Midterm 1 Flashcards
transmembrane potential
definition and
3 factors contributing to it
Unequal distribution of ions across the plasma membrane because
1. ECF and ICF have different ionic composition
2. Cells have selectively permeable membranes
3. Membrane permeability varies by ion
Intracellular fluid (ICF)
fluid in the cytosol of cells, typically more negative than ECF due to greater amounts of K+ and negatively charged proteins
Extracellular fluid (ECF)
fluid outside the cells; includes intravascular and interstitial fluids, typically more positive than ICF
Passive forces acting across the membrane
chemical and electrical gradients
Common ions in the body
Na+, K+, Cl-, Ca2+
Potassium ion gradients
- K+ predominantly inside of cell
- Chemical Gradient tends to push K+ out of cell
- Electrical Gradient tends to push K+ out of cell
= Net Gradient (Electrochemical Gradient) - forces K+ outside of cell
Sodium ion gradients
-Na+ predominantly outside of the cell
- at the normal resting potential, chemical and electrical gradients combine to drive sodium ions into the cell
Why doesn’t more Na+ move into cell at rest?
There are very few Na+ leak channels into the cell
Resting potential of a nerve cell
-70 mV, with more sodium outside the cell and more potassium inside the cell with negatively charged proteins
Active force acting across the membrane
sodium-potassium ATPase (exchange pump)
Sodium-potassium exchange pump
for each ATP molecule used, 3 Na+ are pumped out of the cell while 2 K+ are pumped into the cell, helps to repolarize the membrane after an action potential, maintain the concentration of potassium and sodium ions across the PM
Why does the transmembrane potential exist across the plasma membrane?
because of a difference in ionic and chemical composition between the cytosol (ICF) and ECF due to locations of sodium and potassium and negatively charged proteins in the cell
Leak channels
channels that are always open and allow ions to move along their gradient
Gated channels
a transmembrane protein channel that opens or closes in response to a particular stimulus, ie. chemically-gated, voltage-gated and mechanically-gated
Chemically gated channels
open with binding of a specific neurotransmitter such as ACh
Voltage-gated channels
open and close in response to changes in transmembrane potential, is closed when membrane potential at resting potential of -70 mV, opens when membrane depolarizes to -60 mV, and is inactivated at +30 mV
Mechanically-gated channels
open and close in response distortion of the membrane
Graded potential
any stimulus that opens a gated channel, involve three steps: depolarization, repolarization and hyperpolarization
Graded potential in a neuron: steps
resting membrane potential is -70 mV with closed chemically-gated sodium ion channels -> membrane exposed to chemical that opes the sodium ion channels -> spread of sodium ions inside the PM produces a local current that depolarizes adjacent portions of the PM
Action potential
propagated changes in transmembrane potential that affect the entire membrane
Threshold for action potential
Level of stimulation needed to trigger a neural impulse, typically -60 mV
All-or-none principle
Refers to the fact that the action potential in the axon occurs either full-blown or not at all.
Axolemma
plasma membrane of an axon, contains both voltage-gated sodium channels and voltage-gated potassium channels that are closed when the membrane is at the resting potential
Depolarization to threshold
The stimulus that initiates an action potential is a graded depolarization large enough to open voltage-gated sodium channels, occurs at -60mV, the threshold.