CHAPTER 1.2 THE NERVE IMPULSE

Question 1

ACTION POTENTIAL

Answer 1

Messages sent by axons are called ACTION POTENTIALS

When the membrane is sufficiently depolarized to reach the cell’s threshold, sodium and potassium channels
open. Sodium ions enter rapidly, reducing and reversing the charge across the membrane. This event is known as the action potential.
transmits information without loss of intensity over distance. The cost is a delay between
the stimulus and its arrival in the brain.

Question 2

NEGATIVE CHARGE NEURON

Answer 2

The inside of a resting neuron has a negative charge
with respect to the outside, mainly because of negatively
charged proteins inside the neuron. The sodium–potassium
pump moves sodium ions out of the neuron, and
potassium ions in.

Question 3

ELECTRICAL GRADIENT/CONCENTRATION GRADIENT

Answer 3

> > When the neuron is at rest, two forces act on sodium, both tending to push it into the cell.

1. Electrical gradient & Concentration Gradient

> > When the membrane is at rest, both the electrical gradient and the concentration gradient would act to move sodium ions into the cell, except that its gates are closed.
The electrical gradient tends to move potassium ions into the cell, but the concentration gradient tends to move it out. The two forces almost balance out, but not quite,
leaving a net tendency for potassium to exit the cell.

Question 4

ALL OR NONE LAW

Answer 4

For any stimulus greater than the
threshold, the amplitude and velocity of the action
potential are independent of the size of the stimulus that initiated it.
The all-or-none law puts constraints on how an axon
can send a message. To signal the difference between a weak stimulus and a strong stimulus, the axon cannot send bigger
or faster action potentials. All it can change is the timing. By
analogy, you might send signals to someone by flashing the
lights in your room on and off, varying the speed or rhythm
of flashing.

Question 5

AFTER ACTION POTENTIAL

Answer 5

After the peak of the action potential, the membrane
returns toward its original level of polarization because
of the outflow of potassium ions.

Question 6

REGENERATION OF ACTION POTENTIAL

Answer 6

The action potential is regenerated at successive points along the axon as sodium ions flow through the core of the axon and stimulate the next point along the axon to its threshold. The action potential maintains a constant magnitude as it passes along the axon.

In axons that are covered with myelin, action potentials form only in the nodes that separate myelinated segments.
Transmission in myelinated axons is faster than
in unmyelinated axons.

Question 7

REFRACTORY PERIOD

Answer 7

Immediately after an action potential, the membrane
enters a refractory period during which it is resistant to
starting another action potential.

Question 8

LOCAL NEURONS

Answer 8

Local neurons are small, with no axon. They convey

information over short distances.

Question 9

ALL OR PART OF BRAIN?

Answer 9

Contrary to a popular belief, people use all of their brain,

not some smaller percentage.

Question 10

RESTING POTENTIAL

Answer 10

When at rest, the membrane maintains an electrical
gradient, also known as polarization—a difference in electrical charge between the inside and outside of the cell.
The electrical potential inside the membrane is slightly negative with respect to the outside, mainly because of negatively charged proteins inside the cell.
This difference in voltage iscalled the resting potential.

> > why a resting potential?
excitation of the neuron opens channels that allow sodiumto enter the cell rapidly. Because the membrane did its work in advance by maintaining the concentration gradient for sodium, the cell is prepared to respond vigorously to a stimulus.

Think of a resting potential as a poised bow and arrow > ready to fire!

Question 11

SODIUM POTASSIUM PUMP

Answer 11

*a protein complex, repeatedly
transports three sodium ions out of the cell while
drawing two potassium ions into it. The sodium-potassium pump is an active transport that requires energy.
As a result > of the sodium–potassium pump, sodium ions are more than 10 times more concentrated outside the membrane than inside, and potassium ions are more concentrated inside than outside.

When sodium ions are pumped
out, they stay out.

Question 12

HYPERPOLARISATION

Answer 12

Hyperpolarization is when the membrane potential becomes more negative at a particular spot on the neuron’s membrane,

Question 13

DEPOLARIZATION

Answer 13

Depolarization is when the membrane potential becomes less negative (more positive).
Note that any depolarization that reaches or passes the threshold produces an action potential

Question 14

3 PRINCIPLES OF CHEMICAL EVENTS BEHIND THE ACTION POTENTIAL

Answer 14

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 potassium
   channels in the membrane open.
3. At the peak of the action potential, the sodium channels
   close.

Question 15

PROPOGATION OF THE ACTION POTENTIAL

Answer 15

The term propagation of the action potential describes
the transmission of an action potential down an axon
An action potential always starts in an axon and propagateswithout loss from start to finish.

Question 16

ACTION POTENITAL REVIEW

Answer 16

● When an area of the axon membrane reaches its threshold of
excitation, sodium channels and potassium channels open.
●● At first, the opening of potassium channels produces
little effect.
●● Opening sodium channels lets sodium ions rush into
the axon.
●● Positive charge flows down the axon and opens voltagegated
sodium channels at the next point.
●● At the peak of the action potential, the sodium gates snap
shut. They remain closed for the next millisecond or so,
despite the depolarization of the membrane.
●● Because voltage-gated potassium channels remain open,
potassium ions flow out of the axon, returning the membrane
toward its original depolarization.
●● A few milliseconds later, the voltage-dependent potassium
channels close.
All of this may seem like a lot to memorize, but it is not.
Everything follows logically from the facts that voltage-gated
sodium and potassium channels open when the membrane is
depolarized and that sodium channels snap shut at the peak
of the action potential.