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Lecture 1

# Cellular Physiology Lecture One organized, clearly explained notes on lecture one 'electric currents for communication and ion channels' including introductory lecture notes and basic physics concepts - includes conversions, pictures as well as in-class e

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School
Department
Physiology
Course
Physiology 3140A
Professor
Donglin Bai
Semester
Fall

Description
INTRODUCTION – ELECTRIC CURRENTS FOR COMMUNICATION Electric Potential - - the electric potential (electrostatic potential) at a point in space is the electrical potential energy divided by charge at is associated with the static electric field - Measured in volts - electric field exerts a force on charged objects, accelerating them in the direction of the force - In the same or opposite direction of the electric field - if the object has a positive charge the force and acceleration will be in the direction of the field Electric Current - - the flow of electric charges or the rate of flow of electric charges; in a wire (conductor) its carried by electrons, in an electrolyte its carried by ions and in plasma its carried by both ions and electrons - measured in amperes (charge flowing at one coulomb per second) Electrical Resistance - - a measure of an objects opposition to a steady electric current; an object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross sectional area (and of course proportional to the resistivity of the material) - measured in ohms - resistance is measured as the proportion of voltage to current (R = V/I) Capacitance - - the ability of a body to hold an electric charge; its also a measure of the amount of electrical energy stored (or separated) for a given electrical potential - measured in farads (one coulomb/volt)  Potential (volts) – mV (10-3 V), uV (10-6 V)  Current (amperes) – nA (10-9 A), pA (10-12 A)  Resistance (ohms) - m (10^6), G (10^9)  Conductance (Siemens) – nS (10-9 S), pS (10-12 S) DIRECTION OF ELECTRIC CURRENT - the movement of positive charges - these move from a point of (high) electric potential to low potential - larger potential (voltage) difference, the larger the current - current is also dependent on the medium that is carrying the current (ie smaller dismeter cable offers a higher resistance than a larger diameter cable) Ball Rolling Down Hill Analogy – this is an analogy for current flowing from high to low potential; the height is the potential, the speed of the ball reproduces the current and the surface the ball rolls on is the resistance – here voltage is seen as analogous to pressure and current is analogous to water flow OHM’S LAW (applies to electric circuits) - - the current (I) passing through a conductor is directly proportional to the voltage difference (V) across the conductor and inversely proportional to the resistance (R) of the conductor Therefore I = V/R (or V = IR) and in some cases we use G (conductance) to replace resistance: G = 1/R Therefore ohm’s law can be written I = GV - - - - must know how to convert the units V I R and G for the exams LECTURE ONE – ION CHANNELS What we know about the brain and the spinal cord is that they are the central nervous system. As early as the 1900s we have known that the brain is organized by neurons, and there are many different shapes – unipolar, multipolar or bipolar. Functionally these take different roles. But fundamentally, all brain activity requires ion channels to function. Single Channel Recording was the breakthrough describing how the ion channel works on the cell surface. Cell Membranes: - consist of both the phospholipid bilayer and ion channels made of protein - ions ONLY go through the channels and cannot pass through the membrane itself - ion channels therefore determine resting and active properties of the membrane - recall: K+ high concentration inside and Cl, Na and Ca high concentration outside cell Ion channels have several features: 1) selectivity filter in the pore to allow ions to pass through the cell membrane, meaning they select for one ion specifically – you name the channel according to the filter 2) they have a gate, this controls the opening and closing of the ion channel – 3) this gate has a sensor which senses activity around the ion channel which makes it either open or close Working Hypothesis for Ionic Channels - - ionic channels are transmembrane structures (they span the membrane, sticking out on either side) allowing charged ions to flow through the normally impermeable membrane; they are selective in the sense of only allowing one type of ion through, named according to this selectivity and are dynamically regulated to be open
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