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Chapter 48

A simple nervous system
includes sensory information, an integrating center, and effectors.
Most of the neurons in the human brain are
The nucleus and most of the organelles in a neuron are located in the
cell body.
In certain large animals, this type of neuron can extend beyond 1 meter in length.
a sensory neuron
The somatic nervous system can alter the activities of its targets, the skeletal muscle fibers, because
its signals bind to receptor proteins on the muscles.
The point of connection between two communicating neurons is called
the synapse.
In a simple synapse, neurotransmitter chemicals are released by
the presynaptic membrane.
In a simple synapse, neurotransmitter chemicals are received by
the dendritic membrane.
In the communication between a motor neuron and a skeletal muscle,
the motor neuron is considered the presynaptic cell and the skeletal muscle is the postsynaptic cell.
For a neuron with an initial membrane potential at -70 mV, an increase in the movement of potassium ions out of that neuron's cytoplasm would result in
the hyperpolarization of the neuron.
Although the membrane of a "resting" neuron is highly permeable to potassium ions, its membrane potential does not exactly match the equilibrium potential for potassium because the neuronal membrane is also
slightly permeable to sodium ions.
The operation of the sodium-potassium "pump" moves
sodium ions out of the cell and potassium ions into the cell.
A cation that is more abundant as a solute in the cytosol of a neuron than it is in the interstitial fluid outside the neuron is
The membrane potential that exactly offsets an ion's concentration gradient is called the
equilibrium potential.
ATP hydrolysis directly powers the movement of
Na+ out of cells.
Two fundamental concepts about the ion channels of a "resting" neuron are that the channels
open and close depending on stimuli, and are specific as to which ion can traverse them.
Opening all of the sodium channels, with all other ion channels closedwhich is an admittedly artificial settingon an otherwise typical neuron should move its membrane potential to
+62 mV.
A graded hyperpolarization of a membrane can be induced by
increasing its membrane's permeability to K+.
Self-propagation and refractory periods are typical of
action potentials.
The "selectivity" of a particular ion channel refers to its
permitting passage only to a specific ion.
A "resting" motor neuron is expected to
exhibit a resting potential that is more negative than the "threshold" potential.
The "threshold" potential of a membrane
is the minimum depolarization needed to operate the voltage-gated sodium and potassium channels.
Action potentials move along axons
more rapidly in myelinated than in non-myelinated axons.
A toxin that binds specifically to voltage-gated sodium channels in axons would be expected to
prevent the depolarization phase of the action potential.
After the depolarization phase of an action potential, the resting potential is restored by
the opening of voltage-gated potassium channels and the closing of sodium channels.
The "undershoot" phase of after-hyperpolarization is due to
sustained opening of voltage-gated potassium channels.
Immediately after an action potential passes along an axon, it is not possible to generate a second action potential; thus, we state that the membrane is briefly
An action potential can start in the middle of an axon and proceed in both opposite directions when
only the middle section of the axon has been artificially stimulated by an electrode.
The primary means by which a neuron can communicate to a second neuron is by
the frequency of its action potentials.
In the sequence of permeability changes for a complete action potential, the first of these events that occurs is
the opening of voltage-gated sodium channels.
Saltatory conduction is a term applied to
jumping from one node of Ranvier to the next in a myelinated neuron.
The surface on a neuron that discharges the contents of synaptic vesicles is the
presynaptic membrane.
Neurotransmitters are released from axon terminals via
The fastest possible conduction velocity of action potentials is observed in
thick, myelinated neurons
Neural transmission across a mammalian synaptic gap is accomplished by
impulses causing the release of a chemical signal and its diffusion across the gap.
One possible disadvantage to a nerve net is that it might conduct impulses in two directions from the point of the stimulus. Most of the synapses in vertebrates conduct information in only one direction
because only the postsynaptic cells can bind and respond to neurotransmitters.
The release of acetylcholine from the terminal of a motor neuron is most directly linked to
the entry of calcium into the axon terminal.
The observation that the acetylcholine released into the junction between a motor neuron and a skeletal muscle binds to a sodium channel and opens it is an example of
a ligand-gated sodium channel.
An inhibitory postsynaptic potential (IPSP) occurs in a membrane made more permeable to
potassium ions.
following steps refer to various stages in transmission at a chemical synapse.
1. Neurotransmitter binds with receptors associated with the postsynaptic membrane.
2. Calcium ions rush into neuron's cytoplasm.
3. An action potential depolarizes the membrane of the axon terminal.
4. The ligand-gated ion channels open.
5. The synaptic vesicles release neurotransmitter into the synaptic cleft.
Which sequence of events is correct?
A) 1 → 2 → 3 → 4 → 5
B) 2 → 3 → 5 → 4 → 1
C) 3 → 2 → 5 → 1 → 4
D) 4 → 3 → 1 → 2 → 5
E) 5 → 1 → 2 → 4 → 3
The activity of acetylcholine in a synapse is terminated by
its degradation by a hydrolytic enzyme on the postsynaptic membrane.
Adjacent neurons with direct (non-neurotransmitter) action potential transfer are said to have electrical synapses, based on the presence of
gap junctions at their point of contact.
Ionotropic receptors are found at synapses operated via
ligand-gated ion channels.
An example of ligand-gated ion channels is
acetylcholine receptors at the neuromuscular junction.
An example of the action of metabotropic receptors is when
acetylcholine-gated sodium channels open.
Neurotransmitters categorized as inhibitory are expected to
hyperpolarize the membrane.
When several EPSPs arrive at the axon hillock from different dendritic locations, depolarizing the postsynaptic cell to threshold for an action potential, this is an example of
spatial summation
When several IPSPs arrive at the axon hillock rapidly in sequence from a single dendritic location, hyperpolarizing the postsynaptic cell more and more and thus preventing an action potential, this is an example of
temporal summation.
Assume that a single IPSP has a negative magnitude of -0.5 mV at the axon hillock, and that a single EPSP has a positive magnitude of +0.5 mV. For a neuron with an initial membrane potential of -70 mV, the net effect of the simultaneous arrival of six IPSPs and two EPSPs would be to move the membrane potential to
-72 mV.
Receptors for neurotransmitters are of primary functional importance in assuring one-way synaptic transmission because they are mostly found on the
dendritic membrane.
Functionally, which cellular location is the neuron's "decision-making site" as to whether or not an action potential will be initiated?
axon hillocks
The primary neurotransmitter from the parasympathetic system that influences its autonomic targets is
The major inhibitory neurotransmitter of the human brain is
The major excitatory neurotransmitter of the human brain is
A neuropeptide that might function as a natural analgesic is
) An amino acid that operates at inhibitory synapses in the brain is
The botulinum toxin reduces the synaptic release of
The heart rate decreases in response to the arrival of
A chemical that affects neuronal function but is not stored in presynaptic vesicles is
nitric oxide.
Action potentials are normally carried in only one direction: from the axon hillock toward the axon terminals. If you experimentally depolarize the middle of the axon to threshold, using an electronic probe, then
two action potentials will be initiated, one going toward the axon terminal and one going back toward the hillock
Assume that excessive consumption of ethanol increases the influx of negative chloride ions into "common sense" neurons whose action potentials are needed for you to act appropriately and not harm yourself or others. Thus, any resulting poor decisions associated with ethanol ingestion are likely due to
decreased membrane depolarization of "common sense" neurons.
What happens when a resting neuron's membrane depolarizes?
The neuron's membrane voltage becomes more positive.
A common feature of action potentials is that they
are triggered by a depolarization that reaches the threshold.
Where are neurotransmitter receptors located?
the postsynaptic membrane
Temporal summation always involves
multiple inputs at a single synapse.
Why are action potentials usually conducted in one direction?
The brief refractory period prevents reopening of voltage-gated Na+ channels.
Which of the following is a direct result of depolarizing the presynaptic membrane of an axon terminal?
Voltage-gated calcium channels in the membrane open.