Section 9.2 Assignment Electrochemical Impulse S.Dosman Biology 12

1. What is the difference between an ECG and an EEG? An ECG is known as an electrocardiogram and it records the transmission of electrical impulses in heart muscle. An EEG is known as electroencephalograph and it is used to measure brain-wave activity.

2. In the early 1900s, scientists realized that nerve impulses were actually electrochemical messages created by the movement of ions through the membrane of nerve cells. Differentiate between an action potential and a resting potential. When nerves become excited there is a rapid change in the electrical potential across the membrane. The resting membrane of a neuron has a potential of approximately -70 mV. This is known as the resting potential. When the neuron becomes excited, the potential inside the membrane measures +40 mV. The reversal of the membrane potential in the neuron is known as the action potential. The action potential does not last very long (no more than a few milliseconds) and the membrane returns to its resting potential (-70 mV).

3. Why do negative ions do very little to create a charged membrane in neurons? Neurons have a significant supply of both positive and negative ions located in the intracellular and extracellular environment of the cell. The negative ions are generally very large and are located inside the cell and cannot cross the membrane. It is the unequal movement of positive ions (sodium and potassium) into and out of the cell that causes the electrochemical event (charged membrane).

4. Summarize the movement of potassium and sodium ions across the resting neuron membrane. Potassium ions (K+) are in high concentration inside the neuron and have a tendency to move out of the cell. Sodium ions (Na+) are in high concentration outside the neuron and have a tendency to move inside the cell. Although K+ is moving in as Na+ is moving out, the rate of exchange is unequal. The membrane of the neuron is 50 times more permeable to K+ than Na+ which means there is an unequal distribution of ions and the exterior of the cell becomes positive relative to the interior of the cell.

5. What happens to the movement of sodium and potassium ions when a nerve cell becomes excited (excitation)? When excitation occurs in a neuron, it becomes more permeable to sodium than potassium. Sodium gates in the membrane open while potassium gates close. The increase in Na+ ions inside the cell depolarizes (reverses the charge) the internal environment. The sodium gates will close once the internal environment is positive and the movement of Na+ ions into the cell is stopped.

6. Explain how the sodium potassium pump works to repolarize the nerve membrane. After the neuron has been depolarized by the flow of sodium ions it must be returned to its original polarity through a process known as repolarization. There are sodium-potassium pumps embedded in the membrane of the neuron that utilize active transport (ATP) to pump sodium ions out of the cell and potassium ions back into the cell at a ratio of 3 Na+ ions for every 2 K+ ion. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

7. Action potentials are created when a neuron is depolarized 7. Action potentials are created when a neuron is depolarized. Explain how the action potential moves along a nerve cell. An action potential is an electrical pulse that travels along a neuron once it has been “excited”. It begins with the opening of the sodium channels and the flow of Na+ into the neuron. The diffusion of the Na+ into the cell causes the depolarization of the membrane. This means that the internal environment is now positive and the external environment is negative.

7. Action potentials are created when a neuron is depolarized 7. Action potentials are created when a neuron is depolarized. Explain how the action potential moves along a nerve cell. It is important to remember that K+ is still diffusing out of the cell but at a slower rate. The positive and negative charged ions are attracted to one another on either side of the membrane and this allows for the flow of the action potential toward the end of the neuron. As the action potential slides along the neuron it causes the sodium channels to open in the adjoining area of the neuron which continues to cause depolarization and the action potential slides along until it reaches the end of the axon.

8. What is the difference between a presynaptic neuron and a postsynaptic neuron? B – synapse C – postsynaptic neuron A presynaptic neuron is a neuron that is carrying an impulse to the synapse (region between two neurons). A postsynaptic neuron is a neuron that carries an impulse away from the synapse. In other words, a presynaptic neuron passes an impulse across a synapse to a postsynaptic neuron.

9. Why do scientists believe you react more quickly during a reflex arc than during problem solving? Problem solving requires many synapses. The transmission of nerve impulses slows as it moves across the synapse. Diffusion is a slow process so the more synapses involved, the slower the overall process will be. Reflex arcs have very few synapses and therefore occur much faster than problem solving.

10. Explain the role of acetylcholine and cholinesterase in synaptic transmission. The presynaptic neuron will release neurotransmitters from the end plate when the impulse reaches the end of the axon. Acetylcholine is one type of an excitatory neurotransmitter that is found in neurons. It is released via exocytosis into the synaptic cleft and attaches itself to the postsynaptic neuron. Acetylcholine causes sodium ion channels to open in the postsynaptic neuron which in turn cause the depolarization of the postsynaptic neuron.

10. Explain the role of acetylcholine and cholinesterase in synaptic transmission. The depolarization of the postsynaptic neuron causes an action potential in the neuron and the impulse continues along the neuron. The sodium channels will remain open (and the neuron remains depolarized) as long as acetylcholine is still present. The postsynaptic neuron will release cholinesterase, an enzyme that destroys the acetylcholine, and the sodium channels will close allowing the neuron to repolarize.

http://www.youtube.com/watch?v=90cj4NX87Yk

11. How does an inhibitory neurotransmitter work? Inhibitory neurotransmitters make the postsynaptic neuron more permeable to potassium. The potassium ion gates open and potassium ions diffuse out of the cell. Due to the movement of the K+ there is an increase in the positive ions outside of the cell creating a more negative environment inside the resting membrane which can be referred to as hyperpolarized. This prevents the postsynaptic neuron from becoming active; in other words it inhibits the transmission of an action potential.

12. How does the interaction of excitatory and inhibitory neurotransmitters allow you to throw a ball? The action of throwing a ball requires the coordination of the triceps muscle on the back of the arm and the bicep muscle on the front of the arm. During the throwing action, the triceps muscle receives excitatory impulses which cause the contraction of the muscle while the bicep receives inhibitory impulses which cause the muscles to relax. The coordination of excitation and inhibition means the muscles will not pull against one another.

Helpful Animations http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html# http://bcs.whfreeman.com/thelifewire/content/chp44/4402002.html http://www.youtube.com/watch?v=dSkxlpNs3tU