9.2 Electrochemical Impulse

Late 18 th Century: Luigi Galvani  leg muscle of dead frog could be made to twitch under electrical stimulation. 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