Nervous System

What does acetylcholine bind to

Acetylcholine binds to cholinergic receptors.

How do changes in membrane potential occur during an action potential

As a result of time-dependent opening/closing of Na+ and K+ voltage-gated channels

Why is a single EPSP not sufficient to depolarize a postsynaptic neuron to threshold

Because the membrane potential at the axon hillock must be depolarized to the threshold of -55mV for an action potential to be triggered.

How do nicotinic cholinergic receptors work

Binding of acetylcholine opens channels that allow both Na+ and K+ to move down their respective electrochemical gradients, generating an excitatory postsynaptic potential (EPSP) in the postsynaptic neuron or muscle cell. However, the electrochemical gradient for Na+ is greater, so the net movement of Na+ is greater.

What is required for Na+ to enter the neuron through a channel

Both gates being open

What are voltage-gated Na+ channels

Channels in the axon membrane that are responsible for the time-dependent opening and closing of Na+ channels during an action potential

What are voltage-gated ion channels

Channels in the cell membrane that open and close in response to changes in membrane potential

Are chemical synapses unidirectional or bidirectional

Chemical synapses are typically unidirectional, meaning that the signal only travels in one direction from the presynaptic neuron to the postsynaptic neuron.

What are inhibitory postsynaptic potentials (IPSPs)

IPSPs are hyperpolarizing graded potentials that occur at inhibitory synapses.

What is the equilibrium potential of Na+

+60 mV

What happens when a 10 msec threshold stimulus is applied to a neuron

A single action potential will be generated.

How do voltage-gated Na+ channels open during depolarization

A threshold level of depolarization causes a conformational change in the channel, allowing it to open and permit the influx of Na+ ions

Will a subthreshold stimulus generate an action potential

No, a subthreshold stimulus will not generate an action potential.

When can the channel return to a resting state

Once repolarization has occurred

What are ion channels

Proteins that span the cell membrane and allow ions to pass through in response to a stimulus

What is transported via fast retrograde transport

Recycled materials are transported via fast retrograde transport.

Where can synapses occur

Synapses can occur at dendrites (axodendritic), at the cell body (axosomatic), or with another axon near the axon terminal (axoaxonic).

What is transported via fast anterograde transport

Synaptic vesicles, enzymes, and other similar products are transported via fast anterograde transport.

What happens during temporal summation

The EPSPs overlap and summate to reach the threshold at the axon hillock, which can produce an action potential.

What can the G protein do

The G protein can cause opening or closing of ion channels, changing the electrical properties of the postsynaptic neuron.

What does the G-protein do in slow EPSP

The G-protein activates adenylate cyclase, which generates cAMP.

What is the result of the inactivation gate closing

The channel becomes closed and incapable of opening in response to a second depolarization

What is the state of the channel when the inactivation gate is closed

The channel is closed and incapable of opening in response to a second depolarization

How does Na+ rush into the neuron during depolarization

The channel is open when both gates are open, allowing Na+ to rush into the neuron through the channel

What happens to the choline after acetylcholine is degraded

The choline is actively transported back into the presynaptic neuron and recycled, while the acetate enters the bloodstream.

What is the axon hillock

The cone-shaped region of a neuron where the axon originates and where action potentials are generated

How does the depolarization resulting from stimulation above threshold potential propagate in unmyelinated axons

The depolarization is restricted to a small region of the axon at any one time, with the inside becoming temporarily positive at the axon hillock.

How does the electrical signal transfer in an electrical synapse

The electrical signal of one neuron is directly transferred to a second neuron as ions flow through the gap junctions.

How do voltage-gated K+ channels open during repolarization

The increased intracellular concentration of Ca2+ ions triggers the opening of voltage-gated K+ channels, allowing K+ ions to flow out of the cell and repolarize the membrane

What is required to generate a second depolarization during the absolute refractory period

The majority of Na+ channels must have returned to their resting state, which is near the end of repolarization.

What happens during the depolarization phase of the action potential

The membrane potential changes from -70 mV to +30 mV due to an influx of sodium ions (Na+) rushing into the cell down their electrochemical gradient and approaching the Na+ equilibrium potential of +60 mV

What happens during the repolarization phase of the action potential

The membrane potential is restored to its resting potential of -70 mV due to an efflux of potassium ions (K+) out of the cell

What is the resting potential

The membrane potential of a neuron when it is not actively transmitting a signal, typically around -70 millivolts (mV) relative to the outside of the cell

What do the motor proteins do

The motor proteins carry the products being transported and 'walk' along the microtubules using energy from ATP hydrolysis.

What are nicotinic cholinergic receptors

They are a type of cholinergic receptor that is ionotropic, meaning they are directly coupled to ion channels.

What are muscarinic cholinergic receptors

They are a type of cholinergic receptor that is metabotropic, meaning they are indirectly coupled to ion channels through G-proteins.

What are chemical synapses

They are a type of synapse in which one neuron, the presynaptic neuron, releases neurotransmitter into the extracellular space in response to an action potential.

What are monoamine oxidase inhibitors (MAOIs)

They are antidepressant drugs that block the action of monoamine oxidase.

What are excitatory postsynaptic potentials (EPSPs)

They are depolarizing graded potentials that can be either fast or slow response.

What are electrochemical gradients

They are differences in ion concentrations and electrical charges across a membrane, which create an electrochemical potential that can drive the movement of ions across the membrane.

What are endogenous opioids

They are neuropeptides such as enkephalins and endorphins that produce morphine-like analgesic effects.

What are refractory periods

They are periods during and immediately after depolarization when the membrane of a neuron is less excitable and unresponsive to further stimulation.

What are adrenergic receptors

They are receptors for norepinephrine and epinephrine that produce slow responses via G-proteins.

What are cholinergic receptors

They are receptors that respond to the neurotransmitter acetylcholine.

What are graded potentials

They are small, short-lived changes in the membrane potential of a neuron that occur in response to stimuli.

What neuropeptides are included in this class

They include "neurohormones" such as thyrotropin-releasing hormone (TRH) and other hypothalamic tropic hormones, antidiuretic hormone (ADH), and oxytocin. Other neuropeptides include substance P, which decreases gastrointestinal (GI) motility, and cholecystokinin, which is involved in the regulation of gall bladder contraction.

How are voltage-gated ion channels involved in generating an action potential

They open in response to depolarization, allowing ions to move across the cell membrane and generate an electrical signal

Why does the stimulus need to be stronger during the relative refractory period

This is because the increased K+ permeability that occurs during the repolarization phase continues into the after-hyperpolarization phase, making it more difficult to depolarize the membrane. Additionally, more Na+ channels, which are now in their resting state, need to open to counter the outflow of K+.

What is the purpose of the inactivation gate

To prevent the channel from opening again too soon after depolarization

What happens when a 20 msec threshold stimulus is applied to a neuron

Two action potentials will be generated because the stimulus is applied longer than the relative refractory period.

What happens to the current flow in a myelinated axon

Very little current flows across the membrane insulated by myelin.

What ion channels are involved in repolarization

Voltage-gated K+ channels

What ion channels are involved in depolarization

Voltage-gated Na+ channels

What sort of channels does action potentials use

Voltage-gated ion

Can IPSPs (inhibitory postsynaptic potentials) from different synapses summate

Yes, they can summate, resulting in hyperpolarization.

How does the second messenger system work

In the second messenger system, the neurotransmitter binds to a metabotropic receptor that is coupled with a G protein (G protein-linked receptor). The G protein then activates or inhibits a second messenger system, which may open or close ion channels or produce other cellular responses.

What happens to the Na+ channel after approximately 1 millisecond

Inactivation gate closes, and the channel is now closed and incapable of opening until returning close to the resting membrane potential of the neuron

What is the state of inactivation and activation gates in the resting state of a neuron

Inactivation gate is open, and activation gate is closed

What happens to Na+ channels during repolarization

Inactivation gates close during repolarization

What are inhibitory synapses

Inhibitory synapses bring the membrane potential of the postsynaptic neuron further from threshold, which means that they hyperpolarize the neuron.

What is the role of the axon hillock in neural integration

It acts as an integrator, summing up all the convergent signals arriving at the postsynaptic neuron.

What is the result of presynaptic facilitation

It increases the strength of the resulting EPSP in the postsynaptic neuron to threshold, generating an action potential.

What is serotonin

It is a biogenic amine neurotransmitter in the CNS that regulates emotions and sleep and is converted to melatonin.

How does alcohol affect neurotransmission

It is a depressant that inhibits the release of glutamate, leading to decreased excitatory activity in the CNS.

What is suprathreshold

It is a membrane potential that is depolarized to a level greater than the threshold, resulting in an action potential being initiated.

What is the absolute refractory period

It is a period of time during and immediately after an action potential when the membrane of a neuron is completely unresponsive to further stimulation and cannot generate a second action potential.

What is the relative refractory period

It is a period of time immediately following the absolute refractory period that lasts for 5-15 milliseconds.

What is presynaptic inhibition

It is a process by which activity in a presynaptic neuron decreases the release of neurotransmitter from its postsynaptic neuron.

What is presynaptic facilitation

It is a process in which activity in a presynaptic neuron enhances the release of neurotransmitter from its postsynaptic neuron.

What is spatial summation

It is a process in which two or more EPSPs (excitatory postsynaptic potentials) originating at different synapses overlap and summate to reach the threshold at the axon hillock, which can then produce an action potential.

What is presynaptic modulation

It is a regulatory process that occurs through axoaxonic synapses, which are modulator synapses that regulate communication.

What is fast axonal transport

It is a type of axonal transport that occurs at a rate of 20 to 400 millimeters per day.

What guides materials along the axon

Microtubules act as guides for materials being transported along the axon.

How is acetylcholine synthesized

Acetylcholine is synthesized in the axoplasm of the axon terminal from acetyl CoA and choline, which comes from the diet. This process is catalyzed by choline acetyl transferase.

What is acetylcholine

Acetylcholine is the most abundant neurotransmitter in the PNS and is also found in the CNS.

How is stimulus intensity coded in neurons

Action potentials cannot sum, unlike graded potentials, because the absolute refractory period prevents overlap of action potentials. Instead, information regarding stimulus intensity is encoded by changes in the frequency of action potentials generated in a given period of time.

How does an action potential propagate in a myelinated axon

Action potentials propagate via saltatory conduction, which means the action potential jumps from node of Ranvier to node of Ranvier, resulting in faster conduction velocities.

How do Na+ channels become activated during depolarization

Activation gates open during depolarization

How does repolarization occur

After approximately 1 millisecond, Na+ permeability decreases reducing Na+ inflow, simultaneously K+ permeability increases, K+ rushes out of the cell (down electrochemical gradient), repolarizing the membrane back to the resting potential

What happens to K+ during repolarization

K+ permeability increases, causing K+ ions to rush out of the cell (down the electrochemical gradient) and repolarize the membrane back to the resting potential

What happens to K+ during after-hyperpolarization

K+ permeability remains elevated, causing the membrane potential to become more negative as it approaches the K+ equilibrium potential of -94 mV

What is the role of kinesin in axonal transport

Kinesin is a motor protein involved in anterograde transport.

What happens to Na+ during depolarization

Na+ permeability increases, allowing Na+ ions to rush into the cell (down the electrochemical gradient) and approach the Na+ equilibrium potential of +60 mV

What is the range of neuron lengths in humans

Neurons can range in length from less than 1 millimeter to 1 meter.

When do the special K+ 'leak' channels reopen

The special K+ 'leak' channels remain closed until cAMP is degraded.

What is propagation in the context of the action potential

The spread of the action potential along the axon without degrading

How wide is the synaptic cleft

The synaptic cleft is only 30-50 nanometers wide.

Where is the neurotransmitter stored

The synaptic vesicles store neurotransmitter in the axon terminal of the presynaptic neuron.

What can spread through electrical synapses

Depolarizing or hyperpolarizing current can spread through electrical synapses.

How much does the membrane potential change during depolarization

From -70 mV to +30 mV

What is the threshold potential that must be reached to initiate an action potential

-55 mV

What is the resting potential of a neuron

-70 mV

What is the equilibrium potential of K+

-94 mV

How does axon terminal depolarize

1) An action potential depolarizes the axon terminal and opens voltage-gated Ca2+ channels. 2) Ca2+ flows into the axon terminal due to its electrochemical gradient. 3) Ca2+ causes synaptic vesicles to fuse with the plasma membrane, releasing neurotransmitter into the synaptic cleft. The voltage-gated Ca2+ channels then close, and Ca2+ is pumped out of the cell. This process ceases neurotransmitter release. 4) The neurotransmitter diffuses to the receptor on the postsynaptic neuron. 5) The receptor binding induces a response in the postsynaptic neuron. 6) The binding is brief and reversible, and the neurotransmitter is degraded by enzymes on the plasma membrane of the postsynaptic neuron or within the interstitial fluid of the synaptic cleft. This prevents a continual response. 7) Some neurotransmitters are actively transported back into the presynaptic neuron to be degraded and recycled. 8) Some neurotransmitters or their degraded products diffuse away from the synaptic cleft and are degraded by adjacent glial cells. The synaptic delay is 0.5 - 5 msec, the time lag between the arrival of an action potential and the occurrence of a response, mostly resulting from the synaptic delay. What is signal transduction: Signal transduction is the mechanism by which a messenger (neurotransmitter) produces a response in a cell.

What are the three phases of action potentials

1. Depolarization 2. Repolarization 3. After-hyperpolarization

What is large, rapid depolarization

A brief increase in the membrane potential from -70 mV to +30 mV within 1 millisecond

What is depolarization

A change in the membrane potential that makes the inside of the cell less negative (more positive) than its resting potential

What is hyperpolarization

A change in the membrane potential that makes the inside of the cell more negative than its resting potential

What is repolarization

A change in the membrane potential that restores the inside of the cell to its resting potential after depolarization or hyperpolarization

What is a graded potential

A change in the membrane potential that varies in amplitude and can be either depolarizing (making the cell more likely to fire an action potential) or hyperpolarizing (making the cell less likely to fire an action potential)

What is an action potential

A neural impulse that is generated when graded potentials reach threshold at the axon hillock

What is after-hyperpolarization

A period following repolarization during which the K+ permeability remains elevated for 5-15 msec

How does an action potential propagate down an axon

A positive charge depolarizes an adjacent area of the axon, which opens adjacent voltage-gated channels in front and generates an action potential. This process continues with one action potential setting up local currents that create another action potential in adjacent areas all the way to the axon terminals.

How is acetylcholine released

Acetylcholine is released via exocytosis.

How is acetylcholine stored

Acetylcholine is stored in vesicles.

How is acetylcholine degraded

Acetylcholine is degraded by acetylcholinesterase into acetate and choline.

How long does it take for the inactivation gate to close after depolarization

Approximately 1 millisecond

When does the inactivation gate close

Approximately 1 millisecond after depolarization

What happens during a fast response

During a fast response (within milliseconds), neurotransmitter binds to an ionotropic receptor (channel-linked receptor), opens a channel allowing specific ions to enter postsynaptic neuron, changes membrane potential (produces a postsynaptic potential, PSP = graded potential), and the rapid channel closes as soon as neurotransmitter unbinds.

What is the role of dynein in axonal transport

Dynein is a motor protein involved in retrograde transport.

What are electrical synapses

Electrical synapses are formed when the plasma membrane of adjacent neurons (or neuron and glial cell) are linked by gap junctions.

Where are electrical synapses found

Electrical synapses are found in the retina and brainstem that regulates breathing.

What type of transmission is associated with electrical synapses

Electrical synapses have very rapid bidirectional transmission.

What is the process for Na+/K+ pumps to work

For every ATP hydrolyzed, 3 Na+ ions are pumped out of the cell, while 2 K+ ions are pumped into the cell.

How much does the membrane potential change during repolarization

From +30 mV back to -70 mV

What are activation and inactivation gates of Na+ channels

Gates that control the opening and closing of Na+ channels

What happens when cAMP is generated

It activates a protein kinase, which then phosphorylates and closes K+ ('special' leak) channels.

What is monoamine oxidase

It is an enzyme that degrades catecholamines.

What is the name for the process of summing up convergent signals arriving at the postsynaptic neuron

It is called summation or neural integration. What is temporal summation: It is the process by which two or more EPSPs (excitatory postsynaptic potentials) are generated in rapid succession at the same synapse so that the first EPSP does not have time to completely dissipate before the second EPSP is generated.

What is the result of the channel returning to a resting state

It is capable of opening again in response to depolarization

What is membrane permeability

It is the ease with which ions can move across the cell membrane, which is crucial for generating and propagating action potentials in neurons.

What is threshold

It is the minimum level of depolarization needed to trigger an action potential in a neuron.

What is glutamate

It is the most common excitatory neurotransmitter in the CNS and produces fast excitatory postsynaptic potentials (EPSPs) in the postsynaptic neuron.

What is gamma-aminobutyric acid (GABA)

It is the most common inhibitory neurotransmitter in the CNS, and receptor binding produces inhibitory postsynaptic potentials (IPSPs) that depress the activity of the CNS.

What is retrograde transport

It is the movement of products from the neuron's axon terminal back to its cell body. Retrograde transport only occurs at fast rates.

What is anterograde transport

It is the movement of products from the neuron's cell body to its axon terminal. Anterograde transport can occur at both slow and fast rates.

What is the all-or-none principle

It is the principle that states an action potential either occurs completely or not at all, depending on whether the membrane potential of a neuron reaches a certain threshold.

What is axonal transport

It is the process by which materials are transported along the length of a neuron's axon.

What is neural integration

It is the process by which neurons combine and process incoming information from other neurons.

What is amplitude

It is the strength or size of a signal, in this case, the size of the action potential.

What is convergence in neural integration

It is when one neuron receives communication from thousands of other neurons.

What is divergence in neural integration

It is when the axon of one neuron communicates with several other neurons through collateral axons.

What is the duration of the absolute refractory period

It lasts for 1-2 milliseconds and includes all of the depolarization and most of the repolarization phase.

How does a presynaptic modulator neuron affect its postsynaptic neuron

It produces neurotransmitters that bind to receptors on or near the axon terminal of its postsynaptic neuron.

What happens at the beginning of repolarization in the absolute refractory period

Na+ inactivation gates are closed and incapable of opening.

What happens to Na+ during repolarization

Na+ permeability decreases, reducing the inflow of Na+ ions

What are the two types of response that neurotransmitters can induce

Neurotransmitters can induce a fast or slow response.

What is the mechanism behind slow responses that take msecs to minutes

One mechanism is via direct coupling, in which a neurotransmitter binds to a metabotropic receptor that is coupled with a G protein (G protein-linked receptor).

What is the resting state of the Na+ channel

Open inactivation gate and closed activation gate

What is the state of the gates in the resting state of the channel

Open inactivation gate and closed activation gate

How do muscarinic cholinergic receptors work

The G-protein of the postsynaptic neuron can open or close ion channels or activate secondary messengers in response to the binding of acetylcholine. The action of the neurotransmitter does not depend on the nature of the messenger, but rather on the signal transduction mechanism of the receptor that is activated by the bound messenger.

What re-establishes proper electrochemical gradients following action potentials

The Na+/K+ pumps re-establish proper electrochemical gradients.

What is the difference between the activation gate and the inactivation gate

The activation gate determines whether the channel is open or closed, while the inactivation gate determines whether the channel can open again after depolarization

Does the amplitude of hyperpolarization increase or decrease as more neurotransmitters bind to more receptors

The amplitude of hyperpolarization increases as more neurotransmitters bind to more receptors. However, the hyperpolarization decreases as it moves away from the binding site.

What does the axonal terminal axoplasm contain

The axonal terminal axoplasm contains enzymes that synthesize most (but not all) neurotransmitters, which are actively transported into vesicles for storage.

What happens during after-hyperpolarization

The membrane potential becomes more negative as it approaches the K+ equilibrium potential of -94 mV

What is the after-hyperpolarization phase of the action potential

The membrane potential briefly becomes more negative than the resting potential (-70 mV) before returning to its resting state

What are the motor proteins involved in axonal transport

The motor proteins involved in axonal transport are kinesin and dynein.

What happens when a neurotransmitter binds to an inhibitory synapse

The neurotransmitter causes K+ ion channels to open, and K+ rushes out of the neuron, hyperpolarizing it.

What happens during depolarization in the absolute refractory period

The opening of Na+ channels that has been set in motion will proceed to conclusion.

What happens when the permeability of K+ decreases

The permeability of Na+ (via 'leak channels') remains unchanged, which causes more Na+ to enter the neuron than K+ leaving the neuron, resulting in depolarization. This depolarization can last for seconds to minutes.

How does a positive charge travel in a myelinated axon

The positive charge must flow all the way to the next node of Ranvier, where it depolarizes the area to threshold and initiates an action potential.

What is the postsynaptic neuron responsible for

The postsynaptic neuron possesses the receptors.

What is the presynaptic neuron responsible for

The presynaptic neuron releases the neurotransmitter.

What does the refractory period do

The refractory period prevents action potentials from travelling backwards, ensuring unidirectional propagation of the action potential.

What happens during a chemical synapse

The released neurotransmitter diffuses across the synaptic gap and binds to a ligand-receptor on a second neuron, the postsynaptic neuron, triggering an electrical signal that may or may not generate an action potential.

What is the function of serotonin transporters

They actively transport serotonin back into the presynaptic neuron, and drugs that inhibit this transport (e.g., selective serotonin reuptake inhibitors or SSRIs) are used as antidepressants.

What are benzodiazepines

They are a class of anti-anxiety drugs such as Valium, Ativan, etc., that increase the affinity of GABA receptors for GABA, enhancing its inhibitory effect on the CNS.

What are biogenic amine neurotransmitters

They are a class of neurotransmitters that are derived from amino acids and possess an amine group.

What are amino acid neurotransmitters

They are a class of neurotransmitters that are derived from amino acids, the building blocks of proteins, and are the most abundant neurotransmitters in the central nervous system (CNS).

What are neuropeptide neurotransmitters

They are a class of neurotransmitters that are produced in the endoplasmic reticulum (ER) and packaged into vesicles by the Golgi apparatus in the cell body of the neuron. They are then transported anterograde to the terminal.

What are catecholamines

They are a type of biogenic amine neurotransmitter that includes dopamine (CNS), norepinephrine (CNS and PNS), and epinephrine (CNS and released from chromaffin cells of the adrenal medulla).

What are excitatory synapses

They are synapses that bring the membrane potential of the postsynaptic neuron closer to threshold, thereby depolarizing it.

What is the effect of these neurotransmitters on the postsynaptic neuron

They bind with and regulate the opening and closing of voltage-gated Ca2+ channels located on the terminal regions of the postsynaptic neuron, thus altering the amount of neurotransmitter released into the synapse.

What happens when an EPSP from one synapse and an IPSP from another synapse occur at the same time

They can cancel each other out.

What happens to positively charged ions, such as Na+, when they enter the cell during propagation of action potentials in unmyelinated axons

They follow their concentration gradient and repel each other, moving toward adjacent negatively charged regions inside the cell.

How does presynaptic inhibition work in the example of neurons F and H

When both neurons are active, neurotransmitter from neuron H decreases the amount of neurotransmitter released from neuron F, preventing it from reaching threshold when summated with neuron G.

How does activity in two neurons lead to presynaptic facilitation

When both neurons are active, the neurotransmitter released by one neuron (e.g., neuron E) enhances the release of neurotransmitter from the other neuron (e.g., neuron C) by allowing more Ca2+ to enter the terminal of the latter.

How does presynaptic inhibition work in the example of neurons F and G

When both neurons are active, the summation of EPSPs generates an action potential.

When are action potentials generated

When graded potentials reach a certain threshold at axon hilock

What happens during a fast EPSP

When neurotransmitters bind to their receptors, ion channels open, causing Na+ to rush in and K+ to rush out. However, the net movement of Na+ is greater due to its larger electrochemical gradient, which leads to depolarization that lasts several milliseconds. The amplitude of depolarization increases as more neurotransmitters bind to more receptors, but depolarization decreases as the neurotransmitter moves away from the binding site. (A fast EPSP is one type of EPSP.) What is a slow EPSP: It is a type of excitatory postsynaptic potential that occurs when a neurotransmitter binds with a receptor, activating a G-protein.

How long does it take for an action potential to occur

Within 1 millisecond

Can IPSPs (inhibitory postsynaptic potentials) also summate

Yes, IPSPs can also summate, resulting in hyperpolarization.

Can a strong, long-lasting graded potential generate a burst of action potentials

Yes, a strong, long-lasting graded potential may generate a burst of action potentials.

Can a suprathreshold stimulus generate multiple action potentials

Yes, a suprathreshold stimulus can generate multiple action potentials because it is strong enough to produce a second action potential during the relative refractory period of the first action potential. What does a stimulus need to do in order to generate multiple action potentials: The stimulus must be strong enough to open enough Na+ channels (that have returned to resting state) to overcome the elevated K+ outflow. How does the strength of a stimulus affect the timing of action potentials: The stronger the stimulus, the closer the first and second action potentials are in time. However, this is always limited by the absolute refractory period.

Can presynaptic inhibition prevent the generation of an action potential

Yes, if enough neurotransmitter is not released to reach threshold, the postsynaptic neuron will not generate an action potential.

Is it possible to generate a second action potential during the relative refractory period

Yes, it is possible, but the stimulus must be much stronger than during the absolute refractory period.

Can neurons also form synapses with glands or muscle cells

Yes, neurons can also form synapses with effector organs such as glands or muscle cells.

Does the conduction velocity of an action potential vary between axons

Yes, the conduction velocity is faster in large diameter axons with less resistance.