Chapter 48 Mastering Biology

The "information receiving" section of a neuron is its _____.

dendrites (The dendrites have receptor proteins that receive information by binding chemical messages called neurotransmitters. Together with the cell body, the dendrites receive signals from other neurons.)

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

cell body.

Choose the set that includes the most charged compounds that are more abundant inside neurons, in the cytosol, than outside the neurons, in the extracellular fluid.
chloride ions and proteins
sodium and chloride ions
sodium and potassium ions
proteins and sodium ions
potassium ions and proteins

potassium ions and proteins (Proteins are abundant in all cells of the body and are mostly anions, thus contributing significantly to the membrane potential; and potassium ions are actively pumped into the cytosol of all cells in the body.)

Ions move in the direction opposite to that favored by the chemical concentration gradient when _____.

they are pumped by proteins that require ATP hydrolysis and when the electrical charge gradient repulses or attracts them (Active pumping moves ions against their chemical concentration gradients and the Nernst equation describes how membrane potentials can yield electrical gradients to motivate ion movement.)

Acetylcholine receptors on skeletal muscles are described as being "ionotropic" receptors because _____.

binding of acetylcholine to the receptor protein converts the protein to an open ion channel (Ionotropic receptors have both binding sites for their messenger ligands and the capacity to alter shape enough to become an open ion channel, allowing an increase in cross-membrane traffic of ions, thus altering the membrane potential.)

At the neuromuscular junction, the arrival of acetylcholine on the muscle most immediately causes _____.

a graded depolarization (The acetylcholine receptor, after it binds acetylcholine, increases membrane permeability to sodium ions, and the sodium influx causes a graded depolarization.)

A nerve poison that blocks acetylcholine receptors on dendrites would _____.

reduce the binding of acetylcholine to its receptors on the postsynpatic membrane (The binding of neurotransmitter molecules to their specific receptors on the postsynaptic membrane opens ion channels, which completes the transmission of the impulse to the receiving cell. A nerve poison that blocked the acetylcholine receptors would prevent reception of the signal.)

Select the choice that describes neurons with the fastest conduction velocity for action potentials.

thick, myelinated neurons (Increased diameter and myelination yields very high conduction velocities for action potentials.)

In a neuron, during the depolarization phase that may trigger an action potential _____.

some voltage-gated sodium channels are open (When a stimulus depolarizes the membrane, some gated sodium channels open, causing further depolarization. If the threshold is reached, most gated sodium channels open, triggering an action potential.)

The simultaneous arrival of graded depolarization and a graded hyperpolarization of equal but opposite magnitude at a particular location on the dendritic membrane is likely to _____.

cancel each other out, making it appear as if there was no change in membrane potential (Grade potentials are summable, so a depolarization and a hyperpolarization can offset each other to make it appear that there were no changes in membrane potential at that location.)

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 postsynaptic membrane

Which term describes the difference in electrical charge across a membrane?

Membrane potential. (Membrane potential is the difference in electrical charge across a membrane.)

Resting neurons are most permeable to which of the following ions?

K+ (Resting neurons are most permeable to K+ ions.)

True or false? The potential energy of a membrane potential comes solely from the difference in electrical charge across the membrane.

False (The potential energy of a membrane potential comes both from the difference in electrical charge and from the concentration gradient of ions across a membrane.)

Which channel is mainly responsible for the resting potential of a neuron?

Potassium leak channel. (K+ ions flow along their concentration gradient to maintain the resting potential of a neuron.)

Which term describes an electrical signal generated by neurons?

Action potential. (An action potential is a rapid electrical signal generated by neurons.)

Which channel maintains the concentration gradients of ions across a neuronal membrane?

The sodium-potassium pump moving Na+ ions out and K+ ions in. (This channel maintains the ion concentration gradients across a neuronal membrane.)

What behavior is observed if the voltage across a neuronal membrane is set to -20 mV?

The sodium channel opens, and Na+ ions flow in. (Sodium ions flow into the cell when the membrane potential is between -20 mV and 30 mV.)

The operation of the sodium-potassium "pump" moves _____.

sodium ions out of the cell and potassium ions into the cell

In a resting potential, an example of 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+

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

See graph on pg. 18
The membrane’s permeability to sodium ions is at its maximum at label _____.

B

See graph on pg. 19
The minimum graded depolarization needed to operate the voltage-gated sodium and potassium channels is indicated by the label _____.

A

Self-propagation and refractory periods (states) are typical of _____.

action potentials

A toxin that binds specifically to voltage-gated sodium channels in axons would be expected to _____.

prevent the depolarization phase of the action potential

Why are action potentials usually conducted in one direction?

The brief refractory period prevents reopening of voltage-gated Na+ channels.

Why do Na+ ions enter the cell when voltage-gated Na+ channels are opened in neurons?

because the Na+ concentration is much higher outside the cell than it is inside, and the Na+ ions are attracted to the negatively charged interior

Which of the following will increase the speed of an action potential moving down an axon?
I) Action potentials move faster in larger diameter axons.
II) Action potentials move faster in axons lacking potassium ion channels.
III) Action potentials move faster in myelinated axons.

only I and III

Ionotropic receptors found at synapses are operated via _____.

ligand-gated ion channels