Biology 12

• 
  
  
  
  • − − + +
  
  
  
  

/

pump

− − + + − −

impulse

gated

channel

gated

channel

Na+ gated

channel

Na+ Na+ Na+

K+ gated

channel

K+

K+

K+

K+

− − − − − −

impulse

gated channel

gated

channel

Na+ Na+

3Na+ K

+

2K+

/K+

pump

− − + + +

outside cell

inside cell

+ + −− −

impulse

gated channel

gated

channel

Na+

Na+

Na+ Na+

K+

K+

plasma

membrane

148 MHR • Unit 2 Homeostasis

stimulus is like the trigger of a gun.

Once the pressure on the trigger is

strong enough, the bullet is on its way

— pulling harder on the trigger has no

effect on the speed of the bullet.

Depolarization

As mentioned above, when a neuron

is sufficiently stimulated, a wave of

depolarization is triggered. When this

occurs, the gates of the K+channels

close and the gates of the Na+channels

open. Sodium ions move into the axon.

This input of positively charged ions

neutralizes the negative charge in the

axon. This change in charge is called the

action potential. The depolarization of

one part of the axon causes the gates of

the neighbouring Na+channels to open,

and this depolarization continues along

the length of the axon. Note that action

potentials can occur in the dendrites

and in the cell body as well.

Repolarization

Any specific region of the axon is only

depolarized for a split second. Almost

immediately after the sodium channels

have opened to cause depolarization,

the gates of the K+channels re-open

and potassium ions move out. The Na+

channels close at the same time. This

process, combined with rapid active

transport of Na+out of the axon by the

Na+/K+pump, re-establishes the polarity

of that region of the axon. The speed

with which this process occurs allows

an axon to send many impulses along

its length every second, if sufficiently

stimulated. The brief time between the

triggering of an impulse along an axon

and when it is available for the next

impulse is called the refractory period.

For many neurons, the refractory period

is approximately 0.001 s.

One advantageous effect of a wave of

depolarization is that an impulse can

move along the entire length of a neuron

and the strength of the signal does not

dissipate. The signal moves at about

2 m/s. However, in some cases it is

important that a wave of depolarization

Figure 5.11The membrane of a neuron contains open as well as gated

channels that allow movement of sodium ions (Na+) and potassium ions (K+)

into and out of the cell. The gated channels open and close as a wave of

depolarization moves down the axon of a neuron.

Gated sodium ion channels open, allowing sodium ions to enter and make

the inside of the cell positively charged and the outside negatively charged.

A

As the impulse passes, gated sodium ion channels close, stopping the

influx of sodium ions. Gated potassium ion channels open, letting

potassium ions out of the cell. This action repolarizes the cell.

B

As gated potassium ion channels close, the Na+/K+pump restores the

ion distribution.

C