chapter 4.4

Question 1

Voltage-activated ion channels

Answer 1

ions channels that open or close in response to changes in the level of the membrane potential.

Question 2

When the membrane potential of the axon is depolarized to the threshold of excitation by an EPSP

Answer 2

the voltage-activated sodium channels in the axon membrane open wide, and Na+ ions rush in, suddenly driving the membrane potential from about -70 to +50 mV.

Question 3

The rapid change in the membrane potential associated with the influx of Na+ ions triggers

Answer 3

the opening of voltage-activated potassium channels. K+ ions near the membrane are driven out of the cell through these channels, first by their relatively high internal concentration and then, when the AP is near its peak, by the positive internal charge.

Question 4

After about 1 millisecond, the sodium channels close,

Answer 4

which marks the end of the rising phase of the AP and the beginning of repolarization by the continued efflux f K+ ions.

Question 5

Once repolarization has been achieved

Answer 5

the potassium channels gradually close. Although, too many K+ ions flow out of the neuron, and it is left hyperpolarized for a brief period of time.

Question 6

AP only involves the ions right next to the membrane

Answer 6

A single AP has little effect on the relative concentrations of various ions inside and outside the neuron, and the resting ion concentrations next to the membrane are rapidly reestablished by the random movement of ions.

Question 7

Sodium-potassium pumps

Answer 7

play only a minor role in the reestablishment of the resting potential.

Question 8

Absolute refractory period

Answer 8

brief period of about 1 to 2 milliseconds after the initiation of an AP during which it is impossible to elicit a second one.

Question 9

Relative refractory period

Answer 9

follows the absolute refractory period; period during which it is possible to fire the neuron again but only by applying higher-than-normal levels of stimulation. End of this period is the point at which the amount of stimulation necessary to fire a neuron returns to baseline.

Question 10

(1) The relative refractory period is responsible for

Answer 10

responsible for the fact that action potentials normally travel along axons in only one direction. Portions of an axon over which an AP has just traveled are left momentarily refractory, making it impossible for an AP to reverse direction.

Question 11

(2) The relative refractory period is responsible for

Answer 11

Responsible for the fact that the rate of neural firing is related to the intensity of the stimulation. High level of continual stimulation causes the neuron to fire again as its absolute refractory period is over. If stimulation is just sufficient to fire the neuron, then the neuron does not fire again until both the absolute and the relative refractory periods are over.

Question 12

Conduction of APs differ from the conduction of EPSPs and IPSPs by:

Answer 12

(1) È The conduction of APs along an axon is nondecremental; APs do not grow weaker as they travel along the axonal membrane.
(2) APs are conducted more slowly than postsynaptic potentials.

Question 13

The conduction of EPSPs and IPSPs is

Answer 13

passive, whereas the axonal conduction of APs is largely active.

Question 14

The wave of excitation triggered by the generation of an AP near the axon hillock always

Answer 14

spreads passively back through the cell body and dendrites of the neuron.

Question 15

Antidromic conduction

Answer 15

if electrical stimulation of sufficient intensity is applied to the terminal end of an axon, an AP will be generated and will travel along the axon back to the cell body.

Question 16

Orthodromic conduction

Answer 16

axonal conduction in the natural direction – from cell body to terminal buttons.

Question 17

In myelinated axons

Answer 17

ions can pass through the axonal membrane only at the nodes of Ranvier. Axonal sodium channels are also concentrated at the Nodes of Ranvier.

Question 18

When an AP is generated in a myelinated axon

Answer 18

the signal is conducted passively – instantly and decrementally – along the first segment of myelin to the next node of Ranvier. Still strong enough to open the voltage-activated sodium channels at the node and can generate another AP.

Question 19

Saltatory conduction

Answer 19

transmission of APs in myelinated axons.

Question 20

Conduction is faster

Answer 20

in large diameter axons and faster in those that are myelinated.

Question 21

Mammalian motor neurons

Answer 21

synapse on skeletal muscles; large and myelinated and thus can conduct at speeds of 100 meters per second.

Question 22

The maximum velocity of conduction in human motor neurons

Answer 22

is about 60 meters per second.

Question 23

Conduction in mammalian interneurons

Answer 23

is typically passive and decremental.

Question 24

Hodgkin-Huxley model

Answer 24

provided a simple effective introduction to what we now understand about the general ways in which neurons conduct signals. Based on the study of squid motor neurons.

Question 25

Motor neurons

Answer 25

are simple, large, and readily accessible in the PNS.

Question 26

Properties of cerebral neurons that are not shared by motor neurons:

Answer 26

(1) Many cerebral neurons fire continually even when they receive no input.
(2) Axons of some cerebral neurons can actively conduced both graded signals and APs.
(3) APs of different classes of cerebral neurons vary greatly in duration, amplitude, and frequency.
(4) Many cerebral neurons do not display APs.
(5) The dendrites of some cerebral neurons can actively conduct APs.

Question 27

Postsynaptic potentials (PSPs)

Answer 27

are elicited on the cell body and dendrites. PSPs are conducted decrementally to the axon. When the summated PSPs exceed the threshold of excitation at the axon, an AP is triggered. The AP is conducted nondecrementally down the axon to the terminal button. Arrival of the AP at the terminal button triggers exocytosis.