Chapter 4.1-4.8 Neural Conduction And Synaptic Transmission

Question 1

What is the membrane potential?

Answer 1

Difference in electrical charge between the inside and outside of the cell

Question 2

What are microelectrodes?

Answer 2

Intracellular electrodes

-tips are less than one-thousandth of a mm in diameter

Question 3

What is the potential inside the resting neuron relative to outside the neuron?

Answer 3

The potential inside the resting neuron is about 70mV less than outside

Question 4

What is the membranes resting potential?

Answer 4

-70 mV

Question 5

What is the term given to a neuron n its resting state?

Answer 5

Polarized

Question 6

In resting neurons, there are more _____ outside the cell than inside, and more _____ inside than outside

Answer 6

Na+ ; K+

Question 7

There is substantial pressure on Na+ ions to enter the resting neuron. This pressure is of two types, describe:

Answer 7

1. Electrostatic Pressure: from the resting membrane potential.
   - because opposite charges attract, the -70mV charge attracts eh positively charged Na+ ions into resting neurons
2. Random Motion: for Na+ ions to move down their concentration gradient = evenly distributed

Question 8

Why don’t Na+ ions diffuse into the neuron despite electrostatic pressure and random motion?

Answer 8

The sodium ion channels in resting neurons are closed

Question 9

In a resting neuron are potassium channels open or closed?

Answer 9

Open. Very few potassium ions leave the neuron, however, because of the negative resting membrane potential

Question 10

Some Na+ ions do manage to enter the resting neuron an some K+ ions do exit, so why does the resting membrane potential remain constant? Who discovered this?

Answer 10

Hodgkin and Huxley

-At the same rate that Na+ ions leaked into resting neurons, other Na+ ions were actively pumped out and at the same time that K+ ions leaked out of the resting neurons, other K+ ions were actively pumped in

Question 11

What is a sodium-potassium pump?

Answer 11

Transporter that actively exchanges three Na+ ions inside the neuron for two K+ ions outside. maintains membrane potential

Question 12

when neurons fire, what happens?

Answer 12

Neurotransmitters are released from the terminal buttons and diffuse across the synaptic cleft
- interact with specialized receptor molecules on the receptive membranes of The next neurons in the circuit

Question 13

Neurotransmitters bind to postsynaptic receptors and have one of two effects:

Answer 13

1. Depolarize

2. Hyperpolarize

Question 14

What is depolarization?

Answer 14

Neurotransmitters bind to postsynaptic receptors and the receptor membrane potential is decreased. (Eg Goes from -70 to -67 mV)

Question 15

What is hyperpolarization?

Answer 15

Neurotransmitters bind to postsynaptic receptors and increase the resting membrane potential (eg goes from -70 to -72 mV)

Question 16

What is another term given to postsynaptic depolarizations? Why?

Answer 16

Excitatory Postsynaptic Potentials (EPSPs)

-because they increase the likelihood that the neuron will fire

Question 17

Postsynaptic Hyperpolarizations are given what term? Why?

Answer 17

Inhibitory postsynaptic Potentials (IPSPs)

-because they decrease the likelihood that the neuron well fire

Question 18

What does it mean to say that EPSPs and IPSPs are “graded responses”

Answer 18

the amplitudes of them are proportional to the intensity of the signals that elicit them:

• weak signals = small postsynaptic potentials
• strong signals = large postsynaptic potentials

Question 19

How do EPSPs and IPSPs travel?

Answer 19

Passively from sites of generation (at synapses) usually on the dendrites or cell body
(Like an electrical current through a cable)

Question 20

What are two important characteristics of the transmission of a postsynaptic potential?

Answer 20

1. It is rapid: almost instantaneous
   Duration =/= rate of transmission
2. Transmission of EPSPs and IPSPs is “decremental”
   - decrease in amplitude as they travel through the neuron, (just as a sound wave loses amplitude (fainter))
   - means that most don’t travel very far along an axon at risk of fading out

Question 21

Whether a neuron fires depends on he balance between what?

Answer 21

The excitatory and inhibitory signals reaching the axon

Question 22

What is the axon hillock?

Answer 22

The conical structure at the junction between the cell body and the axon
-once believed to be the site where action potentials were generated

Question 23

What is the axon initial segment?

Answer 23

Adjacent section of the axon where action potentials are generated

Question 24

What is the threshold of excitation?

Answer 24

The level of neural depolarization that is necessary to generate an action potential. Usually about -65mV

-*the sum of the depolarizations and hyperpolarizations reaching the axon initial segment must be sufficient enough to depolarize the membrane to a certain level ie the excitation threshold

Question 25

What is an action potential (AP)

Answer 25

Massive but momentary (~1 millisecond) reversal of the membrane potential from about -70mV to about +50mV

Question 26

Are action potentials graded responses?

Answer 26

No.

• magnitude is not related to intensity of stimuli
• They are “All or None responses”

Question 27

What does it mean to say “All-or-None Response”?

Answer 27

They either occur to their full extent or not at all

Question 28

What is integration?

Answer 28

Adding or combining a number of individual signals into one overal signal

Question 29

Neurons integrate incoming signals I n two ways:

Answer 29

1. Over time

2. Over space

Question 30

there are three possible combinations of spatial summation:

Answer 30

1. Local EPSPs produced simultaneously = greater EPSP
2. Simultaneous IPSPs= greater IPSP
3. Simultaneous IPSP and EPSP = Cancel out

Question 31

What is temporal summation?

Answer 31

Postsynaptic potentials produced in rapid succession at the same synapse sum to form a greater signal

Question 32

What are voltage-activated ion channels?

Answer 32

• ion channels that open or close in response to changes in the level of the membrane potential
• Produce action potentials and conduct action potentials along th axon

Question 33

What are the three phases of an action potential?

Answer 33

1. Rising phase
2. Repolarization
3. Hyperpolarization

Question 34

Describe an action potential

Answer 34

Rising Phase:

• membrane potential is depolarize to excitation threshold
• voltage-activated Na+ channels in axon membrane open
• Na+ rush in
• Membrane potential goes from -70 to +50mV
• rapid change in membrane potential an influx of Na+ = opening of voltage-activated K+ pumps
• K+ ions are driven out of the cell through these channels by diffusion and by push of positive charge inside cell
• sodium chnnels close

Repolarization

• continued efflux of K+
• Potassium channels close = slow = too many K+ flow out
• leaves neuron hyperpolarized

Question 35

What is the absolute refractory period?

Answer 35

A brief period about 1 to 2 milliseconds after the initiation of an action potential during which it is impossible to elicit a second one

Question 36

What is a relative refractory period?.

Answer 36

Follows the absolute factory period
Is the period during which it is possible to fire the neuron again but only by applying higher than normal levels of stimulation

Question 37

The refractory period is responsible for two important characteristics of neural activity what are they?

Answer 37

1. It is responsible for the fact that action potentials normally travel along axons in only one direction
2. Refractory period is responsible for the fact that the rate of neural firing is related to the intensity of stimulation

Question 38

The conduction of action potentials along an axon differs from the conduction of EPSPs and IPSPs in 2 important ways:

Answer 38

1. the conduction of action potential’s along an axon is non-decremental (meaning they do not grow weaker as they travel along the axonal membrane)
2. Action potentials are conducted more slowly than postsynaptic potential

Question 39

What is the reason for the two differences in the conduction of EPSPs and IPSPs versus the conduction of the action potential’s?

Answer 39

The conduction of EPSPs and IPSPs is passive, whereas the axonal conduction of action potentials is largely active.

Question 40

What is antidromic conduction?

Answer 40

-The direction of action potential moving along the axon back to the cell body

If electrical stimulation of sufficient intensity is applied to the terminal end of an axon, an action potential will be generated and will travel along the axon back to the cell body

Question 41

What is orthodromic conduction?

Answer 41

Axonal conduction in the natural direction i.e. from cell body to terminal buttons

Question 42

In myelinated axons, Ions can pass through the axonal membrane only at the___

Answer 42

Nodes of ranvier

Question 43

What are the nodes of Ranvier?

Answer 43

The gaps between adjacent myelin segments

-where axonal sodium channels are concentrated

Question 44

When an action potential is generated in a myelinated axon, the signal is conducted_____

Answer 44

Passively: instantly and decrementally

-along the first segment of myelin to the next node of ranvier = jumps

Question 45

What is saltatory conduction?

Answer 45

Transmission of action potentials in myelinated axons

Question 46

The speed that action potentials are conducted along an axon depends on two properties of the axon:

Answer 46

1. Conduction is faster in large-diameter axons
2. Conduction is faster in myelinated axons

Motor neurons are large and myelinated = 100m/s

Question 47

Action potentials are the means by which axons conduct _____________ signals ___________ over relatively _______ distances

Answer 47

All or none;
Nondecrementally;
Long

Question 48

Conduction in interneurons (which have short axons or no axons) is typically _______ and ________

Answer 48

Passive ; decremental

Question 49

What are some properties of central neurons that aren’t shared by motor neurons and thus make the Hodgkin-Huxley model somewhat unreliable when applied to cerebral neurons.

Answer 49

1. Many cerebral neurons fire continually even with no input
2. Axons of some cerebral neurons can actively conduct both graded signals and action potentials
3. Action potentials of different classes of cerebral neurons vary in duration, amplitude and frequency
4. Many cerebral neurons don’t display action potentials
5. The dendrites of some cerebral nouns can actively conduct action potentials