4: Signalling

Question 1

What is charge?

Answer 1

• • a force between objects that acts at a distance; the source of this force has been given the name charge.
• • There are two types of electric charge.
    1) Opposite types of charge attract
    2) Like types of charge repel
• • in the brain the charge comes from water

Question 2

What is current?

Answer 2

• • charges that move
• • Reported as the number of charges per unit time passing through a boundary.
• • The SI unit for current is the ampere (A)
• • An ampere is one coulomb per second

Question 3

What are conductors?

Answer 3

• • Made of atoms whose outer electrons have relatively weak bonds to their nuclei (many free electrons); because these virtually unbound electrons are free to leave their respective atoms and swarm around in the space between adjacent atoms, they are often called free electrons; a very small electric force can make the electron swarm move. Copper, gold, silver, and aluminum are good conductors.
    ex. salwater
• • This relative mobility of electrons within a material is known as electric conductivity
• • Conductivity is determined by the types of atoms in a material and how the atoms are linked together with one another

Question 4

What are insulators?

Answer 4

• • Materials whose outer electrons are tightly bound to their nuclei
• • Modest electric forces are not able to pull these electrons free; when an electric force is applied, the electron clouds around the atom stretch and deform in response to the force, but the electrons do not depart.
• • Glass, plastic, stone, and air are insulators, rubber
    ex. lipids (fats)

Question 5

How is saltwater a good conductor?

Answer 5

• • Current is carried by both positive and negative charges in saltwater
• • If we put ordinary table salt in water, it becomes a good conductor
• • Table salt is sodium chloride, NaCl.
• • The salt dissolves in water, into free-floating Na+ and Cl- ions.
• • Both ions respond to electric force and move through the saltwater solution, in opposite directions

Question 6

Why are good conductors needed for currents?

Answer 6

– Electrons can flow only when they have the opportunity to move in the space between the atoms of a material (Water can flow through the tube if and only if the tube is open and there is no blockage)

Question 7

What is static and dynamic electricity?

Answer 7

• • Static: “pumping” electrons to create an electric charge imbalance results in a certain amount of energy being stored in that imbalance. At rest, a neuron has a resting membrane potential (static) because the inside is largely negatively charged while the outside is largely positively charged
• • Dynamic: providing a way for electrons to flow back to their original “levels” results in a release of stored energy. Channels across the membrane allows for dynamic electricity (like a wire in a circuit allows for electrons to move from the negative region of a battery to the positive region of a battery)

Question 8

What is potential energy?

Answer 8

• • When electrons are poised in that static condition (just like water sitting still, high in a reservoir), the energy stored there is called potential energy, because it has the possibility (potential) of release that has not been fully realized yet
• • stored in the form of an electric charge imbalance and capable of provoking electrons to flow through a conductor

Question 9

What is voltage?

Answer 9

– a measure of potential energy per unit charge of electrons
– work required to move a unit charge from one location to another, against the force which tries to keep electric charges balanced.
– The potential energy available for moving electrons from one point to another is relative to those two points.
– Therefore, voltage is always expressed as a quantity between two points
V = (I)(R)

Question 10

What drives current?

Answer 10

• • When a voltage source is connected to a conductor, it applies a potential difference V that creates an electric field
• • The electric field in turn exerts force on charges, causing current

Question 11

What is resistance?

Answer 11

• • As electrons work their way through wires, they encounter opposition to motion
• • This opposition to electric current depends on the type of material, its cross-sectional area (large vs small diameter axon), and its temperature (colder it is, the slower the charged particles move).
• • It is technically known as resistance (R): conductors have low resistance and insulators have very high resistance.
• • serves to limit the amount of current through the circuit with a given amount of voltage

Question 12

What is Ohm’s law?

Answer 12

– The current that flows through most substances is directly proportional to the voltage V (I α V)
– The electric property that impedes current is called resistance R; resistance is defined as inversely proportional to current (I α 1/R)
– The law states that the amount of current (I; measured in amperes or amps) flowing in a conductor is related to the potential difference (voltage; V; measured in volts) applied to it
– The constant R (measured in ohms) is resistance of the wire
I = V/R

Question 13

What are capacitors?

Answer 13

• • Capacitors introduce a time element into current flow; they accumulate and store electrical charge up to a certain point before discharging and when they are added to a circuit, current and voltage changes are no longer simultaneous.
• • when the voltage is reduced, the capacitor discharges in the opposite direction
• • Consist of two conducting plates separated by an insulator
• • Because capacitors are able to store electrical energy, they act like small batteries and can store or release the energy as required

Question 14

How do you charge a capacitor?

Answer 14

• • Charging the capacitor stores energy in the electric field between the capacitor plates.
• • If Ic is charging current through the capacitor then Ic is maximum at the beginning and it starts getting smaller until the capacitor is fully charged or the Potential difference built across capacitor is equal to the supply voltage V
• • The charging current asymptotically approaches zero as the capacitor becomes charged up to the battery voltage.

Question 15

How is a capacitor discharged?

Answer 15

• • The discharging current will flow in the opposite direction of the charging current
• • If we take the direction of charging current as positive then the discharging current is taken negative as it flows in opposite direction

Question 16

How does the cell membrane act as a capacitor?

Answer 16

• • The membrane of a neuron is related to a capacitor because of its ability to store and separate a charge; it can build up charge when the positive sodium ions enter around the membrane as the resistance goes down (channel opens)
• • When one conducting region accumulates a charge, an electric field is created, which pushes the charge off from the conducting region

Question 17

How are ion channels resistors?

Answer 17

– Channels allow current in the form of charged ions (passive)
– When more channels are open, more ions move
Represents a decreased resistance; an increase in conductance.

Question 18

What are intracellular and extracellular recordings?

Answer 18

• • Extracellular recording: the electrode is placed just outside the neuron of interest
• • Intracellular recording: the electrode is inserted inside the neuron of interest

Question 19

What are patch clamp techniques?

Answer 19

– the electrode is closely apposed to the neuronal membrane, forming a tight seal with a patch of the membrane.

Question 20

What is diffusion?

Answer 20

– Movement of ions from an area of higher concentration to an area of lower concentration through random motion

Question 21

What is a concentration gradient?

Answer 21

– Differences in concentration of a substance among regions of a container that allows the substance to diffuse from an area of higher concentration to an area of lower concentration

Question 22

What is voltage gradient?

Answer 22

– Difference in charge between two regions that allows a flow of current if the two regions are connected
(Opposite charges attract, similar charges repel)
– Ions will move down a voltage gradient from an area of higher charge to an area of lower charge

Question 23

What is the Nernst Equation?

Answer 23

• • denotes the the equilibrium potential (A point in which forward and reverse reaction rates are equal in an electrolytic solution) which depends on the relative contributions of the particular ions inside and outside of the neural membrane
• • It is this differential distribution of ionic concentrations that gives rise to the resting membrane potential and thereby, voltage difference when measuring outside versus inside
• • The resting potential is somewhere between the equilibrium potentials of these three ions.

Question 24

What is the resting potential?

Answer 24

• -Electrical charge across the cell membrane in the absence of stimulation
• • A store of negative energy on the intracellular side relative to the extracellular side
• • The inside of the membrane at rest is (in general) −70 millivolts relative to the extracellular side (there are exceptions, however, because If you change the concentration of any of the ions (Na, K, Cl, Ca), then that resting membrane potential will change)

Question 25

How is the resting potential maintained?

Answer 25

-- Four charged particles take part in producing the resting potential >> Sodium (Na+) and chloride (Cl−) Higher concentration outside cell >> Potassium (K+) and large proteins (A−) Higher concentration inside cell -- Because the membrane is relatively impermeable to large molecules, the negatively charged proteins remain inside the cell (no membrane channels are large enough to allow these proteins to leave) and their negative charge alone is sufficient to produce transmembrane voltage, or a resting potential -- The gates for potassium and chloride are leaky so they are constantly moving in and out of the membrane, however the gates for Na+ are ordinarily closed -- Cells accumulate potassium ions (K+) about 20 times as many inside the cell relative to outside; Because the internal concentration ([K+]i) is much higher than [K+]e, K+ are drawn out of the cell by the concentration gradient, but they go back into the cell because of their voltage gradient (inside of the cell is more negative so they will be drawn back in) -- Because there are more Cl- outside the cell, they will diffuse across the membrane and move down their concentration gradient, but then they will go back out of the cell because of their voltage gradient (because outside of the cell is more positive) -chloride ions contribute very little to resting potential; the equilibrium point, at which Cl- concentration gradient equals its voltage gradient, is approximately the same as the membrane’s resting potential -- Na+ kept out to the extent that about 10 times as many Na+ on the outside as on the inside; Difference contributes to the membrane’s resting potential. However, the channels for sodium (and calcium) are gated meaning when they are not leaky and do not allow sodium to freely move in and out of the cell. Sodium is constantly being pushed in because of both concentration and voltage gradient but it is being kept out by these gated channels –this is what really maintains that resting membrane potential

Question 26

How does proprioception work?

Answer 26

- - Receptors sensitive to the stretch of muscles and tendons and the movement of joints - - Na+ channels can be opened by forces conveyed through lipid tension

Question 27

What are graded potentials?

Answer 27

- - If the concentration of any of the ions across the cell membrane changes, the membrane voltage changes. - - These graded potentials are small voltage fluctuations across the cell membrane

Question 28

What is hyperpolarization?

Answer 28

-- Membrane potential becomes more negative (inside relative to outside) -- Usually due to the inward flow of chloride ions (Cl-) or outward flow of potassium ions (K+)

Question 29

What is depolarization?

Answer 29

-- Membrane potential becomes more positive (inside relative to outside) -- Depolarization can be produced by an influx of sodium ions and is produced by the opening of normally closed gated sodium channels -- Depolarization also occurs with influx of Ca++ (In some cases, depolarization is mediated by influx of K+) -- Channels/receptors can be opened in many ways (tension, temperature, etc)

Question 30

What are EPSPs and IPSPs?

Answer 30

-- Excitatory Postsynaptic Potential (EPSP) >> Brief depolarization of a neuron membrane in response to stimulation >> Neuron is more likely to produce an action potential -- Inhibitory Postsynaptic Potential (IPSP) >> Brief hyperpolarization of a neuron membrane in response to stimulation >> Neuron is less likely to produce an action potential -- Both EPSPs and IPSPs last only a few milliseconds before they decay and the resting potential is restored -- EPSPs and IPSPs Are Summed (Temporal/Spatial)

Question 31

What is temporal summation and spatial summation?

Answer 31

-- Temporal Summation >> Pulses that occur at approximately the same time on a membrane are summed >> Stimuli are introduced in rapid succession from a single source that produce graded potentials that sum -- Spatial Summation >> Pulses that occur at approximately the same location on a membrane are summed >> However, inputs closer to the cell body overshadows the other signals

Question 32

How do temporal/spatial summation add up to action potentials?

Answer 32

- - Sodium wave will go down it’s concentration gradient as it is entering the cell in one area and leak out until it hits the axon hillock - - The influx of sodium ions (or efflux of potassium ions) accompanying one EPSP is added to the influx of sodium ions (or efflux of potassium ions) accompanying a second EPSP if the two occur close together in time and space; If the two influxes are remote in time or in space or in both, no summation is possible. - - The threshold potential is the voltage on a neural membrane at which an action potential is triggered and causes opening of Na+ and K+ voltage-sensitive channels - - The axon hillock is the junction of cell body and axon and rich in voltage-sensitive channels (where EPSPs and IPSPs are integrated and action potentials are initiated). - - Na+ channels are double gated but K+ are not –the Na+ channels have one gate that is voltage sensitive but another gate that is not. You want the voltage-insensitive gate to close so that action potential stays and goes in the correct direction - - As the temporal/spatial summation adds up and the wave of Na+/Ca+ goes down it’s concentration gradient towards the axon hillock (reaches the threshold potential of -50 or -40mv), the voltage-gated Na+ channels (gate 1) detects this change in membrane potential (voltage) and open up causing all these Na+ ions to rush in and the cell becomes depolarized and reaches +30mV (action potential). - - The voltage-insensitive gates (gate 2) on the Na+ channels (inside the cell side) then close as these Na+ ions are rushing in to prevent any Na+ from leaving (from going backwards and triggering the channel to open) and ensuring that the action potential only travels in one direction (this also limits how high or how positive the action potential will get) - - The voltage sensitive K+ channels detects that reversal in polarity (when the Na+ channels opened and Na+ entered) and open which cause K+ to leave the cell (because all the Na+ entering the cell make the inside of the cell more positive which repels the K+) - - These K+ leaving cause the cell to become hyperpolarized. This is also because the K+ channels stay open for longer compared to the Na+ channels and also the Na+ channels open first because they are more sensitive than K+ channels - - Thus, the entering of Na+ causes the cell to become depolarized, but the exiting of K+ after it causes the cell to become hyperpolarized

Question 33

What is the absolute and relative refractory period?

Answer 33

- - Absolute: The state of an axon in the repolarizing period during which a new action potential cannot be elicited (with some exceptions) because voltage-insensitive gate 2 of sodium channels is closed - - relative refractory period: The state of an axon in the later phase of an action potential during which increased electrical current is required to produce another action potential (potassium channels still open)- during this time the cell can become depolarized but because it is much more negative than usual it requires a much greater input. - - To get this cell from this hyperpolarized state back to resting membrane potential, the Na+ voltage insensitive gate opens to let out all the Na+ (voltage-sensitive gates closed to prevent any Na+ from entering)

Question 34

What is the importance of refractory periods?

Answer 34

- - Although an action potential can travel in either direction on an axon, refractory periods prevent it from reversing direction and returning to its point of origin. - - Refractory periods thus produce a single, discrete impulse that travels away from the initial point of stimulation.

Question 35

How does the myelin sheath speed up neural impulse (saltatory conduction)?

Answer 35

- - Myelin prevents the Na+ from leaving (except at Node of Ranviers –which enables saltatory conduction) - - Saltatory conduction allows more Na+ ions to enter and regenerate that “wave” of Na+ entering the cell (this is how the action potential “jumps” from one Node of Ranvier to the next)