Neurophysiology
NEUROTRANSMISSION
Chapter 12
Chemical Synapse Vocabulary
• Presynaptic neuron
• Postsynaptic neuron
• Synapse – means of communication between each
neuron and the next cell; space
• Excitatory neurotransmitters – chemicals that cause
nerve impulses (e.g. ACh, glutamate)
• Inhibitory neurotransmitters – chemicals that inhibit
nerve impulses (e.g.gama aminobutyric acid – GABA)
Steps of Synaptic Transmission
Step 1: Action potential arrives at the axon terminal
Step 2: Release of neurotransmitter from a vesicle
Step 3: Neurotransmitter binds to receptor site on ion
channel on postsynaptic neuron
Step 4: Ions cross the membrane through open
channels
Step 5: The influx of ions causes action potential in
postsynaptic neuron (details are coming up - )
Step 6: Removal of neurotransmitter
• www.mind.ilstu.edu/flash/synapse_1.swf
Major Neurotransmitters in the Body
• Acetylcholine – regulates muscles and memory; mostly
excitatory
• Dopamine – produces feelings of pleasure; mostly
inhibitory
• GABA – major inhibitory neurotransmitter in the brain
• Glutamate - major excitatory neurotransmitter in the
brain
• Seratonin – involved in many functions including mood,
appetite, and sensory perception; inhibitory in pain
pathways
• Norephinephrine – regulates normal brain processes and
is a part of the fight-or-flight response; usually excitatory
Resting Membrane Potential (RMP)
The outside of the
cell is more positive
(Na+) than the
inside (K+)
Electrical charge gradient associated with
the cell membrane; typically -70
millivolts
Protein Channels
•
•
•
•
Ion protein channels are chemically, mechanically
or voltage regulated
Ion channels open in response to the particular
stimulus and allow ions to flow in or out the cell
Flow of ions changes the membrane
potential/voltage
Sudden change in membrane potential that
accompanies activity = action potential (nerve
impulse)
A Special Ion Channel: Na+/K+
ATPase
• When a neuron is at rest, there is a slow leakage of
Na+ into the cell & K+ out of the cell (along
concentration gradient)
• Na+/K+ ATPase pumps 3 Na+ out and 2 K+ in per ATP
hydrolysis & thus prevents reaching equilibrium of
Na+ & K+ ions
Steps of Action Potential
•
•
•
•
Step 1: Resting
Step 2: Depolarization
Step 3: Repolarization
Step 4: Return to normal permeability
Step 1: Resting (-70 mV)
Step 2: Depolarization
• Neurotransmitters bind to their receptors on a
postsynaptic neuron, chemically gated Na+ channels
open, Na+ flows in, and local potential reaches a
threshold limit (-55mV)
• Then, voltage-gated Na+ channels open and Na+ ions
rush into the cell
• Membrane potential reaches +30 mV
Step 3: Repolarization
•
•
Na+ channels close when the inside of the axon
becomes sufficiently positive (30 mV)
Voltage-regulated K+ channels open & K+ flows out
Step 4: Return to resting potential
• Ion movements drive the membrane potential back
toward resting membrane potential value
• Na+/K+ ATPase continues pumping ions, adjusting
levels back to resting equilibrium levels
• Hyperpolarization - briefly the exterior of the
membrane is more negative than resting potential
voltage level
• Refractory period - the time during which a nerve
cell cannot generate another action potential despite
stimulation
And Another Look
• www.blackwellpublishing.com/matthews/cha
nnel.html
• www.blackwellpublishing.com/matthews/acti
onp.html
Closer Look at Action
Potential
http://itc.gsw.edu/faculty/gfisk/anim/actionpot
ential.swf