PT15 LEC: Membrane & Action Potentials

charge of ECF and ICF at rest

positive, negative

electrophysiology

study of the electrical activity of the cells

electrical potentials

- difference in the concentration of charged particles between one point & another
- form of potential energy that can produce a current

nerve cells

highly specialized to transmit nerve impulses from one part of the body to another

functional properties of nerve cells

irritability and conductivity

voltage

electrical potential energy from separation of oppositely charged particles (ions)

resting membrane potential

charge difference across the plasma membrane

diffusion potentials

- created by conc gradient of ions across membrane
- block further net diffusion of K+ out of the cell

membrane potential

voltage across a membrane that exists due to unequal distribution of ions between ECF & ICF

current

flow of charged particles from one point to another

Nernst Potential

- explains relationship of diffusion potential & concentration gradient
- only for selectively permeable membrane with single transport of ions
- the greater the ratio, the greater the tendency for the ion to diffuse in one direction, & therefore the greater the Nernst potential required to prevent additional net diffusion

Nernst equation

can be used to calculate the Nernst potential of one single ion

Goldman-Hodgkin-Katz Equation

- multiple ions w/ non-selective membrane
- diffusion potential depends on polarity of electrical charge, permeability of membrane, and concentration of ions inside & outside of the membrane

Na+, K+, chloride ions

most important ions involved in the development of membrane potentials in nerve & muscle fibers, & neuronal cells in the nervous system

permeability of Na+ and K+ channels

undergoes rapid changes during transmission of nerve impulse

ion which has the greatest influence on RMP

K+ ion

combined effects that result in RMP

- diffusion down conc gradient
- permeability of membrane
- electrical attraction of cations & anions

local potentials

short-range change in membrane voltage

action potentials

rapid change in which the membrane potential goes through when given a stimulus in an area

characteristics of LP

graded, decremental, reversible, excitatory or inhibitory

phases of AP

RMP, depolarization, repolarization, RMP

threshold

point for stimulation need to open sodium voltage-gated channels; irreversible once reached & generates an action potential

depolarization

upward-rising of AP curve due to transient increase in Na+ permeability followed by Na+ impermeability

repolarization

shift of membrane voltage back to negative

Na+ K+ pump

returns ionic gradient

hyperpolarization

negative undershoot when membrane voltage drops (more negative than original RMP)

properties of AP

- depolarization must reach threshold to trigger AP
- all-or-none
- self-propagates without decrement (irreversible and does not get weaker with distance)

plateau in AP

prolonged depolarization of an AP that often occurs in cardiac muscles

calcium-sodium channels

- found in membrane of cardiac muscle cell
- facilitates movement of Ca into cardiac muscle cell
- slow

nerve conduction of AP

if a neuron is to communicate with another cell, a signal must travel to the end of the axon

influences speed of conduction

diameter of fiber, temperature, myelin presence or absence

continuous conduction

- UNMYELINATED nerve fiber
- AP travels in all directions away from the stimulus, even along all the branches of a nerve fiber, until the entire membrane has become depolarized

saltatory conduction

- MYELINATED nerve fiber
- transmission of AP from node-to-node (too thick for myelin sheaths except at Nodes of Ranvier w/c are unmyelinated)

importance of saltatory conduction

- "jumps" over areas of myelin; faster than continuous
- conserves energy
- allow repolarization with very little ion transfer

refractory period

a period during an AP & few milliseconds thereafter, when it is difficult or impossible to stimulate that region of a neuron to fire again

absolute refractory period

AP cannot be elicited no matter how strong stimulus is

relative refractory period

larger-than-normal stimulus can initiate AP