Transcript Transmission of Nerve Impulses

Transmission of Nerve
Impulses
Section 9.2 in your Bio 12 textbook
Nerve Impulse
 An “electrochemical” signal
 Nerves use cellular energy (ATP) to
generate a current & move the signal down
the length of the neuron
Cellular chemistry…
 There are + and – ions in both the external
& internal environments around the nerve
cell
 At rest:
 outside of cell has a net + charge
 inside of cell has a net – charge
 more + ions outside of the cell than inside
 Difference in voltage = resting potential
 At rest: cell has a potential of -70 mV
 As a nerve impulse passes along the length
of the axon, the voltage changes
 Excited:
 outside of cell has a net – charge
 inside of cell has a net + charge
 more + ions inside the cell than outside
 Difference in voltage = action potential
 When excited: cell has a potential of +40
mV
What causes the difference in
charge?
 - ions stay put
 too large to cross the cell membrane
 + ions pass through the membrane
 unequal concentrations produce the
difference in charge
 K+ tend to diffuse: inside  outside
 Na+ tend to diffuse: outside  inside
 Diffusion of both types of ions
happens simultaneously
 But: at rest, cell membrane is more
permeable to K than to Na
 therefore: there is an unequal
concentration of + ions
 this is why the outside is + compared to
the inside
Polarization
 Membrane is “polarized” when at rest
 Polarized = charged
 Question: Can you hypothesize the
effect on the charge of the resting
membrane if there were a greater
concentration difference of + ions?
Depolarization
 When the cell is excited…
 An electrical impulse runs down the
length of the axon
 “gates” for Na open & more Na+ diffuse
rapidly into the cell
 Cell is more permeable to Na than K now
 Cell is now “depolarized” at that area 
causes a reversal in charge
 Once the inside becomes + relative to
the outside, the Na gates close
Repolarization
 Resting potential is restored
 Achieved via sodium-potassium
pumps
 Move 3 Na+ out
 For every 2 K+ in
 Pumps are powered with ATP
How often can an action potential
be initiated?
 Depends on the length of the
refractory period
 The time that it takes (1-10 ms) for the
resting potential to be restored
Movement of Action Potential
 Copy Figure 6, page 421 in your
textbook
 Impulse moves along the cell’s axon,
away from the cell body, toward the
adjacent cell
 Impulse moves from the area of
depolarization to the next, resting,
region
Threshold level
 Minimum amount of stimulus required
to produce a nerve impulse
All-or-nothing!
 Neurons only “fire” at one intensity
 Once the threshold level is reached, the
impulse is transmitted
 Larger stimulus does not cause a larger
or faster impulse
 Neurons fire or they don’t, there is
nothing in-between
 This is the “all-or-none” response
How do we feel the difference in
stimuli?
 E.g.  a warm stove vs. a hot stove
 ?????
 The frequency of the nerve impulse
OR
 The number of nerves stimulated
McGraw-Hill Animation
 http://highered.mcgrawhill.com/sites/0072495855/student_vi
ew0/chapter14/animation__the_nerv
e_impulse.html
So… Once the action potential reaches the
end of the axon, how does it get to the
next neuron?
 Axon terminals are (almost) in
contact with the dendrites of other
neurons
 Synapse: space between neurons
Chemical signals
 Transmission of the signal is chemical
rather than electrical
 Chemicals: neurotransmitters
 Speed of impulse transmission decreases
during this chemical phase  because
diffusion is a relatively slow process
 More synapses: slower the speed of the
nerve impulse
 Reflex arc has few synapses (very quick
responses)
Acetylcholine
 Common neurotransmitter
 Excitatory
 Causes the postsynaptic neuron to
become more permeable to Na
 Depolarization
Cholinesterase




Enzyme (enzymes always end in –ase)
Released from postsynaptic neuron
Destroys acetylcholine
Na gates close, neuron can recover
McGraw-Hill Animation
 http://highered.mcgrawhill.com/sites/0072495855/student_vi
ew0/chapter14/animation__transmiss
ion_across_a_synapse.html
More about neurotransmitters…
 If a neurotransmitter is inhibitory
rather than excitatory, membrane
becomes more permeable to K
 K+ leave the cell
 Cell is “hyperpolarized”
 Postsynaptic neurons become inactive
Summation
 >1 neuron can provide an impulse at
a time
 Branching, axon terminals of >1
neuron may be in contact with the
next neuron
 The effect produced by
neurotransmitters from >1 neuron is
“summation”
What are some other
neurotransmitters?





Serotonin
Dopamine
Gamma-aminobutyric acid (GABA)
Glutamic acid
Norpinephrine/noradrenaline
Homework/Practice questions
 Section 9.2, Questions, page 426
 #1-11