Factor(s) Which Determine the Equilibrium Potential for Sodium - Engineering Assignment Help

Assignment Task:

1. Describe how Na+ and K+ contribute to resting membrane potential in a neuron by answering the sub questions below, accounting for both active and passive movement of the ions:

• Describe the factor(s) which determine the equilibrium potential for sodium, and how those factors affect sodium’s movement across the cell membrane at resting membrane potential. 
• Describe the factor(s) which determine the equilibrium potential for potassium, and how those factors affect potassium’s movement across the cell membrane at resting membrane potential. No need to repeat basic principles you have already described in part a. 
• Describe how the both the passive and active movement of sodium and potassium across the membrane determine resting membrane potential in a cell: which equilibrium potential RMP is closest to, and why. There is no need to re-state what you have already described in parts a and b. 

2. You have been called in by the Super-Secret Society for the Study of Hushed Hidden Happenings (SSSSHHH) to study a recently discovered alien organism. The organism has a similar structure to humans, but the ions found in the body are entirely different.
Here is what we know:

• The organism’s neurons have an active-transport pump which pumps the negatively charged ion Iq- into the cell, and the negatively charged ion Af- out of the cell (Iq-/Af- pump). 
• The cell membrane has leak channels for both ions but is 75 times more permeable to Iq- at rest compared to Af-.
• The cell has voltage-gated channels for both ions which are triggered at the same voltage.
• Voltage-gated Af- channels have an activation gate and an inactivation gate and behave like voltage-gated sodium channels.
• Voltage-gated Iq- channels are single-gated and behave like voltage-gated potassium channels.
• Proteins in this creature are positively charged.

a) Considering the concentrations for the ions set by the Iq-/Af- pump, would the equilibrium potentials for each of these ions be positive or negative? Explain your reasoning.

b) Based on the other information provided, at what voltage would you predict resting membrane potential to be relative to the two ions’ equilibrium potentials? You do not need to provide actual values for this, just where RMP would lie relative to the two ions’ equilibrium potentials (more positive, more negative). You may invent values if it helps your answer, as long as they are correct relative to each other. 

Explain your reasoning, including the directions and rate at which each ion would leak across the membrane.

c) This question is exactly what you were afraid it would be: Describe how these neurons would fire an action potential by addressing the points below. Remember that an action potential is a rapid, transient change in membrane voltage. The direction of that change (positive or negative) doesn’t matter as long as there is a change. You may draw a diagram if it helps in your answer, but it is not required. Ensure you also describe the process in words.

• What would threshold voltage be for the channels relative to RMP? Again, only relative (more positive, more negative). But you can invent theoretical values if it aids your answer.
• When threshold voltage is reached, in what order would the channel gates open and close in order to generate an action potential and quickly re-polarise the neuron? 
• Describe how this would affect permeability, which direction the ions would move, and how this would impact membrane potential over the course of the action potential.
• You also discover an ionotropic receptor which is a ligand-gated Af- channel. Would activation of this receptor result in an IPSP or an EPSP? Explain your reasoning in 1-2 sentences. 

3. We discussed several ways in which certain skeletal muscles are equipped to exert more precision than others throughout the range of motion.

• Describe how muscle spindles enhance precision of movement, and how this may vary between muscles requiring high power vs those requiring more precision. You do not need to describe alpha/gamma coactivation.
• Now consider precision in the force of a contraction. For example, playing the piano requires great force precision regarding how hard we press the keys, but jumping high requires less precision in the amount of force generated. 

For a given muscle, how would having a greater number of muscle fibres within each motor unit impact the precision of force exerted? Describe this in the context of motor unit recruitment. Hint: this is not about slow vs fast twitch fibres; consider how much force would be produced for each additional motor unit recruited. 2 marks 

4. Consider a drug which blocks a proportion (~25%) of the Ca2+ binding sites on troponin proteins. How would this impact the maximum contractile force of a muscle fibre? Explain your reasoning in the context of the crossbridge cycle. 

5. Cardiac Glycosides are a class of drugs used to increase the contractility of the heart in patients with irreversible heart failure. These drugs selectively inhibit the sodium/potassium pump in cardiac contractile cells. Describe how inhibiting the sodium/potassium pump in ventricular myocytes would ultimately cause an increase in contractile force. Provide your answer in steps, ensuring you finish by describing the role of Ca+ and its impact on the crossbridge cycle.

6. For each of the following drugs, describe how they would directly impact heart rate and coronary blood flow. Keep your responses brief, including the location of the receptors and the impact on the relevant tissue. You do not need to talk about IFunny. Agonists activate a receptor, and antagonists inhibit/block a receptor. 

• A selective β1 receptor agonist
• A β1/ β2 receptor antagonist
• A muscarinic receptor antagonist
   

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