Ch.11 Nervous
How is an action potential propagated along an axon?
An influx of sodium ions from the current action potential depolarizes the adjacent area.
What is the role of calcium in synaptic activity?
Calcium influx into the synaptic terminal causes vesicle fusion.
What type of conduction takes place in unmyelinated axons?
Continuous conduction
What changes occur to voltage-gated Na+ and K+ channels at the peak of depolarization?
Inactivation gates of voltage-gated Na+ channels close, while activation gates of voltage-gated K+ channels open.
Where do most action potentials originate?
Initial segment
During the hyperpolarization phase of the action potential, when the membrane potential 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 closed state.
Which of the following is the clearest example of a neuronal membrane's selective permeability?
K+ ions can diffuse across the membrane more easily than Na+ ions.
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 cell through voltage-gated channels
The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____.
K+, Na+
Sodium and potassium ions can diffuse across the plasma membranes of all cells bc of the presence of what type of channel?
Leak channels.
The concentrations of which two ions are highest outside the cell?
Na+ and Cl-
During the action potential of a neuron, which ion is primarily crossing the membrane during the depolarization phase, and in which direction is the ion moving?
Na+ is entering the cell.
The Na+-K+ pump actively transports both sodium and potassium ions across the membrane to compensate for their constant leakage. In which direction is each ion pumped?
Na+ is pumped out of the cell and K+ is pumped into the cell
What prevents the Na+ and K+ gradients from dissipating?
Na+-k+ATPase
What is the direct 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 characterizes repolarization, the second phase of the action potential?
Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV.
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).
What is the difference between TLC and VC?
TLC includes RV; VC does not include RV.
What best describes the Na+ and K+ concentrations across a neuron's plasma membrane?
The Na+ concentration is higher outside the cell compared to inside. The K+ concentration is higher inside the cell compared to outside.
Why does regeneration of the action potential occur in one direction, rather than in two directions?
The inactivation gates of voltage-gated Na+ channels close in the node, or segment, that has just fired an action potential.
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.
What characterizes depolarization, the first phase of the action potential?
The membrane potential changes from a negative value to a positive value.
What event triggers the generation of an action potential?
The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV.
What is the function of the myelin sheath?
The myelin sheath increases the speed of action potential conduction from the initial segment to the axon terminals.
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.
Hypothetically, what would be the most immediate effect of doubling the number of Na+ leakage channels in the plasma membrane?
The resting membrane potential would become less negative (more positive).
Suppose a drug is developed that blocks K+ leakage channels. The drug prevents ions from passing through those channels. If this drug was applied to a neuron, what would be the most immediate effect on that neuron?
The resting membrane potential would become less negative (more positive).
The plasma membrane is much more permeable to K+ than to Na+. WHY?
There are many more K+ leak channels than Na+ leak channels in the plasma membrane.
What opens first in response to a threshold stimulus?
Voltage-gated Na+ channels
What is the first change to occur in response to a threshold stimulus?
Voltage-gated Na+ channels change shape, and their activation gates open.
What type of channel on the postsynaptic membrane binds neurotransmitter?
a chemically gated channel
A postsynaptic cell can be a neuron, a muscle cell, or a secretory cell. What is an example of a presynaptic cell?
a neuron
The velocity of the action potential is fastest in which of the following axons?
a small myelinated axon
The mechanism by which the neurotransmitter is returned to a presynaptic neuron's axon terminal is specific for each neurotransmitter. Which of the following neurotransmitters is broken down by an enzyme before being returned?
acetylcholine
Where are action potentials regenerated as they propagate along a myelinated axon?
at the nodes of Ranvier
Where in the neuron is an action potential initially generated?
axon hillock
During depolarization, which gradient(s) move(s) Na+ into the cell?
both the electrical and chemical gradients
Which component has a role in the postsynaptic cell during synaptic activity?
chemically gated channels
Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.
chemically gated; postsynaptic
An action potential is self-regenerating because __________.
depolarizing currents established by the influx of Na+ flow down the axon and trigger an action potential at the next segment
Binding of the neurotransmitter to its receptor causes the membrane to __________.
either depolarize or hyperpolarize
Neurotransmitter is released from presynaptic neurons through what mechanism?
exocytosis
With emphysema, the muscles that contract with normal exhalation include which of the following?
internal intercostals and abdominal muscles
What is the major role of the Na+-K+ pump in maintaining the resting membrane potential?
maintaining the concentration gradients for Na+ and K+ across the cell membrane
Which of the following would increase the membrane permeability to K+?
more K+ leakage channels
A molecule that carries information across a synaptic cleft is a
neurotransmitter.
In a synapse, neurotransmitters are stored in vesicles located in the __________.
pre-synaptic neuron
How do action potential propagation speeds compare in myelinated and unmyelinated axons?
propagation is faster in myelinated axons
In addition to diffusion, what are two other mechanisms that terminate neurotransmitter activity?
reuptake and degradation
The small space between the sending neuron and the receiving neuron is the
synaptic cleft.
Imagine you changed the concentration of K+ outside a neuron such that the resting membrane potential changed to -80 mV (from the normal resting value of -70 mV). What have you changed?
the electrical gradient for K+ and the concentration gradient for K+
The repolarization phase of an action potential results from __________.
the opening of voltage-gated K+ channels
When calcium ions enter the synaptic terminal
they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron.
An action potential releases neurotransmitter from a neuron by opening which of the following channels?
voltage gated Ca2+ channels
Events that occur during synaptic activity are listed here, but they are arranged in an incorrect order. Choose the correct order of these events below. (a) Voltage-gated calcium channels open (b) Neurotransmitter binds to receptors (c) Action potential arrives at axon terminal (d) Neurotransmitter is removed from the synaptic cleft (e) Neurotransmitter released into synaptic cleft (f) Graded potential generated in postsynaptic cell
(c) Action potential arrives at axon terminal (a) Voltage-gated calcium channels open (e) Neurotransmitter released into synaptic cleft (b) Neurotransmitter binds to receptors (f) Graded potential generated in postsynaptic cell (d) Neurotransmitter is removed from the synaptic cleft
What is the value for the resting membrane potential for most neurons?
-70 mV
What is the magnitude (amplitude) of an action potential?
100 mV
Tetraethylammonium (TEA) blocks voltage-gated K+ channels such that K+ cannot pass even when the channels are open. However, TEA leaves K+ leakage channels largely unaffected. How would you expect the action potential to change if you treated a neuron with TEA?
The action potential would depolarize as usual, but the repolarization phase would take longer, causing the action potential to be more broad in time.
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.
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
What is the electrochemical gradient of an ion?
the sum of the electrical and concentration gradients for that ion
The node-to-node "jumping" regeneration of an action potential along a myelinated axon is called ____________.
saltatory conduction
What is the amount of air that is normally ventilated in one breath?
tidal volume
Which of the following describes the volume of air exhaled during passive breathing?
tidal volume
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
Which volumes are combined to provide the inspiratory capacity?
tidal volume (TV) and inspiratory reserve volume (IRV)
Which of the following would increase with exercise?
tidal volume, frequency of breathing and minute volume
The depolarization phase of an action potential results from the opening of which channels?
voltage-gated Na+ channels
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
What is the driving force for the movement of air into the lungs?
the pressure gradient
If a signal from a sending neuron makes the receiving neuron more negative inside
the receiving neuron is less likely to generate an action potential.
During an acute asthma attack, the obstruction is caused by _______.
bronchiole smooth muscle spasms
Which of the following is a passive process?
expiration
In which type of axon will velocity of action potential conduction be the fastest?
Myelinated axons with the largest diameter
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)
Why is normal exhalation passive?
Respiratory muscles are relaxing and elastic tissue in the lungs recoils.
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
When neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron,
ion channels in the plasma membrane of the receiving neuron open.
When the diaphragm contracts, _______.
it flattens out