Download The Importance of the Nervous System

Nerve Signals
11.2
Nerve Impulses
• there are about 100 billion neurons in
the human brain
• neurons can transmit 10-100 nerve
impulses per second
Potential Difference
• caused by relative
concentrations of
positive ions (Na+
and K+) on either
side of the
membrane
Neuron Membrane
Resting Potential (-70 mV)
• inside of cell is more negative (-70 mV)
than outside
• membrane is “polarized”
• Na+/K+ active transport pump
establishes resting potential, pumping
out 3 Na+ for every 2 K+ in
Na+/K+ Pump
Action Potential
depolarization
+ repolarization
+ restoration of resting potential
= action potential
Depolarization (+40 mV)
• stimulus causes
depolarization
• if threshold
potential reached,
Na+ channels
open
• Na+ ions enter,
causing rapid
depolarization
Repolarization (-70+ mV)
• Na+ channels shut
• K+ channels open
• K+ moves out of cell
• slow closure of K+
channels causes
hyperpolarization
Action Potential
Animation & Quiz
• The dry but informative
McGraw-Hill narrated animation
& quiz:
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter14/
animation__the_nerve_impulse.html
Refractory Period
• from the peak of an action potential until
the membrane reaches resting potential
again
• a few milliseconds
• ensures action potential travels in one
direction only
All-Or-Nothing
• intensity of stimulus is indicated by the
frequency of action potentials, not the
size
• 10-100 action potentials per second
• rate of conduction increases with
diameter of nerve (up to 25 m/s)
Myelinated Axons
What happens when the action
potential reaches the axon
terminals?
Chemical Synapse
Synaptic Transmission
• Ca2+ ions actively pumped out of
neurons
• action potential in presynaptic neuron
causes calcium channels to open
• Ca2+ ions flow in and cause synaptic
vesicles to fuse with plasma membrane
Synaptic Transmission
• neurotransmitters
released into the
synaptic cleft
• diffuse across cleft
and bind to
receptors on
postsynaptic cell
Neurotransmitters
• acetylcholine (excitatory) causes Na+
channels to open in postsynaptic
neuron, propagating the action potential
• many drugs interfere with action of
neurotransmitters
Anti-anxiety drugs
• depressants such as
diazepam (Valium)
act by increasing the
amount of inhibitory
neurotransmitters at
synapses
Endorphins
endorphin
= endogenous
morphine
(http://dictionary.reference.com/browse/endorphin)
Animation & Quiz
• Quick McGraw-Hill narrated animation
with quiz:
• http://highered.mcgrawhill.com/sites/0072943696/student_view0/chapter8/animation__
chemical_synapse__quiz_1_.html