Flashcards in Chapter 3 Deck (74):
How do neurons solve the problem of conducting info over long distances?
Using electrical signals that sweep along the axon.
How does electrical charge in the cytosol of the axon get carried?
- By electrically charged atoms (ions) instead of free electrons.
- This makes cytosol far less conductive than copper wire.
- Also, axon is not especially well insulated and is bathed in salty extracellular fluid which conducts electricity.
(Nerve impulse) A special type of signal that overcomes biological constraints. They do not diminish over distance (unlike passively conducted electrical signals). They are signals of fixed size and duration.
Cells capable of generating and conducting action potentials.
What does it mean when a cell is at rest?
The cytosol along the inside surface of the membrane has a negative electrical charge compared to the outside.
What is the difference in the electrical charge across the membrane?
The membrane resting potential. In terms of the resting potential, the action potential is a brief reversal of this condition (inside is positive).
Atoms or molecules that have a net electrical charge.
Electrical attraction of oppositely charged atoms.
Electrical charge of an atom depends on...
...the difference between the number of protons and electrons.
Give examples of hydrophilic substances.
Ions, polar molecules. These dissolve in water.
Give examples of hydrophobic substances.
- Hydrophobic: Any compound held together by nonpolar covalent bonds (no net electrical charge).
- Lipids (important to structure of cell membranes).
Lipids of neuronal membrane contribute to resting and action potentials by...
...forming a barrier to water soluble ions and to water itself.
- The main chemical building blocks of cell membranes.
- Contain long nonpolar chains of carbon atoms bonded to hydrogen atoms, as well as a polar phosphate groups (phosphorous bonded to three oxygen atoms) attached to one end of the molecule.
How are phospholipids arranged (the head and tail)?
They have a polar head (containing phosphate) that is hydrophilic, and a nonpolar tail (containing hydrocarbon) that is hydrophobic.
Which side of the phospholipid faces the outer and inner watery environments?
Hydrophilic heads face outer and inner watery environments. Hydrophobic tails face each other.
Neuronal membrane consists of...
...a sheet of phospholipids, two molecules thick. This is the phospholipid bilayer, and it effectively isolates the cytosol of the neuron from the extracellular fluid.
...routes for ions to cross the neuronal membrane.
Describe the physical characteristics of amino acids.
- All have a central carbon atom (alpha carbon) which is covalently bonded to four molecular groups.
- A hydrogen atom, an amino group, a carboxyl group, and a variable group called the R group.
What is the difference between amino acids?
Differences between amino acids result from differences in the size and nature of R groups.
How are amino acids assembled into a chain?
They are connected by peptide bonds, which join the amino group of one acid to the carboxyl group of the next.
Proteins made of a single chain of amino acids.
Regions where nonpolar R groups are exposed will be...
...hydrophobic and will tend to associate readily with lipid.
Regions with exposed polar R groups will be...
...hydrophilic and will tend to avoid a lipid environment.
Suspended in a phospholipid bilayer, with its hydrophobic portion inside the membrane and its hydrophilic ends exposed to the watery environments on either side.
Specified by diameter of pore and nature of R groups lining it.
Channels with this property can be opened and closed by changes in the local microenvironment of the membrane.
- Membrane spanning proteins that come together.
- Enzymes that use energy released by breakdown of ATP to transport certain ions across the membrane.
Ionic movement through channels are influenced by two factors:
Diffusion and electricity
- Constant motion of ions and molecules dissolved in water is temp dependent and random.
- This allows for net movement of ions from region\s of HIGH concentration to LOW. This is called diffusion.
Do ions pass through the phospholipid bilayer directly?
No, but diffusion causes ions to be pushed through channels in the membrane.
A difference of concentration (high to low).
Driving ions across the membrane by diffusion happens when...
- the membrane possesses channels permeable to ions.
- AND there is a concentration gradient across the membrane.
Cathode vs. Anode
- Cathode: Negative terminal attracts cations (positive ions).
- Anode: Positive terminal attracts anions (negative ions).
- The movement of electrical charge.
Which flow of current is positive?
In the positive charge movement direction.
Two factors determine how much current will flow:
- Electrical potential (voltage): The force exerted on a charged particle, and it reflects the difference in charge between the anode and cathode (represented by V).
- Electrical conductance: The relative ability of an electrical charge to migrate from one point to another (Represented by g and measured in siemens).
What does electrical conductance depend on?
- Depends on number of particles available to carry electrical charge and the ease with which these particles can travel through space.
- The relative inability of an electrical charge to migrate (same thing as conductance just phrased differently).
- Represented by R.
Conductance vs. Resistance
R = 1/g
The relationship between potential (V) conductance (g) and the amount of current (i) that will flow.
I = gV
Driving an ion across the membrane electrically requires that:
- The membrane possesses channels permeable to that ion, AND
- There is an electrical potential difference across the membrane.
Ions can cross the membrane ONLY by...
...way of protein channels.
The voltage across the neuronal membrane at any moment.
What is the resting potential of a typical neuron?
Ionic equilibrium potential
The electrical potential difference that exactly balances an ionic concentratjion gradient.
What do you need to generate a steady electrical potential difference across a membrane?
All you need is an ionic concentration gradient and selective ionic permeability.
Large changes in membrane potential are caused by...
...miniscule changes in ionic concentrations.
The net difference in electrical charge occurs...
...at the inside surfaces of the membrane.
Because phospholipid bilayer is so thin...
...it is possible for ions on one side to interact electrostatically with ion on the other side.
Where does the bulk of the negative and postiive charges inside and outside the neuron occur?
They are mutually attracted to the cell membrane. So the net negative charge inside the cell is localized at the inner face of the membrane.
When the membrane stores electrical charge.
Ions are driven across the membrane...
...at a rate proportional to the difference between the membrane and equilibrium potential.
Ionic driving force
The difference between the real membrane potential and the equilibrium potential for a particular ion.
Takes into consideration the change of the ion, the temp, and the ratio of the external ion concentrations to give the exact value of an equilibrium potential in mV.
E = -2.303(RT/zF) log(ion conc outside/ion conc inside).
- E = ionic equilibrium potential
- R = gas constant
- T = abs temp
- Z = chagre of the ion
Explain why Equilibrium is proportional to temp in the nernst equation.
Increasing thermal energy of each particle increases diffusion and will therefore increase the potential difference achieved at equilibrium.
Explain why equilibrium is inversely proportional to the charge of the ion in the Nernst equation.
Increasing the electrical charge of each particle will decrease the potential difference needed to balance diffusion.
The neuronal membrane potential depends on...
...ionic concentrations on either side of the membrane.
Explain where potassium, sodium, and calcium ions are most concentrated.
- K is more concentrated inside
- Sodium and calcium are more concentrated on the outside of the cell.
Ionic concentration gradients are established by...
...the actions of ion pumps in the neuronal membrane.
Sodium Potassium pump
- Enzyme that breaks down ATP in the presence of internal Na.
- Chemical energy released by reaction drives pump, exchanges internal Na for external K.
- Ensure that K is concentrated inside the neuron and that Na is concentrated outside.
- Pump pushes these ions across the membrane against their concentration gradients (requires expenditure of metabolic energy).
- Expends as much as 70% of total ATP utilized by brain.
- Enzyme that actively transports Ca out of the cytosol across the cell membrane.
What other mechanisms decrease intracellular calcium ions?
- Calcium binding proteins and organelles, such as mitochondria and types of ER that sequester cytosolic calcium ions.
Without ion pumps...
...there would be no resting membrane potential.
...ionic concentration gradients across the neuronal membrane.
NOTE: An equilibrium potential for an ion is the membrane potential that results if....
...a membrane is selectively permeable to that ion alone. In reality neurons are not permeable to only a single type of ion.
Formula that takes into consideration the relative permeability of the membrane to different ions.
What is the significance of potassium channel permeability?
It is a key determinant of the resting membrane potential and therefore neuronal function.
Selectivity for potassium ions derives from...
...the arrangement of amino acid residues that line the pore regions of the channels.
Most potassium channels are made up of...
...four subunits that form a pore.
A region which contributes to the selectivity filter that makes the channel permeable mostly to potassium ions.
Consequence of high potassium ion permeability is that...
...the membrane potential is particularly sensitive to changes in the concentration of extracellular potassium.
- A change in membrane potential from the normal resting value to a less negative value.
- Increasing extracellular potassium depolarizes neurons.
Blood brain barrier
A specialization of the walls of brain capillaries that limits the movement of potassium (and other bloodborne substances) into the extracellular fluid of the brain.