Question 1

A DRUG BLOCKS VOLTAGE-GATED SODIUM CHANNELS IN A NEURON. WHAT HAPPENS TO ITS ACTION POTENTIALS?

Accepted Answer

THEY CANNOT FIRE. The rising phase depends on sodium rushing in through voltage-gated channels. With those channels blocked, the membrane cannot depolarize rapidly past threshold, so no action potential fires. Local anesthetics such as lidocaine work this way.

Question 2

A DRUG BLOCKS VOLTAGE-GATED POTASSIUM CHANNELS. WHICH PART OF THE ACTION POTENTIAL IS MOST SLOWED?

Accepted Answer

REPOLARIZATION. Repolarization is driven by potassium leaving through voltage-gated potassium channels. Blocking them slows the return to rest and widens the action potential.

Question 3

THE POTASSIUM CONCENTRATION OUTSIDE A NEURON RISES SHARPLY. WHAT HAPPENS TO ITS RESTING MEMBRANE POTENTIAL?

Accepted Answer

IT BECOMES LESS NEGATIVE. The resting potential sits near the potassium equilibrium potential. Raising outside potassium shrinks the gradient driving potassium out, so less positive charge leaves and the membrane depolarizes.

Question 4

A RESTING NEURON SITS NEAR -70 mV MAINLY BECAUSE ITS MEMBRANE LEAKS ___ IONS MORE THAN ANY OTHER ION.

Accepted Answer

POTASSIUM. At rest the membrane is far more permeable to potassium, which leaks out down its gradient, so the resting potential lies close to the potassium equilibrium potential (about -90 mV).

Question 5

ON A MYELINATED AXON, THE ACTION POTENTIAL SEEMS TO JUMP FROM ONE NODE TO THE NEXT. THIS IS ___ CONDUCTION.

Accepted Answer

SALTATORY. The signal is regenerated only at the gaps in the myelin, so it skips the insulated stretches. The name comes from the Latin for "to leap".

Question 6

A STRONGER-THAN-NORMAL STIMULUS ARRIVES DURING THE RELATIVE REFRACTORY PERIOD. WHAT HAPPENS?

Accepted Answer

A NEW ACTION POTENTIAL CAN FIRE. In the relative refractory period some sodium channels have recovered but the membrane is still hyperpolarized, so a larger stimulus can reach threshold. Only the absolute refractory period blocks firing completely.

Question 7

WHY DOES AN ACTION POTENTIAL NORMALLY TRAVEL IN ONLY ONE DIRECTION ALONG AN AXON?

Accepted Answer

THE MEMBRANE IT HAS JUST PASSED IS REFRACTORY. Behind the action potential the sodium channels are inactivated, so that patch of membrane cannot fire again straight away. Current spreading backwards therefore triggers nothing, and only the membrane ahead fires.

Question 8

MULTIPLE SCLEROSIS DESTROYS MYELIN IN THE BRAIN AND SPINAL CORD. WHAT HAPPENS TO SIGNALS IN THE AFFECTED AXONS?

Accepted Answer

CONDUCTION SLOWS OR FAILS. Without myelin, current leaks across the long internode membranes, which have few sodium channels. The signal weakens before reaching the next node, so conduction slows or is blocked.

Question 9

THE UNMYELINATED GAPS WHERE A MYELINATED AXON REGENERATES ITS ACTION POTENTIAL ARE THE NODES OF ___.

Accepted Answer

RANVIER. Voltage-gated sodium channels are concentrated at these gaps, so each one fires a fresh action potential.

Question 10

A STIMULUS JUST ABOVE THRESHOLD AND ONE FAR ABOVE IT GIVE THE SAME SIZE ACTION POTENTIAL. THIS IS THE ALL-OR-___ PRINCIPLE.

Accepted Answer

NONE. Once threshold is reached, the sodium-channel feedback loop runs to completion, so every action potential in a given neuron has the same size.

Question 11

A STIMULUS MOVES A NEURON'S MEMBRANE POTENTIAL FROM -70 mV TO -60 mV. ITS THRESHOLD IS -55 mV. WHAT HAPPENS?

Accepted Answer

NO ACTION POTENTIAL FIRES. The change is a graded potential that stops 5 mV short of threshold. Action potentials are all-or-none, so nothing fires and the membrane drifts back to rest.

Question 12

TWO INPUTS AT ONE SYNAPSE, EACH TOO WEAK TO REACH THRESHOLD ALONE, ARRIVE IN QUICK SUCCESSION AND TOGETHER TRIGGER AN ACTION POTENTIAL. WHAT IS THIS CALLED?

Accepted Answer

TEMPORAL SUMMATION. The second input arrives before the first has faded, so their depolarizations add up over time at the same place. Spatial summation adds inputs arriving at different synapses at once.

Question 13

A NEUROTRANSMITTER OPENS CHLORIDE CHANNELS ON A POSTSYNAPTIC NEURON, AND CHLORIDE FLOWS IN. WHAT IS THE EFFECT?

Accepted Answer

AN INHIBITORY POSTSYNAPTIC POTENTIAL. Negative ions entering make the inside more negative, moving the membrane away from threshold. GABA acting at GABA-A receptors works this way.

Question 14

A DRUG BLOCKS VOLTAGE-GATED CALCIUM CHANNELS IN THE AXON TERMINAL. WHAT FAILS?

Accepted Answer

NEUROTRANSMITTER RELEASE. When the action potential reaches the terminal, calcium entry triggers vesicles to fuse with the membrane and release transmitter. The axon can still conduct, but the signal is not passed on.

Question 15

A DISEASE STRIPS MYELIN FROM PERIPHERAL NERVES ONLY. WHICH CELLS MADE THE MYELIN IT DESTROYS?

Accepted Answer

SCHWANN CELLS. Schwann cells myelinate axons in the peripheral nervous system; oligodendrocytes do it in the brain and spinal cord. Guillain-Barré syndrome is an example of peripheral demyelination.

Question 16

MOST INCOMING SIGNALS REACH A NEURON THROUGH ITS BRANCHED ___, WHICH CARRY GRADED POTENTIALS TOWARD THE CELL BODY.

Accepted Answer

DENDRITES. Dendrites receive synaptic input and pass the resulting graded potentials to the soma and axon hillock, where they are summed.

Neurons and Action Potentials