What causes a nerve signal to move from one end of a neuron along the length of the neuron to the other end? What is a nerve signal, exactly? Why can't it go backward? How is a nerve signal transmitted from one neuron to the next across a synapse? Write a short paragraph that answers these questions.

Which of these statements about myelination in neurons is/are correct? Select True or False for each statement.

T/FIt speeds propagation by increasing the density of voltage-gated channels all along the axon.

T/FMultiple sclerosis is characterized by disrupted myelination of certain neurons in the central nervous system.

T/FIt speeds propagation by preventing cations from leaking out across the membrane as they spread down the axon.

T/FIt is more commonly observed in vertebrates than in invertebrates.

Where are neurotransmitter receptors located?

a. The nuclear membrane

b. The nodes of Ranvier

c. The postsynaptic membrane

d. Synaptic vesicle membranes

In a neuron, what creates the electrochemical gradient favoring the outflow of K⁺ when the cell is at rest?

a. Na⁺/K⁺-ATPase

b. Voltage-gated K⁺ channels

c. Voltage-gated Na⁺ channels

d. Ligand-gated Na⁺/K⁺ channels

Why are action potentials usually conducted in one direction?

a. Ions can flow along the axon in only one direction

b. The brief refractory period prevents the reopening of voltage-gated Na⁺ channels

c. The axon hillock has a higher membrane potential than the terminals of the axon

d. Voltage-gated channels for both Na⁺ and K⁺ open in only one direction.

Which of the following is the most direct result of depolarizing the presynaptic membrane of an axon terminal?

a. Voltage-gated calcium channels in the membrane open

b. Synaptic vesicles fuse with the membrane

c. Ligand-gated channels open, allowing neurotransmitters to enter the synaptic cleft

d. An EPSP or IPSP is generated in the postsynaptic cell

Explain the difference between a ligand-gated K⁺ channel and a voltage-gated K⁺ channel.

Suppose a particular neurotransmitter causes an IPSP in postsynaptic cell X and an EPSP in postsynaptic cell Y. A likely explanation is that

a. The threshold value in the postsynaptic membrane is different for cell X and cell Y

b. The axon of cell X is myelinated, but that of cell Y is not

c. Only cell Y produces an enzyme that terminates the activity of the neurotransmitter

d. Cells X and Y express different receptor molecules for this particular neurotransmitter

Describe the role of summation in postsynaptic cells.