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_uUKD18No&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

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