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Flashcards in Excitable Cells Deck (19)
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A healthy young man drinks a sports supplement containing glucose and creatine which is advertised to “enhance muscle endurance by increasing intracellular ATP and Na-pump activity”. Assuming this “Na-pump” refers to the Na /K /ATPase, which ONE of the
following options is MOST CORRECT?


An increase in Na /K /ATPase activity would be expected to lead to a decrease in the intracellular free Ca2+ concentration due to an increased activity of a secondary active transporter that exchanges Na influx for Ca2+ efflux

An increase in Na /K /ATPase activity should result in increased pumping out of Na thus generating a greater electrochemical force for Na / Ca exchange. In fact, blocking the Na /K /ATPase can be used to increase intracellular Ca and aid cardiac muscle contraction in congestive heart disease.


A nerve cell contains intracellular K+ and Na+ concentrations of 140 mM and 10 mM, respectively. It is surrounded by an extracellular solution containing K+ and Na+ concentrations of 4 mM and 150 mM, respectively. The only open ion channels in the
nerve cell are selective for K+ . Given that R= the Gas constant, T = temperature in Kelvins, F = Faraday’s constant and z = valence, the nerve cell’s membrane potential can be calculated as:


(RT) / (zF) multiplied by loge (4 mM / 140 mM)


A sensory nerve cell in the skin of the finger is quiescent (not firing action potentials) with a steady resting membrane potential of -70 mV. This nerve cell detects when the finger touches something, responding to touch by firing a burst of action potentials. This signal is then transmitted to the somatosensory cortex in the brain where the sense of touch is perceived. In regard to this sensory nerve cell, which ONE of the following options is most
likely to be CORRECT?


The touch causes the membrane potential of this sensory nerve cell to cross the threshold for activation of voltage-dependent Na+ and K+ channels.

The fact that the nerve fired an action potential in response to touch indicates that it must have been depolarized beyond the threshold for activation of
the voltage-dependent Na and K channels


Consider a skeletal muscle or nerve cell that is bathed in an extracellular and intracellular solution containing the same concentration of a cation, i.e. the cation concentration inside the cell = its concentration outside the cell. Regarding the electrochemical forces on this
cation, which ONE of the following options is CORRECT?


When the membrane potential of the cell is +30 mV, an electrochemical force acts on the ion to favour cation efflux.

There is an electrochemical force exerted on the cation which acts to push the cation out, ie efflux.


Which ONE of the following statements about ion channels is INCORRECT?


Ion channels allow transport of an ion against its electrochemical gradient.

Ion channels are passive (facilitated diffusion)
transporters so that ions can only flow down their electrochemical gradients.


Consider you work in a health / sports clinic and a client comes to discuss with you their symptoms of muscle weakness. Their neurologist has diagnosed that the motor nerve EMG is normal and that the muscle weakness results from an abnormal sequence of events occurring at the neuromuscular junction. Which ONE of the following options MOST CORRECTLY lists the sequence of events that occur at the neuromuscular junction, and in the correct order:


Activation of nicotinic acetylcholine receptors, generation of an end-plate potential, enzymatic breakdown of the neurotransmitter.


Which ONE of the following statements regarding action potential propagation in nerve cells is CORRECT?


Action potentials are initiated in the axon hillock and can then propagate both forward towards the axon terminal and backwards into the nerve dendrites.

Action potentials start in a specialized region of the axon close to the soma called the axon hillock, and can propagate towards the nerve terminal and also back into the nerve dendrites (“back propagation”).


Which ONE of the following statements regarding communication between cells of the body is CORRECT?


At the mammalian neuromuscular junction, enzymatic breakdown plays a major role in terminating the actions of acetylcholine on its receptors.

The enzyme acetylcholinesterase breaks down acetylcholine and is the main mechanism by which the neurotransmitter’s actions are terminated (there is also some lateral diffusion away from the synapse).


At the neuromuscular junction, the motor nerve communicates with the skeletal muscle by releasing a chemical neurotransmitter. In regard to this neuromuscular transmission which ONE of the following typically occurs?


Acteylcholinesterase helps to terminate the response by metabolising the neurotransmitter


The schematic diagram below shows a model of epithelial transport in the kidney proximal tubule. The cell is at rest with typical ion concentrations in the extracellular and intracellular solutions. The membrane transport protein labelled “X”, which transports Na+ ,
K+ and Cl- into the cell as shown, is an example of a:


Secondary active transporter (co-transporter).

The transporter uses a Na gradient set up by active transport to transport K ions against their electrochemical gradient. Na and K are transported in the same direction (hence co-transport)


Consider an experiment where a small amount of blue-coloured copper sulphate (CuSO4) salt is added to a mixture of water and paraffin oil and the solution is shaken so that the molecules making up the salt become hydrated by some of the other molecules in this solution. Complete the sentence with ONE of the following options: The CuSO4 becomes hydrated by:


Polar water molecules giving rise to Cu2+ cations and SO4 2- anions.


Which ONE of the following statements regarding membrane transport processes is CORRECT?


A K+ ion can only move through an ion channel in a direction that is down its electrochemical gradient

An ion channel is an example of facilitated diffusion, so the substance always moves down its chemical gradient (if solute is uncharged) or electrochemical gradient (if solute is charged).


If the K+ equilibrium (Nernst) potential is -90 mV and a nerve cell has a resting membrane potential of -60 mV, then:


Opening of more K channels will hyperpolarize the membrane.

More K leaves and Vm tends towards the Nernst potential for K .


Which ONE of the following statements regarding electrical signals in excitable cells is CORRECT?


The plasma membrane can be represented electrically as a resistor and capacitor in parallel.

The lipid bilayer gives rise to the capacitive component while the ion channels give rise to the resistive component.


If a substance is moving from inside a cell to outside via simple diffusion then which ONE of the following is CORRECT?


Its transport rate is directly proportional to the substance’s lipid solubility (partition co-efficient) -> Fick’s Law


The cell membrane is composed of a lipid bilayer with membrane proteins embedded within. In regards to the cell membrane, which ONE of the following statements is


The lipids within the cell membrane include phospholipids and sphingolipids


The diagram below shows a typical Action Potential in a nerve cell in the brain. In regards to this action potential, which ONE of the following is MOST CORRECT?


The duration of the action potential is about 1-3 milliseconds, with an amplitude of about 100 mV.


Choose the MOST CORRECT option. With regard to the ion channels underlying the action potential in a typical nerve axon:


Voltage-dependent K+ channels are open during the repolarization phase.

Efflux of K through these open channels is what causes the repolarization.


The cell membrane contains a population of K+ -selective ion channels and a separate population
of Na+ -selective ion channels as indicated.
An electrode is inserted into the cell that measures the membrane potential (Vm).
The Nernst equation for ion x at 37°C, can be simplified to: Ex = 62 mV log10 [ion_outside] / [ion_inside].
Extracellular concentrations:
[K+]outside = 14 mM
[Na+]inside = 140 mM
Intracellular concentrations:
[K+]outside = 140 mM
[Na+]inside = 14 mM

If initially (at rest) only the K+ channels are open, and then the Na+ channels also open, which ONE of the following options MOST CLOSELY describes the membrane potential (Vm):


Resting Vm = -62 mV, when the Na+ channels open, Vm becomes depolarized