Nervous System test 1

The nervous systems of vertebrates have numerous __________ cells, or support cells, which nurture, protect, and insulate them.

glial gliocyte astrocyte schwann ependymal

Electrical signals in the dendrites and cell body of a neuron are called:

graded potentials.

Sensory neurons are nerve cells that conduct impulses _____ the central nervous system.

toward

As membrane potential is approaching _____, the membrane is getting less polarized (hypopolarized or depolarized).

zero

A neurobiologist is studying the cable properties of two different axons. She finds that axon A has a larger length constant than axon B. What does this mean?

A change in membrane potential degrades less with distance in axon A vs. axon B.

This part of the neuron acts as a branching network of neuronal extensions which receive incoming signals:

Dendrite

Is a typical efferent neuron (as shown in Figure 4.18b) multipolar, bipolar or unipolar?

Efferent neurons are typically multipolar.

Cells which line the fluid-filled cavities of the central nervous system:

Ependymal cell

_____ potentials are mainly produced by voltage-gated sodium and potassium channels.

Graded

Explain in your own words why increasing the density of voltage-gated Na+ channels decreases the threshold potential of a neuron.

In order for an action potential to occur, the positive feedback of Na+ entry must be triggered, which occurs when the stimulus is greater than the threshold potentials. A high density of voltage-gated Na+ channels results in more Na+ channels being able to open at a low stimulus intensity, causing more Na+ to enter, and increasing the probably of triggering the positive feedback loop.

What is the importance of Otto Loewi's experiment with the frog hearts?

Loewi proved that synaptic transmission occurs through chemical messengers instead of electrical signals. The Vagus nerve stimulated the release of acetylcholine which slowed the heart by way of muscarinic receptors.

Phagocytic glial cell which removes debris and dead cells:

Microglial cell

__________ are junctions between a neuron and another cell where the neuron releases chemical messengers to communicate with these target cells.

Synapse

The phenomenon of synaptic __________ is evidenced when a neuron increases the amount of neurotransmitter released, due to a buildup of calcium in response to repeated stimulation.

facilitation plasticity potentiation

The larger the diameter of an axon, the _____ it can conduct an action potential.

faster

Many vertebrate axons are wrapped with an insulating material called __________, which speeds up the conduction of nerve impulses.

myelin

At the neuromuscular junction and at many central synapses, acetylcholine binds to the __________ ACh receptor, an ionotropic receptor that results in membrane depolarization.

nicotinic muscarinic

The first phase of the action potential, called the depolarization phase, is produced through the:

opening of voltage-gated sodium channels.

How do neurons alter their membrane potential?

Neurons selectively alter the permeability of their membranes to ions by opening and closing gated ion channels in the membrane.

Support cell which produces the myelin sheath of neurons in the peripheral nervous system:

Schwann cell

Draw a diagram to illustrate the relationship between the states of the various voltage-gated ion channels, membrane permeability, and the phases and refractory periods of the action potential.

See Figure 4.10 on page 155 of your textbook.

Describe the primary types of glial cells in the vertebrates. What are their functions?

The primary types of glial cells in the vertebrates are Schwann cells, oligodendrocytes, astrocytes, microglia and ependymal cells.

_____ hormones are NOT a major class of neurotransmitters.

Thyroid

_____, a chemical that is used as a neurotransmitter in animals, has been detected in organisms that do not have nervous systems (e.g. bacteria, algae, protozoans, and plants).

Acetylcholine

Why can action potentials be conducted across long distances along the axon without degrading, when graded potentials die out within a few millimeters?

An action potential will depolarize adjacent areas of membrane beyond threshold and thus initiate a new action potential that will do the same in turn. Graded potentials do not open new channels, unless, of course, they initiate an action potential, and thus decrease with distance from the source of stimulus.

Often the longest part of a neuron, this branch transmits action potentials to target cells:

Axon

Compare and contrast electrical and chemical synapses.

Electrical synapses provide a rapid and direct form of transmission with little room for signal interpretation and amplification. Chemical synapses are slower but, by acting via indirect transmission on membrane receptors, can elicit a wider variety of responses.

Which glial cells have cilia that, by beating, help to circulate cerebrospinal fluid through the CNS of vertebrates?

Ependymal cell.

_____ potentials vary in magnitude, whereas _____ potentials are always the same magnitude (in a given cell type).

Graded action

_____ do not have neurons; however, they do express voltage-gated ion channels that are used for electrical communication.

Plants

______________ detect environmental signals and convey this information to the central nervous system. _____________ convey signals from one neuron to another and ___________ convey signals from the central nervous system to effector organs.

Sensory neurons; Interneurons; efferent neurons

What are the fundamental evolutionary innovations that allow neural signaling in animals?

The fundamental evolutionary innovations that allow neural signaling in animals include: voltage-gated channels (seen in protistans, prokaryotes, algae and plants as well as in animals), voltage-gated Na+ channels (unique to animals), and synaptic transmission (unique to animals).

What sets the speed of action potential conduction, and why?

The speed of action potential conduction is determined by the cable properties of the axon. In particular the membrane resistance, the intracellular resistance and the membrane capacitance.

What event occurs after an action potential arrives at an axon terminal?

Voltage-gated calcium channels open.

Action potentials that are "_____" in nature.

all-or-none

The term "salutatory conduction" refers to:

conduction of an action potential along a myelinated axon.

Most nerve cells have branching extensions called __________ that function in receiving input from other nerve cells or external stimuli.

dendrites

Gap junctions are _____ synapses; they do not release neurotransmitters.

electrical

Synapses that are formed with gap junctions are known as __________ synapses.

electrical

The opening of a potassium ion channel will typically produce a membrane _____.

hyperpolarization

The phenomenon of post-tetanic potentiation, by which a post-synaptic response to a single presynaptic impulse is increased after frequent stimulation, is due to:

increased availability of neurotransmitter vesicles.

Compared to transmission across chemical synapses, transmission across an electrical synapse is:

mediated by gap junctions.

A neurotransmitter, which acts via a receptor and second messenger cascade to affect its target cell, is called:

metabotropic

Graded potentials can occur in dendrites and nerve cell bodies, and can add up (_____).

summate

Synaptic facilitation, synaptic depression, and post-tetanic potentiation are examples of:

synaptic plasticity.

cellular polarity

the concept that different structures and/or functions exist in different parts of the cell

The _____ potential is the membrane potential required for an axon to generate an action potential.

threshold

Graded potentials are mainly produced by _____.

voltage-gated sodium and potassium channels

How do action potentials code information about the intensity of the incoming signal?

Action potentials code information about the intensity of the incoming signal through their frequency.

Which part of a neuron is a region with a high density of voltage-gated sodium channels, at which action potentials initiate?

Axon hillock.

In some neurons, such as those in jellyfish, a voltage-gated _____ channel is involved in the action potential.

Ca2+

_____ synapses are essentially one-way, while most _____ synapses allow signals to go in either direction

Chemical electrical

_____ potentials occur in dendrites and the cell body, whereas _____ potentials occur in the axons.

Graded action

A lipid membrane inulating wrapping around the cell's axon; speeds up the speed of action potentials:

Myelin sheath

Axons have cable properties, i.e. they act like electric cables. Axons with bigger diameters have _____ resistance to current flow.

lower

Action potentials are always the same _____ (all-or-none) and are produced only after the _____ potential is exceeded.

magnitude threshold

Immediately following the depolarization phase of an action potential, during the __________ period, a neuron cannot fire again because its sodium channels are in an inactivated state.

refractory

Action potentials can only proceed in one direction because the _____ of the voltage gated sodium channels prevents the action potential from going backwards

refractory period

Only _____ express voltage-gated Na+ channels.

metazoans

A student is eating at the lab bench (in clear violation of laboratory policy), and mistakenly sprinkles tetrodotoxin on his fries. Given that this substance inhibits voltage-gated Na+ channels, indicate whether the following statement concerning this student is true or false. Explain your answers, and consider the time course of the response. a) "It will be more difficult for the student's neurons to generate action potentials." b) "The student's neurons will fire more frequently, since membrane potential will be brought closer to threshold." c) "The effect on the membrane potential of the student's neurons could be predicted by the Nernst equation, which factors in the effects of both ion concentration and ion permeability."

(a) This is true: if voltage-gated Na+ channels are inhibited, depolarization, and triggering a positive feedback loop, will be more difficult. (b) This is false, it will have the opposite effect: fewer functional voltage-gated Na+ channels will be available, thus making cells less responsive by increasing the threshold voltage. (c) This is false, because the Nernst equation does not take into account permeability (it assumes that only one ion is permeable, or at least far more permeable than the others). In addition, the effect of tetrodotoxin will not be to directly change the permeability of membranes, but to inhibit changes in permeability.

How did Sherrington decide that there must be some sort of communication across synaptic gaps instead of the continuation of an action potential through his work with reflexes?

-He noted a delay in the reflex that would not occur if the signal simply went at the speed of an action potential. -He showed temporal and spatial summation. -He showed excitation and inhibition through the same pathway.

What is a gated ion channel? Why are gated ion channels important in neural signaling?

A gated ion channel is a channel that, while closed at rest, will open and allow facilitated diffusion of the relevant ion in response to an appropriate stimulus. Voltage-gated Na+ and K+ channels are responsible for the generation of action potentials in nerve axons. Ligand-gated ion channels are for many nerves the source of graded potentials that initiate signaling.

Why are acetylcholinesterase inhibitors effective in the treatment of myasthenia gravis?

Acetylcholinesterase is an enzyme of the synaptic cleft that breaks down acetylcholine to terminate the signal at chemical synapses. Inhibitors of acetylcholinesterase prevent this breakdown, and therefore increase the concentration of acetylcholine in synapses. This is beneficial for people suffering from myasthenia gravis, which causes muscle weakness because of a decreased number of acetylcholine receptors on muscle cells; by increasing acetylcholine concentration, acetylcholinesterase inhibitors compensate for the reduced number of receptors and therefore reduce muscle weakness.

_____ potentials are always the same magnitude (all-or-none) and are produced only after the threshold potential is exceeded.

Action

Differences between Action Potentials and Postsynaptic Potentials

Action Potential Post-Synaptic Potential All-or-none, regenerative Graded potential, variable in size and duration Has threshold No threshold Voltage gated channels Chemically-gated channels Specific ion channels Non-specific ion channels Always depolarizing Can be depolarizing or hyperpolarizing

Compare and contrast giant axons and myelinated axons as strategies for increasing the speed of signal conduction.

Action potentials are faster in giant axons as membrane resistance (which is proportional to Λ) decreases more slowly with an increase in axonal radius than the intracellular resistance (which is inversely proportional to Λ). Action potentials are also faster in myelinated axons than in unmyelinated ones as the insulation decreases membrane capacitance and increases membrane resistance.

What would happen to action potential generation in an axon if you applied a drug that caused voltage-gated K+ channels to remain open constantly?

Action potentials could not be produced. The resulting increase in K+ permeability would do two things: it would hyperpolarize the axon, making it less likely to cross the threshold voltage, and it would cause a "short-circuit" that would cancel, at least partially, changes in Na+ permeability that normally occur during action potentials (an influx of Na+ would result in an increase in K+ leaving the cell).

Support cell which nourishes neurons and helps to guide neuronal development:

Astrocyte

Why can't graded potentials be propagated across long distances in neurons?

Because of leakage of charged ions across the cell membrane, the electrical resistance of the cytoplasm, and the electrical properties of the membrane, graded potential decrease in amplitude very rapidly with distance. Therefore, the signal would fade away before its destination was reached.

Imagine a postsynaptic neuron that is contacted by two different excitatory presynaptic neurons. One of these presynaptic neurons (neuron A) contacts the cell body of the postsynaptic cell next to the axon hillock, whereas the other presynaptic neuron (B) contacts a dendrite of the postsynaptic cell on the side of the cell body farthest away from the axon hillock. Explain why repeated firing of neuron A at slightly below the threshold potential could cause the postsynaptic neuron to initiate an action potential, while firing of neuron B at exactly the same intensity and frequency might not.

Conduction of signals along dendrites and the cell body occurs in the form of electrotonic transmission, which degrades rapidly with distance. Therefore, neuron A can produce EPSPs that, after undergoing temporal summation, will exceed the threshold. On the other hand, by the time the EPSPs from neuron B reach the axon hillock (where integration occurs), they may have lost so much intensity that, even if they undergo temporal summation, the overall depolarization will be lower than the threshold.

_____ are fine, branching extensions of a neuron, originating at the cell body, where graded potentials form in response to external inputs.

Dendrites

Why do only the ions Na+, K+ and Cl- appear in the Goldman equation as formulated for a neuron at rest?

For an ion to be important in setting the membrane potential, it must be permeable across the cell membrane and have different concentrations on each side of the membrane. Normally, only Na+, K+ and Cl- are permeable under resting conditions, and therefore only these three ions contribute to the membrane potential.

_____ potentials are caused by the opening and closing of many kinds of channels, whereas _____ potentials are caused by the opening and closing of voltage-gated ion channels.

Graded action

_____ potentials vary in duration, whereas _____ potentials are always the same duration (in a given cell type).

Graded action

How do graded potentials code information about the intensity of the incoming signal?

Graded potentials code information about the intensity of the incoming signal through their magnitude.

Compare and contrast action potentials and graded potentials.

Graded potentials vary in magnitude and duration. They decay with distance and occur in dendrites and cell bodies. They may be caused by the opening and closing of many kinds of ion channels. Action potentials are always the same magnitude and duration for a given cell type. They can be transmitted over long distances and occur in axons. They are caused by the opening and closing of voltage-gated ion channels.

Are typical interneurons and sensory neurons (as shown in Figure 4.18b) multipolar, bipolar or unipolar?

Interneurons are typically multipolar. Sensory neurons are also typically multipolar. Most examples of bipolar and unipolar neurons in vertebrates are, however, sensory neurons.

A neurobiologist is studying the cable properties of two different axons. She finds that axon A has a larger time constant than axon B. What does this mean?

It takes axon A longer to reach a given membrane potential than axon B.

Explain why a myelinated neuron conducts signals more rapidly than an equivalent unmyelinated neuron.

Myelinated axons transmit signals using the process of saltatory conduction, which involves the alternative use of electrotonic transmission of graded potentials under the sheath of myelin, with action potentials to regenerate the signal strength at the nodes of Ranvier. Because the spread of electrotonic current is much faster than generating action potentials, and because electrotonic currents can travel further with less degradation on myelinated axon (by increasing the length constant), the speed of signal conduction is faster in myelinated axons. In addition, myelination decreases the capacitance of the membrane, this will tend to reduce the time constant, which also results in an increase in the speed of electrotonic transmission of signals.

Glial cell which forms the myelin sheath of central nervous system neurons:

Oligodendrocyte

_____ neurons are nerve cells that conduct impulses toward the central nervous system.

Sensory

This part of the neuron contains the nucleus and much of the neuron's metabolic machinery:

Soma

List the structures of a typical neuron, and summarize their functions.

Structures that should be described here include the dendrites, the cell body, the axon hillock, the axon and the axon terminal.

The junction of a neuron and another cell (neuron, muscle, or gland). This junction may be chemically or electrically mediated:

Synapse

Twelve neurons synapse on one postsynaptic neuron. At the axon hillock of the postsynaptic neuron, 10 of the presynaptic neurons produce EPSPs of 2 mV each and the other two produce IPSPs of 4 mV each. The threshold potential of the postsynaptic cell is -60 mV (resting membrane potential is -70 mV). Will an action potential be produced? Justify your answer.

Ten EPSPs of 2mV will cause a combined depolarization of 20mV. Two IPSPs of 8mV will cause a combined hyperpolarization of 8mV. Altogether, these signals will cause a combined depolarization of 12mV, which is enough from the membrane voltage to cross the threshold (it will move from -70mV to -58mV) and initiate an action potential.

Why does the membrane potential become positive during the depolarization phase of the action potential?

The increase in Na+ permeability caused by the opening of the voltage-gated Na+ channels allows Na+ to flow into the cell fast enough to depolarize the membrane to a positive value before the channels close again.

Draw a diagram of the shape of an action potential in a neuron that expresses voltage-gated K+ channels compared to the action potential in a neuron that does not express voltage-gated K+ channels, assuming that all other factors are similar between the neurons. Explain the reasoning behind any differences that you indicate in shape between the two action potentials.

The action potential in a neuron that expresses voltage-gated K+ channels would be similar to the action potential shown in Figure 4.7b. In the absence of voltage-gated K+ channels, the depolarization phase of the action potential would be similar (all other things being equal), but the repolarization phase would take longer because the neuron would rely on the closing of Na+ channels and the actions of the Na+/K+ ATPase to restore the resting potential, which would be slow compared to the rapid movement of K+ through voltage-gated K+ channels.

Drugs called selective serotonin reuptake inhibitors (SSRIs), which affect the reuptake of neurotransmitter by presynaptic cells, are used for the treatment of depression. Serotonin normally causes an excitatory postsynaptic potential. What effect would the administration of an SSRI have on the response of these postsynaptic cells, and why?

The effect of these drugs is an increase in the concentration of serotonin in the synaptic cleft, increasing the probability of binding between the neurotransmitter and its receptor on the postsynaptic cell. More receptors on the post-synaptic cell will be activated, leading to an increased depolarization by spatial summation, resulting in an increase in the probably that the post-synaptic cell will produce action potentials, or increase the frequency of action potentials produced by the post-synaptic cell.

What limits the frequency of action potentials?

The frequency of action potentials is limited by the strength of the stimulus and the refractory periods. A second action potential cannot be stimulated within the absolute refractory period. It can be stimulated within the relative refractory period but whether and when it will do so is dependent on whether and the degree to which the stimulus overcomes the elevated threshold value associated with this period.

Which type of neuron would you expect to have more dendrites: an afferent (sensory) neuron or an interneuron? Justify your answer.

The function of dendrites is to receive information. Both afferent and interneurons must, obviously, be able to receive information, but interneurons can have the largest number, especially in the brain, to allow them to receive and integrate information from a multitude of other neurons.

What is the difference between temporal and spatial summation? Can spatial summation occur without temporal summation?

The integration of graded potentials occurring at slightly different times is called temporal summation. The integration of graded potentials occurring on different sites on the receiving neuron is called spatial summation. Spatial summation cannot occur without temporal summation in the sense that two signals being received on two different areas must be received close enough in time to interact; if the time difference between the two is too large, the first one will fade away before integrating with the second one.

Why does the opening of a Na+ channel cause a neuron to depolarize?

The membrane voltage will always move toward the equilibrium potential for the ion for which the permeability is increased. Because the equilibrium potential for Na+ is highly positive in relation to the resting potential of a neuron, opening Na+ channels will cause the membrane potential to increase (a depolarization).

You have discovered a drug that blocks voltage-gated Ca2+ channels. What are the likely effects of this drug at the synapse?

The opening of voltage-gated Ca++ channels in axon terminals is an essential step in synaptic transmission, as an increase in intracellular Ca++ is responsible for triggering the exocytosis of synaptic vesicles. Therefore, the drug would prevent the release of neurotransmitters, and prevent synaptic transmission.

Describe how membrane permeability and ion concentrations affect the membrane potential.

The opening of voltage-gated Na+ channels greatly increases the membrane's permeability to Na+ causing a rapid influx of Na+ and a consequent depolarization of the membrane. The closing of these channels and the opening of voltage-gated K+ channels reduces the membrane's permeability to Na+ and increases its permeability to K+, slowing the influx of Na+ and facilitating a rapid efflux of K+ which repolarizes and hyperpolarizes the membrane.

What factors would you expect to be important in determining the maximum spacing between Nodes of Ranvier in a myelinated neuron, and why?

The primary determinant will be the length constant. If the internodes exceed Λ, the action potential will not be propagated. The thickness of the axon, the effectiveness of the myelin sheathing and any other factors that affect membrane resistance, the intracellular resistance and the membrane capacitance, will play a role in determining the maximum spacing of the nodes.

Describe the relationship between the after-hyperpolarization phase of the action potential and the relative refractory period. Why is the relative refractory period important for neural signaling?

The relative refractory period occurs during the after-hyperpolarization phase. During this period, voltage-gated Na+ channels gradually become reset from the "inactivated" state to the "closed" state. As more and more voltage-gated Na+ channels are reset, it becomes gradually easier for new action potentials to be generated (the threshold voltage gradually decreases). As a consequence, only strong signals can generate new action potentials early during the relative refractory period, while weaker signals can only produce new action potentials later, when more voltage-gated Na+ are reset. This results in the encoding of signal strength into action potential frequencies: the stronger the signals, the higher frequency of action potentials (within some limits).

What causes the shape of action potentials to vary among neurons?

The shape of the action potentials varies among neurons as a result of the different isoforms of K+ and Na+ channels found in the axons and differences in the densities of these channels.

What would happen if you experimentally stimulated an axon close to both the axon hillock and the axon terminal at the same time?

Two action potentials will be initiated that will travel in opposite directions. One will start at the axon hillock and travel toward the axon terminal, as they occur normally. The other will start near the axon terminal and travel toward the axon hillock (it will also travel to the axon terminal); it can travel "backwards" because there is no structural feature that forces action potentials to travel in any direction. When the two action potentials meet, somewhere near the middle of the axons length, they will both disappear as they will both be exposed to areas of the membrane in the absolute refractory period (inactivated voltage-gated Na+ channels).

What molecular properties of the ion channels involved in action potentials cause unidirectional propagation of action potentials along the axon, and why?

Voltage-gated Na+ channels become spontaneously inactivated after opening, which prevents them from re-opening until the absolute refractory period is over. As a result, the region of an axon immediately upstream from an action potential is in its absolute refractory period because it has voltage-gated Na+ channels that cannot be opened; only those that are downstream, ahead of the action potential can open. This causes action potentials to move in only one direction along axons.

What are the four main functional zones of a neuron?

Zones of signal reception, signal integration, signal conduction and signal transmission.

An action potential results if:

a graded potential exceeds the threshold potential at the axon hillock.

The primary neurotransmitter of the neuromuscular junction in vertebrates is _____.

acetylcholine

Many protozoa are capable of generating action potentials based on _____ flux.

calcium ion

When a neurotransmitter binds to a(n) __________ receptor, the receptor initiates a second messenger cascade inside the cell.

metabotropic ionotropic

In animals, the voltage-gated K+ channels display _____ diversity (in molecular sequence and function) than voltage-gated Na+ channels.

more

Neurons which conduct impulses from the periphery toward the central nervous system are called __________ neurons.

sensory afferent efferent

In animals of various species, the _____ and the _____ of an action potential can differ significantly.

shape speed

Action potentials can only occur in regions of a nerve cell that contain:

the appropriate voltage-gated ion channels, i.e. usually only in the axon

Name 4 main factors that influence the speed of action potential propagation:

the degree of myelination of the axon. the diameter of the axon. the kinetics of the voltage-gated channels. the length constant of the axon.

Hyperpolarization means that:

the inside of the cell becomes more negatively charged. This may be brought about by potassium ions leaving the cell.

In general, there is a correlation between the complexity of the nervous system and the total number of isoforms of _____.

voltage-gated ion channels