Transcript Nerve Impulses - HRSBSTAFF Home Page

Biology 12
Neuron Function


Neurons carry messages as electrical
impulses
It comes from:
▪ Another neuron
▪ A sensory receptor

Here we are looking at how an impulse
travels within one neuron, down the axon!

For an impulse to be sent there must be a
difference in electrical charge between the
inside and the outside of the cell.

Also called an ACTION POTENTIAL


Sodium (Na+)
Potassium (K+)
 Found inside and
outside of neurons

Since both are
positive…. It`s all
relative!
 Positive = more
positive ions
 Negative = fewer
positive ions

Not sending an impulse
Neuron is polarized
When a neuron receives a stimulus, the cell is
ready/armed to transmit the signal by having
a charged difference across the membrane

Neuron impermeable to Na+


 Na+ collects outside

K+ moves freely (permeable)

Outside is positive
 All Na+ and some K+

Inside is negative
 Some K+

Polarized

When a neuron is excited by a stimulus, the
cell membrane become highly permeable to
Na+ ions

Na+ channels (gates) open
 Na+ diffuse into cell
 The negative charge become positive inside the
cell

Polarity reversed
 Inside positive
 Outside negative

Depolarized

Note: Na+ gates are only open for a fraction
of a second
 Na+ gets trapped inside the cell

Na+/K+ pumps returns membrane to rest
 Na+ moves out
 K+ moves in

Repolarized
 Outside now +
 Inside now –

Neuron is now returned to “resting potential”


K+ moving out of the cell (closes Na+ gates)
Depolarization occurs in a small area
 Affects adjacent gates
▪ Creates “wave” of electricity
▪ Travels length of axon

Minimum strength stimulus required for
action potential to occur

NOT all stimuli can cause action potential
 Different for each neuron

Impulses are all alike once threshold reached

Different strength of stimuli are detected
depending only on number of impulses sent
 Greater the number of impulses reaching the
brain the great the response
Na+
channel
closes
Na+
channel
opens

Resting Potential:
http://www.youtube.com/watch?v=YP_P6bYvEjE




Action Potential
http://www.youtube.com/watch?v=pbg5E9GCNVE
http://www.youtube.com/watch?v=R0TdXkxBOkE
http://www.youtube.com/watch?v=ifD1YG07fB8

Read pages 418-422
 Page 426 #3, 4, 6, 7, 8, 10, 11