Lesson 23: Cell Excitability 1

Lipid soluble

Molecules that can easily dissolve through a membrane's lipid portion, such as steroids.

Hydrophobic

Molecules that resist or fear water.

Lipid insoluble

Molecules that cannot dissolve through a membrane's lipid portion.

Hydrophilic

Molecules that are attracted to water.

Concentration gradient

The difference in concentration of a substance between two areas; a steeper gradient results in a faster diffusion rate.

Diffusion

The passive movement of a molecule down a concentration gradient from high to low concentration.

Osmosis

The net movement of water across a semipermeable membrane from high concentration of pure water to low concentration of pure water.

Ion channels

Pore-forming proteins that allow the flow of ions across membranes.

Leak ion channels

Constantly permeable, non-gated ion channels.

Ligand-gated ion channels

Ion channels that open when a chemical ligand binds to the protein.

Voltage channels

Ion channels that open and close in response to changes in membrane potential.

Mechanically gated channels

Ion channels that open in response to physical deformation of the receptor.

Facilitated diffusion

Diffusion of solutes through transport proteins in the plasma membrane, still considered passive transport.

Symport

Proteins that move two molecules in the same direction across the membrane.

Antiport

Proteins that move two molecules in opposite directions across the membrane.

Primary Active Transport Mechanism

Transport that occurs against the electrochemical gradient, powered by metabolic energy such as ATP hydrolysis.

Secondary Active Transport Mechanism

Transport that uses the energy stored in electrochemical gradients created by primary active transport to move other substances against their gradients.

Active Transport

Against concentration gradient

energy required

transport is saturable

Passive transport

Simple vs facilitated

no energy required

Simple diffusion NOT saturable

Facilitated diffusion IS saturable

Primary active transport

Uses ATP directly as a source

Mechanism: substrate binds to carrier protein which passes the membrane and releases it on the other side

Secondary Active Transport

Energy is used to generate a concentration gradient of a carrier, driving force for transport

Mechanism: carrier ad substrate bind to carrier protein, passes through membrane and releases onto opposite side

Another name for leak ion channels

Non-gated ion channels

Basic experiment measurement

After penetration of the cell membrane, the electrode records a negative potential of -70mV

In tissue, intracellular potential is negative hence negative membrane potential

Electroneutrality Potential

Total number of positive and negative charge in the extracellular and intracellular fluid is equal

There is only a accumulation of + charge at ECF and - at ICF in immediate vicinity of the cell membrane

How do positive ions(cation create a negative membrane potential

Ion concentration gradient causes effluc of potassium and influx of sodium

k ions diffuse more easily than Na ions because more leak channels

Sodium potential pump

Electrogenic

membrane potential based on difference in positive charges

Moves Na out of cell and K inside

Exchanges 3k for 2Na but some K can leak back into due to chemical gradient

Nernst Equation

EMF = 61/z * log(Xout/Xin) where EMF is the electromotive force, z is the valence of the ion, and X represents the concentration of the ion outside (Xout) and inside (Xin) the cell.

Depolarization

Membrane potential difference decreases

Membrane potential is less negative, more positive

Influx of cations

Hyperpolarization

Membrane potential difference increases

Membrane potential is more negative or less positive

Efflux of cations or Influx of anions

Events in action potential

1. Resting membrane potential

2. Depolarizing stimulus

3. Membrane depolarizes to the threshold. Voltage-gated Na+ and K + channels begin to open

4. Rapid Na+ entry depolarizes the cell,

5. Rapid Na+ channels close (auto inactivation), and slower K+ channels open

6. K+ moves from cell to extracellular fluid

7. K+ channels remain open and additional K+ leaves cell, hyperpolarizing it

8. Voltage-gated K+ channels close, less K+ leaks out of the cell

9. Cell returns to resting ion permeability and resting membrane potential

Graded potential

variable strength signals that travel over short distances and loose strength as they travel.

Short distance communication

Action potential

brief large depolarizations that travel long distances wothout loosing strength

Rapid signaling over long distance

Depolarizations

Influx of Na- ions depolarize from -70mV to 0

Repolarizations

Efflux of K+ ions repolarize cell membrane towards -70 mV

Absolute refractory period

period of ongoing action potential during which no other action potential can be generated

Relative refractory period

Follows absolute refractory period during which a strong stimulus can create another action potential

All or none

Depolarization has no affect or full action potential

Normal EC concentration for K+

4 mmol/L

Hyperkalemia

EC: 8 mmol/L

Depolarize cell membrane

Hypokalemia

EC: 2 mmol/L

Hyperpolarization of cell membrane