The time during which an excitable membrane cannot respond to further stimulation regardless of the stimulus strength is the
absolute refractory period.
The period of time during which an excitable membrane can respond again, but only if the stimulus is greater than the initial stimulus is the
relative refractory period.
How would the absolute refractory period be affected if voltage-gated sodium channels remained inactivated?
It would be longer than normal.
What would happen to the membrane potential if a cell suddenly becomes more permeable to Na+?
Depolarize
During an action potential, activation of voltage-gated sodium and potassium channels occurs at different rates. What is the effect of this difference on ion flow across an axon membrane?
Initially, Na+ flows into the cell followed by K+ flowing out of the cell.
What happens if a graded stimulus is of sufficient strength to reach threshold at the trigger zone?
An action potential occurs.
What would the sudden increase in axonal permeability to sodium cause?
The rising phase of an action potential
Why is an action potential conducted in only one direction, from an axon hillock to an axon terminal?
The membrane channels upstream are refractory and cannot open.
Which two properties determine the conduction velocity in a mammalian neuron?
Axon diameter and the leak resistance of the membrane
The rising phase of the action potential is due to
Na+ flow into the cell only.
The falling phase of the action potential is due primarily to
K+ flow out of the cell only.
The point during an action potential when the inside of the cell has become more positive than the outside is known as the
overshoot.
The absolute refractory period of an action potential
ensures one-way travel down an axon, allows a neuron to ignore a second signal sent that closely follows the first, and prevents summation of action potentials.
In order to signal a stronger stimulus, action potentials become
more frequent only.
All of the following must occur before a second action potential can begin, EXCEPT
the Na+ and K+ ions that moved in/out of the cell must move back to their original compartments.
Voltage-regulated channels are located
in the membranes of dendrites, in the membranes of axons, and on the neuron cell body.
The all-or-none principle states that
all stimuli great enough to bring the membrane to threshold will produce action potentials of identical magnitude.
When voltage-gated Na+ channels of a resting neuron open,
Na+ enters the neuron and the neuron depolarizes.
When voltage-gated K+ channels of a resting neuron open,
K+ leaves the neuron.
Which of the following will best increase the conduction rate of action potentials?
Increase the diameter of the axon, increase the resistance of the axon membrane to ion leakage.
Which of the following does NOT influence the time necessary for a nerve impulse to be conveyed by a particular neuron?
whether axon is sensory or motor
Some neurotoxins work essentially the same way as some local anesthetics, which is to
bind to Na+ channels and inactivate them and prevent depolarization by blocking Na+ entry into the cell.
In the first phase of triggering an action potential in a neuron, Na+ ions flow in and
only trigger a positive feedback loop.
What stops the rising phase of the action potential?
The Na+ inactivation gate closes.
The inactivation gate.
is coupled to the movement of the activation gate, but is much slower.
During the relative refractory period, an initial threshold-level depolarization is usually not sufficient to trigger an action potential. Why?
Some Na+ channels have returned to their resting position and K+ channels are still open, so Na+ entry is offset by K+ loss.
Conduction occurs along an axon because
inflow of Na+ triggers the adjacent channels to open.
Tetrodotoxin is a toxin that blocks voltage-gated sodium channels. What effect does this substance have on the function of neurons?
The neuron is not able to propagate action potentials.
Graded potentials that arrive at postsynaptic neurons are called ________ if they make that cell more likely to fire.
excitatory
Graded potentials that arrive at postsynaptic neurons are called ________ if they make that cell less likely to fire.
inhibitory
What initiates exocytosis of synaptic vesicle contents in an axon terminal?
Opening of voltage-gated Ca2+ channels
The total amount of neurotransmitter released at the axon terminal is directly related to
the total number of action potentials.
To increase the amount of neurotransmitter released onto a postsynaptic cell, the presynaptic cell would have to
send action potentials with higher frequency.
Once the action potential reaches the axon terminal, what happens next?
1)exocytosis of a neurocrine
2)release of the neurotransmitter into the synaptic cleft
3)release of a neurohormone into the blood
Calcium is important in the synapse because it
signals the exocytosis of the neurotransmitter.
When more action potentials arrive at the axon terminal, how are neurotransmitters affected?
More molecules are released into the synapse.
When more action potentials arrive at the axon terminal, how is the postsynaptic cell affected?
Neurotransmitter release increases, thereby increasing the frequency or magnitude of graded potentials in the postsynaptic cell
A stronger stimulus to a neuron results in ________.
larger voltage changes in graded potentials and greater frequency of action potentials produced in response
Whether or not a neuron produces an action potential at a given moment depends on ________.
the total potential change reaching threshold voltage at the trigger zone
When two or more graded potentials arrive at the trigger zone, which of the following could happen?
An excitatory and inhibitory signal can cancel each other out; two excitatory stimuli may be additive, and summation could occur; and two inhibitory stimuli may be additive, resulting in lower excitability.
Spatial summation refers to
multiple graded potentials arriving at one location simultaneously.
The addition of stimuli arriving in rapid succession to produce an action potential is called ________ summation.
Temporal
The addition of several stimuli arriving from different locations on the same cell to produce an action potential is called ________ summation.
Spatial
In a synapse, neurotransmitters are stored in vesicles located in the __________.
presynaptic neuron
Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.
chemically gated; postsynaptic
An action potential releases neurotransmitter from a neuron by opening which of the following channels?
voltage-gated Ca2+ channels
Binding of the neurotransmitter to its receptor causes the membrane to __________.
either depolarize or hyperpolarize
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 in the neuron is an action potential initially generated?
axon hillock
The depolarization phase of an action potential results from the opening of which channels?
voltage-gated Na+ channels
The repolarization phase of an action potential results from __________.
the opening of voltage-gated K+ channels
Hyperpolarization results from __________.
slow closing of voltage-gated K+ channels
What is the magnitude (amplitude) of an action potential?
100mV
How is an action potential propagated along an axon?
An influx of sodium ions from the current action potential depolarizes the adjacent area.
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.
The velocity of the action potential is fastest in which of the following axons?
a small myelinated axon
The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____.
K+; Na+
During depolarization, which gradient(s) move(s) Na+ into the cell?
both the electrical and chemical gradients
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.
The concentrations of which two ions are highest outside the cell.
Na+ and Cl-
The sympathetic and parasympathetic nervous systems are divisions of which system?
Autonomic nervous system
Neurotransmitter is stored and released from
axon terminals and axon varicosities
Repair of damaged neurons can be assisted by certain neurotrophic factors secreted by the
Schwann cells only.