2005 exam 1

Question 0

The following cells are responsible for myelin formation in the nervous system.

Answer 0

Schwann cells and oligodendrocytes

• Conduction velocity of APs… myelination increases it and is reduced in MS

Question 1

The movement of the following ion(s) down its/their concentration gradient during a graded potential would make a neuron more likely to fire an action potential?

Answer 1

Na+ and Ca++

Question 2

Which of the following statements is/are true?

Answer 2

:: The rising phase of the action potential is due to increase cell permeability to Na+

:: The falling phase of the action potential is due to increase cell permeability to K+

Question 3

At which phase of the action potential do the inactivation gates of the sodium channel being to close?

Answer 3

At the peak of the action potential.

Question 4

The following statement(s) are true regarding the absolute and relative refractory periods.

Answer 4

:: During the absolute refractory period, no action potential can be triggered no matter how large the stimulus

:: During the relative refractory period, a stronger than normal stimulus is needed to trigger an action potential

:: The absolute refractory period ensures that action potentials travel in one direction from the point of stimulus

• The absolute refractory period parallels the time course for Na+ activation/deactivation and lasts about 1 msec in an axon.
• The relative refractory period parallels the time course for changes in gK+ and lasts about 3-4 msec in an axon.

Question 5

Nerve conduction along myelinated fibers is faster than along unmyelinated fibers because:

Answer 5

Saltatory conduction requires Na+ channel opening only at nodes resulting in faster conduction than when channels must open along the entire length of the axon.

Question 6

Which ion is the primary determinant of the resting membrane potential in neurons?

Answer 6

K+ (The high K+ conducatance is primarily responsible for the RMP).

Question 7

Assuming the intracellular concentration of K+ remains constant, under what conditions would a graded potential be most likely to trigger an action potential?

Answer 7

Hyperkalemia [K+]o = 7 mM, Ek = -80 mV (caused by kidney failure; depolarizes the membrane, making it easier for heart cells to initiate an action potential).

• Hypokalemia [K+]o = 2 mM, Ek = -109 mV (may be caused by diuretics [drugs that increase urine output]; hyperpolarizes the membrane and makes it more difficult for heart cells to initiate an action potential).
• Both hyperkalemia and hypokalemia cause cardiac arrythmias and cardiac arrest.

Question 8

Tetanus and botulinum toxin (Ach) exert their action by:

Answer 8

Inhibiting specific proteins of the presynaptic exocytotic apparatus.

Question 9

Which maternal nutrient has been found to significantly reduce the incidence of neural tube defects?

Answer 9

Folic acid

Question 10

The after-hyperpolarization phase of the action potential is due to:

Answer 10

K+ efflux through slow voltage-gated K+ channels.

Question 11

A student wants to experiment on a neuron where the equilibrium potential for Cl- is -61 mV. The intracellular concentration of Cl- is 10 mM. What should the concentration of Cl- be in the extracellular solution?

Answer 11

100 mM

Question 12

The following statement is FALSE regarding graded potentials:

Answer 12

None are false. These are all true:

:: They occur when ion channels open or close causing ions to enter or leave the neuron

:: They can be inhibitory or excitatory

:: They lose strength as they travel through the cytoplasm due to current leak or resistance to current flow

Question 13

Glycine is an inhibitory neurotransmitter in the CNS which acts by increasing neuronal membrane permeability to:

Answer 13

Cl-

Question 14

A neuron has a resting membrane potential of -70 mV and a threshold potential of -55 mV. It has input from both excitatory and inhibitory presynaptic neurons. Which of the following combination of stimuli would be most likely to result in a postsynaptic action potential?

Answer 14

1 IPSP of 10 mV and 1 EPSP of 30 mV

Question 15

A patient is given 2L of half-normal saline which has an osmolarity of 154 mosm/L. Which of the following statement is NOT true?

Answer 15

:: Total body water will increase by 2L

:: The osmolarity of the extracellular fluid will decrease

:: Assuming NACL is a non-penetrating solute, water will be forced from the intracellular space to the extracellular space

:: At equilibrium, the osmolarity of the intracellular and extracellular spaces will be the same

Question 16

The following statements refer to graded potentials and action potentials. All are TRUE except:

Answer 16

Graded potentials require a threshold stimulus whereas action potentials require no minimum level to initiate.

True:

• Graded potentials usually occur at dendrites or at the cell body whereas action potentials occur at the trigger zone and axon
• Graded potentials can be summed whereas action potentials cannot be summed
• Graded potentials can be depolarizing or hyperpolarizing whereas action potentials are always depolarizing

Question 17

If you put oubain, a specific inhibitor of the Na+/K+ pump, on a neuron and stimulate it repeatedly, what do you expect to happen to action potentials generated by that neuron?

Answer 17

There is no immediate effect but they diminish over time and eventually disappear.

Question 18

An electrode is placed in the middle of an axon of a resting neuron and a suprathreshold stimulus is applied. Which of the following statements is FALSE?

Answer 18

Current will flow only in the direction away from the cell body

True:

• An action potential will travel in both directions from the point of stimulus
• The first portion of the axon to achieve an absolutely refractory period of at the point of stimulus
• During the relative refractory period the same stimulus could trigger an action potential

Question 19

The resting membrane potential for a neuron is -70 mV but the equilibrium potential for K+ is -90 mV. All of the following statements are TRUE except:

Answer 19

The resting membrane potential strongly favors Na+ efflux from the cell.

These are true:

• At -70 mV, K+ will continuously leak out of the cell.
• If the resting membrane potential is hyperpolarized by a graded potential, K+ efflux will decrease
• The equilibirum potential for K+ is the primary determinant of the resting membrane potential

Question 20

How does the position of the gates of the Na+ channel explain the origin of the absolute and relative refractory periods in a neuron which has fired an action potential?

Answer 20

All are true:

:: During absolutely refractory period all of the inactivation gates are closed

:: During the relative refractory period some of the inactivation gates are closed

:: The total refractory period (absolute and relative) will terminate when both gates are reset to the normal position

Question 21

Amyotrophic lateral sclerosis (Lou Gehrig’s Disease) is a progressive paralytic illness caused by:

Answer 21

Death of motor neurons

Question 22

Botulinum toxin and tetanus exert their effects through similar mechanisms; yet, botulinum toxin causes flaccid paralysis while tetanus toxin causes spastic paralysis. How can this be best explained?

Answer 22

Botulinum toxin blocks the release of the excitatory neurotransmitter Ach whereas tetanus toxin blocks release of the inhibitory neurotransmitter GABA.

Question 23

The maturation arrest theory as it pertains to the generation of malignant gliomas proposes that

Answer 23

Malignant gliomas arise from glial progenitor cells which have not yet transformed into mature glial cells.

Question 24

The graph below was plotted during an experiment with a giant squid axon. What conclusions can you draw from interpreting this graph?

Answer 24

As the extracellular concentration of K+ increases, the resting membrane potential difference decreases.