4b Electrochemical Physiology

Question 1

What is the irritability of a neuron?

Answer 1

The ability to respond to a stimulus and convert it to a nerve impulse

Question 2

What is the conductivity of a neuron?

Answer 2

The ability to transmit the impulse to other neurons, muscles or glands

Question 3

What is a nerve impulse?

Answer 3

Aka action potential
An electrochemical signal involving sodium and potassium ions that cross the cell membrane through ion channels

Question 4

Are ion channels permeable by different types of ions?

Answer 4

No, designed for only 1 type of ion to pass through

Question 5

What is the state in which a neuron is not conducting a nerve impulse?

Answer 5

At rest, inactive, resting membrane porential

Question 6

Describe the conditions of resting membrane potential.

Answer 6

Inner surface is about 70 mV more negative than the outer surface
More Na+, Cl- and Ca+ outside, more K+ inside

Question 7

How does the neuron regulate the concentration of ions that constitute the resting membrane potential and how?

Answer 7

Sodium/potassium pump (Na+/K+ pump)
Pushes Na+, sodium, out of the membrane (3) and brings in K+, potassium, inside (2)

Question 8

Where are Na+/K+ pumps found?

Answer 8

Everywhere on a neuron where you have plasma membrane (axon, cell bodies, dendrites)

Question 9

What are the stages in an action potential?

Answer 9

Resting membrane potential
Threshold
Depolarization
Repolarization
Hyperpolarization

Question 10

What does it mean for a neuron to have a threshold?

Answer 10

Required level (from -70 to -55mV) for there to be a depolarization to fire the impulse
To get to this change, the electrical stimulus must be strong enough
-55 is like the tipping point for depolarizing the cell membrane potential

Question 11

On an ionic level, what does the threshold mean?

Answer 11

There must be a graded potential (push needed to trigger depolarization) which happens when enough Na+ or Ca+ enters the neuron from the dendrites and cell body

Question 12

Where does the actually firing of the nerve impulse begin?

Answer 12

Axon hillock

Question 13

How do the relevant ions enter when trying to reach the threshold and what are they?

Answer 13

Na+ and Ca+
Through receptors

Question 14

What is the all or none law?

Answer 14

The neuron will fire or will not
Can’t have half an action potential, neuron fires or does not

Question 15

Are all neurons in the body have the same threshold? Can the threshold vary within a specific neuron?

Answer 15

No, threshold can vary, not be too much but not fixed value
Within a specific neuron it will always fire at that same threshold

Question 16

What are the basic steps in a nerve impulse?

Answer 16

1. Voltage gated Na+ channels open along the axon membrane
   Na follows the gradient build by the pump and rushes into the neuron
   Peak is reached. Na+ channels close. Next.
2. Voltage gated K+ channels open along the axon membrane
   K follows the gradient and leaves the neuron

Question 17

What happens specifically in the depolarization phase?

Answer 17

Na+ channels are opened by the reaching of the threshold and Na+ rushes INTO the neuron, making it more positive

Question 18

What happens specifically in the repolarization phase?

Answer 18

K+ channels are open and K+ starts rushing OUT of the neuron, reestablishing some of the resting potential conditions by making the inside less positive

Question 19

Are the gated ion channels responsible for maintaining the k+ and Na+ gradient when the neuron is at rest?

Answer 19

No, not a pump, dont control that
Only come into play during the actual action potential

Question 20

Does the strength of the stimulus determine the strength or intensity of the action potential?

Answer 20

No

Question 21

What is the purpose of the refractory period?

Answer 21

To ensure that an action potential will not go backwards, towards the dendrites

Question 22

When is the absolute refractory period? Is it negociable?

Answer 22

After the voltage gated Na+ channels close and become inactive
Region from around stimulus to a little after the peak
Non negotiable, there is no stimulus that can trigger another action potential at this time

Question 23

Why is the absolute refractory period a range and not a specific point in time?

Answer 23

Because the depolarization doesnt occur all at once, it’s more like a chain reaction that starts with a stimulus strong enough to trigger depolarization all along the axon. Not instantaneous, has to make its way and depolarizes in sections
Technically possible for one region of the axon to be repolarizing while another is still depolarizing, thus a range of instances where the channels cannot be reopened

Question 24

When do most Na+ channels come out of the inactive phase?

Answer 24

Somewhere in repolarization in what’s called the relative refractory period

Question 25

Can there be another nerve impulse in the relative refractory period? If so, under what conditions?

Answer 25

Yes, but since the neuron is repolarizing and is more negative than ever, the stimulus would have to be greater than the threshold to reach the -55 mV necessary for depolarization

Question 26

What is teh direction of the nerve impulse, at all times?

Answer 26

From axon hillock -> axon terminals

Question 27

What can cause a graded potential?

Answer 27

A stimulus like a neurotransmitter

Question 28

In the repolarization phase, is teh concentration of Na+ and K+ the same as in the resting membrane potential?

Answer 28

No, though the charge seems similar, the pump must sort out the concentrations

Question 29

Where does the electrical impulse travel?

Answer 29

Along the axon

Question 30

Where does the graded potential happen?

Answer 30

Dendrites and cell body

Question 31

What is a synapse and what happens there in terms of transmission of an electric impulse?

Answer 31

Chemical impulse A function junction where an impulse is transmitted between a neuron and another cell (could be a neuron or not)

Question 32

How does synaptic transmission work?

Answer 32

Communication by neurotransmitters

Question 33

What is a neurotransmitter and what does it do in this context?

Answer 33

Chemical messengers stored and released from axon terminals Transmits or terminates the nerve impulse

Question 34

What are the structures involved in a synapse?

Answer 34

synaptic knob Synaptic cleft Ion channel Synaptic vesicle Post and pre synaptic neuron

Question 35

How wide is the synaptic cleft and what occurs there?

Answer 35

<20 nm wide Neurotransmitters diffuse across the gap Synaptic knob swells at the axon terminal of the presynaptic neuron

Question 36

What are the steps to the release of a neurotransmitter?

Answer 36

1. Nerve impulse reaches synaptic knob or presynaptic neuron 2. This arrival opens Ca+ channels and allows them to move into the synaptic knob 3. Ca2+ causes synaptic vesicles containing neurotransmitter to bind to the presynaptic membrane and release the neurotransmitter into the synaptic cleft 4. NT diffuses across teh cleft and binds with receptors on the membrane of post synaptic neuron 5. Ion channels on the post synaptic neuron open, transmitting the process

Question 37

How long do neurotransmitters stay in the cleft?

Answer 37

A few hundred microseconds

Question 38

How are neurotransmitters removed from the synaptic cleft?

Answer 38

Deactivated by enzymes: acetylcholinesterase removes acetylcholine Pumped back into synaptic knob of the presynaptic axon

Question 39

What are the types of synapses at the post synaptic cell?

Answer 39

Excitatory synapse: excitatory post synaptic potential (EPSP), increases the chance that an action potential will begin Inhibitory synapse: inhibitory post synaptic potential (IPSP), decreasing the chance that an action potential will occur

Question 40

How many synapses (approx) are there potentially between neurons and will they all have the same effect?

Answer 40

Up to 10 000 synapses, some of which have excitatory effects and other inhibitory effects, some even have both

Question 41

What is the summation in an action potential?

Answer 41

The effects of the EPSP and IPSP (sum of graded potentials) determine whether an action potential is generated I.e. if its mostly inhibitory, no transmission, opposite is true This level of integration provides fine control over neuronal responses

Question 42

What are the types of summation?

Answer 42

Temporal: high frequency nerve impulses typically from the same presynaptic neuron allows the release of the same neurotransmitter (either EPSP or IPSP) and if they’re in rapid enough succession, the amount of neurotransmitter can pile up (summation) and lead to a greater overall change in membrane potential in the next neuron Spatial: all at once, multiple presynaptic neurons all firing to the same post synaptic neuron to the point that the effect (either IPSP or EPSP) becomes additive, combined effects from different synapses or different neurons