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Lecture 5

Lecture 5 Reading Notes.doc

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University of Toronto St. George
Human Biology

Lecture 5 Reading Notes: Electrical Signals In Nuerons: Again to understand: When membrane potential is at rest then its voltage is -70mV which is the normal membrane potential - Depolarization: When the sodium levels are high and the membrane potential Is more positive - Hyper polarization: When potassium levels are high and the membrane potential is highly negative - Repolarization: The membrane returns to the resting membrane potential The Goldman Equation describes the resting membrane potential: There are three factors that help the membrane potential of a cell to work: 1) The distribution of ions around the plasma membrane 2) The permeability of the plasma membrane to the ions so they can pass easily 3) The charge of the ions Gated ion channels allow neurons to alter their membrane potentials: - If the membrane potential is not permeable to the ions that pass by, then the ions wont be efficient to work with the membrane potential and signals wont be sent. - However, if the membrane potential is permeable, then ions will work with the membrane potential to send signals which is important. How do neurons depolarize, repolarize and hyperpolarize? It happens by changing the permeability of the membrane potential by the addition of sodium and potassium ions. NOTE: The changing of the permeability of the membrane potential gives electrical signals IMPORTANT NOTE: membrane channel ( adding sodium and potassium) will only happen if the ligand ( stimulus) attaches to the neurotransmitter!! Fact: when a specific gated ion channel opens, the membrane poterntial becomes more permeable to that ion to any other ions. Equilibrium potential: the membrane potential where the electrical and chemical gradients prefers the movement of a particular ion exactly balance each other, and there is no net movement of that ion across the membrane. (meh) Even though the membrane potential is far from this equilibrium potentials of an ion, this ion will still move to the membrane potential because its electrical chemical force is really large. BUT When membrane potential= equilibrium potential, the ion movement will stop obviously because there is no force to move that ion or any reason actually to move that ion. Studying ion channels: - Voltage clamp: it is used for studying ion channels in a single cell The Voltage clamp experience: Its like the squid experiment: They hold the voltage across a membrane by injecting this voltage using an electrode and this makes the membrane changes every time a signal passes by This experiment is used to explain the electrical properties of the membrane in the cell - Aside to the voltage clamp, there is the patch clamp: It studied the properties of single channels So the different between voltage clamp and patch clamp is that voltage clamp studies a whole region of the cell and a large region of the membrane while patch clamp just studied the region of a single channel. Signal in dendrites and cell body: -Vertebrate motor neurons (afferent) receive signals in the form of a chemical transmitter -Membrane-bound receptors in the dendrites or cell body transduce (convert) this incoming chemical signal into an electrical signal in the form of a change in the membrane potential (Just like the lecture, the chemical signal is changed into a electrical signal when it enters the dendrites) binding of neurotransmitter to a ligand causes ion channels in the membrane to open or close, changing the permeability of the membrane and altering the movement of ions. (LIGAND HAS TO BIND INORDER TO WORK) Change in permeability changes the membrane potential and causes an electrical signal These dendrites and cell body in the membrane potential are called GRADED POTENTIALS Graded potential: it is a change in the membrane potential that varied in magnitude when stimulus occurs and helps in the opening or closing of ion channels Graded potentials vary in magnitude Graded potentials vary in magnitude depending on the strength of the stimulus a strong stimulus, such as a high concentration of neurotransmitter will help the increase of the ion channel opening Graded potentials are short-distance signals Graded potentials can travel through the cell but decrease in strength as they get further away from the open ion channel This is called conduction with decrement Example: a neuron with a ligand-gated Na channel on the membrane When a neurotransmitter (ligand) binds to a ligand-gated Na channel, the channel opens and Na+ ions move into the cell Entry of Na+ = depolarization in a small area of the membrane surrounding the opened channel The positive charge spreads along inside the membrane, causing depolarization. This phenomenon is called electrotonic current spread The graded potential signal gets weaker as it travels Several features of the neuron influence why this happens, including leakage of charged ions across the cell membrane, the electrical resistance of the cytoplasm, and the electrical properties of the membrane. Because graded potentials cannot be transmitted across long distances without degrading, neurons
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