Chapter 3 - The neuronal membrane at rest

Question 1

Describe the contents of the CYTOSOL AND EXTRACELLULAR FLUID.

Answer 1

• 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.

Question 2

Why is water important for the resting potential?

Answer 2

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.

Question 3

What are the ions involved in the action and resting potentials?

Answer 3

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.

Question 4

What do the terms MONOVALENT and DIVALENT, etc. mean? What are CATIONS and ANIONS?

Answer 4

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.

Question 5

What are PHOSPHOLIPIDS? What is the phospholipid membrane?

Answer 5

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.

Question 6

What are ENZYMES, CYTOSKELETON and the RECEPTORS’ relation to proteins?

Answer 6

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.

Question 7

Describe briefly the structure of a protein.

Answer 7

1. Central carbon atom (alpha carbon), that has covalent bonds to the following four molecular groups:
2. Hydrogen atom
3. Amino group (NH3+)
4. Carboxyl group (Coo-)
5. Variable group called R (the differences among amino acids relate to these)

Question 8

What are ION CHANNELS?

Answer 8

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.

Question 9

What are ION PUMPS?

Answer 9

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.

Question 10

What is DIFFUSION?

Answer 10

The net movement of ions from regions of high concentration to regions of low concentration.

Question 11

What is ELECTRICAL POTENTIAL?

Answer 11

The same as VOLTAGE, it is the force exerted on a charged particle. The difference in charge between the anode and the cathode.

Question 12

What is ELECTRICAL CONDUCTANCE?

Answer 12

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.

Question 13

What is OHM’S LAW?

Answer 13

The simple relationship between potential (V), conductance (g) and the amount of current (I). Written I = gV.

Question 14

Describe the MEMBRANE POTENTIAL.

Answer 14

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.

Question 15

What is a MICROELECTRODE?

Answer 15

Usually a thin glass tube with an extremely fine tip (0.5 um). Can measure membrane potential.

Question 16

Describe EQUILIBRIUM POTENTIALS?

Answer 16

There is no net movement of ions across the membrane, but there is a charge difference between the two sides. Example used: Selectively permeable channel for movement of K+ in a membrane used in an example.

First, there is no channel = neutral charge.
Then, K+ channel is introduced. K+ moves from inside to outside, but leaves A- inside, so a negative charge is accumulating. This is through DIFFUSION.
After a while, this negative charge will draw K+ ions back inside. This is through ELECTRICAL FORCE. Movements of ions: electrical charge.
Net movement is the same but there is a charge: EQULIBRIUM POTENTIAL.

Question 17

What is the RESTING POTENTIAL of a typical neuron?

Answer 17

About -65 millivolts. Vm = -65 mV.

Question 18

What are the four main points to remember about the EQUILIBRIUM POTENTIAL EXAMPLE in the book?

Answer 18

1. Large changes in membrane potential are caused by minuscule changes in ionic concentrations. 0 mV -> -80 mV, 100 -> 99.99999 mM
2. The net difference in electrical charge occurs at the inside and outside surfaces of the membrane.
3. Ions are driven across the membrane at a rate proportional to the difference between the membrane potential and the resting potential.
4. If the concentration difference across the membrane is known for an ion, the equilibrium potential can be calculated for that ion.

Question 19

How can the exact value of an EQUILIBRIUM POTENTIAL in mV be calculated?

Answer 19

Using the NERNST EQUATION.

Question 20

What is the SODIUM-POTASSIUM PUMP?

Answer 20

An enzyme that breaks down ATP in the presence of internal Na+. The action of this pump ensures that K+ is concentrated inside the neuron and that Na+ is concentrated outside. Transports Na+ and K+.

Question 21

What is the CALCIUM PUMP?

Answer 21

An enzyme that actively transports Ca2+ out of the cytosol across the cell membrane. Additional mechanisms decrease intracellular Ca2+ to a very low level, 0.0002 mM.

Question 22

How can the RESTING MEMBRANE POTENTIAL be calculated?

Answer 22

Using the Goldman equation, which is a mathematic formula that takes into consideration the relative permeability of the membrane to different ions.

Question 23

What are POTASSIUM CHANNELS?

Answer 23

Key determinants of resting membrane potential and neuronal function. Selectivity derives from the arrangement of amino acids.

Mutations involving only a single amino acid in potassium channels can severely disrupt neuronal function. As a consequence of certain mutation, Na+ as well as K+ can pass through a potassium channel. Increased sodium permeability causes the membrane potential of the neurons to become less negative, thus disrupting neuronal function.

Question 24

Why is it important to regulate external potassium concentration?

Answer 24

A tenfold change in K+ concentration outside the cell from 5 to 50 mM would change membrane potential from -65 to -17 mV. Increasing extracellular potassium depolarizes ions.

Question 25

What is POTASSIUM SPATIAL BUFFERING?

Answer 25

A mechanism for the regulation of [K+]0 by astrocytes. When [K+]0 increases, K+ enters the astrocyte through the potassium channels, causing the astrocyte membrane to depolarize.

Question 26

What two functions do proteins in the neuronal membrane perform to establish and maintain the resting membrane potential?

Answer 26

They maintain a phospholipid bilayer, which effectively isolates the cytosol of the neuron from the extracellular fluid.
They also create ion pumps and membrane ion channels, that are permeable to ions.

Question 27

On which side of the neuronal membrane are Na+ ions more abundant?

Answer 27

Outside the membrane.

Question 28

When the membrane is at the potassium equilibrium potential, in which direction (in or out) is there a net movement of potassium ions?

Answer 28

Neither. There is no net movement at equilibrium potential.

Question 29

There is a much greater K+ concentration inside the cell than outside. Why, then, is the resting membrane potential negative?

Answer 29

Because of the accumulation of more Na+ ions outside the cell than potassium ions inside the cell.

Also, potassium ions diffuse out of the cell at a much faster rate than sodium ions diffuse into the cell, since neurons have many more potassium leakage channels than sodium leakage channels.

Question 30

When the brain is deprived of oxygen, the mitochondria within neurons cease producing ATP. What effect would this have on the membrane potential? Why?

Answer 30

The sodium-potassium pumps use ATP, so their function would stop. The resting membrane potential would diminish since the ions would no longer be transported against their concentration gradients.