Chapter 3 - The neuronal membrane at rest Flashcards
Describe the contents of the CYTOSOL AND EXTRACELLULAR FLUID.
- Water is the main ingredient for both the intracellular fluid and the extracellular fluid.
- Electrically charged atoms - ions - that are dissolved in this water are responsible for the resting and action potentials.
Why is water important for the resting potential?
It has an uneven distribution of electrical charge. The covalent bonds mean they share electrons. However, the oxygen atom has a greater affinity for electrons, and they spend more time with oxygen, making the hydrogen net positively charged and the electron net negatively charged. Thus, H2O is a polar molecule, held together by polar covalent bonds.
This makes water an effective solvent of other charged or polar molecules: other polar molecules tend to dissolve in water.
What are the ions involved in the action and resting potentials?
Na+ and Cl- ions. The Na+ ions are covered by water molecules oriented so that the oxygen atoms are facing the ion. The Cl- ions are covered by water molecules oriented in the opposite way.
These clouds of water that surround each ion are called spheres of hydration, and they effectively insulate the ions from one another.
What do the terms MONOVALENT and DIVALENT, etc. mean? What are CATIONS and ANIONS?
The difference between the number of protons and electrons of an atom is named monovalent for difference of 1, divalent for difference of 2, etc. Ions with a net positive charge are CATIONS, and ions with a negative charge are ANIONS.
What are PHOSPHOLIPIDS? What is the phospholipid membrane?
PHOSPHOLIPIDS are the main chemical building blocks of cell membranes. They contain long non-polar chains of carbon atoms bonded to hydrogen atoms.
The neuronal membrane consists of a sheet of phospholipids, two molecules thick.
The hydrophilic heads face the outer and inner watery environments and the hydrophobic tails face each other. This is the PHOSPHOLIPID BILAYER, which effectively isolates the cytosol of the neuron from the extracellular fluid.
What are ENZYMES, CYTOSKELETON and the RECEPTORS’ relation to proteins?
They are all made of proteins. Enzymes catalyze chemical reactions in the neuron, cytoskeletons give it its special shape, and receptors are sensitive to neurotransmitters.
Describe briefly the structure of a protein.
- Central carbon atom (alpha carbon), that has covalent bonds to the following four molecular groups:
- Hydrogen atom
- Amino group (NH3+)
- Carboxyl group (Coo-)
- Variable group called R (the differences among amino acids relate to these)
What are ION CHANNELS?
ION CHANNELS are made of membrane-spanning protein molecules, typically made from 4-6 similar protein molecules that assemble to form a pore between them.
One important property of most ion channels is ION SELECTIVITY. Potassium channels are selectively permeable to K+, sodium channels almost exclusively to Na+, calcium channels to Ca2+, and so on.
Another important property is GATING. Channels with this property can be opened and closed by changes in the local microenvironment of the membrane.
What are ION PUMPS?
Enzymes that use the energy released by the breakdown of ATP to transport certain ions across the membrane. In neuronal signaling, these transport Na+ and Ca2+ from the inside of the neuron to the outside.
What is DIFFUSION?
The net movement of ions from regions of high concentration to regions of low concentration.
What is ELECTRICAL POTENTIAL?
The same as VOLTAGE, it is the force exerted on a charged particle. The difference in charge between the anode and the cathode.
What is ELECTRICAL CONDUCTANCE?
The relative ability of an electrical charge to migrate from one point to another. Represented by the symbol g. Measured in siemens (S). A term that expresses the same property is ELECTRICAL RESISTANCE (R) and measured in ohms. Resistance is the inverse of conductance: R = 1/g.
What is OHM’S LAW?
The simple relationship between potential (V), conductance (g) and the amount of current (I). Written I = gV.
Describe the MEMBRANE POTENTIAL.
The MEMBRANE POTENTIAL is the voltage across the neuronal membrane at any moment. Represented by the symbol Vm. Sometimes at rest, sometimes not. Can be measured by inserting a microelectrode into the cytosol.
What is a MICROELECTRODE?
Usually a thin glass tube with an extremely fine tip (0.5 um). Can measure membrane potential.