Organismal Lec 2 01/20/00 Lect. 2: Nervous System I - Resting Potential
ASST.: READ CH. 5 and Ch 6 pp.163-174
I. Nervous System provided rapid communication throughout body and interaction with environment A. For rapid homeostatic control B. For conscious behavior II. Behavior is dictated by Nervous system A. Everything that is an animal’s behavior is determined by neural activity B. Basic unit is Nerve Cell 1. Pattern of connections with other nerve cells provides 2. Information processing 3. Patterns of activity for Behavior 4. Modulation of those connections Æ Modified behavior C. To understand how nervous system does all this must understand electrical properties of cells and their connections III. First some definitions A. Current (I) 1. Flow of electrical charge 2. Can be electrons (neg. current) 3. Can be positive ions (pos. current) 4. Although physicists generally refer to negative current flow, physiologists prefer to talk about positive ions 5. In analogy to flow of water through a hose, this is the water. B. Resistance (Ohms) 1. Resistance of current 2. In analogy to water, this is diameter of hose; i.e. how much hose restricts flow of water C. Voltage (V) 1. Force which drives current through a resistor 2. Force drops off as current goes through the resistor (IR drop) 3. Sometimes referred to as POTENTIAL DIFFERENCE; i.e. the potential force to
Page 1 Organismal Lec 2 01/20/00 move current 4. In water analogy, this is difference in pressure between two points, can be due to difference in elevation between two points or pressure due to a pump a) Difference is low in gentle stream 5. Difference is very high at water fall 6. In voltage source there is charge separation as in a battery D. Conductor: Structure that allows current to flow easily E. Resistor: Element that reduces current flow F. Insulator: Element that prevents current flow (infinite resistance) G. Capacitor 1. Element that stores charge for current flow 2. In water analogy this is a weak spot in the wall of a hose 3. When pressure is increased all of it is not translated to end of hose 4. Some goes to bulge out wall 5. This stores pressure which can be used when pump is turned off to maintain pressure for short time H. Ohms law 1. Relationship of Current, Resistance and Voltage 2. V = IR IV. Nerve cells use Electrical signals for information A. Electrical potential already present across cell membrane of all cells 1. signals very fast 2. with amplification stations, can go long distances 3. various wave shapes can transport all kinds of information B. Source of MEMBRANE POTENTIAL 1. KCl placed in solution will eventually diffuse to uniform concentration a) Diffusion (1) Molecules are in constant motion (2) this is thermal energy; i.e. heat (3) In a concentration gradient (a) motion in both directions (b) more molecules to move down conc. grad. than up (c) so there is flux in both directions (d) however Net Flux is down conc. grad.
Page 2 Organismal Lec 2 01/20/00 (e) Ultimately molecules will be uniformly distributed in equilibrium
K K
Net Flux
(4) Rate of Diffusion (a) Fick's Law describes factors that affect diffusion rate
(1) C1 - C2 (2) (A) = Cross-Sect. Area
(3) Distance (L) (4) Temperature (5) Particle Size (D)