M2 Topic 4: Electrical signals

Question 1

Resting membrane potential (RMP)

Answer 1

The electrical different between the inside and outside of a neuron

• Difference is caused by imbalance of ions across the neuronal membrane
• At rest, more sodium ion (Na+) outside, more potassium ion (K+) inside

RMP is -70 millivolts

Question 2

Ion permeability

Answer 2

How easily an ion can pass across the membrane

Question 3

Ion concentraion

Answer 3

How much of the ion is present

Question 4

Ion equilibrium potential

Answer 4

Membrane potential at which the ion is balanced, where equal amounts of the ion would move in or out of the neuron

Question 5

Concentrations inside the neuron at RMP

Answer 5

• Potassium (K+) ions at 150mM
• Sodium (Na+) ions at 15mM

Question 6

Concentrations outside the neuron at RMP

Answer 6

• Potassium (K+) ions at 5mM
• Sodium (Na+) ions at 145mM

Question 7

Value of potassium and sodium at equilibrium potential

Answer 7

• Potassium (K+) = -90mV
• Sodium (Na+) = +60mV

Question 8

Graded potentials

Answer 8

Changes in membrane potential that vary according to size of stimulus

• Require channels to open/close from resting state (gated-channels)
• If strong enough when it gets across body and reaches segment/trigger zone in axon hillock, they initiate an AP
• Can be excitatory or inhibitory

Question 9

Gated-channels

Answer 9

Control ion permeability across the membrane

• Mechanical gate, chemical gate, voltage gated

Threshold to open/close voltage gated channels varies from one type to anther

Question 10

Local current flow and application to graded potentials

Answer 10

Wave of positive (or negative) charge that moves through cell

• Graded potentials lose strength as they move through cell
• Become smaller due to current leak (ion escape across membrane) & cytoplasmic resistance

Question 11

Depolarisation

Answer 11

Membrane potential more positive (moving towards 0 mV)

Question 12

Hyperpolarisation

Answer 12

Membrane potential more negative (moving away from 0 mV)

Question 13

Axon hillock

Answer 13

Section of neuron between cell body (soma) and axon

• AKA initial segment of axon
• Segment/trigger zone

Question 14

Subthreshold graded potential

Answer 14

Graded potential starts above threshold at initiation point, but decreases in strength as it moves through cell body

• Once it hits trigger zone, it is below threshold, thus does not initiate AP

Question 15

Suprathreshold graded potential

Answer 15

Stronger stimulus that is still above threshold by the time it reaches the trigger zone

• Initiates AP

Question 16

Action potentials (AP)

Answer 16

Rapid sequence of changes in voltage across membrane

• All-or-none electrical impulse
• Required for neurons to send information from one part of the brain/body to another

Question 17

Ionic basis of the action potential

Answer 17

1. RMP
2. Depolarising stimulus
3. Membrane depolarises to threshold, voltage-gated Na+ and K+ channels begin to open
4. Rapid Na+ entry depolarises cell
5. Na+ channels close and slower K+ channels open
6. K+ moves from cell to extracellular fluid
7. K+ channels remain open, additional K+ leaves the cell, hyperpolarising it
8. Voltage-gated K+ channels close, less K+ leaks out of cell
9. Cell returns to resting ion permeability and RMP

Question 18

Summary of graded potentials

Answer 18

• Input signal
• Occurs usually in dendrites and cell body
• Uses chemical or voltage-gated channels
• Uses Na+, K+ and Ca2+ (calcium) ions
• Depolarising or hyperpolarising signals
• Strength of signal depending on initial stimulus
• No minimum level required to initiate

Question 19

Summary of action potentials

Answer 19

• Output & transmission signal
• Occurs in trigger zone (axon hillock) and along the axon
• Uses voltage-gated channels
• Uses Na+ and K+ ions
• Depolarising signal
• Strength of signal is all-or-none
• Threshold voltage required to initiate

Question 20

Propagation of AP’s

Answer 20

AP travelling along the axon

• Due to a +ve charge spreading along adjacent sections of an axon by local current flow
• Local current flow causes new sections of the membrane to depolarise

Question 21

What is the speed of the AP influenced by?

Answer 21

• Axon diameter
• Resistance of the axon to ion leakage

Question 22

Axon diameter

Answer 22

The larger the axon, the faster

Question 23

Resistance of axon to ion leakage

Answer 23

Myelinated (high resistance) axons

• Faster

Saltatory conduction between the Nodes of Ranvier

• Saltatory = jumping or hopping movement
• Node of Ranvier = small gaps in myelination
• If myelin is missing, conduction is reduced when current leaks out of previously insulated regions between nodes

Question 24

Process of propagation of action potentials

Answer 24

1. Graded potential above threshold reaches trigger zone
2. Voltage-gated Na+ channels open, Na+ enters axon
3. +ve charge flows into adjacent sections of the axon by local current flow
4. Local current flow from active region causes new sections of membrane to depolarise
5. Refractory period prevents backward conduction, loss of K+ from cytoplasm repolarises membrane