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CELLULAR ELECTRICITY

Learning Objectives, Topics, & Key Words

Fall Semester, 2004


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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 EK

membrane 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
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Last updated 20 September 2004