AP Biology Chapter 37

A simple nervous system

includes sensory information, an integrating center, and effectors.

Most of the neurons in the human brain are

interneurons.

The nucleus and most of the organelles in a neuron are located in the

cell body.

The point of connection between two communicating neurons is called the

synapse.

In a simple synapse, neurotransmitter chemicals are released by

the presynaptic membrane.

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

K+.

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 closed�which is an
admittedly artificial setting�on an otherwise typical neuron should move its membrane potential
to

+62 mV.

The "selectivity" of a particular ion channel refers to its

permitting passage only to a specific ion.

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.

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 "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 nonmyelinated 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

refractory.

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 fastest possible conduction velocity of action potentials is observed in

thick, myelinated neurons.

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.

Two fundamental principles that characterize gated ion channels in the neuronal membrane
are that the channels

open and close depending on stimuli and are specific as to which ion can traverse them.

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.

In a simple synapse, neurotransmitter chemicals are received by

the dendritic membrane.

The surface on a neuron that discharges the contents of synaptic vesicles is the

presynaptic membrane.

Neurotransmitters are released from axon terminals via

exocytosis.

Neural transmission across a mammalian synapse is accomplished by

impulses causing the release of a chemical signal and its diffusion across the synapse.

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.

The following steps refer to various stages in transmission at a chemical synapse.

3 ? 2 ? 5 ? 1 ? 4

The activity of acetylcholine in a synapse is terminated by its

degradation by a hydrolytic enzyme on the postsynaptic membrane.

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.

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 IPS

-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

Neurotransmitters affect postsynaptic cells by

All of these options are correct.

The major inhibitory neurotransmitter of the human brain is

GABA.

A neuropeptide that might function as a natural analgesic is

endorphin.

An amino acid that operates at inhibitory synapses in the brain is

GABA.

The botulinum toxin reduces the synaptic release of

acetylcholine.

The heart rate decreases in response to the arrival of

acetylcholine.

A chemical that affects neuronal function but is not stored in presynaptic vesicles is

nitric oxide.

Motor neurons alter skeletal muscle activities by releasing neurotransmitter because

their signals bind to receptor proteins on the muscles.

Most of the synapses in vertebrates conduct information in only one direction

because only the postsynaptic cells can bind and respond to neurotransmitters.

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 threshold.

Where are neurotransmitter receptors located?

the postsynaptic membrane

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.

Suppose a particular neurotransmitter causes an IPSP in postsynaptic cell X and an EPSP in
postsynaptic cell Y. A likely explanation is that

cells X and Y express different receptor molecules for this particular neurotransmitter.