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UWL BIO 312 - Nervous system II

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Lecture 9Lecture Outline:I Nervous System continueda. Permeability of membraneb. Ion channelsII Types of electrical signalsa. Graded potentialsb. Action potentialsMembrane Potentials- Permeability of membrane to ionso Ions pass through membrane by diffusion only through channel proteins (not through bilayer) Ion channels are ion specific Vast majority are subject to regulation (gated) Several different types of gated ion channels whose opening/closing produce changeso Chemically Gated Ion channels: requires the binding of some specific chemical signal to that ion channel for it to open (neurotransmitter, drug) Involved in graded potentials Four types: cation (Na+, K+), K+, Cl-, Ca +2  Opening of cation channels results in depolarization because gradient for Na is steeper than Ko Voltage Gated ion channels: activated by the voltage of the membrane potential Three types for Na, K and Cao Mechanically Gated ion channels: rely on an outside mechanical force to open Most are cation channels and occur in membrane of certain types of sensory receptorsTypes of Electrical Signals:- Graded (local) potentials: function is to stimulate or inhibit action potential formationo Occur on the membrane of the sensory receptor in unipolar neurons and on dendrites, cell body and axon hillock of multipolar neuronso Can be excitatory (action potential is more likely, causes depolarization) or inhibitory (action potential less likely, causes hyperpolarization)o Graded potential weakens as they move over distance (decremental conduction)o Amplitude of potential is proportional to the intensity of stimuluso Graded potentials can be summated if two or more occur close enough together in time Temporal summation: several stimuli occurring simultaneously to produced summed potential Spatial summation: two stimuli with some space in between to produce a summed potential- Action Potential: long distance signals of the nervous system, used by neurons to communicate with other neurons, muscle, and gland cellso Triggered by gated potentialso Triggered at axon hillock, conducted down axon without decremento Large and rapid depolarization of the membrane followed by large and rapid repolarization Entire event lasts only 2-3 milliseconds Amplitude is about 100 mvBio 312o Results from large and rapid sequential changes in permeability of membrane first to Na+ then to K+1. Depolarization caused by Na+ permeability being increased and diffusing into the cell down its electrochemical gradient2. Repolarization caused by K permeability being increased and diffusing out of cell down its electrochemical gradient3. Hyperpolarization caused by K permeability being increased for a little longer than necessaryo Voltage gated Na+ channel: two doors open at same time, and close fully in about 0.5 millisecondso Voltage gated K+ channel: one door closed at resting potential, action potential triggers opening of K+ channel but is slower and does not fully open until around the time of peak depolarization (from Na channel being opened)o A single action potential has no effect on the ICF and ECF concentrations of Na and K because amount ofions used in potential is so much lower than total amount of ions in the ECF and ICFo Na and K channels can speed up or slow down to control concentrations of ions in ECF and ICFo What triggers an action potential: some stimulus has to depolarize a membrane a certain critical amount which then causes feedback opening of voltage gated Na channels in the stimulated patch of membraneo Threshold potential: amount of depolarization needed to activate positive feedback opening of all voltage regulated Na+ channels in a given patch of membraneo As long as stimulus reaches threshold potential, action potential will always be of the same magnitude (all or nothing)o Refractory period: period of time during which the membrane is unresponsive to a threshold level stimulus. Occurs immediately after action potential starts and lasts until complete repolarization.o Absolute refractory period allows no AP relative refractory period requires stronger than normal stimulus to allow APo Time frame of opening/closing of gated Na channels is what causes absolute refractory periodo Time spent in hyperpolarization is what requires stronger stimulus to reach threshold potentialo Absolute refractory period is what sets limit of amount of APs that can fire per secondo Transmission of APs down the axon: mechanism and velocity depends on myelination of axono Initiation of AP at axon hillock causes chain reaction of permeability and diffusion of ions down the entire length of the axono New action potential generated at each new patch of membrane down the axono Forward current flow occurs because area behind is still in refractory periodo Myelination of axon causes patches where there is no membrane potential essentially shortening the length of the axon to only the areas that are non-myelinated (nodes of ranvier) AP chain reaction travels from node to node through the myelin Requires less initiation of APs Referred to as saltatory conduction (saltare: latin for leap or hop) Larger diameter axon gives less resistance, allows AP to travel


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