The Nerve Impulse

Question 1

polarization

Answer 1

a difference in electrical charge between the inside and outside of the cell

Question 2

resting potential.

Answer 2

electrical potential inside the membrane is slightly nega- tive with respect to the outside, mainly because of negatively charged proteins inside the cell

difference in voltage is called the resting potential

Question 3

selective permeability

Answer 3

some chemicals pass through it more freely than others do

Oxygen, carbon dioxide, urea, and water cross freely through channels that are always open

sodium, potassium, calcium, and chloride, cross through membrane channels (or gates) that are sometimes open and sometimes closed

Question 4

sodium–potassium pump

Answer 4

a protein complex, repeatedly transports three sodium ions out of the cell while drawing two potassium ions into it.

sodium ions are more than 10 times more concentrated outside the membrane than inside, and potassium ions are more concentrated in- side than outside

Question 5

forces acting on membrane at rest

Answer 5

electrical gradient

concentration gradient

Question 6

electrical gradient

Answer 6

Sodium is positively charged and the inside of the cell is negatively charged. Opposite electrical charges attract, so the electrical gradient tends to pull sodium into the cell

Question 7

concentration gradient

Answer 7

difference in distribution of ions across the membrane.

Sodium is more concentrated outside than inside, so just by the laws of prob- ability, sodium is more likely to enter the cell than to leave it.

Question 8

forces acting on potassium

Answer 8

positively charged and the inside of the cell is negatively charged, so the electrical gradient tends to pull potassium in

potassium is more concentrated inside the cell than outside, so the concentration gradient tends to drive it out

Question 9

forces acting on sodium

Answer 9

electrical and concentration tend to pull sodium in

Question 10

maintaining negative charge at rest

Answer 10

Negatively charged pro- teins inside the cell sustain the membrane’s polarization

Chloride ions, being negatively charged, are mainly outside the cell. When the membrane is at rest, the concentration gradient and electrical gradient balance, so opening the chloride channels would produce little effect. However, chloride does have a net flow when the membrane’s polarization changes.

Question 11

action potentials

Answer 11

Messages sent by axons

Question 12

hyperpolarization

Answer 12

when the inside of the cell increases its negative charge;

ie, increased polarization

Question 13

depolarization

Answer 13

reduce its polarization toward zero

Question 14

threshold of excitation

Answer 14

stimulation beyond this produces massive depolarization of the membrane

Question 15

all-or-none law

Answer 15

the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it, provided that the stimulus reaches the threshold

Question 16

Molecular Basis of the Action Potential

Answer 16

1. At the start, sodium ions are mostly outside the neuron, and potassium ions are mostly inside.
2. When the membrane is depolarized, sodium and potas- sium channels in the membrane open.
3. At the peak of the action potential, the sodium channels close.

Question 17

voltage-gated channels

Answer 17

axon channels regulating sodium and potas- sium

their permeability depends on the voltage difference across the membrane

At the resting potential, the sodium channels are fully closed and the potassium channels are almost closed, allowing only a little flow of potassium

As the membrane becomes depolarized, both the sodium and the potassium channels begin to open, allowing freer flow.

Question 18

peak of action potential

Answer 18

at the peak of the action potential, the sodium gates snap shut

Question 19

Local anesthetic

Answer 19

attach to the sodium channels of the membrane, preventing sodium ions from entering

Question 20

propagation of the action potential

Answer 20

the transmission of an action potential down an axon

Question 21

back propagation

Answer 21

action potential “back-propagates” from the axon into the cell body and dendrites

hey pas- sively register the electrical event that started in the nearby axon

the dendrite be- comes more susceptible to the structural changes responsible for learning

Question 22

myelin

Answer 22

insulating material composed of fats and proteins

Question 23

myelinated axons

Answer 23

those covered with a myelin sheath

found only in vertebrates

interrupted periodically by short sections of axon called nodes of Ranvier, each one about 1 micrometer wide

After an action potential occurs at a node, sodium ions enter the axon and diffuse, push- ing a chain of positive charge along the axon to the next node, where they regenerate the action potential

Question 24

saltatory conduction

Answer 24

jumping of action potentials from node to node

This flow of charge moves considerably faster than the regeneration of an action potential at each point along the axon.

Question 25

multiple sclerosis

Answer 25

immune system attacks myelin sheaths. An axon that never had a myelin sheath conducts im- pulses slowly but steadily, but an axon that has lost its myelin is not the same, because it lacks sodium channels where the myelin used to be, meaning the action potential dies out

Question 26

refractory period

Answer 26

period during which it resists the production of further action potentials

absolute and relative

Question 27

absolute refractory period

Answer 27

membrane cannot produce another action potential, regardless of the stimulation

Question 28

relative refractory period

Answer 28

a stronger- than-usual stimulus is necessary to initiate an action potential

Question 29

time of refractory periods

Answer 29

absolute refractory period is about 1 millisecond (ms), and the relative refractory period is another 2 to 4 ms.

Question 30

local neurons

Answer 30

Neurons without an axon that exchange information with only their closest neighbors

Question 31

graded potential

Answer 31

a membrane potential that varies in magnitude in proportion to the intensity of the stimulus.

ie, it does not function by the all or none principle

change in membrane poten- tial is conducted to adjacent areas of the cell, in all directions, gradually decaying as it travels.