Homework/Lab #8

What is meant by spatial summation?

Multiple synapses generate EPSPs and IPSPs on the postsynaptic cell that are added together as they revel toward the axon hillock.

The diffusion of what ion, across the neuronal membrane, is responsible for the local currents that depolarize regions of the axon to threshold?

Na+ (sodium) Sodium enters the cell during the beginning of an action potential. Not only does this (further) depolarize the membrane where those channels are located, but it also sets up local currents that depolarize nearby membrane segments. In the case of myelinated axons, these local currents depolarize the next node, some 1 to 2 millimeters away.

What prevents the Na+ and K+ gradients from dissipating?

Na+ -K+ ATPase Also known as the Na+-K+ pump, or simply the pump, this transporter moves three Na+ out of the cell and two K+ into the cell for every ATP it hydrolyzes. This pumping action prevents the Na+ and K+ gradients from running down as these ions passively move through leak channels.

During the action potential of a neuron, which ion is primarily present during the depolarization phase, and in which direction is the ion moving?

Na+ is entering the cell K+ is leaving During the depolarization phase of the action potential, open Na+ channels allow Na+ ions to diffuse into the cell. This inward movement of positive charge makes the membrane potential more positive (less negative). The depolarization phase is a positive feedback cycle where open Na+ channels cause depolarization, which in turn causes more voltage-gated Na+ channels to open.

What is the role of neurotransmitter at a chemical synapse?

Neurotransmitter binds to receptors on the postsynaptic cell membrane and allows ions to diffuse across the membrane.

What is serotonin?

Serotonin is one of many types of neurotransmitters that are released in chemical synapses in the nervous system.

What is meant by Na+ channel inactivation?

The Na+ channel no longer allows Na+ ions to pass through it

The velocity of the action potential is faster in which kind of axon?

a large myelinated axon the myelination acts as insulation and the action potential is generated only at the nodes of Ranvier. Also, the large myelinated axon has a faster conduction velocity than the small myelinated axon because axon diameter is related to conduction velocity as well. Propagation along myelinated axons is known as saltatory conduction.

The resting membrane potential depends on two factors that influence the magnitude and direction of Na+ and K+ diffusion across the plasma membrane. Identify these two factors.

The presence of concentration gradients and leak channels.

During action potential propagation in an unmyelinated axon, why doesn't the action potential suddenly "double back" and start propagating in the opposite direction?

The previous axonal segment is in the refractory period. A propagating action potential always leaves a trail of refractory membrane in its wake. The trailing membrane takes some time to recover from the action potential it just experienced. This is largely because its voltage-gated sodium channels are inactivated. By the time this membrane segment is ready to (re)generate another action potential, the first propagating action potential is long gone.

The myelin on myelinated neurons can be degraded or destroyed in diseases such as multiple sclerosis. If a myelinated neuron was affected by demyelination, how would this affect action potentials in that neuron?

The speed of action potential propagation would be slower

The plasma membrane is mush more permeable to K+ than to Na+. Why?

There are many more K+ leak channels in the plasma membrane.

What type of channel on the postsynaptic membrane binds neurotransmitter?

a chemically gated channel Chemically gated channels bind a specific chemical, which causes the channel to open. At chemical synapses, neurotransmitter molecules are released by the presynaptic neuron and bind to chemically gated channels on the postsynaptic cell membrane. The opening of these channels allows ions to diffuse across the membrane, causing a graded potential in the postsynaptic cell.

A postsynaptic cell can be a neuron, a muscle cell, or sec rectory cell. What is an example of a presynaptic cell?

a neuron A neuron is the only type of presynaptic cell. Neurons release neurotransmitters, effectively changing an electrical signal or action potential into a chemical signal that can communicate across the synaptic cleft to the postsynaptic cell.

During what part of the action potential do voltage-gated Na+ channels begin to inactivate? (their inactivation gates close)

at the end of the depolarization phase, as the membrane potential approaches its peak value

When is the specific period during which potassium ions diffuse out of the neuron due to a change in membrane permeability?

during repolarization

When can an exceptionally strong stimulus trigger a response?

during the relative refractory period

Neurotransmitter is released from presynaptic neurons through what mechanism?

exocytosis Neurotransmitter molecules are released from vesicles that fuse with the plasma membrane through exocytosis. Note that "exo-" means "outside" and "cytosis" means "cell." Once released, neurotransmitter diffuses across the synaptic cleft.

Neurons can communicate with (form synapses on) ________.

neurons, muscles, and glands Neurons can form synapses on, and therefore communicated with, other neurons, muscle cells, and glands. Neurons can even form synapses with themselves at synapses called "autapses."

Which area of the axon does the plasma membrane exhibit chemically gated ion channels?

the receptive region

Which region of an axon receives stimuli?

the receptive region

Which region of the axon releases neurotransmitters into the extracellular space?

the secretory region

During the hyperpolarization phase of the action potential, when the membrane is more negative than the resting membrane potential, what happens to voltage-gated ion channels?

K+ channels close. Na+ channels go from an inactivated state to a close state.

During an action potential, hyperpolarization beyond (more negative to) the resting membrane potential is primarily due to _________.

K+ ions diffusing through voltage-gated channels.

The repolarization phase of the action potential, where voltage becomes more negative after the +30mV peak, is caused primarily by ________.

K+ ions leaving the cells through voltage-gated channels.

Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?

Leak channels Leak channels for Na+ and K+ are ubiquitous, and they allow for the diffusion of these ions across plasma membranes.

What is the approximate magnitude of an action potential?

100mV

How is an action potential propagated along an axon?

An influx of sodium ions from the current action potential depolarized the adjacent area

Which is considered the conductive portion of the neuron?

Axon The axon is considered the conductive part of the neuron because this is where the action potential occurs and is propagated.

What is the role of calcium in the synaptic activity?

Calcium influx into the synaptic terminal causes vesicle fusion with the presynaptic membrane.

What is meant when we say the action potential is an "all-or-none" event?

For a given neuron, once the action potential is generated, it maintains the same amplitude and duration as it travels down the axon.

Ions are unequally distributed across the plasma membrane of all cells. This ion distribution creates an electrical potential difference across the membrane. What is the name given to this potential difference?

Resting membrane potential (RMP) The resting membrane potential is the baseline potential that can be recorded across the plasma membrane of an excitable cell prior to excitation.

What is the name of the cell that forms the myelin sheath in the peripheral nervous system?

Schwann cell The Schwann cell is the cell in the peripheral nervous system that produces the myelin sheaths around some axons. (In the central nervous system, the oligodendrocyte makes myelin.) It takes many Schwann cells to insulate a single axon, because each Schwann cell wraps only a small portion of the axon. Neighboring Schwann cells do not touch each other, and the gaps between Schwann cells are called Nodes of Ranvier. We will learn that these nodes have an important function during propagation of the action potential in myelinated axons.

What must happen before an action potential is generated?

The membrane potential at the axon hillock must reach a trigger point called threshold potential.

Where are action potentials regenerated as they propagate along a myelinated axon?

at the nodes of Ranvier In myelinated axons, voltage-gated sodium channels are largely restricted to the nodes between myelinated segments. This means that action potentials can only be regenerated at these locations. Myelin prevents leakage of charge from the axon and ensures that currents generated at one node will quickly bring the next node to threshold even though it may be a millimeter away.

What is the are where nerve impulse is generated?

axon hillock

Where in the neuron is an action potential initially generated?

axon hillock This region (first part of the axon) receives local signals (graded potentials) from the soma and dendrites and has a high concentration of voltage-gated Na+ channels. If the graded potentials are large enough when they reach the axon hillock, an action potential is generated.

Which area of the axon does the plasma membrane exhibit voltage-gated Ca2+ channels?

axon terminals

What is a nerve?

cable like bundle of parallel axons in the peripheral nervous system

What is considered the receptive portion(s) of the neuron?

dendrites and cell body (soma) Axon terminals from other neurons can form synapses on the dendrites and cell body (soma), so both of these regions are considered the receptive region of the neuron. These areas "receive" the incoming signals. These areas also integrate the incoming signals, so they can also be considered the "integrative" region. We will learn more about integration of incoming signals later in the lab in the Synaptic Potentials and Cellular Integration module.

What are three main structural components of neurons?

dendrites, cell body (soma), axon

When does the interior of the cell become less negative due to an influx of sodium ions?

during depolarization

How do action potential propagation speeds compare in myelinated and unmyelinated axons?

propagation is faster in myelinated axons The insulated segments of myelinated axons allow local currents to travel quickly between nodes where the action potential is regenerated. This "leaping" of action potentials from node to node is several-fold faster than the continuous propagation found in unmyelinated axons. In addition, myelinated axons tend to have larger diameters, which also enhances propagation speed.

In addition to diffusion, what are two other mechanisms that terminate neurotransmitter activity?

re-uptake and degredation To terminate neurotransmitter effects, neurotransmitter molecules must be removed from the synaptic cleft. Reuptake does this by moving neurotransmitter back into the presynaptic neuron. Diffusion causes neurotransmitter to drift away from the synaptic cleft. Degradation occurs when enzymes break down neurotransmitter. As long as neurotransmitter molecules remain in the synaptic cleft, the chemically gated channels on the postsynaptic cell will continue to bind them and cause graded potentials.

The node-to-node "jumping" regeneration of an action potential along a myelinated axon is called ____________.

saltatory conduction

What is known as an axon terminal forming a synapse on a dendrite and firing several action potentials in quick succession, resulting in multiple EPSPs in the postsynaptic cell that can be added together to create a greater depolarization of the postsynaptic membrane?

temporal summation When an axon terminal forms a synapse on a dendrite and fires several action potentials in quick succession, resulting in multiple EPSPs in the postsynaptic cell, these EPSPs will be added together or summated to create a greater depolarization of the postsynaptic membrane. This is known as temporal summation. Temporal summation can occur with any combination of synapses - they can be excitatory (EPSPs), inhibitory (IPSPs) or a combination of both.

When can the neuron not be able to respond to a second stimulus, no matter how strong?

the absolute refractory period

What is also called a nerve impulse that is transmitted by axons?

the action potential

Why does the action potential only move away from the cell body?

the areas that have had the action potential are refractory to a new action potential.

Which region of the axon does the plasma membrane exhibits voltage-gated Na+ and K+ channels?

the conducting region

An action potential in one segment of axon causes adjacent sections of axon membrane to reach threshold through what mechanism?

the generation of local currents An action potential at the axon hillock generates local currents that depolarize nearby sections of axon to threshold. This change in membrane potential causes voltage-gated Na+ channels to open in the adjacent axon segment. This (re)generates the action potential in the adjacent segment, causing the action potential to propagate away from the axon hillock.

On average, the resting membrane potential is -70mV. What does the sign and magnitude of this value tell you?

the inside surface of the plasma membrane is much more negatively charged than the outside surface.

In a myelinated axon, how do the nodes of Ranvier differ from other segments of the same axon?

the nodes are more permeable to ions In myelinated axons, voltage-gated channels are largely confined to the nodes of Ranvier. This means the nodes are much more permeable to ions than the myelinated segments where voltage-gated channels are absent.

Which type of channel has an "inactivation" phase?

voltage-gated Na+ channels

What is happening during the depolarizing phase of the action potential?

voltage-gated Na+ channels are opening

Action potentials are due to which type of ion channels?

voltage-gated channels Action potentials are generated when voltage-gated Na+ and voltage-gated K+ channels are activated in the axon. Voltage-gated channels are only located in the axon of a neuron. Voltage-gated channels are opened by a change in the membrane potential. The voltage-gated channels that we have learned about in this course open when the membrane potential is depolarized, and close when the membrane potential is repolarized to resting membrane potential.