CELLULAR ELECTRICITY
Learning Objectives, Topics, & Key Words
Fall Semester, 2004
Readings: Martini, 6th ed.: 96, 99 - 100; 398 - 409; 5th ed.: pp 68 - 76; pp 373 - 386.
Objective: Understand how cells behave as batteries
fundamental electrical concept is charge (plus or minus)
electrical current is the flow of charged particles
the flow of ions carries the current in biosystems
if do work to separate + and - charges --> difference in voltage (also called, potential)
potential = the push on charges to cause current
transmembrane potential = difference in potential between inside and outside of cell
fact: most cells at rest have a membrane potential (resting potential, about 70 mV) How?
due to differences in intracellular and extracellular ion concentrations --> "ionic batteries"
ionic oncentration gradients (cause of the "ionic batteries")
potassium ion (high concentration inside, low outside)
sodium ion (low concentration inside, high outside)
ionic equilibrium potentials
potentials exist for each ion for which there is a concentration gradient if the
membrane is permeable to that ion
membrane properties
selective permeability
potassium diffusion and sodium diffusion
active ion transport creates and maintains concentration gradients ("charges up the ionic batteries")
the sodium-potassium pump uses ATP directly to move these ions uphill across the membrane
since resting membrane potential = "polarization"
depolarization means less polarized (potential moves toward zero; more positive)
hyperpolarization means more polarized (potential becomes more negative than normal resting potential)
ionic equilibrium potentials
at equilibrium, a potential exists that balances the concentration gradient
the steeper the concentration gradient, the bigger the potential
sodium ion (ENa~ +66 mV)
potassium ion (EK~ 90 mV)
relative permeability to sodium and potassium ions controls membrane potential
if permeability to sodium ions goes up --> depolarization
cations will enter cell, bringing in positive charge (Em approaches ENa)
if permeability to potassium ions goes up --> hyperpolarization
cations leave cell, losing positive charge (Em approaches EK)
voltage-gated channels in excitable cells
enable the action potential (also called nerve impulse or spike)
Objective: Describe how an action potential is produced
stimulus is an applied depolarization (usually "applied" by soma at initial segment)
if stimulus exceeds threshold --> "all or nothing" action potential
rapid depolarization phase
sodium channel activation (sodium channels open rapidly)
positive feedback: sodium ions enter --> depolarization --> more channels open
-- > more sodium ion entry --> et cetera -->
membrane potential rapidly approaches ENa
repolarization phase
sodium channel inactivation
(sodium channels close spontaneously when kept depolarized)
potassium channels open due to depolarization
hyperpolarization phase
membrane potential approaches E
Kmembrane permeable to potassium ion only
recovery phase
absolute refractory period (all sodium channels inactivated)
relative refractory period (more sodium channels returning to resting state)
Objective: Understand action potential propagation in myelinated and unmyelinated axons.
unmyelinated axons
inward sodium ion current spreads to depolarize nearby membrane (local current flow)
[hot spot "ignites" adjacent axon, like a burning gun powder trail]
myelinated axons
myelin - wrapping of Schwann cell (a type of neuroglial cell)
node of Ranvier - gap between Schwann cell where axon membrane is exposed and sodium channels are concentrated
saltatory conduction (AP "jumps" from node to node)
conduction velocity
determined by axon diameter (approx 6 m/sec/µm diameter)
clinical significance: demyelinating disease slows or blocks action potential
question: what is a disease of this type?
question: what symptoms would you expect from the disturbed anatomy and physiology?
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Durham Technical Community College
Durham, NC 27703
Last updated 20 September 2004