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Neurophysiology of Nerve Impulses Essay

1.Explain why increasing extracellular K+ reduces the net diffusion of K+ out of the neuron through the K+ leak channels. If the extracellular K+ increases then the concentration of intracellular K+ will decrease causing a decrease in the steepness of the concentration gradient and fewer K+ ions would be drawn out.

2.Explain why increasing extracellular K+ causes the membrane potential to change to a less negative value. How well did the results compare with your prediction? If extracellular K+ is increased then the intracellular K+ will decrease. Fewer intracellular K+ ions would result in the membrane potential being less negative. The results of my prediction were the same.

3.Explain why a change in extracellular Na+ did not alter the membrane potential in the resting neuron. There are less Na+ leak channels than K+ leak channels, and more of the K+ channels are open.

4.Discuss the relative permeability of the membrane to Na+ and K+ in a resting neuron. Membrane permeability to Na+ is very low because there are only a few Na+ leak channels. The membrane is more permeable to K+ because of the higher number of K+ leak channels.

5.Discuss how a change in Na+ or K+ conductance would affect the resting membrane potential. The resting membrane potential depends on the intracellular and extracellular concentrations of the Na+ and K+ ions. Conductance would change the concentration gradient causing either Na+ or K+ to flow into or out of the cell which would change the resting membrane potential.

ACTIVITY 2 Receptor Potential
1.Sensory neurons have a resting potential based on the efflux of potassium ions (as demonstrated in Activity 1). What passive channels are likely found in the membrane of the olfactory receptor, in the membrane of the Pacinian corpuscle, and in the membrane of the free nerve ending? Chemical and pressure channels.

2.What is meant by the term graded potential?
Graded potential are changes in the transmembrane potential that cannot spread far from the site of stimulus.

3.Identify which of the stimulus modalities induced the largest amplitude receptor potential in the Pacinian corpuscle. How well did the results compare with your prediction? High Pressure, my prediction was correct.

4.Identify which of the stimulus modalities induced the largest-amplitude receptor potential in the olfactory receptors. How well did the results compare with your prediction? The moderate intensity chemical modality would induce a receptor potential of the largest magnitude in the olfactory receptors.

5.The olfactory receptor also contains a membrane protein that recognizes isoamyl acetate and, via several other molecules, transduces the odor stimulus into a receptor potential. Does the Pacinian corpuscle likely have this isoamyl acetate receptor protein? Does the free nerve ending likely have this isoamyl acetate receptor protein? The Pacinian corpuscle and the free nerve ending are not likely to have this receptor protein because they did not respond to chemical stimuli in activity 2.

6.What type of sensory neuron would likely respond to a green light? Photosensory neurons would respond to a green light.

ACTIVITY 3 The Action Potential: Threshold
1.Define the term threshold as it applies to an action potential. Threshold is the voltage that must be reached in order to generate an action potential.

2.What change in membrane potential (depolarization or hyperpolarization) triggers an action potential? Depolarization in the membrane potential results in an action potential. The membrane potential must become less negative in order to trigger an action potential.

3.How did the action potential at R1 (or R2) change as you increased the stimulus voltage above the threshold voltage? How well did the results compare with your prediction? The action potential didn’t change as the stimulus voltage increased. This is because once the threshold is met, it is all or none, not graded.

4.An action potential is an “all-or-nothing” event. Explain what is meant by this phrase. This means that once the threshold is met, an action potential occurs. If the stimulus is too small an action potential does not occur.

5.What part of a neuron was investigated in this activity? The trigger zone was investigated. This is where the axon hillock and the initial segment come together.

ACTIVITY 4 The Action Potential: Importance of Voltage-Gated Na1 Channels 1.What does TTX do to voltage-gated Na+ channels?
TTX blocks the diffusion of Na+ through voltage-gated Na+ channels. This blockage is irreversible.

2.What does lidocaine do to voltage-gated Na+ channels? How does the effect of lidocaine differ from the effect of TTX? Lidocaine blocks the diffusion of Na+ through voltage-gated Na+ channels. The difference between TTX and lidocaine is that lidocaine’s effect is reversible.

3.A nerve is a bundle of axons, and some nerves are less sensitive to lidocaine. If a nerve, rather than an axon, had been used in the lidocaine experiment, the responses recorded at R1 and R2 would be the sum of all the action potentials (called a compound action potential). Would the response at R2 after lidocaine application necessarily be zero? Why or why not? With a compound action potential, the results would not necessarily be zero because some axons could remain unaffected.

4.Why are fewer action potentials recorded at R2 when TTX is applied between R1 and R2? How well did the results compare with your prediction? Fewer action potentials are recorded at R2 when TTX is applied between R1 and R2 because it prevents propagation of the action potential by blocking voltage-gated Na+ channels.

5.Why are fewer action potentials recorded at R2 when lidocaine is applied between R1 and R2? How well did the results compare with your prediction? Fewer action potentials are recorded at R2 when lidocaine is applied between R1 and R2 because it prevents propagation of the action potential by blocking voltage-gated Na+ channels.

6.Pain-sensitive neurons (called nociceptors) conduct action potentials from the skin or teeth to sites in the brain involved in pain perception. Where should a dentist inject the lidocaine to block pain perception? Lidocaine should be applied to the receptors to prevent the start of an action potential that would lead to the perception of pain.

ACTIVITY 5 The Action Potential: Measuring Its Absolute and Relative Refractory Periods 1.Define inactivation as it applies to a voltage-gated sodium channel. Voltage-gated Na+ channels are inactivated when they no longer allow Na+ to diffuse through.

2.Define the absolute refractory period.
The absolute refractory period is the time in which no action potential can be generated regardless of the strength of the stimulus.

3.How did the threshold for the second action potential change as you further decreased the interval between the stimuli? How well did the results compare with your prediction? The threshold for the second action potential was higher, requiring a larger depolarization.

4.Why is it harder to generate a second action potential during the relative refractory period? A greater stimulus is required because voltage-gated K+ channels that oppose depolarization are open during this time.


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