W2: 2.3 Transmembrane Transport 2

Question 1

how does the potassium gradient across a nerve cell membrane generate a voltage gradient across it?

Answer 1

• K+ generates a voltage gradient across the nerve cell membrane.
• cell has a high internal K+ con’t due to activity of K+ ion exchange pump
• K+ selective ion channels in the membrane will cause K+ to flow out of the cell down it’s concentration gradient.
• Each K+ ion that moves out of the cell carries one positive charge with it, making the interior one charge more negative
• The interior cell then becomes so negative that the K+ ions stop leaving the cell.
• The tendency of (-) charged interior to attract (+) charged ions back to the cell will stop the flow of K+ out of the cell.

Question 2

What are the main parts of a neuron? What functions do they perform?

Answer 2

Dendrites - receive information from other cells/stimuli/neurons into the cell body

Cell body - nucleus of the neuron + cell machinery

Axon - long processes the cell uses to send info outward to other cells. Some axons are short such as in the brain but other can be feet in length such as those produced by motor neurons

Synapses: specialized junctions between the nerve cells axon terminal and targets

Question 3

Explain the mechanisms responsible for the creation and movement of a depolarization wave in nerve signaling.

Answer 3

Resting potential is -70mV and sodium channels on nerve cell are closed at this voltage.

• Voltage change (+change) or action potential can raise the membrane potential and the Na+ channels open. This happens at -50mV.
• Na+ will rush into the cell rapidly and make the interior of the cell POSITIVE (40mV)
• This change in overall membrane potential (-70mV to 40mV) is DEPOLARIZATION
• At 40mV Na+ channels will close and b/come inactive. (Na+ is pumpked out of cel and K+ gradient is reestablished)

Question 4

What is the trigger that initiates nerve cell depolarization

Answer 4

positive voltage change in resting membrane potential. Can be caused by a pulse of current (voltage) aka Action potential

Question 5

What is myelin sheath, what produces it and what is it’s function?

Answer 5

Produced by schwann cells or oligodendrocytes

-myelin sheat is an electrical and ionic insulator. It allows the wave of depolarization to have a quicker transmission (speeds up to 120meters/sec)

the influx of ions at one node can create changes in the voltage of the membrane potential surrounding it for a much larger distance than would be the case if there was no myelin sheath. Consequently, the movement of the depolarization wave moves along the distance between nodes at the diffusion rate of sodium ions, which is faster than the rate of opening and closing the channels in the intervening segment between the nodes

Question 6

What is long term potentiation?

Answer 6

Stimulatory pathways in the brain that are used heavily become more easily stimulated. the cell becomes sensitive and is able to further input signal and more easily activated. This heightened state of sensitive can last many days or weeks and it’s called long term potentiation.

Question 7

What is the mechanism of long term potentiation?

Answer 7

LTP occurs on any occasion when a presynaptic cell fires (once or more) at a time when the postsynaptic membrane is strongly depolarized (either through recent repetitive firing of the same presynaptic cell or by other means).

Alberts, Bruce. Molecular Biology of the Cell (p. 636). W. W. Norton & Company. Kindle Edition.

Process involves a channel that responds to voltage and neurotransmitter ligand.

• postsynaptic cell can recognize that it is being heavily stimulated b/c neurotransmitter will remain present in the synapse even after membrane is depolarized (an action potential has been produced by the postsynaptic cell).
• Ca++ channels on the post cell will be activated if the membrane is depolarized and neurotransmitter is present.
• Ca++ enters the cell and causes downstream signaling events which lead to the production of more receptors for neurotransmitters on the cell.

Question 8

What is a node of ranvier and what sort of membrane proteins are found there?

Answer 8

node of ranvier are gaps present between the schwan cells

-sodium channels are highly concentrated at these nodes

Question 9

What kinds of transport mechanisms are involved in producing muscle contraction in response to nerve cell signaling?

Answer 9

Depolarizing nerve cells by opening Sodium channels
Action potential will propagate down to the axon terminal
At the terminal calcium channels open and this signals secretory vesicles containing neurotransmitter acetylcholine to be released into synaptic cleft
Acetylcholine opens sodium gated channel on the muscle cell and this creates and acton potential that travels across the muscle cell
finally the muscle cells membrane has invaginations that reach into the cells cytoplasm. There are voltage gated calcium channels on those membranes, which open and calcium into the cytoplasm, the calcium is used to signal the internal components of the muscle cell to contract

Question 10

What is patch clamping and how is it performed?

Answer 10

Patch clamp recording allows the study/examination of a single channel protein in a patch.

• glass micropipette with opened diameter of less than 1 micron is used and positioned against the side of a cells membrane
• small negative pressure (or suction) is applied to the micropipette/needle. This pulls a patch of the membrane against the needle opening.
• an electrode is placed into the needle and many diff kinds of measurements can be made.

Question 11

What are nerve cells

Answer 11

carriers of information

Question 12

In a neuron/nerve cell what is the normal membrane potential AKA resting potential? What does this mean?

Answer 12

• about -70mV
• this means that the inside of the cell is negatively charged relative to the outside of the cell and the membrane is POLARIZED

The charge difference creates the membrane potential

Question 13

What ion has the greatest influence on membrane resting potential?

Answer 13

K+movement /gradient creates the resting potential. Potassium channels are present and open in nerve cell membranes

Question 14

Why is axon potential propagation unidirectional?

Answer 14

B/c sodium channel inactivation prevents action potential to move backwards.

Question 15

How do nerve cells integrate signals from more than one input source?

Answer 15

By the frequency of an action potential

• the response of the postsynaptic cell could be related to how many action potentials it receives & frequency .
• if it receives many over short time, the combined effect, called postsynaptic potential or PSP will be large and the cell will respond
• if only a few action potentials are received in a unit of the the PSP will be small and the postsynaptic cell might not responds

Question 16

How might a postsynaptic neuron respond to a large PSP (postsynaptic potential)?What about a low PSP?

Answer 16

by sending many action potentials of its own high frequency along the axon

-low PSP values might produce less frequent outputs

Question 17

How is equilibrium membrane potential for K+ measured?

Answer 17

in mV when there is no movement of K+

Question 18

What will happen when K+ is leaving the inside of the cell and the interior of the cell becomes extremely negative?

Answer 18

K+ ion will stop leaving the cell and this happens when the concentration gradient that is moving potassium out of the cell is balanced by the tendency of negatively charged interior to attract positively charged ions back into the cell.

Question 19

When K+ is moving out, what are two reason that Na+ might not move across the cell membrane to balance the charge difference created?

Answer 19

1) sodium/potassium pump is working hard to keep sodium concentrations in the cell low.
2) although the potassium channels (K+ leaky channels) are present and open in nerve cell membranes there is no similar open channel for sodium. At -70mV sodium channels are closed

Question 20

When membrane of the nerve cells reach about +40mV what does the cell do to resent the membrane potential?

Answer 20

• Na+ will reach it’s equilibrium and Na+ channels will become inactive
  2) Na+/K+ pump and K+ leaky channels help reset membrane potential

Question 21

Na+ rushing into the interior of the cell making it - charged to + charged, also known as what?

Answer 21

depolarization of the cell

Question 22

The large change in membrane potential away from resting potential is commonly known as?

Answer 22

an action potential and action potentials can travel along nerve processes

Question 23

What prevents action potential from being multidirectional?

Answer 23

Sodium channel inactivation prevents action potential from moving backwards and forces AP to move in one direction

Question 24

What is movement of depolarization in one direction called?

Answer 24

a depolarization wave

Question 25

What is patch clamp recording?

Answer 25

technique that allows examination of a single channel protein in a patch. Seal is tight and current can only enter through that single channel.

Question 26

How does a patch clamp work

Answer 26

1) fine glass needle/pippete created with diameter of less than 1 micron
2) Needle positioned against the side of cell membrane
3) small neg pressure or suction is applied to needle - pulls a patch of membrane against the needle opening
4) Needle can be left at cell or pulled away carrying patch w/it.
5) electrode is then placed into needle and different kinds of measurements can be made.

Question 27

Specialization of fast waves of depolarization occurs where?

Answer 27

peripheral neurons created by supporting cells called schwann cells

Question 28

Why would a depolarized wave be slower if no myelination is present?

Answer 28

B/c the myelination layer is electrical and ionic insulator so the influx of ions at one node can create changes in the voltage of the membrane potential surrounding it for much larger distance. This couldn’t happen w/o myelin

Question 29

What can happen If ligand gated sodium channels are opened on the post synaptic cell?

Answer 29

the post synaptic cell might begin a new action potential on post synaptic cell.

Question 30

What is considered the electrical signal and chemical signal in neuron signaling?

Answer 30

Electrical: pre-synaptic action potential

Chemical: voltage gated calcium channels stimulating synaptic vesicles and releasing their neurotransmitters into synaptic cleft. (diffusion of these to postsynaptic is chemical since neurotransmitter binds to ligand gated ion channels.

Question 31

What are the five ion channels in neuromuscular signaling? List them in order?

Answer 31

1) At end of axon - Voltage gated calcium channels.
2) At synapse - regular calcium channels & release acetylcholine
3) Acetylcholine opens ligand gated Na+ channels on muscle cell
4) New AP is created in this cell and travels opening voltage gated sodium channels
5) also open voltage gated channels

Question 32

How is long term potentiation (learning) generated?

Answer 32

by synthesis of additional neurotransmitter receptors in response to Ca++ signaling.

Question 33

Long term potentiation involves what kind of channel?

Answer 33

one that responds to both voltage and the presence neurotransmitter ligand.