Biol 225 (A&P) - Excitable cells (Obj. #11 - 19) - Dr. Steele

Depolarization

For this phase of an action potential, the ionic events include the opening of Na+ gates and the following influx of Na+ and the positive charges that follow (not offset by the flow of K+, and difference across membrane becomes positive)

Peak

For this phase of an action potential, the ionic events responsible include the opening of voltage-gated K+ channels. Once this is reached, depolarization starts and Na+ gates close and K+ gates open, carrying their positive charge from the inside of the cell

Repolarization

For this phase of an action potential, the ionic events responsible include the reduced influx of Na+ due to the closing of inactivation Na+ channels and voltage-gated K+ channels staying open as well as K+ continuing to leave through leaks and open channels

Hyperpolarization

For this phase of an action potential, the ionic events responsible include the staying open of voltage-gated K+ channels which cause a great loss of K+. The inactivation gate of voltage-gated Na+ channels open which allow Na+ to start to flow back

After-hyperpolarization

For this phase of an action potential, the ionic events responsible include the closing of voltage-gated K+ channels and the membrane potential returning to the resting value

The inactivation of the channel is the determining factor as to if Na+ will pass through. The gated ion channel can be open, and the inactivation gate can be closed. This results in no flow of Na+. The inactivation gate needs to be open to transfer Na+.

The inactivation of the channel prevents it from carrying another impulse. If the Na+ channel is only closed, a big enough stimulus can cause the the transfer of an impulse.

What is the difference between a gated ion channel that is closed and a gated ion channel that is inactivated

Absolute refractory period

This refractory period is the guaranteed return to resting potential where a normal stimulus causes the reaction to occur again

Relative refractory period

This refractory period requires a larger stimulus to return to resting potential and then be activated again - might be able to start another action potential, but need a larger stimulus - this causes action potentials to not be able to sum together

As the action potential transfers down the axon, the positive charge inside spreads. The threshold voltage needed is -55mV, and the peak of threshold is 30mV. This extra voltage above threshold is called overshoot.

Threshold must be met to transfer the signal, and the difference in charge between the threshold and the peak has the purpose of increasing the likelihood of passing the signal by achieving threshold in the next segment

Once the stimulus passes through, depolarization occurs as well as hyper polarization and Na+ channels are inactive. This makes the impulse travel in only one direction

How are action potentials propagated down the axon and the role of positive feedback in impulse propagations?

Myelin is made out of cell membranes (phospholipids) and is basically fat that insulates the axon, protecting the stimulus as it travels along the axon, and speeding up the travel of the impulse

What is the role of myelin in impulse conduction?

Myelin insulates the +30mV action potential and spreads the charge until it reaches the next node - myelin sheath is the perfect length to transfer the action potential to achieve threshold (-55mV) over its distance - Myelin allows the signal to travel much further and quicker as it jumps from node to node (saltatory conduction) during the transfer of the signal

Why does myelin lead to saltatory conduction?

Voltage-gated ion channels

The role is to trigger the release of neurotransmitters

At chemical synapses, an impulse opens this and allows the entry of Ca2+ through its gradient (diffusion) which causes the vesicles filled with neurotransmitters to migrate towards the membrane and eventually release into the synapse

Ligand-gated ion channels

Role is to trigger depolarization

At chemical synapses, the released neurotransmitter causes this to open due to excitatory post-synaptic potentials and depolarization

Exocytosis requires ATP to expel the contents (neurotransmitters) of vesicles (membrane bound sacks) into the synapse. The vesicle and the membrane both have phospholipid bilayers, so they fuse and neurotransmitters are able to be released

How are neurotransmitters secreted through exocytosis?

Either by a reuptake process which returns them to their original location, or an enzyme destroying the neurotransmitter in the synapse

How are neurotransmitters removed from chemical synapses?

Gap junctions allow excess Na+ to pass through and raise the membrane potential of the next cell. If enough electrical charge is Brough into the following cell due to the overshoot, it can create excess positive charge that can trigger the achievement of threshold

What is the role of gap junctions in electrical synapses?

Ligand-gated ion channels

What type of channels are located on the dendrites?

Voltage-gated ion channels

What type of channels are located on the axon?

Electrostatic forces

This is another cause to the flow of ions due to their charge and not necessarily concentration

All action potentials are identical, result in the exact same response every time

Do action potentials differ?

They do not change

During an action potential, what happens to the overall concentration gradients for Na+ and K+?

More permeable to K+

Are cell membranes more permeable to Na+ or K+?

K+ leak channels are always open and pumps are always working

These channels are always open during an action potential

This system is also always working

Co-transportation

Na+ can be transported with molecules like glucose, what is this called?

Activation gate and inactivation gate

What are the two gates of a Na+ voltage gated ion channel?

Activation gate

This gate in a Na+ voltage channel opens to let ions through

Inactivation gate

This gate in a Na+ voltage channel shuts down its function and overpowers the activation gate

Resting

During this part of the action potential process, there is a slight K+ leak, and the cell is more permeable to K+ here. Most Na+ remains outside the cell

Stimulus

During this part of the action potential, both Na+ and K+ gates start to open as it gets closer to threshold (around -55mV) - once threshold is reached, Na+ channels are opened, and Na+ brings in positive charges - voltage gated channels start to open

Rising

During this part of the action potential, there is Abigails influx of Na+ due to the voltage-gated Na+ channels of the trigger zone being opened - there is a net gain of positive charge in the cell (depolarization) which causes the difference across the membrane less pronounced and closer to threshold

Depolarization

During the rising portion of an action potential, they is the net gain of positive charge in the cell

Na+

During depolarization, __________ gates open and there is an influx of __________ with positive charges coming in. This large influx is not offset by the flow of K+, the difference across the membrane is much less (becomes positive)

Falling

During this part of the action potential, no more Na+ is allowed to enter the cell as inactivation gates are closed. K+ voltage channels open and positive charge is lost

Peak phase

At this part of the falling portion of the action potential, voltage-gated K+ channels open, Na+ channels inactivate, and an absolute refractory period occurs

Inactivation gates of Na+ ion channels close it off and do not allow the transfer of a stimulus - K+ channels are dependent on Na+ channels regarding the transfer of an impulse, so nothing can be done when inactivated

How come a stimulus only travels in one direction?

Repolarization phase

At this part of the falling portion of an action potential, there is a reduced influx of Na+ as inactivated Na+ channels close and voltage-gated K+ channels remain open. K+ leaves through open channels and leaks which causes hyper polarization as they pass resting potential

Hyperpolarization phase

This part of the falling portion of an action potential occurs when voltage-gated K+ channels remain open, the inactivation gate of voltage-gated Na+ channels reopen, and a relative refractory period occurs

Inactivation gates

This part of a Na+ channel prevents the start of another action potential when not at the absolute refractory period

After hyperpolarization

This stage of an action potential occurs when Na+/K+ pump restores the charge balance and voltage-gated K+ channels close and membrane potential returns to its resting value

Hyperpolarization

This part of an action potential occurs when the charge becomes more positive inside the cell than normal. K+. voltage gates stay open long enough to go past regular resting potential and the cell is much less responsive during this time

Inactivation gates

What is the reason summation cannot happen?

Refractory period

This is a time of reduced responsiveness in and action potential

Absolute refractory period

During this time, it is completely impossible for nerves to be stimulated and action potentials cannot be summed together - this is in the falling section because Na+ channels are inactivated

Rising

This is the only stage of an action potential where there is a heavy influx of Na+ ions

Beginning of the rising phase

Threshold is reached here in an action potential, where Na+ flows in at a greater rate than K+ flowing out

Inactivate

At the peak of an action potential, Na+ channels __________, meaning they cannot be stimulated

Voltage-gated K+ channels are closing

From falling to resting, K+ flow decreases because voltage gated K+ channels are __________

Activation gates are closed and inactivation gates are open

At rest, describe the position of the gates of a Na+ channel

Depolarization

Subthreshold causes Na+ channels to fully open (Activation gate opens) and Na+ flows in, __________ occurs when the c channel is open

Sodium

At the peak and depolarization, inactivation gates close and shuts down the flow of what ion?

Overshoot

This is the extra voltage above the threshold in action potential propagation

Has the purpose of increasing the likelihood of passing the signal by achieving threshold in the next segment

Difference in charge between threshold and peak has what purpose?

Because K+ channels functions are dependent on Na+ channels and whether they are open or not

What do K+ channels not need inactivation gates?

Voltage gated ion channels

The trigger zone is full of what type of ion channels?

Oligodendrocytes in the CNS and Schwann cells in the PNS

What are the two different ways to insulate axons?

Oligodendrocytes

In the central nervous system, these are cells with a few branches that wrap themselves around the axons to insulate them. Their cell bodies are not fully responsible for myelin creation, but can make several sections of myelin

Schwann cells

In the peripheral nervous system, these are wrapped around and around the neuron to create myelin - these are dedicated to making one section of myelin

1. A neurotransmitter is released via exocytosis from a presynaptic neuron (knob)

2. The neurotransmitter binds to the receptor on the postsynaptic cell

3. Neurotransmitter changes the permeability of the postsynaptic cell's membrane by opening ligand-gated ion channels

4. Action potential stimulates voltage-gated Ca2+ channels which allow diffusion of Ca2+ into the cell (Ca2+ channels are not present at the axon) - concentration gradient and electrostatic forces attract Ca2+ into postsynaptic cell

5. Vesicles are filled with neurotransmitters and are surrounded by cell membranes, so those sacks are expelled from the knob and absorbed into the postsynaptic cell - calcium stimulates the release of neurotransmitters

5.

Explain synaptic transmission

Synapses

These are the ways that cells communicate with one anotehr

Smooth and cardiac muscle

Cell-to-cell transmission of impulses is present in which types of muscle?

Schwann cells

These cells make myelin in the peripheral nervous system

Oligodendrocytes

These cells make myelin in the central nervous system

Saltatory conduction

This is the moving of an impulse from one node to another (jumping) through the myelinated portion of the axon

Nodes

These are the only places that actually depolarize/repolarize/have ionic movement during an action potential

True

True or False?

To transfer the stimulus across an entire neuron, if enough Na+ comes into the cell through one channel, surrounding. voltage gated channels can by stimulated as well

Post-synaptic cells have voltage gated channels on the cell body and elsewhere, so positive feedback can cause stimulation of these channels by continuing the trend of opening more channels

How does positive feedback work on post-synaptic cells?