1-05 The Nerve Impulse (Part 1)

Front 1

Objective

Explain why an axon must regenerate an impulse instead of just conducting it.

Back 1

The electrical impulses produced by neurons have to travel long distances. If the axon did not regenerate them along the way, these signals would eventually weaken.

Front 2

Objective

Describe the forces behind the resting potential.

Back 2

The resting potential of a neuron is -70mV. This means that when the neuron is at rest, there are more positive ions outside the neuron than there are inside, creating a slightly negative charge inside the cell. The following forces act to create this negative charge:

1. Selective permeability- There are channels in the membrane that allow sodium to enter the neuron. When the neuron is at rest, these sodium channels are closed.

2. Sodium-potassium pump- Specific proteins in the membrane use active transport to pump sodium ions out of the neuron and to bring in potassium ions.

3. Electrical gradient- The difference in electrical charge across a membrane will attract positive ions to the negative side of the membrane and vice versa. In this case, the negative charge inside the membrane is attracting sodium ions, although they have no way of entering if the sodium channels are closed and the sodium-potassium pump is not active.

4. Concentration gradient- The difference in concentration of a substance across a membrane will cause that substance to move from an area of higher concentration to an area of lower concentration. When the neuron is at rest, sodium ions are 10 times more concentrated on the outside of the membrane than on the inside. This concentration gradient will also attract sodium into the neuron, although they have no way of entering at this point. Unlike sodium, potassium ions are more concentrated inside the membrane and will be attracted outward by the concentration gradient.

Front 3

Objective

Explain why there needs to be a resting potential.

Back 3

The resting potential prepares the neuron to respond more quickly to the action potential. A good analogy is a bow and arrow that is already poised; the tension on the bow makes it easier for the arrow to be released.

Front 4

Objective

Discuss the all-or-none law and its significance.

Back 4

If a stimulus has enough intensity, it will produce an action potential of a certain intensity and velocity. The action potential is fixed; even if the stimulus has twice as much intensity as it needs to cross the activation threshold, the action potential will have exactly the same intensity and velocity. If the stimulus is not strong enough, no action potential will be generated. This is called the all-or-none law because a stimulus either produces an action potential of fixed intensity and velocity, if the stimulus is strong enough, or it produces no potential.

Front 5

Objective

Describe the molecular basis of the action potential.

Back 5

Here is the chain of events associated with an action potential:

1. A slight depolarization occurs, increasing the negative voltage inside the membrane so that it approaches zero. This is caused by the migration of sodium ions inside the membrane from adjacent areas (see “propagation of an action potential” below).

2. The sodium channels in the membrane are voltage-activated. When the voltage reaches a certain threshold, these channels open.

3. The sodium ions outside the membrane rush into the neuron, driven by both a concentration gradient and an electrical gradient. This rush of sodium ions raises the voltage to +30mV.

4. Potassium ions inside the neuron flow out, drawn by a concentration gradient and no longer held back by an electrical gradient.

5. The sodium-potassium pump will become activated, pumping out excess sodium ions and pumping in potassium ions until the resting potential is re-established.

Front 6

Objective

Explain how the neuron returns to its resting potential.

Back 6

The sodium-potassium pump removes sodium ions until the resting potential is re-established.

Front 7

Objective

Describe the mechanism by which local anesthetics work.

Back 7

Local anesthetics block sodium channels in a way that prevents the action potential from taking place.

Front 8

electrical gradient

Back 8

A difference in charge across the membranes that will attract positive ions to negative areas and vice versa

Front 9

polarization

Back 9

An electrical gradient across a membrane

Front 10

resting potential

Back 10

The electrical potential across a membrane when a neuron is not being stimulated

Front 11

selective permeability

Back 11

the ability of some molecules to pass more freely than others through a membrane

Front 12

sodium-potassium pump

Back 12

A mechanism that uses proteins in a membrane to actively transport sodium channels out of the cell and potassium ions into the cell

Front 13

concentration gradient

Back 13

A difference in the concentration of a substance across a membrane that draws the substance from areas of high concentration to areas of low concentration

Front 14

hyperpolarization

Back 14

An increase in the electrical potential across a membrane

Front 15

depolarization

Back 15

A decrease in the electrical potential across a membrane

Front 16

threshold of excitation

Back 16

The level of depolarization that triggers an action potential

Front 17

action potential

Back 17

A rapid depolarization of a membrane that causes sodium channels to open and sodium ions to rush into the cell

Front 18

voltage-activated channel

Back 18

A channel in a membrane that opens when a threshold potential is reached

Front 19

refractory period

Back 19

A brief period after an action potential when the cell resists the generation of new action potentials

Front 20

absolute refractory period

Back 20

Time immediately after an action potential when sodium channels close and the membrane cannot produce a new action potential, regardless of the intensity of a stimulus

Front 21

relative refractory period

Back 21

Time after an absolute action potential when potassium channels stay open wider than usual, requiring a stronger than usual stimulus to produce a new action potential