The Nervous System: Neurophysiology of Nerve Impulses

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Session 9: The Nervous System - Assignment #2

PhysioEx Assignment :
Exercise 3 Neurophysiology of Nerve Impulses

Activity 1: The Resting Membrane Potential

1. Explain why increasing extracellular K+ reduces the net diffusion of K+ out of the neuron through the K+ leak channels.

Increasing the extracellular K+ causes the membrane potential to change to a less negative value because when K+ ions diffuse out across the membrane they are leaving behind a net negative charge.

2. Explain why increasing extracellular K+ causes the membrane potential to change to a less negative value.

Potassium ions diffuse out across the membrane, leaving behind a net negative charge.

3. Explain why a change in extracellular Na+ did not alter the membrane potential in the resting neuron.

There are less leakage sodium channels than leakage potassium channels, and more of the potassium channels are open.

4. Discuss the relative permeability of the membrane to Na+ and K+ in a resting neuron.

Membrane permeability to sodium is very low because there are only a few sodium leakage channels. Sodium ions do slowly diffuse inward, down their concentration gradient. Left unchecked, such inward leakage of sodium would eventually destroy the resting membrane potential. The sodium potassium pumps offset the small inward sodium leak and outward leak of potassium leak.

5. Discuss how a change in NA+ or K+ conductance would affect the resting membrane potential.

The resting period potential is a potential difference between the inside of the cell and the outside of the cell across the membrane. It depends on the resting permeability of the membrane to ions and on the intracellular and extracellular concentrations of those ions to which the membrane is permeable.

Activity 2: Receptor Potential

1. Sensory neurons have a resting potential based on the efflux of potassium ions. 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?

Passive potassium channels maintain the efflux of potassium ions.

2. What is meant by the term graded potential?

Local potentials are referred to as graded potentials because the magnitude of deviation from the resting membrane potential is proportional to the magnitude of the stimulus.

3. Identify which of the stimulus modalities induced the largest amplitude receptor potential in the Pacinian corpuscle.

For the pressure modality, the largest amplitude receptor was high intensity at an amplitude of 40mV. For the chemical modality the amplitude receptors were all equivalent at an amplitude of zero. The heat modality also showed an amplitude of zero across the board. Light did not appear to have affected the amplitude response either as the amplitude response was zero as well, regardless of the intensity. Therefore, according to the results I could conclude that pressure was the stimulus modality to induce the largest amplitude receptor.

4. Identify which of the stimulus modalities induced the largest amplitude receptor potential in the olfactory receptors.

The pressure modality did not have an effect regardless of the intensity of the stimulus as they each had a response of zero. The chemical modality had the largest amplitude of response at 25 mV. The heat modality did not elicit a response, as the amplitude response of each was zero. The light modality did not elicit a response either as the amplitude response of each was zero as well. Therefore I would conclude that the chemical modality had the greatest amplitude of response at 25 mV.

5. The olfactory receptor also contains a membrane protein that recognizes isogamy 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...
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