 1. Leaky K+ channel: tab that is always open, allowing positively charged potassium to pass through cell membrane out of neuron 2. Voltage gated K+ channel: closed in resting state of neuron and positively charged sodium ions flow into cell in only very low concentrations An action potential is an all-or-nothing event.  Resting voltage of neuron constantly fluctuating somewhere -70mv  Under influences of nearby neurons and random ion flow, large enough in resting charge can occur to reach an important threshold level at -50mv (action potential triggered) Upon reaching threshold, an action potential will be triggered and continue until completion.  The rise o Na+ influx? Channel? Force?  At this point, most Na ions piled outside of cell  Channels now open, force of diffusion causes positively charged Na ions to begin rushing into neuron, causing charge on inside to increase o K+ outflux? Channel? Force?  + charged ion builds up inside cell to point where voltage gated K+ channels open which allow more K+ ions to rush out the cell  Na+ channels are closing  The fall o K+ fast outflux? Overshoot towards -100mv?  Inside of the cell begins losing positive charge and continues to fall and overshoot baseline (-70mv); at this point, the voltage gated potassium channels have completely closed During the refractory period, the neuron is unable to fire another action potential until the cell becomes more positive.  Peak at +40 mV? VG – Na+ channel? o Na+ channels close. Means that Na stops entering cell but let continues rush outward through still open voltage gated K+ channels o Rush of ions complete, cell slowly returns to -70mv and short refractory period occurs, where neuron cant fire another action potential until it settles and recovers from previous cascade The sodium-potassium pump helps to maintain ion balance and restore resting potential following an action potential  Has the role of removing Na from cell and replacing K+  Speed and energy? o Moves slowly and uses extensive energy, playing little role in action potential  Role: o Expels 3 Na+ from intracellular fluid and replaces them with 2 K+ Action potentials cascade via saltatory conduction to neighbouring neurons’ dendrites  Through this process, a signal can travel through a long axon very rapidly without any loss of strength  Myelin: special glial cells coat many axons with a type of fatty, insulating tissue  Saltatory conduction: when action potential reaches a myelin sheath, it jumps across it  Nodes of Ranvier: as the electrical signal jumps through the myelin sheath, it weakens. The signal can be strengthened again through ion channel cascades before continuing along and jumping throug
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