A patient is complaining of recent onset of muscle weakness in her right hand an

Question

A patient is complaining of recent onset of muscle weakness in her right hand and arm that gets progressively worse throughout the day. Her right eyelid and the right side of her mouth also are drooping (ie. when she tries to smile) and her speech is slurred.

First a blood test is run to determine plasma electrolyte concentrations. Results show normal electrolyte concentrations.

Explain what the effect would be on EK and on resting Vm if the ECF [K+] level was abnormally low? Explain how and why this change would affect the likelihood of AP firing? Be clear as to the direction of the equilibrium potential and membrane potential changes compared to normal. (3 pt, 100 words)

Explain what the effect would be on ENa and on resting Vm if the ECF [Na+] level was abnormally high? Explain how and why this change would affect the likelihood of AP firing? Be clear as to the direction of the equilibrium potential and membrane potential change compared to normal. (3 pt, 100 words)

Explanation / Answer

1)

If the extra cellular fluid (ECF) potassium [K+] level was abnormally low, then the Vm (equilibrium potential) has negative value and attains resting potential and further decrease of ECF [K+] results depolarization of membrane.

For example, if the ECF [K+] is 2 mM, then the Vm is -133mV, at ECF [K+] 10 mM then the Vm is -92mV, at ECF [K+] 20 mM then the Vm is -75mV and at ECF [K+] 200 mM then the Vm is -17mV. Thus, as ECF [K+] level the Vm value decreases.

The Ek is calculated by using the formula: EK= RT/zF ×ln [KOut] / [KIn]

Here, the RT/zF = 25mV at 25oC, hence

EK= 25×ln [KOut] / [KIn] -------- EK= 58×log[KOut] / [KIn]

The resting membrane potential is attained when concentration gradient and selective permeability for potassium ions increases across the membrane. At this condition the EK+ becomes undershot. The direction action potential is depolarization, follows increase in g(K+), K+ outflux and then membrane becomes repolarized.

2)

If the extra cellular fluid (ECF) Sodium [Na+] level was abnormally low at resting Vm, then resting Vm shifted towards the depolarization of membrane and then repolarizes by increasing ICF of [Na+] ions.

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