Lecture 3.2: Electrical Communication with a Neuron

Question 1

Neurons continuously receive info:
- PSPs or synaptic unputs can change the ______ of a neuron

• Neurons integrate these inputs
• Sufficient _______ will reach a threshold for an AP

Answer 1

PSPs or synaptic inputs can change the membrane voltage of a neuron

Neurons integrate these inputs

Sufficient excitatory signals will reach a threshold for an AP

Question 2

APs are conducted along ____

Answer 2

APs are conducted along axons both myelinated and unmyelinated

Question 3

When the threshold for an AP is achieved, membrane depolarization is _______, meaning _______

Therefore the AP depends on _____ or ______

In contrast, EPSPs and IPSPS are integrated without amplification; they depend on ______ of the neuron

Answer 3

When the threshold for an AP is achieved, membrane depolarization is amplified, meaning it increases very rapidly over milliseconds, therefore the AP depends on active currents or active electrical properties of the neuron

In contrast, EPSPs and IPSPs are integrated without amplification; they depend on the passive properties of the neuron

Question 4

Explain which receptors mediate fast and slow IPSPS and EPSPs

Answer 4

Ionotropic Receptors (FAST):

• EPSPs are mediated by AMPA
• IPSPs are mediated by GABAa

Metabotropic Receptors (slow):

EPSPs are mediated by many such as mGLUR

IPSPs are mediated by GABAb

Remember, ionotropic is fast, metabotropic is slow

Question 5

Explain the relationship between synaptic voltage, input resistance and current

What happens if Rinput is large?

If Rinput is slow?

Answer 5

Synaptic voltage = input resistance * current

The voltage response to a sunsequent stimulus depends on Rinput

If Rinput is large (closed channels) -> voltage change is larger

If Rinput is small (open channels) then the voltage change is smaller

Question 6

If an input arrives during a long lasting IPSP, it causes _____ in V than if it arrives at resting conditions

Answer 6

If an input arrives during a long lasting IPSP it causes less change in voltage than if it arrives at resting conditions

Question 7

At a given moment, a neuron receives dozens of inputs to different areas of the cell

Explain spatial summation

Explain temporal summation

Neurons are continually summating inputs, concurrently employing ______

Answer 7

Spatial summation: PSPs that happen at the same time can summate over the entire cell surface

Temporal summation: PSPs that occur in one place summate over time

Neurons are continually summating inputs, concurrently employing spatial and temporal summation

Question 8

Define the legnth constant?

What is it in words?

What is the equation?

What two parameters matter?

Answer 8

Length constant is a measure of how much a potential change decreases as it travels

It is the distance that a potential travels before it drops to 37% of its original value

It depends on two parameters : membrane resistance and axial resistance

Equation is length constant = square root of (rm/ra)

Question 9

PSPs travel further if the membrane/axial resistance is high and or the membrane/axial resistance decreases

Axial resistance is high in ___ axons and low in ___ axons

________ axons have larger length constants

Answer 9

PSPs spread further if the membrane resistance is high and the axial resistance is low

Axial resistance is high in thin axons and low in large diameter axons

Large-diameter, myelinated axons have the larger length constants

Question 10

Neurons with long legnth constants do what?

Neurons with short length constants are only affected by ______

Answer 10

Neurons with LONG length constants summate potentials arriving from widely dispersed sites

Neurons with short length constants are only affected by nearby synaptic potentials

Question 11

Define time constant

What two parameters matter for time constant?

What is the equation?

Answer 11

Time constant: the time it takes a potential to rise 63% of its final value

T depends on membrane resistance and membrane capacitance

T = Rm * Cm

T increases as rm increases

T increases as cm increases

Question 12

What does the time constant essentially stand for?

Answer 12

Time constant reflects the time needed to charge the membrane and also denotes the time needed to discharge the voltage

Question 13

Neurons with low capacitance are what size? What happens to their charge and potentials reaching their peak?

What about neurons with high capacitance? Size? Charge speed?

Answer 13

Neurons with low capacitance (small neurons) charge rapidly and potentials reach their peak fast

Neurons with large capacitance (large somas) charge slowly, stretching the synaptic input’s effect over a large period of time

Question 14

Action Potentials are an all or nothing event:

• After reaching its threshold the AP (or spike) reaches roughly ____ mV and rapidly repolarizes to hyperpolarized potentials (undershoot) before going back to restine conditions
• The AP travels ________ distances, “boosting” the signal to convey info
• It results in ____ release

Answer 14

Action Potentials are all or nothing:

• After reaching its threshold the AP reaches roughly 40 mV and rapidly repolarizes to hyperpolarized potentials (undershoot) before going back to resting conditions
• APs travel long distances, “boosting” the signal to convey info
• It results in NT release

Question 15

Action Potentials:
- Rising phase is due to ____ channel activation, its brief duration due to ____

• Repolarization is due to _____ channels towards ___
• ________ also causes the undershoot or after-hyperpolarization (AHP)

Answer 15

Action Potentials:

• Rising phase is due to Na channel activation, its brief duration due to sodium channel inactivation
• Repolarization is due to potassium channel (delayed rectifiers) towards Ek
• Potassium conductance also causes the undershoot or after-hyperpolarization

Question 16

• Sodium permeability increases ____ at the start of an AP
• Potential is dominated by ____, it doesn’t reach it because ______ and the _____
• Threshold depends on the number of channels that are _____
• Only after the ____, Na channels recover from inactivation

Answer 16

• Sodium permeability increases 20x at the start of an AP
• Potential is dominated by Ena= +60, but it doesn’t reach it because rm decreases and the driving force for Na decreases too
• Threshold depends on the number of channels that are inactivated (failure of Na channels to inactivate causes paralysis)
• Only after AHP, Na channels recover from inactivation

Question 17

During the refractory period, Na channels cannot be opened, therefore APs _______ (called the ____)

When Na channels recover from inactivation, they are closed but _____. At the same time, K conductance is very high, resulting in the ____. During this period, a larger input is necessary to reach the threshold and fire an AP. This is called the ______.

Answer 17

During the refractory period, Na channels cannot be opened, therefore APs cannot be elicited (this is called the absolute refractory period or ARP)

When Na channels recover from inactivation, they are closed but available for opening again. At the same time, K channel conductance is very high, resulting in the AHP (hyperpolarization). During this period, a larger input than necessary to reach threshold is needed to fire an AP. This is called the relative refractory period (RRP)

Question 18

The refractory period polarizes AP conduction, providing ____

Answer 18

The refractory period polarizes AP conduction, providing directionality

Question 19

Unmyelinated axons conduct APs at speeds proportional to _____

Answer 19

Unmyelinated axons conduct APs at speeds proportional to their diameter

Question 20

An AP travelling down the axon from point A to B changes the memrbane potential at B until it reaches threshold and fires an AP

The speed (or “______”) depends on _______ (give the equation)

Ra ______ with increasing axon diameter

APs propagate faster in ______ fibers

Answer 20

The speed or “conductance velocity” depends inversely on the axia resistance and membrane capacitance

So conduction velocity = (1/ ra cm)

Ra decreases with increasing axon diameter

APs propagate faster in larger diameter fibers

Question 21

Conduction velocity is proportional to _____

AP propogation is slower in _____ and faster in _____

Answer 21

Conduction velocity is proportional to 1/ ra * Cm

AP propogation is slower in small diameter fibers and faster in larger diameter fibers

Question 22

Rather than increase diameter to increase velocity, the evolutionary strategy to increase AP conduction velocity is to ____ Cm and increase resistance by ______

Glia produce _____ that wraps around the axon and speeds up conduction velocity

Myelin is made up of 80% ____

Myelin has a few channels or ____ specific resitance

Answer 22

Rather than increase diameter to increase conduction velocity: evolution decreased Cm and increase specific resistance by myelination

Glia produce myelin

Myelin is made up of 80% lipids (galactocerebroside is the main lipid)

Myelin has a few channels or “high specific resistance

Question 23

• Each myelin wrap consists of ___ and _____
• The bilayers between wraps are joined together by ______ and _____ which increase the resistance of the myelin wrap
• ______ are the regions of bare axons where Na channels are located and APs occur
• Axons > ____ are myelinated

Answer 23

Each myelin wrap consists of two lipid bilayers and the cytoplasm

• The bilayers between wraps are joined together by adherens junctions and tight junctions, both of which increase the resistance
• Nodes of ranvier are the regions of bare axons where na channels are located and APs occur
• Axons > 1-2 microns are myelinated

Question 24

Demyelinating diseases destroy the myelin wrap:
Name three of these diseases

What are their concequences?

Answer 24

Demyelinating diseases destroy the myelin wrap:

Examples: Guillan-Barre Disease

Multiple Sclerosis

Charcot-Marie-Tooth disease

Concequences: slow conduction, failed AP propagation, high metabolic cost of axon restructuring

Question 25

APs are self-______

APs are uni______

APs allow neurons with processes of up to two meters in length to reliably and rapidly send info gathered at one end of the body all the way to the other end, effectively ensuring survival of the organism

Answer 25

APs are self-perpetuating

APs are uni-directional