Download NERVE IMPULSE

9.2
Electrochemical Impulse
Late 18th Century:
Luigi Galvani  leg muscle of dead frog could
be made to twitch under electrical
stimulation.
http://www.youtube.com/watch?v=eI_uUKD18
No&feature=related
• Since then...
– ECG (Electrocardiogram)
– EEG (Electroencephalograph (EEG)
Difference between Electrical and
Neuron Transmission
• Currents along wire much faster than neuron
• Cytoplasmic core offers great resistance
• Electrical currents diminish as they move
along a wire  neurons do not.
K.S. Cole and H.J. Curtis
• Placed a tiny electrode inside the large nerve
cell of a squid
• Rapid change in electrical potential across
membrane
• From -10 mV  +40 mV
• ACTION POTENTIAL: reversal of potential 
does not last more than a few ms before
returning to RESTING POTENTIAL.
How do Nerve Cell Membranes
become charged?
• Neurons have a rich supply of positive and
negative ions on inside and outside of cell
– Negative ions do not have much to do with action
potential  they are large and cannot cross the
membrane (stay inside of cell)
– Therefore, electrochemical event caused by
unequal conc. Of + ions across membrane
Unequal concentration due to...
• High concentration of K+ inside of the cell
– Tendency to diffuse out.
• High concentration of Na+ outside of cell
– Tendency diffuse in.
THE RESTING MEMBRANE POTENTIAL IS 50 TIMES
MORE PERMEABLE TO POTASSIUM THAN TO
SODIUM  MORE POTASSIUM IONS DIFFUSE OUT
OF NERVE CELL THAN SODIUM DIFFUSE IN.
ION-GATES: Control the movement of ions.
So...
• There is a more rapid diffusion of potassium
ions ______ of the nerve cell than potassium
ions ______.
• Therefore, net __________ of ions.
• During resting potential, net _____________
charge on outside and net ________________
charge inside.
• Resting membrane potential: polarized
membrane.
Electrical Charge Separation
• The separation of electrical charge allows
membrane to have potential to do work.
• A neuron has charge of -70 mV
Depolarization
• Upon excitation, the nerve cell becomes more
permeable to sodium than potassium.
– Sodium gates are opened
• Therefore, sodium ions rush _________ the cell by
diffusion and charge attraction.
– Potassium gates close
• No movement of potassium ions _______ the cell.
DEPOLARIZATION! (NERVE CELL TEMPORARILY _____
CHARGED.
Now the nerve cell is depolarized...
• Sodium gates slam closed and inflow of
sodium is halted.
The Sodium-Potassium Pump
• Restores the resting membrane potential by
– Transporting sodium ions out
– Moving potassium ions in
Na+/K+: three Na+ ions out, two K+ ions in.
Energy supply: ____________.
REPOLARIZATION!
Page 420
Read/make notes
on this very
important
figure.
Be able to
identify
resting
membrane
potential,
depolarization
, action
potential,
repolarization.
Refractory Period
• Nerves cannot be activated again until resting
potential restored.
• Refractory Period: time required for
repolarization.
– Lasts 1-10 ms
Movement of the Action Potential
• Recall: action potential is characterized by the opening
of sodium channels in the nerve membrane.
– Sodium ions rush INTO the cytoplasm
– Causes a charge reversal (depolarization) in that area.
– + ions that rush into nerve cell attracted to adjacent
negative ions, aligned along the inside of the nerve
membrane.
– Similar attraction occurs along the outside of the nerve
membrane; + charged sodium ions of the resting
membrane attracted to the negative charge that has
accumulated along the outside of the membrane.
Threshold Levels and the All-or-None
Response
• Nerve cells respond to changes in pH, changes
in pressure, and to specific chemicals.
• Major: mild electrical shock (intensity can be
regulated)
Classic Experiment
• Single neuron leading to a muscle is isolated
and a mild electrical shock is applied to the
neuron.
• Strength of muscle contraction measured.
• Intensity slowly increased.
• What does this data tell us?
- Threshold level: Critical intensity of a stimulus
to create an action potential.
- All-or-none: Increasing the intensity of the
stimuli above the critical threshold value will
not produce an increased response.
- Neurons fire maximally or not at all.
Detecting the Intensity of a Stimuli
• If neurons do not respond correspondingly to
intensity of stimuli, how do we know the
difference between warm and hot?
• The more intense the stimulus, the greater the
frequency of impulses
– If a warm glass rod is placed in your hand, sensory
impulses sent to brain at slow rate
– If hot glass rod, frequency is greatly increased  a
difference that the brain recognizes.
• Each nerve is
composed of many
individual nerve cells
or neurons.
Different neurons may
have different
threshold levels.
Synaptic Transmission
• Synapse: small spaces between neurons or
neurons and effectors.
– Rarely involve just two neurons.
• Neurotransmitters: located at end plates of axon.
– Impulse reaching axon terminal cause vesicles of
neurotransmitters to be released into synapse.
– Presynaptic neuron: NTs released from here.
– Postsynaptic neuron: effected by presynaptic.
– Nerve transmission slows across this synapse.
• Reflex arc vs. Solving math problems.
Acetylcholine & Cholinesterase
• Example of a neurotransmitter
• Acts as an excitatory neurotransmitter by opening
the sodium channels of a postsynaptic nerve 
causing _______________.
• Problem: if acetylcholine is causing sodium
channels to open, how can nerve respond to next
impulse?
• Cholinesterase: an ________________ released
from postsynaptic membrane  destroys
acetylcholine.
Insecticides
• Block cholinesterase  insect ‘heart’ will
remain in contracted state and never relax.
Hyperpolarization
• Condition in which the inside of the nerve cell
membrane becomes more negative than the
resting potential.
– Many NTs can cause this to happen: open up K+
gates  K+ leaves neuron  negative resting
membrane potential.
Different Neurotransmitters
• Some neurotransmitters are inhibitory:
serotonine, dopamine, gamma-aminobutyric
acid (GABA), glutamic acid
– All found in central nervous system
• Excitatory NTs: norepinephrine  found in
both excitatory and inhibitory NSs.
Coordination of Inhibitory & Excitatory
Responses
• Throwing a ball
– As triceps excited and contracts, biceps receive
inhibitory impulses and relaxes.
– Two muscles do not pull against each other
• Inhibitory impulses in CNS: more important
– Sensory info is received by brain and prioritized.
– Less important info is ignored. Why?
NS diseases
• Parkinson’s
– Characterized by involuntary muscle contractions
and tremors  due to inadequate dopamine
production.
• Alzheimer’s
– Decreased production of acetylcholine.
Seatwork/Homework
• Page 426
• #1-13