Excitable cells

Question 1

A healthy young man drinks a sports supplement containing glucose and creatine which advertised to “ehance 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?

Answer 1

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.

Question 2

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

Answer 2

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

Question 3

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

Answer 3

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

Question 4

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:

Answer 4

Secondary active transport (co-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)

Question 5

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

Answer 5

Opening of more K+ channels will hyperpolarize the membrane

Question 6

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

Answer 6

Ion channels allow transport of an ion against its electrochemical gradient -This is the incorrect statement. Ion channels are passive (facilitated diffusion) transporters so that ions can only flow down their electrochemical gradients.

Question 7

A sensory nerve cell in the skin of the finger is quiescent (not firing action potentials) with a steady resting membrane potential of -70mV. 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 sense of touch is perceived. In regard to this sensory nerve cell, which ONE of the following options is most likely to be CORRECT?

Answer 7

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

Question 8

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:

Answer 8

**polar water molecules giving rise to Cu2+ cations and SO4 2- anions

Question 9

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?

Answer 9

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

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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

Question 12

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:

Answer 12

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

Question 13

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

Answer 13

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

Question 14

The diagram below shows a schematic nerve cell bathed in intracellular and extracellular solutions at 37C and with the concentrations of Na+ and K+ as shown. 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 Nerst equation for ion x at 37C, can be simlified to: Ex=62mVlog₁₀([ion_outside]/[ion_inside].

Extracellular concentraions:

[K+]_outside=14mM

[Na+]_inside=140mM

Intracellular concentrations:

[K+]_outside=140mM

[Na+]_inside=14mM

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

Answer 14

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

Question 15

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 CORRECT?

Answer 15

The lipids within the cell membrane include phospholipids and shingsolipids

Question 16

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?

Answer 16

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

Question 17

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

Answer 17

Actelcholinesterase helps to terminate the response by metabolising the neurotransmitter

Question 18

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

Answer 18

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.

Question 19

A nerve cell contains intracellular K+ and Na+ concentrations of 140mM and 10mM, respectively. It is surrounded by an extracellular solution containing K+ and Na+ concentrations of 4mM and 150mM, 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=valnce, the nerve cell’s membrane potential can be calculated as:

Answer 19

** (RT)/(zF) multiplied by log_e(4mM/140mM)

• The cell is only permeable to K+, so the membrane potential will quickly go to the K+ equilibrium potential given by the Nernst equation.