Chapter 4 - Electrical Signalling

Question 1

Where is dendritic activity summated?

Answer 1

• At the axonal initial segment (AIS), found shortly after the soma

Question 2

Define the terms electricity, battery, voltage, and current.

Answer 2

• Electricity - the flow of electrons from a body that contains a higher charge to a lower charge
• Battery - Source of potential energy
• Voltage - the difference in charge between two points (measured in volts)
• Current - movement of charged particles (measured in amps)

Question 3

T/F: Polar molecules diffuse across the membrane.

Answer 3

• FALSE
• Non-polar molecules diffuse across the membrane because of the hydrophobic tails.
• Polar molecules require protein channels

Question 4

What’s the difference between a concentration gradient and a voltage gradient?

Answer 4

• Concentration gradient - substances move from areas of high concentration to areas of low concentration
• Voltage gradient - ions move from areas of high charge to areas of low charge

Question 5

What does the term electrochemical gradient signify?

Answer 5

• When the chemical and electrical gradients are equal in magnitude (i.e., when the neuron is resting)

Question 6

What electrical term is a neuronal membrane synonymous with?

Answer 6

• Acts as a dam, or rather a charged battery
• The flow of charged particles acts as the water (i.e., the current)

Question 7

What’s the resting potential of a neuron?

Answer 7

• -70mv (more negative on the inside)

Question 8

Which ions are in higher concentration inside and outside the cell?

Answer 8

• Outside: High sodium, high chloride
• Inside: High potassium, high negatively charged proteins

Question 9

How are sodium and potassium ions moved across the neuronal membrane?

Answer 9

• Potassium - Has a leak ion channel and is free to move in and out of the cell when needed
  -Sodium - Has a gated-ion channel with a closed gate (on top) and an activation gate (on bottom). Closed at rest to maintain resting voltage of -70mV

Question 10

What’s the purpose of the sodium-potassium pump?

Answer 10

• Maintains high sodium outside the cell and high potassium inside the cell
• Expels 3 sodium ions for every 2 potassium ions

Question 11

What does a voltmeter do?

Answer 11

• Device that measures the strength of electrical voltage by recording the difference in electrical potential between two points
• One electrode is inside, the other is outside

Question 12

What animal is commonly used to measure neuronal mechanisms?

Answer 12

• A giant squid axon kept in a saline solution
• Discovered by Hodgkins and Huxley in the 1930s
• Won a Nobel prize for discovering the neurons ionically-based electrical activity

Question 13

Where are ion channels highly concentrated along the neuron?

Answer 13

• At the Nodes of Ranvier

Question 14

What are the confirmational states of potassium voltage-gated ion channels?

Answer 14

1) Closed at resting potential
2) Start to open at -50mV where potassium efflux occurs

Question 15

What are the confirmational states of sodium voltage-gated ion channels?

Answer 15

1) Activation gate Closed at resting potential
2) Opens at -50mV where sodium influx occurs
3) non-activation gate closes at +30mV

Question 16

What are the main causes of depolarization and hyperpolarization?

Answer 16

• Depolarization - due to the inward flow of sodium ions due to an action potential
• Hyperpolarization - due to the outward flow of potassium ions or inward flow of chloride ions when a neuron is trying to return to a resting state.

Question 17

T/F: An action potential is an all or none response.

Answer 17

• TRUE
• The action potential has the same amplitude every time no matter the stimulus
• Can only increase the frequency of action potentials occurring at one time, never the amplitude

Question 18

What are the general steps during an action potential?

Answer 18

1) Neuron initially at -70mV resting potential
2) Stimulus triggers membrane potential to increase to -50mV
3) Sodium ion channels open, causing a sodium influx
4) Rapid depolarization occurs and membrane potential shoots up to +30mV
5) The sodium activation gate closes and the potassium channels finally open, causing a potassium efflux
6) Hyperpolarization occurs, and both sodium gates close
7) Neuron returns to resting state as both channels close

Question 19

Why does potassium efflux occur after depolarization occurs?

Answer 19

• Even though potassium channels are triggered to open at -50mV, they just take longer to open
• They are mainly responsible for repolarization

Question 20

What’s the difference between the relative refractory and absolutely refractory period?

Answer 20

• Absolutely refractory period - a new AP cannot be elicited (found during ascending and descending peak on graph)
• Relatively refractory period - During the hyperpolarization phase, a higher intensity stimulation is required to produce another action potential (potassium channels still open but you’re below the resting potential)

Question 21

How does an electrical signal propagate down the neuron?

Answer 21

• AIS > axon terminals
• Influx of sodium ions in one segment triggers the depolarization of the following segment

Question 22

What’s saltatory conduction?

Answer 22

• The “jumping” of ions from one node of Ranvier to another, critical for speeding up the propagation of action potentials

Question 23

What’s multiple sclerosis (MS)?

Answer 23

• Most common autoimmune disease
• Due to loss of myelin formed by oligodendroglia due to a poor immune response
• Plaques form at the sites of myelin loss, making it difficult to propagate signals properly.
• Starts with loss of sensation in motor control in hands and feet

Question 24

T/F: AIS is sparse in terms of ion channels

Answer 24

• FALSE
• It’s rich in voltage-gated ion channels
• These channels help generate the initial action potential

Question 25

What exactly is the AIS summating?

Answer 25

• Excitatory postsynaptic potentials (EPSPs) and Inhibitory postsynaptic potentials (IPSPs)
• Thousands occurring all at once at the dendrites

Question 26

Temporal summation vs. Spatial summation?

Answer 26

• Temporal - Inputs on the same dendritic spine occurring in rapid succession
• Spatial summation - Inputs on different spines that occur at the same time, will summate if the inputs are close in space